messenger mercury orbit insertion press kit

8/7/2019 Messenger Mercury Orbit Insertion Press Kit

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A NASA Discovery Mission

Mercury Orbit InsertionMarch 18, 2011 UTC

(March 17, 2011 EDT)


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Media Contacts

NASA Headquarters

Policy/Program ManagementDwayne C. Brown(202) [email protected]

The Johns Hopkins University Applied Physics LaboratoryMission Management, Spacecraft OperationsPaulette W. Campbell(240) 228-6792 or (443) [email protected]

Carnegie Institution of WashingtonPrincipal Investigator InstitutionTina McDowell(202) [email protected]


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Mission OverviewMESSENGER is a scientic investigation

o the planet Mercury. UnderstandingMercury, and the orces that have shapedit, is undamental to understanding the

terrestrial planets and their evolution.

The MESSENGER (MErcury Surace, Space

ENvironment, GEochemistry, and Ranging)spacecrat will orbit Mercury ollowing three

fybys o that planet. The orbital phase willuse the fyby data as an initial guide to

perorm a ocused scientic investigation othis enigmatic world.

MESSENGER will investigate keyscientic questions regarding Mercurys

characteristics and environment duringthese two complementary mission phases.

Data are provided by an optimized set ominiaturized space instruments and the

spacecrat telecommunications system.

MESSENGER will enter orbit about Mercuryin March 2011 and carry out comprehensive

measurements or one Earth year. Orbitaldata collection concludes in March 2012.

Key Spacecrat Characteristics

Redundant major systems provide critical backup.

Passive thermal design utilizing ceramic-clothsunshade requires no high-temperature electronics.

Fixed phased-array antennas replace a deployable

high-gain antenna.

Custom solar arrays produce power at sae operating

temperatures near Mercury.

MESSENGER is designed to answer sixbroad scientifc questions:

Why is Mercury so dense?

What is the geologic history o Mercury?

What is the nature o Mercurys magnetic eld?

What is the structure o Mercurys core?

What are the unusual materials at Mercurys poles?

What volatiles are important at Mercury?

MESSENGER provides:

Multiple fybys or global mapping, detailed study o high-priority

targets, and probing o the atmosphere and magnetosphere.

An orbiter or detailed characterization o the surace,

interior, atmosphere, and magnetosphere.

An education and public outreach program to produce

exhibits, plain-language books, educational modules, andteacher training through partnerships.

Mission Summary

Launch: 3 August 2004

Launch vehicle: Delta II 7925H-9.5

Earth fyby: 2 August 2005

Venus fybys (2): 24 October 2006,

5 June 2007

Mercury fybys (3): 14 January 2008,

6 October 2008,

29 September 2009

Mercury orbit insertion: 17 March 2011 (EDT)

18 March 2001 (UTC)


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MESSENGER Mercury, the Last Frontier of the Terrestrial Planets

Understanding Mercury is fundamental to understanding terrestrial planet evolution.

Discoveries from MESSENGERs Mercury Flybys:

In addition to providing key gravity assists that enable orbit insertion as well as opportunities to test scientic operations and

command sequences or all payload instruments, MESSENGERs three fybys o Mercury yielded a number o discoveries that have

markedly changed our view o Mercury and infuenced our preparations or orbital operations. These include:

Geology

VolcanismwaswidespreadonMercuryandextendedfrom

beore the end o heavy bombardment to the second hal

o solar system history.

Mercuryexperiencedexplosivevolcanism,indicatingthat

interior volatile contents were at least locally much higher

than thought.

ContractionspannedmuchofMercurysgeologichistory.

Composition and surface-derived exosphere

Mercuryssurfacesilicates,eveninfreshcraterejecta,

contain little or no errous oxide.

Mercurysthermalneutronuxmatchesthatofseveral

lunar maria, indicating that iron and titanium are present in

comparable collective abundances, perhaps as oxides.

MagnesiumandionizedcalciumarepresentinMercurys

exosphere.

Internal structure and dynamics

TheequatorialtopographicreliefofMercury,inagreement

with earlier radar results, is at least 5.5 km.

ThecaseforaliquidoutercoreinMercuryis

greatly strengthened.

Mercurysinternalmagneticeldisdominantlydipolarwith a vector moment closely aligned with the spin axis.

Magnetospheric dynamics

Mercurysmagnetosphereismoreresponsiveto

interplanetary magnetic eld (IMF) fuctuations than those

o other planets.

UndersouthwardIMF,ratesofmagneticreconnection

are ~10 times that typical at Earth.

Loading o magnetic fux in Mercurys magnetic tail can

be so intense that much o Mercurys dayside could be

exposed to the shocked solar wind o the magnetosheath

during such episodes.

On the Web

MESSENGER mission: http://messenger.jhuapl.edu NASA Discovery Program: http://discovery.nasa.gov

FS2 03-11

Mission ManagementPrincipal Investigator: Sean C. Solomon,

Carnegie Institution o Washington

Project Management: The Johns Hopkins University

Applied Physics Laboratory (JHU/APL)

Spacecraft Integration

and Operation: JHU/APL

Instruments: JHU/APL,

NASA Goddard Space Flight Center,

University o Colorado,

University o Michigan

Structure: Composite Optics, Inc.

Propulsion: GenCorp Aerojet

Navigation: KinetX, Inc.

Science Payload

Mercury Dual Imaging System

(MDIS) takes detailed color and

monochrome images o Mercurys

surace.

Gamma-Ray and Neutron

Spectrometer (GRNS) measures

surace elements (including polar

materials).

X-Ray Spectrometer (XRS) maps

elements in Mercurys crust.

Magnetometer (MAG) maps

Mercurys magnetic eld.

Mercury Atmospheric

and Surface Composition

Spectrometer (MASCS)

measures atmospheric

species and surace minerals.

Energetic Particle and

Plasma Spectrometer (EPPS)

measures charged particles in

Mercurys magnetosphere.

Mercury Laser Altimeter (MLA)

measures topography o surace

eatures; determines whether

Mercury has a fuid core.

Radio Science uses Doppler

tracking to determine Mercurys

mass distribution.

Sunshade

EPPS

GRNS

MLA

MASCS

XRSMDIS

MAG

Antenna


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NASAs Mission to Mercury

Table o Contents

Media Services Inormation ............................................................................................................. 7

MESSENGER Quick Facts .................................................................................................................. 8Mercury Orbit Insertion & Station Keeping .................................................................................... 9

Getting into Mercury orbit. ............................................................................................................. 9

.and staying there .........................................................................................................................12

Science orbit: Working at Mercury .................................................................................................... 12

Observing the surace ....................................................................................................................... 13

Orchestrating the observations ......................................................................................................... 14

Mercury at a Glance ........................................................................................................................ 17

Why Mercury? ................................................................................................................................. 18

Question 1: Why is Mercury so dense? ............................................................................................. 18

Question 2: What is the geologic history o Mercury? .......................................................................19

Question 3: What is the nature o Mercurys magnetic eld?............................................................. 20

Question 4: What is the structure o Mercurys core? ........................................................................ 21

Question 5: What are the unusual materials at Mercurys poles? ....................................................... 21

Question 6: What volatiles are important at Mercury?....................................................................... 23

Highlights rom the Mercury Flybys .............................................................................................. 24

The Spacecrat ................................................................................................................................. 41

Science payload ................................................................................................................................ 42

Spacecrat systems and components.................................................................................................46

Hardware suppliers ........................................................................................................................... 49

Mission Summary ............................................................................................................................ 50

Cruise trajectory ............................................................................................................................... 50

Launch ............................................................................................................................................. 52

Earth fyby highlights ........................................................................................................................52

Venus gravity assists .........................................................................................................................55

Flying by Mercury ............................................................................................................................. 58

MESSENGERs deep-space maneuvers ...............................................................................................60

The MESSENGER Science Team ....................................................................................................... 61

Program/Project Management ....................................................................................................... 62

NASA Discovery Program ..................................................................................................................62Other Discovery missions .................................................................................................................. 62


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Media Services Inormation

News and Status Reports

NASA and the MESSENGER team will issue periodic news releases and status reports on mission activitiesand make them available online at http://messenger.jhuapl.eduand http://www.nasa.gov/messenger.

When events and science results merit, the team will hold media briengs at NASA Headquarters in Washington,D.C., or the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Briengs will be carried on NASA TVand the NASA website.

NASA TelevisionNASA Television is carried on the Web and on an MPEG-2 digital signal accessed via satellite AMC-6, at 72 degrees

west longitude, transponder 17C, 4040 MHz, vertical polarization. It is available in Alaska and Hawaii on AMC-7, at137 degrees west longitude, transponder 18C, at 4060 MHz, horizontal polarization. A Digital Video Broadcast-compliant Integrated Receiver Decoder is required or reception. For NASA TV inormation and schedules on the Web,

visit http://www.nasa.gov/ntv.

MESSENGER on the WebMESSENGER inormation including an electronic copy o this press kit, press releases, act sheets, mission

details and background, status reports, and images is available on the Web at http://messenger.jhuapl.edu.MESSENGER multimedia les, background inormation, and news are also available at http://www.nasa.gov/messenger.


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MESSENGER Quick Facts

Spacecrat

Size: Main spacecrat body is 1.44 meters (57 inches) tall, 1.28 meters (50 inches) wide, and 1.85 meters (73 inches)deep; a ront-mounted ceramic-abric sunshade is 2.54 meters tall and 1.82 meters across (100 inches by 72 inches);two rotatable solar panel wings extend about 6.14 meters (20 eet) rom end to end across the spacecrat.

Launch weight: Approximately 1,107 kilograms (2,441 pounds), including 599.4 kilograms (1,321 pounds) opropellant and 507.6 kilograms (1,119 pounds) o dry spacecrat and instruments.

Power: Two body-mounted gallium arsenide solar panels and one nickel-hydrogen battery. The power system generatedabout 490 watts near Earth and will generate its maximum possible output o 720 watts in Mercury orbit.

Propulsion: Dual-mode system with one bipropellant (hydrazine and nitrogen tetroxide) thruster or large maneuvers;4 medium-sized and 12 small hydrazine monopropellant thrusters or small trajectory adjustments and attitude control.

Science investigations: Mercury Dual Imaging System (MDIS), with wide-angle color and narrow-angle monochrome

imagers; the Gamma-Ray and Neutron Spectrometer (GRNS); the X-Ray Spectrometer (XRS); the Magnetometer (MAG);the Mercury Laser Altimeter (MLA); the Mercury Atmospheric and Surace Composition Spectrometer (MASCS); theEnergetic Particle and Plasma Spectrometer (EPPS); and the radio science experiment.

MissionLaunch: August 3, 2004, rom Pad B o Space Launch Complex 17 at Cape Canaveral Air Force Station, Fla., at 2:15:56a.m. EDT aboard a three-stage Boeing Delta II rocket (Delta II 7925H-9.5).

Gravity assist ybys: Earth (1) August 2005; Venus (2) October 2006, June 2007; Mercury (3) January 2008, October2008, September 2009.

Enter Mercury orbit: March 18, 2011 UTC (March 17, 2011 EDT).

Total distance traveled rom Earth to Mercury orbit: 7.9 billion kilometers (4.9 billion miles). Spacecrat circles theSun 15.2 times rom launch to Mercury orbit insertion.

Primary mission at Mercury: Orbit or one Earth year (equivalent to just over our Mercury years, or two Mercurysolar days), collecting data on the composition and structure o Mercurys crust, its topography and geologic history, thenature o its thin atmosphere and active magnetosphere, and the makeup o its core and polar materials.

ProgramCost: Approximately $446 million (including spacecrat and instrument development, launch vehicle, mission operationsand data analysis).


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Mercury Orbit Insertion & Station Keeping

Getting into Mercury orbit.

On March 18, 2011 UTC (March 17, 2011 EDT), ater almost ve years in development and more than six and ahal years in cruise toward its destination, NASAs MErcury Surace, Space ENvironment, GEochemistry, and Ranging(MESSENGER) spacecrat will execute a 15-minute maneuver that will place it into orbit about Mercury, making it therst crat ever to do so, and initiating a one-year science campaign to understand the innermost planet. The MercuryOrbit Insertion maneuver and subsequent orbital activities are described in the next ew pages.

Just over 33 hours beore the main Mercury orbit insertion event, two antennas rom the Deep Space Network one main antenna and one backup will begin to track the MESSENGER spacecrat continuously. Nearly thirty-onehours later, at 6:30 p.m. EDT on March 17, 2011, the number o antennas tracking MESSENGER will increase to ve our o these are arrayed together in order to enhance the signal coming rom the spacecrat, and a th will be used orbackup.

About two and a hal hours later, at 8:00 p.m. EDT, the solar arrays, telecommunications, attitude control, and

autonomy systems will all be congured or the main thruster ring (known as a burn), and the spacecrat will beturned into the correct orientation or MESSENGERs Mercury orbit insertion maneuver.

In order to slow the spacecrat down suciently so that it can be captured into orbit around Mercury, the mainthruster will begin ring at 8:45 p.m. and will continue or 15 minutes until 9:00 p.m. About 31% o the spacecratsoriginal allotment o propellant is required or Mercury orbit insertion, and MESSENGERs thrusters must slow thespacecrat by just over 0.86 kilometers (0.53 miles) per second. As the spacecrat approaches Mercury, the largestthruster must re close to the orward velocity direction o the spacecrat. Ater the thruster has nished ring, thespacecrat will be turned toward Earth and recongured or normal post-maneuver operations. Data will be collected byDeep Space Network antennas and transerred to the Mission Operations Center at APL to be analyzed. It is expectedthat by 10:00 p.m. EDT the Mission Operations Team will be able to conrm that MESSENGER has been successullycaptured into orbit around Mercury.

Approximately one and a hal hours ater the maneuver is complete, the DSN coverage will be stepped back to twostations. At 2:47 a.m. EDT on March 18, the spacecrat will begin its rst ull orbit around Mercury (as measured romthe highest point in the orbit). About 10 hours later, the Deep Space Network coverage will be urther reduced tocontinuous coverage with only one station.

The MESSENGER spacecrat will continue to orbit Mercury once every twelve hours or the duration o its primarymission. The rst two weeks rom orbit insertion will be ocused on ensuring that the spacecrat systems are all workingwell in the harsh thermal environment o orbit; this interval is known as the orbital commissioning phase. Starting onMarch 23 the instruments will be turned on and checked out, and on April 4 the science phase o the mission will beginand the rst orbital science data rom Mercury will be returned.

The table on the next page summarizes the spacecrat events surrounding Mercury orbit insertion. Note that thetimes given in the rst column are ground receipt times, which are approximately 9 minutes ater a maneuver isexecuted on the spacecrat.


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Ground Receipt Time* Spacecrat Time

EventEastern Daylight Time (EDT)

CoordinatedUniversal Time (UTC)

{DOY-hh:mm}

Time Relativeto Burn Start

{hh:mm}

TuesdayMarch 15 8:54 p.m. 75-00:45 48:00 Start initial pre-burn propulsion systemconguration

WednesdayMarch 16

11:40 a.m. 75-15:31 33:14Start critical Deep Space Network coverage(two stations, one primary and one backup)

8:54 p.m. 76-00:45 24:00Spacecrat commanded to pre-critical burn

conguration

ThursdayMarch 17

5:00 p.m. 76-20:51 03:54Start conguration or DSN burn coverage

(our stations arrayed together)

6:30 p.m. 76-22:21 02:24Finish conguration or DSN burn coverage

(backup 70-m antenna)

7:45 p.m. 76-23:37 01:09 Start nal pre-burn propulsion system conguration

8:09 p.m. 77-00:00 00:45 Start RF conguration or burn execution

8:21 p.m. 77-00:12 00:33 Complete RF conguration or burn execution

8:24 p.m. 77-00:15 00:30Turn spacecrat to burn attitude and congure

attitude control or burn execution

8:34 p.m. 77-00:25 00:20 Congure solar arrays or burn execution

8:49 p.m. 77-00:40 00:05Congure spacecrat ault protection or

burn execution

8:54 p.m. 77-00:45 00:00Mercury orbit insertion (MOI)

engine ignition

9:09 p.m. 77-01:00 00:15 Engine shutdown

9:21 p.m. 77-01:12 00:27Turn spacecrat to Earth and acquire

post-maneuver data

9:32 p.m. 77-01:23 00:38Re-congure spacecrat systems or normal

post-maneuver operations

10:25 p.m. 77-02:16 01:31End DSN burn coverage

(back to critical coverage with 2 stations)

FridayMarch 18

2:56 a.m. 77-06:47 06:02First orbital apoapse passage

(start orbit #1)

12:40 p.m. 77-16:31 15:46End DSN critical coverage

(back to 1 station continuous coverage)

2:57 p.m. 77-18:48 18:03Second orbital apoapse passage

(start orbit #2)

MondayMarch 21

12:56 p.m. 80-16:48Orbital commissioning period begins

(spacecrat checkout)

TuesdayMarch 22 Start Gamma-Ray Spectrometer (GRS) cooler

WednesdayMarch 23

Turn on all instruments and congure or operations(except imagers)

MondayMarch 28

3:51 p.m. 087-19:45Continue orbital commissioning period

(Instrument checkout imagers turned on)

MondayApril 4

4:20 p.m. 094-20:15 Mercury science observations begin

*Ground Receipt Time adjusted or one-way light time, which gradually decreases through the reporting period.

-- Events without specic execution times are initiated by direct commands rom the ground.


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Three views o MESSENGER's insertion into orbit about Mercury are shown above; they include a view romthe direction o Earth, a view rom the direction o the Sun, and a view rom over Mercurys north polelooking down toward the planet. Time is given in Coordinated Universal Time (UTC). The 15-minute orbitalinsertion maneuver is shown in light blue in the gures and places the spacecrat into the primary scienceorbit, which is shown in dark blue. The bright areas near the poles indicate portions o the surace notimaged by either Mariner 10 or MESSENGER during their respective fybys.


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.and staying thereAter MESSENGER arrives in its primary science orbit, small orces, such as solar gravity the gravitational attraction

o the Sun slowly change the spacecrats orbit. Although these small orces have little eect on MESSENGERs12-hour orbit period, they can increase the spacecrats minimum altitude, orbit inclination, and latitude o the surace

point below MESSENGERs minimum altitude. Let uncorrected, the increase in the spacecrats minimum altitude wouldprevent satisactory completion o several science goals.

To keep the spacecrats minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur atleast once every Mercury year one complete revolution around the Sun, or 88 Earth days. The rst, third, and thmaneuvers ater Mercury orbit insertion will occur at the arthest orbital distance rom Mercury, where a minimumamount o propellant will be used to slow the spacecrat just enough to lower the minimum altitude to 200 kilometers(124 miles). The act o lowering the spacecrats altitude in this way has an unavoidable side eect o also lowering orbitperiod by about 15 minutes.

The second and ourth maneuvers ater orbit insertion will increase the orbit period back to about 12 hours byspeeding up the spacecrat around the time when it is closest to Mercury. Because the sunshade must protect the mainpart o the spacecrat rom direct sunlight during propulsive maneuvers, the timing o these maneuvers is limited to a

ew days when Mercury is either near the same point in its orbit as it was during Mercury orbit insertion, or near thepoint where Mercury is on the opposite side o the Sun rom that or orbit insertion.

Science orbit: Working at MercuryThe MESSENGER mission has six specic science objectives.

Providemajor-elementmapsofMercuryto10%relativeuncertaintyonthe1000-kmscaleanddeterminelocal

composition and mineralogy at the ~20-km scale.

Provideaglobalmapwith>90%coverage(monochrome,orblackandwhite)at250-maverageresolutionand

> 80%oftheplanetimagedstereoscopically.Alsoprovideaglobalmulti-spectral(color)mapat2km/pixelaverage

resolution, and sample hal o the northern hemisphere or topography at 1.5-m average height resolution.

Provideamulti-polemagnetic-eldmodelresolvedthroughquadrupoletermswithanuncertaintyoflessthan~20%in the dipole magnitude and direction.

Provideaglobalgravityeldtodegreeandorder16anddeterminetheratioofthesolid-planetmomentofinertiato

the total moment o inertia to ~20% or better.

Identifytheprincipalcomponentoftheradar-reectivematerialatMercurysnorthpole.

Providealtitudeprolesat25-kmresolutionofthemajorneutralexosphericspeciesandcharacterizethemajorion-

species energy distributions as unctions o local time, Mercury heliocentric distance, and solar activity.

To accomplish these science goals, the MESSENGER spacecrat must obtain many types o observations rom dierentportions o its orbit around Mercury. Some major constraints must be met, including completing the observations withintwo Mercury solar days (equivalent to one Earth year) and keeping the spacecrat sunshade acing the Sun at all times.

The observation plan must also take into account MESSENGERs orbit around Mercury. The orbit is highly elliptical(egg-shaped), with the spacecrat passing 200 kilometers (124 miles) above the surace at the lowest point and morethan 15,193 kilometers (9,420 miles) at the highest. At the outset o the orbital phase o the mission, the plane o thespacecrats orbit is inclined 82.5 to Mercurys equator, and the lowest point in the orbit is reached at a latitude o 60North.

The spacecrats orbit is elliptical rather than circular because the planets surace radiates back heat rom the Sun. Atan altitude o 200 km, the re-radiated heat rom the planet alone is 4 times the solar intensity at Earth. By spending onlya short portion o each orbit fying this close to the planet, the temperature o the spacecrat can be better regulated.


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Observing the suraceMESSENGERs 12-month orbital phase covers two Mercury solar days; one Mercury solar day, rom sunrise to sunrise,is equal to 176 Earth days. This means that the spacecrat passes over a given spot on the surace only twice during themission, 6 months apart, making the time available to observe the planets surace a precious resource. The rst solarday is ocused on obtaining global map products rom the dierent instruments, and the second ocuses on specictargets o scientic interest and completion o a global stereo map.

As Mercury moves around the Sun, the spacecrats orbit around the planet stays in a nearly xed orientation thatallows MESSENGER to keep its sunshade toward the Sun. In eect, Mercury rotates beneath the spacecrat and thesurace illumination changes with respect to the spacecrat view. At some times, the spacecrat is traveling in an orbitthat ollows the terminator the line that separates day rom night. These are known as dawn-dusk orbits and aregood or imaging surace eatures such as craters, as shadows are prominent and topography and texture can be clearly

seen. At other times, the spacecrat ollows a path that takes it directly over a ully lit hemisphere o Mercury, then overa completely dark hemisphere. These are called noon-midnight orbits and are good or taking color observations onthe dayside, because there are ewer shadows to obscure surace eatures.

Some instruments, such as Mercury Laser Altimeter (MLA), can operate whether the surace is lit or not, but others,such as Mercury Dual Imaging System (MDIS), need sunlight in order to acquire data. The low-altitude segments othe orbit over the northern hemisphere will allow MESSENGER to conduct a detailed investigation o the geology andcomposition o Mercurys giant Caloris impact basin the planets largest known surace eature, among other goals.

As Mercury moves around the Sun, the MESSENGER spacecrat stays in an approximately xedorientation with its sunshade acing the Sun, so eectively the planet rotates beneath the spacecrat.Dierent parts o the surace are illuminated depending on where Mercury is in its year, so thespacecrat can view the surace o the planet under every possible lighting condition.


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Orchestrating the observations

Dierent instruments are given priority in determining spacecrat pointing at dierent portions o the spacecrat orbitand as a unction o the parts o Mercurys surace that are illuminated at any given time. For example, MLA drivesthe spacecrat pointing whenever its laser can range to the planets surace (less than ~1500 km altitude), UVVS controlsthe pointing when no other instruments can see the planet, and MAG and EPPS primarily ride along and collectdata regardless o what else is going on, since they generally dont need to point at the planets surace. The two MDISimagers are mounted on a common pivot, and so they can oten look at the surace or at other targets when the rest othe instruments are pointed in a dierent direction.

MESSENGER will operate in orbit around Mercury or one Earth year, equivalent to our Mercury years ortwo Mercury solar days. Dierent portions o the orbit are used by dierent instruments to acquire data.

This image shows a typical view romMESSENGERs science planning sotwaretool. The picture on the let shows theorientation o the spacecrat with respectto Mercury, and the table on the right

shows details o the spacecrats orbit atthat time. Views such as this one allowscientists to decide how best to take datato accomplish their science goals.


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To meet the mission science objectives while taking into consideration the constraints associated with spacecratsaety and orbital geometry, the MESSENGER Project has planned the entire year o observations in advance o theorbital phase. Because o the large number o dierent science observations required to meet the science objectives, aspecial sotware tool has been developed to help carry out the complicated process o maximizing the scientic return

rom the mission and minimizing conficts between instrument observations. This task is particularly challenging becausemost o the instruments are xed on the spacecrat and are pointed in the same direction, but the dierent instrumentsmay need to be pointed toward dierent locations at dierent times to meet the science goals.

Some observations also must be taken under specic observing conditions (such as taking color images when the Sunis high overhead), and the sotware tool works by nding the best opportunities or each o the instruments to maketheir measurements and then analyzing how those measurements contribute toward the science goals o the entiremission. Many iterations are necessary beore a solution is ound that satises all the science goals while staying withinthe limitations associated with the spacecrats onboard data storage and downlink capacity.

Although a baseline plan or the entire year has been ormulated, commands to execute the plan will be sent upto the spacecrat on a weekly basis. Each command load contains all the commands that the spacecrat will needto execute during a given week. Because each command load is dierent and contains many tens o thousands ocommands, the mission operations engineers start each load three weeks ahead o time. This schedule permits thecommand load to be thoroughly tested and reviewed beore it is sent up to the spacecrat. Because o this process,mission operations personnel at any given time will be working on several command loads, each o which is at adierent stage o development.

This planning-tool view shows MDIS image ootprints (boxes) on Mercurys suraceater one orbit. The ootprints vary in size depending on where the spacecrat is in itsorbit and which o the two imagers are used. Here, the ootprints are smallest at high

northern latitudes, when MESSENGER is closest to the planet, and are largest nearthe bottom o the view because at that time the spacecrat is much artherrom Mercury.


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The Science Team has also developed the capability to regenerate the plan at short notice in order to respond to anyanomalies that might occur in fight, such as an instrument problem, or on the ground, such as a missed Deep SpaceNetwork track.

Under this plan, each instrument will obtain the data needed to ulll MESSENGERs science objectives. Once in orbit,

MDIS will build on the imaging it acquired during the three Mercury fybys to create global color and monochromeimage mosaics during the rst six months o the orbital mission phase. Emphasis during the second six months will shitto targeted, high-resolution imaging with the NAC and repeated mapping at a dierent viewing geometry to create astereo map. MLA will measure the topography o the northern hemisphere over our Mercury years. GRNS and XRS willbuild up observations that will yield global maps o elemental composition. MAG will measure the vector magnetic eldunder a range o solar distances and conditions. VIRS will produce global maps o surace refectance rom which suracemineralogy can be inerred, and UVVS will produce global maps o exospheric species abundances versus altitude.EPPS will sample the plasma and energetic particle population in the solar wind, at major magnetospheric boundaries,and throughout the environment o Mercury at a range o solar distances and levels o solar activity. The radio scienceexperiment will extend topographic inormation to the southern hemisphere by making occultation measurements oplanet radius, and the planets obliquity and the amplitude o the physical libration will be determined independentlyrom the topography and gravity eld.

Each orbit is 12 hours in duration, so MESSENGER orbits Mercury twice every Earth day. Once a day, the spacecratstops making measurements and turns its antenna toward Earth or 8 hours, in order to send data back to the DeepSpace Network, rom which it will be sent on to the MESSENGER Mission Operations Center.


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General Oneofveplanetsknowntoancientastronomers;

in Roman mythology Mercury was the feet-ootedmessenger o the gods, a tting name or a planet thatmoves quickly across the sky.

TheclosestplanettotheSun,Mercuryisalsothe

smallest planet in the Solar System.

PriortoJanuary2008,Mercuryhadbeenvisitedbyonly

one spacecrat; NASAs Mariner 10 viewed less than halthe surace (~45%) in detail during its three fybys in1974 and 1975.

Physical characteristics Mercurysdiameteris4,880kilometers(3,032miles),

about one-third the size o Earth and only slightly largerthan our Moon.

ThedensestplanetintheSolarSystem(whencorrected

or compression), Mercurys density is 5.3 times greaterthan that o water.

ThelargestknownfeatureonMercuryspockmarked

surace is the Caloris basin (1,550 kilometers or 960miles in diameter see http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=&gallery_

id=2&image_id=149), likely created by an ancientasteroid impact.

Mercuryssurfaceisacombinationofcraters,smooth

plains, and long, winding clis.

Thereispossiblywatericeonthepermanently

shadowed foors o craters in Mercurys polar regions.

Anenormousironcoretakesupatleast60%ofthe

planets total mass twice as large a raction asEarths.

Environment MercuryexperiencestheSolarSystemslargestswing

in surace temperatures, rom highs above 700 Kelvin(about 800 Fahrenheit) to lows near 90 Kelvin(about 300 Fahrenheit).

Mercurysextremelythinatmospherecontains

hydrogen, helium, oxygen, sodium, potassium,calcium, and magnesium.

TheonlyinnerplanetbesidesEarthwithaglobal

magnetic eld, Mercurys eld is about 100 timesweaker than Earths (at the surace).

Orbit MercurysaveragedistancefromtheSunis58million

kilometers (36 million miles), about two-thirds closer tothe Sun than Earth is.

Thehighlyelliptical(elongated)orbitrangesfrom

46 million kilometers (29 million miles) to 70 millionkilometers (43 million miles) rom the Sun.

MercuryorbitstheSunonceevery88Earthdays,

moving at an average speed o 48 kilometers (30 miles)per second and making it the astest planet in theSolar System.

BecauseofitsslowrotationMercuryrotatesonitsaxis once every 59 Earth days and ast speed aroundthe Sun, one solar day on Mercury (rom noon to noonat the same place) lasts 176 Earth days, or two Mercuryyears.

MercurysdistancefromEarth(duringMESSENGERs

orbit) ranges rom about 87 million to 212 millionkilometers, about 54 million to 132 million miles.

Mercury at a Glance


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Why Mercury?Mercury, Venus, Earth, and Mars are the terrestrial (rocky) planets. Among these, Mercury is an extreme: the smallest,

the densest (ater correcting or sel-compression), the one with the oldest surace, the one with the largest dailyvariations in surace temperature, and the least explored. Understanding this end member among the terrestrialplanets is crucial to developing a better understanding o how the planets in our Solar System ormed and evolved. Todevelop this understanding, the MESSENGER mission, spacecrat, and science instruments are ocused on answering sixkey questions.

Question 1: Why is Mercury so dense?Each o the terrestrial planets consists o a dense iron-rich core surrounded by a rocky mantle, composed largely o

magnesium and iron silicates. The topmost layer o rock, the crust, ormed rom minerals with lower melting points thanthose in the underlying mantle, either during dierentiation early in the planets history or by later volcanic or magmaticactivity. The density o each planet provides inormation about the relative sizes o the iron-rich core and the rockymantle and crust, since the metallic core is much denser than the rocky components. Mercurys uncompressed density(what its density would be without compaction o its interior by the planets own gravity) is about 5.3 g/cm3, by arthe highest o all the terrestrial planets. In act, Mercurys density implies that at least 60% o the planet is a metal-richcore, a gure twice as great as or Earth, Venus, or Mars. To account or about 60% o the planets mass, the radius oMercurys core must be approximately 75% o the radius o the entire planet!

There are three major theories to explain why Mercury is so much denser and more metal-rich than Earth, Venus,and Mars. Each theory predicts a dierent composition or the rocks on Mercurys surace. According to one idea,beore Mercury ormed, drag by solar nebular gas near the Sun mechanically sorted silicate and metal grains, with thelighter silicate particles preerentially slowed and lost to the Sun; Mercury later ormed rom material in this region andis consequently enriched in metal. This process doesnt predict any change in the composition o the silicate mineralsmaking up the rocky portion o the planet, just the relative amounts o metal and rock. In another theory, tremendousheat in the early nebula vaporized part o the outer rock layer o proto-Mercury and let the planet strongly depleted involatile elements. This idea predicts a rock composition poor in easily evaporated elements like sodium and potassium.The third idea is that a giant impact, ater proto-Mercury had ormed and dierentiated, stripped o the primordial crustand upper mantle. This idea predicts that the present-day surace is made o rocks highly depleted in those elementsthat would have been concentrated in the crust, such as aluminum and calcium.

William Hartmanns depiction o the early solarnebula shows the time when the terrestrial planetswere orming. Processes such as nebular gas drag,vaporization in the hot early nebula, and giantimpacting collisions have all been suggested ashaving possible eects on the bulk composition oMercury. (During the Formation o the Terrestrial

Planets, painting, copyright 1999 by William K.Hartmann. This and additional artwork associatedwith Mercury and the early solar system is availablerom Dr. Hartmann at [email protected]).


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MESSENGER will determine which o these ideas is correct by measuring the composition o the rocky surace. X-ray,gamma-ray, and neutron spectrometers will measure the elements present in the surace rocks and determine i volatileelements are depleted or i elements that tend to be concentrated in planetary crusts are decient. A visible-inraredspectrometer will determine which minerals are present and will permit the construction o mineralogical maps o the

surace. Analysis o gravity and topography measurements will provide estimates o the thickness o Mercurys crust. Tomake these challenging measurements o Mercurys surace composition and crustal characteristics, these instrumentswill need to accumulate many observations o the surace. MESSENGERs three Mercury fybys provided opportunitiesto make preliminary observations, but numerous measurements rom an orbit around Mercury are needed to determineaccurately the surace composition. Once in orbit, these measurements will enable MESSENGER to distinguish amongthe dierent proposed origins or Mercurys high density and, by doing so, gain insight into how the planet ormed andevolved.

Question 2: What is the geologic history o Mercury?Prior to MESSENGER, only 45% o Mercurys surace had been seen by spacecrat during the Mariner 10 mission.

Combining the Mariner 10 photos with the images rom MESSENGERs three Mercury fybys, about 98% o the surace

o Mercury has been seen in detail. It is now possible or the rst time to begin to investigate Mercurys geologic historyon a global basis.

Much o Mercurys surace appears cratered and ancient, with a resemblance to the surace o Earths Moon.Slightly younger, less cratered plains sit within and between the largest old craters. Many o these plains are volcanic,on the basis o their age relative to nearby large impact eatures and other indicators o volcanic activity. Data romMESSENGERs fybys indicate that volcanism on Mercury persisted or at least the rst hal o the planets history, and

that the style o volcanism included both eusive andexplosive eruptions.

Mercurys tectonic history is unlike that o anyother terrestrial planet. On the surace o Mercury,the most prominent eatures produced by tectonic

orces are long, rounded, lobate scarps or clis, someover a kilometer in height and hundreds o kilometersin length. These giant scarps are believed to haveormed as Mercury cooled and the entire planetcontracted on a global scale. Understanding theormation o these scarps thus provides the potentialto gain insight into the thermal history and interiorstructure o Mercury.

Once in orbit, MESSENGER will bring a varietyo investigations to bear on Mercurys geology inorder to determine the sequence o processes that

have shaped the surace. The X-ray, gamma-ray,and visible-inrared spectrometers will determinethe elemental and mineralogical makeup o rockunits composing the surace. The cameras will imageMercurys surace in color and at a typical imagingresolution that surpasses that o most Mariner 10pictures. Nearly all o the surace will be imaged instereo to determine the planets global topographicvariations and landorms; the laser altimeterwill measure the topography o surace eatureseven more precisely in the northern hemisphere.

A portion o the long, lobate scarp named Beagle Rupes (rightside o this image) deorms an impact craters seen in the upperright. This image was taken during MESSENGERs rst fyby oMercury, and the width o the image is about 110 km. (Courtesyo NASA, JHU/APL, CIW.)


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Comparing the topography with the planets gravity eld, measured by tracking the MESSENGER spacecrat, will allow

determinations o local variations in the thickness o Mercurys crust. This diversity o high-resolution data returned by

MESSENGER will enable the reconstruction o the geologic history o Mercury.

Question 3: What is the nature o Mercurys magnetic feld?Mercurys magnetic eld and the resulting magnetosphere, produced by the interaction o Mercurys magnetic eld

with the solar wind, are unique in many ways. Perhaps one o the most noteworthy observations about Mercurys

magnetic eld is that the small planet possesses one at all. Mercurys magnetic eld is similar in its dipole shape to

Earths magnetic eld, which resembles the eld that would be produced i there was a giant bar magnet at the center

o the planet. In contrast, Venus, Mars, and the Moon do not show evidence or intrinsic dipolar magnetic elds, but the

Moon and Mars have evidence or local magnetic elds centered on dierent rock deposits.

Earths magnetosphere is very dynamic and constantly changes in response to the Suns activity, including both solar

storms and more modest changes in the solar wind and interplanetary magnetic eld. We see the eects o these

dynamics on the ground as they aect power grids and electronics, causing blackouts and intererence with radios and

telephones. Mercurys magnetosphere was shown by Mariner 10 to experience similar dynamics; understanding thosevariations will help us understand the interaction o the Sun with planetary magnetospheres in general.

Although Mercurys magnetic eld is thought to be a miniature version o Earths, Mariner 10 didnt measure

Mercurys eld well enough to characterize it. There was even considerable uncertainty in the strength and source o

the magnetic eld ater Mariner 10. MESSENGERs Mercury fybys conrmed that there is a global magnetic eld on

Mercury, and that the eld has a strong dipolar component nearly aligned with the planets spin axis. Mercurys magnetic

eld most likely arises rom fuid motions in an outer liquid portion o Mercurys metal core. There is debate, however,

about the molten raction o the core as well as whether the eld is driven by compositional or thermal dierences.

These dierent ideas or the driving orce behind Mercurys magnetic eld predict slightly dierent eld geometries, so

careul measurements by spacecrat can distinguish among current theories.

MESSENGERs

magnetometer will characterize

Mercurys magnetic eld in

detail rom orbit over our

Mercury years (each Mercury

year equals 88 Earth days) to

determine its precise strength

and how that strength varies

with position and altitude.

The eects o the Sun on

magnetospheric dynamics will

be measured by MESSENGERs

magnetometer and by theenergetic particle and plasma

spectrometer. MESSENGERs

highly capable instruments

and broad orbital coverage

will greatly advance our

understanding o both the

origin o Mercurys magnetic

eld and the nature o its

interaction with the solar wind.

The dierent components o Mercurys magnetosphere result rom the complex anddynamic interactions between Mercurys magnetic eld and the solar wind. (Courtesyo Jim Slavin, NASA Goddard Space Flight Center.)


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Question 4: What is the structure o Mercurys core?As discussed in Questions 1 and 3, Mercury has a very large iron-rich core and a global magnetic eld; this

inormation was rst gathered by the Mariner 10 fybys. More recently, Earth-based radar observations o Mercury have

also determined that at least a portion o the large metal core is still liquid. Having at least a partially molten core means

that a very small but detectable variation in the spin rate o Mercury has a larger amplitude because o decouplingbetween the solid mantle and the liquid core. Knowing that the core has not completely solidied, even as Mercury has

cooled over billions o years since its ormation, places important constraints on the planets thermal history, evolution,

and core composition.

However, these constraints are limited because o the low precision o current inormation on Mercurys gravity eld

rom the Mariner 10 and MESSENGER fybys. Fundamental questions about Mercurys core remain to be explored, such

as its composition. A core o pure

iron would be completely solid

today, due to the high melting

point o iron. However, i other

elements, such as sulur, are also

present in Mercurys core, even

at only a level o a ew percent,

the melting point is lowered

considerably, allowing Mercurys

core to remain at least partially

molten as the planet cooled.

Constraining the composition

o the core is intimately tied to

understanding what raction o

the core is liquid and what raction

has solidied. Is there just a very

thin layer o liquid over a mostly

solid core, or is the core completely

molten? Addressing questions

such as these can also provide

insight into the current thermal

state o Mercurys interior, which

is very valuable inormation or

determining the evolution o the

planet.

Using the laser altimeter in orbit, MESSENGER will veriy the presence o a liquid outer core by measuring Mercurys

libration. Libration is the slow, 88-day wobble o the planet about its rotational axis. The libration o the rocky outerpart o the planet will be twice as large i it is foating on a liquid outer core than i it is rozen to a solid core. By radio

tracking o the spacecrat in orbit, MESSENGER will also determine the gravity eld with much better precision than can

be accomplished during fybys. The libration experiment, when combined with improved measurements o the gravity

eld, will provide inormation on the size and structure o the core.

Question 5: What are the unusual materials at Mercurys poles?Mercurys axis o rotation is oriented nearly perpendicular to the planets orbit, so that in polar regions sunlight strikes

the surace at a near-constant grazing angle. Some o the interiors o large craters at the poles are thus permanently

shadowed and perpetually very cold. Earth-based radar images o the polar regions show that the foors o large craters

The radius o the core o Mercury is approximately 75% o that o the entire planet,which is a much larger raction o the plan than or Earth. Like Earth, Mercury has acore that is at lest partially liquid. However, unlike Earth, the size o the solid innercore is not known. (Courtesy o NASA, JHU/APL, CIW.)


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are highly refective at radar wavelengths, unlike the surrounding terrain. Furthermore, the radar-bright regions are

consistent in their radar properties with the polar cap o Mars and the icy moons o Jupiter, suggesting that the material

concentrated in the shadowed craters is water ice. The idea o water ice being stable on the surace o the planet closest

to the Sun is intriguing.

The temperature inside these permanently shadowed craters is believed to be low enough to allow water ice tobe stable or the majority o the observed deposits. Ice rom inalling comets and meteoroids could be cold-trapped

in Mercurys polar deposits over millions to billions o years, or water vapor might outgas rom the planets interior

and reeze at the poles. A ew craters at latitudes as low as 72 N have also been observed to contain radar-bright

material in their interiors, and at these warmer latitudes, maintaining stable water ice or longer periods o time may

be more dicult; a recent comet impact, in the last ew million years, may be required to satisy all radar observations.

Alternatively, it has been suggested that the radar-bright deposits are not water ice but rather consist o a dierent

material, such as sulur. Sulur would be stable in the cold traps o the permanently shadowed crater interiors, and the

source o sulur could be either meteoritic material or the surace o Mercury itsel. It has also been proposed that thenaturally occurring silicates that make up the surace o Mercury could produce the observed radar refections when

maintained at the extremely low temperatures present in the permanently shadowed craters.

MESSENGERs three fybys o Mercury passed nearly over the equator and did not allow or viewing o the planets

poles. Once in orbit around Mercury, however, MESSENGERs neutron spectrometer will search or hydrogen in any polar

deposits, the detection o which would suggest that the polar deposits are water-rich. The ultraviolet spectrometer and

energetic particle and plasma spectrometer will search or the signatures o hydroxide or sulur in the tenuous vapor ove

the deposits. The laser altimeter will provide inormation about the topography o the permanently shadowed craters.

Understanding the composition o Mercurys polar deposits will clariy the inventory and availability o volatile materials

in the inner Solar System.

A radar image o the north polar region o Mercury shows radar-bright regions concentrated in circularfoors o craters with permanently shadowed interiors. The radar-bright material might be water ice, butalternative suggestions have also been proposed. (Courtesy o John K. Harmon, Arecibo Observatory.)


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Question 6: What volatiles are important at Mercury?Mercury is surrounded by an extremely thin envelope o gas. It is so thin that, unlike the atmospheres o Venus, Earth

and Mars, the molecules surrounding Mercury dont collide with each other and instead bounce rom place to place onthe surace like many rubber balls. This tenuous atmosphere is called an exosphere.

Seven elements are known to exist in Mercurys exosphere: hydrogen, helium, oxygen, sodium, potassium, calcium,and, as discovered by MESSENGER, magnesium. The observed exosphere is not stable on timescales comparable to theage o Mercury, and so there must be sources or each o these elements. High abundances o hydrogen and helium arepresent in the solar wind, the stream o hot, ionized gas emitted by the Sun. The other elements are likely rom materialimpacting Mercury, such as micrometeoroids or comets, or directly rom Mercurys surace rocks. Several dierentprocesses may have put these elements into the exosphere, and each process yields a dierent mix o the elements:vaporization o rocks by impacts, evaporation o elements rom the rocks in sunlight, sputtering by solar wind ormagnetospheric ions, or diusion rom the planets interior. Strong variability in the composition o Mercurys exospherehas been observed, suggesting an interaction o several o these processes.

MESSENGER will determine the composition o Mercurys exosphere using its ultraviolet spectrometer and energeticparticle and plasma spectrometer. The exosphere composition measured by these instruments will be compared with

the composition o surace rocks measured by the X-ray, gamma-ray, and neutron spectrometers. As MESSENGER orbitsMercury, variations in the exospheres composition will be monitored. The combination o these measurements willelucidate the nature o Mercurys exosphere and the processes that contribute to it.

During MESSENGERs rst fyby o Mercury, the distribution o neutral sodium in the

tail o Mercurys exosphere was measured. (Courtesy o NASA, JHU/APL, CIW.)


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Highlights rom the Mercury FlybysMESSENGER has completed three fybys o Mercury, utilizing the planets gravity to alter its trajectory and bring its

orbit about the Sun closer to that o the innermost planet. All three fybys were executed fawlessly.

To make ull use o this opportunity, the Science Team developed a comprehensive plan to conduct observationsthroughout the encounters. The data recorded by MESSENGER during its fybys have been used to ne-tune theobservation strategy or the prime orbital phase o the mission. Even now these data are providing exciting new insightsinto the history and dynamics o our Solar Systems innermost planet.

MESSENGERs rst fyby o Mercury on January 14, 2008, was a resounding success. Measurements were made thathad never been possible with Mariner 10 or rom ground-based telescopes. These include plasma ion measurements,laser altimetry, high-resolution surace spectroscopy, spacecrat elemental chemical remote sensing, high-spatial-resolution observations o both known and new species in Mercurys exosphere, and eleven-color imaging. In additionto these observations, complementary measurements were made to those rom Mariner 10 by the MESSENGERMagnetometer, and 21% o the previously unseen hemisphere was imaged or the rst time, bringing to 66% the totalsurace area o the planet imaged by spacecrat.

During MESSENGERs second fyby on October 6, 2008, MDIS images lled in a urther 24% o the previously unseenhemisphere, so that 90% o the planet had at that point been observed by spacecrat. Much o the hemisphere imagedby Mariner 10 was viewed by MESSENGER under dierent lighting conditions or in color, allowing new discoveries.MESSENGER became the rst spacecrat to fy over the planets western hemisphere, making the rst measurements oMercurys internal magnetic eld above that portion o the planet. All instruments took data during the fyby, and anemerging picture o Mercurys global environment and history continued to take shape.

On September 29, 2009, MESSENGER few by Mercury or the third and nal time prior to orbit insertion in March2011. An additional 6% o the surace was imaged, completing the equatorial coverage by spacecrat and leaving onlythe polar regions yet to be seen by spacecrat. Shortly beore closest approach to the planet, as the spacecrat passedinto eclipse, an unexpected conguration o the power system caused the ault protection system to halt the sciencecommand sequence. Although the spacecrat was never at risk and continued through the needed gravity assist, a

number o planned science observations were not made. Despite the truncated set o measurements, new discoverieswere made about the innermost planet, including the rst observations o emission rom an ionized species in Mercurysexosphere, indications o a surprisingly complex distribution o exospheric species over the north and south poles, newinormation about magnetic substorms, and evidence or younger volcanism than had been previously anticipated.


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Mapping an old worldInitial mapping o the innermost world o the Solar System is now almost complete. MESSENGER has now seen 91%

o Mercury rom its three encounters with Mercury. In the accompanying map o imaging coverage, the narrow regionsare the sunlit crescents seen as MESSENGER approached Mercury prior to each fyby, and include the area imaged during

the approach to fyby 3 (yellow outline). The larger areas are the sunlit portions o the surace seen as MESSENGERdeparted the Solar Systems innermost planet on its rst and second fybys. Between Mariner 10 and MESSENGER, morethan 98% o Mercurys surace has been mapped at a resolution o 1 kilometer or better. Because o the ast encountervelocity and Mercurys slow rotation, the lighting angle within the global mosaic varies rom high noon to just over thehorizon, resulting in a non-uniorm look at the planet. Ater MESSENGER enters orbit about Mercury in 2011, a higher-resolution (on the average o 250 meters/pixels) global mosaic will be built up with more uniorm illumination.

MESSENGER has now seen 91% o Mercury rom its three encounters with the planet. Combining images rom MESSENGERs

rst (outlined in blue), second (red), and third (yellow) Mercury fybys with photos obtained rom Mariner 10s three fybys in1974-75 (outlined in green) yields nearly total coverage o Mercurys surace with the exception o the regions poleward o60 N or 60 S. Along with revealing intriguing geologic eatures in previously unseen terrain, completion o this nearly globalmap o Mercurys surace, ree o gaps, has been valuable or planning MESSENGERs orbital operations, which begin inMarch 2011.


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Mercury in color!During the fybys, the Mercury Dual Imaging System (MDIS) Wide-Angle Camera (WAC) snapped images o Mercury

through 11 dierent narrow-band color lters, which range rom violet in the visible (395 nm) to the near-inrared (1040nm). The specic colors o the lters were selected to discriminate among common minerals, and statistical methods

that utilize all 11 lters in the visible and near-inrared are used to enhance subtle color dierences and aid geologistsin mapping regions o dierent composition. What do the exaggerated colors tell us about Mercury? The nature ocolor boundaries, color trends, and brightness values help MESSENGER geologists understand the distinct regions (orgeological units) on the surace. This color inormation has shown Mercurys surace to be composed o a variety omaterials with dierent color characteristics, such as smooth volcanic plains; darker material excavated rom depth byimpact craters; younger, less space-weathered material; reddish deposits near volcanic vents; and very bright material onsome crater foors. The color images are complemented by images rom the MDIS Narrow-Angle Camera (NAC), whichprovides higher resolution views o areas with interesting color properties.

These images are orthographic projections o Mercury created with WAC enhanced-colorimages. The orthographic projection produces a view that has the perspective that onewould see rom deep space. The WAC enhanced color uses a statistical analysis o imagesrom all 11 WAC lters to highlight subtle dierences in the color o crustal rocks onMercurys surace. The top view uses images rom Mercury fyby 1, with the thin crescento Mercury imaged during approach orming the right portion o the globe and the ullerdeparture view showing Caloris basin orming the let side and majority o the view. Theblack strip between the approach and departure images is a portion o Mercurys suracenot viewed by MESSENGER during the fyby. Similarly, the approach and departureimages obtained during Mercury fyby 2 yielded the bottom view. The top and bottomprojections are centered on 180 and 0 longitude, respectively.


27/6427


From the color images alone it is not possible to determine unambiguously the minerals that comprise the rockso each unit. During the brie fybys, MESSENGERs other instruments sensitive to composition lacked the timeneeded to build up adequate signal or gain broad areal coverage, so only MESSENGERs cameras were able to acquirecomprehensive measurements. Once in orbit about Mercury, MESSENGERs ull suite o instruments will be brought to

bear on the newly discovered color units, and the results will provide inormation about Mercurys composition and theprocesses that acted on Mercurys surace.

First clues to mineralogy: Clear dierences rom Earths MoonHigh-resolution, ultraviolet-to-inrared spectra o Mercurys surace acquired by the Mercury Atmospheric and Surace

Composition Spectrometer (MASCS) revealed dierences in color rom the Earths Moon that, despite some generalsimilarities, indicate a dierent composition. For mineralogical identication, spectral dierences o a ew percent aresignicant, and the dierences ound by MASCS are up to 20%, indicating that the surace o Mercury has a number oimportant surprises in store. Identiying the classes o minerals consistent with the observed spectra will require extensiveanalysis and comparison with the color imaging rom MDIS and laboratory measurements o the refectance o mineraland rock mixtures, but it is already clear that these spectra will play a key role in siting out the geologic history o the

range o materials evident on the surace and, ultimately, in telling the story o Mercurys unique history.

Volcanism on MercuryThe role volcanism played in shaping the landscape o Mercury was a subject o scientic debate ater the fybys o

Mariner 10. From MESSENGERs fybys, high-resolution images combined with complementary color inormation haveled to the rst identication o volcanic vents on Mercury. The vents are seen as irregularly shaped, rimless depressions,which distinguish them rom impact craters. Smooth, bright, diusely distributed deposits surround the vents, similar tomaterial seen around explosive volcanoes on Earth and other planets. The characteristics o this bright material suggestthat it was erupted explosively rom magma that contained substantial amounts o gas, or volatiles. By measuring howar the material ell rom the source vent, scientists can estimate the speed at which it was erupted, and what kinds

Close-up o the short-wavelength portion o thespectrum comparing theMASCS spectra rom the Moonand Mercury. Note the sharpdeparture o the Mercuryspectrum rom that o the Moonat wavelengths below 300 nm.


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o volatiles were present in Mercurys interior at that time. This inormation provides insight into how Mercury wasassembled during its ormation and how its interior has evolved.

High-resolution MDIS images reveal many examples o impact craters that have been fooded and embayed byvolcanic lava. By measuring the shallow depth o craters fooded by volcanic processes, lava fow thicknesses as greatas 5 km have been estimated. Further evidence or volcanic processes comes rom high-resolution color images romMDIS, which can indicate variations in composition and age o dierent eatures. Many impact basins (including Caloris)have foors that are covered with material o a dierent composition and younger age, implying that they were foodedby later volcanism. The impacts that ormed some craters have punched through the surace material to excavate oldermaterial rom the subsurace. Impacts make it possible to assess how Mercurys crust varies with depth and ultimatelyhow the crust evolved through time. Thus, results rom MESSENGERs fybys indicate that volcanism was an importantprocess in the geologic history o Mercury, and additional MESSENGER data will urther elucidate the extent o volcanismon the Solar Systems innermost planet.

Other evidence or volcanism on Mercury comes rom craters that contain rimless, oten irregularly shaped pits withintheir foors. These pits display no associated ejecta or lava fows. They are thought to be evidence o shallow magmaticactivity and may have ormed when retreating magma caused an unsupported area o the surace to collapse, creating apit. The discovery o multiple pit-foor craters augments a growing body o evidence that volcanic and magmatic activityhas been a widespread process in the geologic evolution o Mercurys crust.

MESSENGER identied volcanic vents on Mercury or the rst time. The irregularly shaped depression marking the vent in thelet image is about 20 km across in its longest dimension and is surrounded by a bright, smooth deposit with diuse margins.The bright material is believed to consist o pyroclastic deposits ejected during explosive volcanic eruptions at the vent. Afooded impact crater (right image), about 60 km in diameter, shows urther evidence that volcanism has shaped Mercuryssurace.


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This enhanced-color image was created by usinghigh-resolution images taken in all 11 WAC ltersand comparing and contrasting them to accentuatedierences on Mercurys surace. Here, smoothreddish plains material near Rudaki crater shows clear

boundaries with bluer, more cratered terrain, indicatingthat the two units have dierent compositions. The rimo and ejecta surrounding Calvino, the 68-km-diametercrater in the center o the image, is more orange thanthe surrounding plains, indicating that this craterexcavated material diering in composition rom theplains. Furthermore, its central peak is comparativelyblue, indicating that material o a third compositionwas excavated rom still greater depth during thecraters ormation. Impacts make it possible to assesshow Mercurys crust varies with depth and ultimatelyhow the crust evolved through time.

Lermontov crater (~150 km in diameter) was rstobserved by Mariner 10 and seen more recently by

MESSENGER during its second fyby o Mercury. Thecrater foor is somewhat brighter than the surroundingsurace and is smooth with several irregularly shapeddepressions. Such eatures may be evidence o pastexplosive volcanic activity on the crater foor.


30/6430


The duration o volcanic activityPrior to MESSENGER, it was thought that the volcanism on Mercury had ceased relatively early in its history,

probably around 3.8 billion years ago. Images rom MESSENGERs fybys showed basins that were very sparsely cratered

compared to some other terrains on the planet, implying that they are much younger. On the inner foor o one basin,

Rachmanino, there is a volcanic deposit that contains even ewer craters per area than the remaining portions o thebasin foor, suggesting modication by volcanism some time ater the basin had ormed. By counting the craters per area

in the inner foor deposit, the MESSENGER Science Team has estimated that the volcanic deposit is probably younger

then 2 billion years. Volcanism on Mercury thus continued over a much longer time span than was previously thought,

probably at least hal o the history o the Solar System. This discovery means that Mercurys interior remained hotter or

longer than had been predicted. Once in orbit, other examples o relatively young volcanism may be discovered, and it

will be possible to reconstruct Mercurys surace history in greater detail.

The great Caloris impact basinIt was known rom Mariner 10 photos that Mercurys Caloris basin is a large, well preserved impact basin, but

MESSENGER images showed the true extent o the eature or the rst time. From Mariner 10 photos, only a portion

o the eastern hal o Caloris was visible, and the diameter o Caloris was estimated at 1,300 km. MESSENGERsimages o the entire Caloris basin show that the structure is larger than previously believed, with a diameter o about

1,550 km.ThedensityofsuperposedsmallercratersinsideandoutsideCalorisbasinshowsthatthedepositsformedat

the same time as the basin date rom airly early in the history o the Solar System, likely around 3.8 billion years ago.

Plains interior and exterior to the basin, however, have a lower density o impact craters, indicating that they postdate

the basin and consist o volcanic deposits. Near the center o Caloris basin, a set o over 200 narrow troughs, named

Pantheon Fossae, radiate outward in a pattern unlike anything previously seen on Mercury. Structures interpreted

as volcanic vents are seen around the margins o the great basin. Craters with intriguing dark- and light-color

characteristics are ound on the basin foor. Overall, understanding the ormation and evolution o this giant basin will

provide insight into the early history o major impacts in the inner Solar System, with implications not just or Mercury,

but or all the rocky planets, including Earth.

This enhanced-color image o the Rachmaninobasin (~290 km in diameter) highlights dierencesin refectance, color, and surace orm between thesmooth plains within the basins inner ring and the

surrounding surace. MESSENGER team membershave documented evidence that these interiorsmooth plains are products o relatively youngvolcanism, the youngest documented on Mercuryto date. Whereas interpretations o Mariner 10images prior to the MESSENGER fybys were thatvolcanism on Mercury ended early in the planetshistory, MESSENGERs images o Rachmaninoreveal that some volcanism extended well beyondthat time, probably into the second hal o SolarSystem history.


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High-precision topography rom laser rangingThe very rst laser ranging by the Mercury Laser Altimeter (MLA) to Mercurys surace yielded topographic proles

across multiple craters, smooth plains, and other terrain. During the rst fyby, MLA was within range only over the night

side o Mercury, and the surace within view o MLA had not been imaged by Mariner 10, so the ranging results werecorrelated with Earth-based, radar images o the surace. During the second fyby, topographic measurements weremade across territory that was photographed in high resolution by the MDIS NAC as well as other areas that had beenimaged during the rst fyby.

The MLA ranging provided the rst denitive observations o terrain slopes and crater depths on Mercury, showingthat the slopes are more gradual and older craters shallower than those on the Moon, presumably the result o volcanicinlling. The results also clearly show that there is great variation in the surace roughness o crater foors, suggestingdierences in ages or in the geologic processes that have operated in dierent craters. From orbit, MESSENGER will beable to construct a topographic map o Mercurys northern hemisphere, enabling a better understanding o the shapeand depth o impact craters and how they vary on Mercury compared with other bodies.

This mosaic o multiple MDIS images shows the Caloris basin in its entirety. The Calorisbasin was discovered in 1974 rom Mariner 10 images, but when Mariner 10 few byMercury, only the eastern hal o the basin was in daylight. During MESSENGERs rstMercury fyby, the spacecrat was able to acquire high-resolution images o the entirebasin, revealing its ull extent or the rst time.


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Rembrandt A newly discovered impact basinImpact basins are ormed by the impact o objects much larger than those that orm craters, resulting in much

larger structures as well as multiple rings o elevated terrain ormed during the impact process. MESSENGERs secondfyby revealed a basin not previously known, Rembrandt, which has a diameter o 715 km (440 miles). The numbero impact craters superposed on Rembrandts rim indicates that it is one o the youngest basins on Mercury. The fooro Rembrandt appears to be lled with volcanic material, which is overprinted with a system o wrinkle-ridges andtroughs in radial or concentric shapes, lending the basin an unusual wheel and spoke appearance. The troughs bear asimilarity to the extensional troughs o Pantheon Fossae, imaged near the center o Caloris basin. From an examinationo relationships among the dierent eatures within Rembrandt, the relative timing o volcanism, deormation, andcratering within this basin is being revealed.

This gure shows a 400-kilometer-long(250-mile-long) section o the MLAtopographic prole rom MESSENGERs secondMercury fyby superposed on a high-resolutionNAC departure mosaic acquired during the

same encounter. The blue dots indicate thespacecrat ground track, and the yellow dotsshow the altimetry data points; the blue arrowshows the spacecrats direction o travel. Nearthe center o this prole, the MLA track crossestwo craters o comparable sizes but dierentdepths. The deeper crater in the center o thetrack is Machaut crater, and the unnamedcrater to Machauts east is considerably

shallower and has probably been lled by volcanic material. By making such measurements systematically over the surace, itwill be possible to measure the volumes o volcanic material erupted over Mercurys history.

NAC mosaic o the newly discovered Rembrandt impact basin (let) with a diameter o ~715 kilometers (444 miles),slightly less than hal the diameter o the Caloris basin. To put the size o Mercurys Rembrandt basin into a amiliarcontext, a NAC mosaic o the basin is overlaid on an Advanced Very High Resolution Radiometer image o the eastcoast o the United States (right). Such a eature, i ormed at this location on Earth, would encompass the cities oWashington, D.C., and Boston, Massachusetts, and everything in between. The basin contains an unusual patterno troughs and ridges in its center and appears to be one o the youngest impact basins on Mercury.


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The probe was within Mercurys magnetosphere, the volume o space within which the magnetic eld is dominatedby that o the planet, or about 30 minutes during each fyby. The second Mercury fyby provided the only data to daterom the planets western hemisphere, and those data are thereore key to constraining the geometry o the planetsinternal magnetic eld. Magnetic eld measurements showed that the planets eld is that o a magnetic dipole, similar

in strength and direction to that measured by Mariner 10 over three decades earlier. The planetary magnetic momentis very nearly centered within the planet and is strongly aligned with the rotation axis, to within a tilt o 2. The dipolenature o the eld avors an active dynamo in Mercurys molten outer core as the source o the eld.

Solar wind control o Mercurys magnetosphereThe Sun has been relatively quiet during and between MESSENGERs three transits o Mercurys magnetosphere,

and the maximum measured strength o Mercurys internal magnetic eld was comparable during each o the fybys.Nonetheless, magnetospheric activity varied greatly romone fyby to the next as a result o small dierences inthe interplanetary magnetic eld (IMF). The north-southcomponent o the eld outside the magnetosphere, in the

solar wind, was northward or fyby 1, southward or fyby2, and varied rom northward to southward and back duringfyby 3. The northward IMF during the rst fyby produceda very quiet magnetosphere, and MESSENGER measuredsteady magnetic elds and registered the presence o onlyvery-low-energy charged particles.

The second fybys southward IMF resulted in amagnetosphere whose outer boundary was highly porous tosolar wind charged particles as a consequence o magneticreconnection between interplanetary and planetarymagnetic elds. Huge bundles o twisted magnetic fux,somewhat resembling fux bundles ejected rom the Sun

in coronal mass ejections ollowing solar fares, wereobserved to emanate rom Mercurys magnetosphere. Thisdynamic interaction creates magnetic linkage over the polarregions o the planet and provides open windows orthe entry o solar wind charged particles. Once inside themagnetosphere, these charged particles impact the suraceo Mercury where they give up their energy to atoms,such as sodium, which are ejected to resupply the planetsatmosphere.

The most extreme magnetospheric conditions wereobserved in response to the variable northsouth

component o the IMF observed during the third andnal fyby. MESSENGER documented the rapid buildupo magnetic energy in Mercurys magnetic tail ollowedby its rapid release in magnetic substorms. Althoughqualitatively similar to magnetospheric substorms at Earth,these events at Mercury were much aster, lasting only a ewminutes rather than the ew hours at Earth, and the relativeeect on the conguration and intensity o Mercurysmagnetic eld was at least a actor o 10 greater than seenat Earth.

The top gure shows the angle that the magnetic eldmade with the northward direction or the outboundpasses through the magnetopause and bow shock orthe missions rst (blue) and second (orange) Mercuryfybys. The bottom gure illustrates the proounddierence in magnetic connection between Mercury andthe solar wind when the magnetic eld in the solar windis southward (let) as or fyby 2 versus northward (right)as or fyby 1. These views rom the Sun show a notionalcross section o the magnetic lines o orce in the dawn-dusk meridian plane.


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Taken together, the magnetospheric measurements rom the three MESSENGER fybys indicate that Mercurysmagnetosphere responds more strongly to the direction o the IMF than that o any other planet with an internalmagnetic eld. MESSENGER measurements collected rom orbit will be necessary to resolve the question o whyMercurys magnetic elds are so dynamic.

MESSENGER also made the rst measurements o the planetary ions that interact with Mercurys magnetosphere,revealing a remarkable richness in the species present and providing inormation about the complex interaction othe plasma, solar wind, and surace. In addition to the solar wind protons that make up the bulk o the solar wind,MESSENGERs Fast Imaging Plasma Spectrometer discovered that Mercurys magnetosphere is host to a wide varietyo heavy ions. This richness o ion species links to material driven o the surace, providing another opportunity ordeducing, albeit indirectly, Mercurys surace composition.

Mercurys exosphere as never seen beoreMercurys exosphere an atmosphere so tenuous that particles are more likely to hit the surace than to collide

with each other was discovered during the Mariner 10 fybys. Ground-based telescopic observations over the past25 years have added to our knowledge and shown that Mercury also has an extended exospheric tail, a result o solar

radiation pressure eects (basically sunlight pushing exospheric atoms in the antisunward direction). But these ground-based observations are both dicult to make and limited by the Earths atmosphere. With the Mercury Atmospheric andSurace Composition Spectrometer (MASCS) on MESSENGER, however, Mercurys exosphere has been observed withunprecedented wavelength coverage and spatial resolution.

During the three fybys, the Ultraviolet and Visible Spectrometer (UVVS) channel o MASCS obtained the mostdetailed measurements o the exosphere and tail ever made, yielding maps o emission rom several species present inthe exosphere, including sodium, calcium, and magnesium. Although both sodium and calcium in Mercurys exospherehad previously been observed with ground-based telescopes on Earth, the fybys were the rst time that measurementso the two species were obtained simultaneously. Observations o magnesium were a rst or MESSENGER becausethe emission rom magnesium atoms occurs at ultraviolet wavelengths and is thereore blocked rom ground-basedobservation by Earths atmosphere.

Atoms in Mercurys exosphere that are heavier than hydrogen and helium predominantly originate rom the suraceo Mercury. The detection o magnesium in the exosphere thus provided evidence that magnesium is an importantcomponent o surace material, something that had long been expected but never proven. A number o processescontribute to the release o exospheric species rom the surace, and dierences in their distributions in both time andspace provide insight into the relative importance o the processes that generate and maintain the exosphere. Theobserved spatial distributions or all three species dier rom one another, indicating that the source and loss processescontrolling the distributions aect each species in distinct ways or that there are other, currently unknown, processesthat play a role.


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Schematic summary o the processes that generate and maintain the exosphere o Mercury.

Processes at Work in Mercurys Exosphere

Atoms released with

low energy generallyreturn to the surface

Atoms released with

higher energy can make

it to higher altitudes These more energetic atoms

are airborne long enough

to be aected by solar

radiation pressure

Dierent atoms are accelerated

in the anti-sunward direction

to varying degrees; those

most strongly accelerated

can form an extended tail

Some atoms will be

photoionized and be removed

via the magnetic eld

Atom Trajectories

Source Processes

Photon-Stimulated

Desorption and

Thermal Evaporation

Low-EnergyProcesses

Meteoroid

Vaporization

Ion Sputtering

++

+

High-Energy

Processes

Sun


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The histograms in the upper part o this gure represent typical observations o emission in Mercurysexosphere rom magnesium (let), calcium (center), and sodium (right) atoms. Known as spectrallines, these emissions have been scaled to approximately the same peak level or ease o comparison;however, the sodium emission is much brighter than that o either magnesium or calcium. Eachemission occurs at a characteristic wavelength. Sodium (which is actually two lines known as D1and D2) and calcium are in the visible portion o the spectrum, at approximately yellow and bluewavelengths, respectively, whereas magnesium alls in the ultraviolet portion o the spectrum thatcannot be observed by ground-based telescopes because o blockage by Earths atmosphere. The threepanels in the lower part o the gure show the spatial distributions o emission in the polar and tailregions o Mercury rom these three elements during MESSENGERs third fyby. In these panels, therainbow color scale represents brightness rather than wavelength. Individual rectangles indicate the

relative brightness o each measurement and the region over which the measurement was made. Thesodium emission shows strong peaks over the polar regions with a rapid all-o in the tail region. Incontrast, the calcium emission shows a more gradual all-o toward the tail but relatively more emissionconcentrated near the equatorial regions. The magnesium emission is dierent rom either o the othertwo in that it appears to have a less rapid all-o than sodium between the polar and tail regions butlacks the equatorial concentration within the tail region o calcium. These dierences indicate that theprocesses controlling these exospheric distributions act on each species in distinct ways.

ultraviolet

InstrumentCounts

Calcium

Wavelength

Magnesium

Wavelength

Sodium

Wavelength

D2 D1

Simultaneous Observations of Multiple Species

Calcium

~12,000 miles ~12,000 miles

SodiumMagnesium

~12,000 miles


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complete with seasonal variations The emissions observed by the UVVS are primarily solar resonance lines, so-called because the atoms in Mercurys

exosphere absorb sunlight at specic wavelengths and then re-radiate a raction o that ligh

messenger mercury orbit insertion press kit

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