ast 248, lecture 18 · lunar chronology i moon formed 4.4{4.5 byrs ago, by impact of minor planet...

AST 248, Lecture 18

James Lattimer

Department of Physics & Astronomy449 ESS Bldg.

Stony Brook University

April 13, 2020

The Search for Intelligent Life in the [email protected]

James Lattimer AST 248, Lecture 18

Meteorite Types

There are about 1050 observed falls and about 31,000 finds.I Stony meteorites

I Chondrites (85.7% of falls, 52% of finds) oldest, 4.55 Byrs old.I Ordinary contain both volatile and oxidized elements, formed in inner

belt, and are the most common type.I Carbonaceous contain more volatiles, formed in outer belt.I Enstatite contain more refractory elements, formed in the inner SS.

I Achondrites (7.1% of falls, 1% of finds) show evidence of igneousprocesses.

I HED groupI SNC group (Martian)I AubritesI UrelitesI Eucrites (Vesta)

I Stony iron (1.5% of falls, 5% of finds)I PallasitesI Mesosiderites

I Iron (5.7% of falls, 42% of finds) formed in meteoriteparent body cores.


Meteorite PropertiesI primitive: unaltered (mostly chondrites, esp. carbonaceous

chondrites)I differentiated: altered, products of melting, separation

Interesting discoveries:I Organic compounds in carbonaceous chondrites (Allende,

Murchison) are equally left- and right-handedI Isotopic anomalies in carbonaceous chondrites: 16,17,18O, 26Mg

(from 26Al, halflife 720,000 yrs)I SNC meteorites traced to MarsI Possible microfossils, complex hydrocabons, biominerals in SNC

meteorite ALH 84001I Eucrites traced to VestaI Sayh al Uhaymir 169 is a meteorite from the MoonI The meteorite Kaiduri is possibly from Mars’ moon Phobos

On September 15 2007 a chondriticmeteorite hit near Carancas, Peruand poisioned about 30 villagers thatapproached the impact site (600more had provoked psychosomaticailments). The impact caused releaseof arsenic-contaminated groundwater from natural deposits. Thelocal province is turning this into a tourist attraction.


Tidal Forces

Ft =GMm

(R − r)2− GMm

(R + r)2

= GMm

[(R + r)2 − (R − r)2

(R − r)2(R + r)2

]≈ GMm4Rr/R4 = GMm4r/R3.

Tides generate a tidal bulge along the equator. As the object rotates, thebulge is dragged around the object opposite to the spin. The pushing andpulling of the planetary material generates frictional heat and produces atorque (a force that changes angular momentum). The strong tides havesynchronized, or tidally locked, the rotation of the Moon with its orbitalperiod, 28 Earth days. Tides are pushing the Moon away.


Effects of Tidal ForcesCause tides or tidal bulges, such as changing sea levels, also induceresonances: rotations and orbits become synchronized in integer ratios.

I Mercury in a 3:2 rotation:orbit resonanceI Moon locked to Earth; a 1:1 rotation:orbit resonanceI Io, Europa, Ganymede, Callisto and 4 others with 1:1 rotation:orbit

resonance with Jupiter; Mimas, Enceladus, Tethys, Dione, Rhea,Titan, Iapetus and 8 others with Saturn; Miranda, Ariel, Umbrieland Titania with Uranus; Proteus and Titania with Neptune; Phobos& Deimos with Mars

I Io, Europa & Ganymede locked into a 1:2:4 Laplace orbital resonanceI Gleise 876e, b & c locked into 1:2:4 orbital resonance with Gleise

876aI Charon and Pluto locked to each other; Nix & Hydra in 1:4 and 1:6

resonancesI Pluto and many Kuiper Belt Objects in a 3:2 orbital resonance with

NeptuneI Tau Bootis locked to an orbiting giant planetI Asteroids: Kirkwood gaps at 3:1, 5:2, 7:3 and 2:1; clumps at 3:2,

4:3 and 1:1I In Saturn’s rings, Cassini division at 2:1 resonance with Mimas;

Encke and Keeler gaps at 1:1 resonance with moons Pan andDaphnis


Tidal Heating

Tides can also induce heating of the interior of orbitingobjects.

Satellites spin at a constant speed, but in an eccentric orbitthe orbital speed varies with distance from planet (Kepler’s2nd Law).

Tidal bulges are pulled back and forth; the resulting frictionheats the interior.

I An extreme case is the Io, Europa and Ganymede systemwhich is in a 4:2:1 orbital resonance that makes Iotremendously volcanically active and maintains a liquidshell under Europa’s icy crust.

I Other situations may lead to heat energy sources for icegeysers on some moons of Saturn, Uranus and Neptune.


Resonances and Kirkwood Gaps

Io

Ganymede

Europa

Wikipedia

Outside 3.3 AU, asteroidsclump at resonance points3:2, 4:3 and 1:1 (Hilda family,279 Thule, Trojans)


I Roche limit: Minimum distance from a planet that a solid satellitecan orbit without tides pulling it asunder.Equate tidal force and self-gravity of 2 moonlets of mass m andradius r :

GMm4r

R3R

= Gm2

4r2, or RR = 24/3r

(M

m

)1/3

.

Planet’s average density is ρ⊕ = 3M⊕/(4πR3⊕)

moon’s average density is ρm = 3m/(4πr3)

Implies that the Roche limit is

RR = 24/3

(ρ⊕ρm

)1/3

R⊕ ≈ 2.5 − 3R⊕.

I Rings and satellites: Large satellites must orbit outside the RocheLimit. Rings can extend to the Roche Limit.

I All the Jovian planets have rings; Outer edge of Saturn’s A ring is at2.3RSaturn

I Eventual loss of prograde satellites: Frictional loss of angularmomentum leads to expanding orbits.

I Destruction of retrograde satellites: Retrograde satellites haveshrinking or decaying orbits and will eventually fall within the RocheLimit and be destroyed.

I Triton will be destroyed in 100 Myr to 1 Gyr.


The MoonI The moon is in synchronous rotation.I Only 1 side of moon visible from Earth.I Bright regions are heavily cratered highlands.I Dark regions, called mare, are lava-flooded lowlands.I Near side of moon is 31% mare; far side is 2% mare.I Craters due to impact; largest in solar system is the far side

South-Pole Aitken basin, 2240 km diameter and 13 km deep.I The moon’s surface (regiolith) is composed of broken rocks and dust.

Near Far


Lunar Interior Determined From SeismometryI Crust—50 km thick solid magma ocean

I Mantle—to depths of about 800 km

I Core—small, semi-liquid or plastic (as indicated by moon’stime-variable rotation)

Wikipedia


Lunar Surface CompositionI No sedimentary rocks (limestone, shale) on the Moon

I All lunar rocks are igneous (cooling lava)

I Nearly complete lack of water in lunar rocks, with the exception of asmall amount of water hidden in deep craters at lunar north andsouth poles (discovered by Clementine and Lunar Prospector)

I General lack of volatile elements (Hg, Na, K, . . . ) in lunar rocks

I General lack of iron and iron-loving (siderophile) elements (Ni, Mn,. . . ) in lunar rocks.


Theories of Moon’s OriginMust explain lack of iron and lack of volatiles and similarities of otherchemical and isotopic properties; also the large angular momentum ofEarth-moon system.

I Daughter, or fission. Needs too much spin, cannot explain lack ofvolatiles. Also predicts moon should orbit in equatorial plane ofEarth.

I Sister, or co-accretion. Cannot explain lack of iron.

I Capture theory. Difficult to make regular orbit, cannot explainisotopic similarity to Earth. Statistically improbable.

I Collision of a Mars-sized object(called Theia) with early Earth.Explains lack of volatiles andiron, preserves other similarities.Thought to occur 30-50 millionyears after Earth’s formation.

http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question38.html


Lunar Chronology

I Moon formed 4.4–4.5 Byrs ago, by impact of minor planetwith Earth (?)

I Original rocks obliterated by extreme meteoroidbombardment; oldest surviving rocks date to 4.3 Byrs.

I Highland regions are oldest and provide evidence ofextensive meteoroid bombardment until 3.9 Byrs ago.

I Radioactive heating melted interior of Moon, semi-liquidregions reaching depths of 200 km 3–3.5 Byrs ago.

I Last large impacts fractured crust, producing massive lavaflows from interior which filled large impact basins 3–3.5Byrs ago. This created the lunar mare.

I Moon has been geologically quiet since 3.1 Byrs ago, withthe exception of alterations caused by occasionalmeteoroid impacts, solar wind, cosmic rays and humanspacecraft. The portion of the interior that is semi-liquidis shrinking and is almost gone now.


MercuryI The innermost and smallest planet.I Rotation rate is slow: 59 Earth days, exactly 2/3 of the Mercury

year (the orbital period about the Sun), 88 days.I Heavily cratered surfaceI Surface similar to moon’s

with extensive mare regionsand heavy cratering.

I Cratering indicates extremelyold surface and long-timegeologic inactivity.

I Temperatures on Mercuryare extreme; 90 K at nightto 700 K by day (but stillnot quite as hot as Venus!).

I Tides have forced Mercury’spole perpindicular to orbitalplane; polar crater bottomsare permanently in shade withtemperatures near 150 Kallowing survival of ice.


I Largest impact basin is theCaloris Basin 1300 km in diameter(top). At the antipode of this basinis “weird terrain” (bottom) thoughtto be due to shock convergence.

I Mercury’s density is large,its uncompressed density is 20%larger than Earth’s, indicatingmassive iron content (up to 70%).

I Molten iron core with a size equalto 3/4 the planet’s radius; mantleabout 20%; crust about 5%.

I Large iron content indicatescatastrophic, collisional originfor Mercury. Over half its massmust have been lost.


Mercury’s RotationI Rotation rate deduced using radar from Earth. Rotation causes a

Doppler shift in radar’s frequency.

I Rotation is tidally locked from a strong tidal resonance due to Sun.

I The “solar” day is 2 Mercury years long, causing strange seasons.

I The 3:2 resonance is a consequence of the fact that at perihelion,the rotational surface velocity at the point nearest the Sun almostcancels the orbital velocity. At noon at perihion, the Sun appears tohang nearly motionless overhead.


VenusI Venus’ mass and radius are nearly the same as Earth.I Rotation rate is slow and retrograde: 243 Earth days, longer than its

year (225 Earth days). Perhaps caused by solar tides, atmosphericfriction or collisional impact. Has a near 13:8 orbital resonance withEarth, known to Babylonians in 1900 BC and the Maya, and a 13:5resonance between its synodic and orbital periods.

I Atmosphere is thick (90 times ⊕, equal to 900 m below sea level).I Highly reflective clouds nearly cancels Venus’ proximitiy to Sun, so

Venus absorbs only 5% more solar energy than the Earth.I 750 K temperature due to Greenhouse effect, raising temperature by

470 K (45 K on the Earth, and 5 K on Mars, for comparison).I Surface obscured by sulfuric acid clouds high in Venus’ atmosphere.I Near surface, atmosphere is clear, but atmosphere causes peculiar

bending of light so that it looks like the bottom of a fishbowl.I Surface is probed by radar from Earth and orbiting satellites, and is

90% basalt. It has no lakes, some craters and no mountains otherthan millions of volcanoes, some active, since surface is young.

I No plate tectonics, due to lack of water in crust, deduced by lowabundance of atmospheric Ar40 (from K40 decay).

I Evidence for “snow” made of tellurium or lead sulfide on volcanoes.I Water has disappeared but was abundant in past, as deduced from

the high D/H ratio in atmosphere, 100 times Earth’s value.I Venus evolved divergently from Earth because it’s too close to Sun.


Atmosphere of Venus

Atmospheric composition: 96.5% CO2, 3.5% N2,.015% SO2 (forms visible clouds with H2SO4),.007% Ar, .002% H2O, .0017% CO, .0012% He, .0007% Ne


Greenhouse Effect

Important greenhouse gases: H2O (36%), clouds(25%), CO2 (20%), O3

(2%), CH4 (1%), N2O (1.5%), fluorocarbons (0.5%)Clathrate greenhouse: release of CH4

from Siberian tundra could occur.

Runaway greenhouse effect on Venus:Rising temperature boils oceans, increases H2O concentration inatmosphere. Higher temperatures then sublimate CO2 from rocks,putting more CO2 in atmosphere.


How We Know Water Was Lost on Venus

Deuterium is heavy hydrogen and weights twice as much as normalhydrogen.Velocity of gas molecules in atmosphere is proportional to the square rootof their masses.Normal hydrogen atoms are thus preferentially lost from dissociatedwater molecules.

http://www.astronomynotes.com/solarsys/s9.htm


Radar Map of Venus


MagellanTopographicalMap


I Successful spacecraft exploration included Mariner 2, 5 and 10;Venera 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16; PioneerVenus Orbiter and Pioneer Venus Multiprobe; Vega 1 and 2;Magellan; Venus Express; Messenger; Atkatsuki (2015); Parker SolarProbe (flybys until 2024); BepiColombo (flybys in 2020 and 2021);Solar Orbit (8 flybys from 2020 to 2030).

I Planned exploration includes Shukrayaan-a (India, 2023); Venera-D(late 2020s)


Impact crater

Over 1600 craters known, none smallerthan 3km because of atmosphere shield.

Pristine condition of most volcanosindicates cyclic resurfacing fromcatastrophic subduction about 500 Myr ago.

Pancake volcanos

SO2 concentrations changed by10 between 1978 and 1986,indicating active volcanism.


Head et al., J. Geophys. Res. 97, 13153 (1992)


Volcanoes on VenusI Venus has more volcanoes than any planet in the solar system, over

55,000 larger than 1 km, and possibly a million in total.I Volcanoes form in fields, as a group, 100–200 km wide, which are

former hot spots where magma rising from mantle broke throughcrust.

I No active volcanoes are known, but some may be active. Evidencefor recent activity includes:

I variations in SO2 levels, a gas that is rapidly depleted from theatmosphere by photodissociation, have been seen by ESA’s VenusExpress

I two orbiters have detected radio bursts similar to lightning dischargeswhich often occur in connection with volcanic activity.

I High atmospheric pressure, lack of winds and water, make volcanicfeatures appear fresh.

I There is no obvious evidence for plate tectonics: linear volcanicchains, subduction zones or continents.

I Volcanic style is not eruptive, but involve fluid flows:I atmospheric pressure too high, requires very high gas content to

eruptI lack of water, the usual gas driving lava explosions on the EarthI no subduction zones, where explosive volcanism is common on the

EarthJames Lattimer AST 248, Lecture 18

These planets meet 5 times during 8 Earth years and during13 Venus years. Note the start of Fibonacci sequence 5, 8, 13...

Venus’ spin and rotation are nearly tidally locked by the Earth.Venus’ sidereal period is 243.0185 Earth-days, and its solar day is116.75 days. In 5 solar days, Venus and Earth are again closest.After 13 orbits, Venus meets Earth 5 times and rotates 25 times.


ast 248, lecture 18 · lunar chronology i moon formed 4.4{4.5 byrs ago, by impact of minor planet...

Documents