PTYS 395a Mercury: Open Questions and New Data

Shane Byrne – shane@lpl.arizona.eduBackground is from NASA Planetary Photojournal PIA02418 and PIA00404

Overview

PYTS 395a – Mercury Overview 2

Surface activity on the Moon and Mercury mostly died off about 3 Ga

Surface history of Venus is only available from ~1.0 Ga onward

0.38 RE 0.39 AU

Surface history of Mars spans its entire existence

…as opposed to…

Surface activity and history of Earth destroyed by very active processes

Rocky innermost planet

Half way between the Moon and Mars (radius 2400km)

Massive temperature extremes -180C at night +430C during the day

PYTS 395a – Mercury Overview 3

Orbital period ~88 days is 3/2 times the rotational period

Orbit is eccentric (e=0.21)

Obliquity near 0 No seasons

Leads to hot and cold poles on the equator

Surface is lunar-like but with important differences

Surface units: Intercrater plains Smooth plains Caloris basin Global tectonic features

Introduction to Mercury

PYTS 395a – Mercury Overview 4

Mariner 10 had three fly-bys in 1974/5

Equatorial pass @ 700 km (on dark side)

South polar pass @ 50,000 km North polar pass @ 400 km

Ironically the mission was not really designed for photogeology

Outgoing Incoming

45% photographic coverage of variable resolution and illumination

Discovery of a dipole magnetic field

Mariner 10

PYTS 395a – Mercury Overview 5

PYTS 395a – Mercury Overview 6

Themes to cover Formation and History

Interior – a very abnormal planet…. Large core Magnetic field Still molten?

Surface – like the Moon… but not really… Tectonics Volcanics Cratering Composition

Atmosphere – exotic ice and metals Volatiles baked out of rocks Unusual material (probably water ice) in polar craters

PYTS 395a – Mercury Overview 7

Mercury’s uncompressed density (5.3 g cm-3) is much higher than any other terrestrial planet.

For a fully differentiated core and mantle Core radius at least ~75% of the planet Core mass at least ~60% of the planet

3 possibilities Differences in aerodynamic drag between

metal and silicate particles in the solar nebula.

Differentiation and then boil-off of a silicate mantle from strong disk heating and vapor removal by the solar wind.

Differentiation followed by a giant impact which can strip away most of the mantle.

Mercury’s Abnormal Interior

PYTS 395a – Mercury Overview 8

Core freezes into a solid inner core over time Slowed by sulfur Causes planetary contraction

Core still liquid? Cooling models say probably not

Unless there’s a lot of (unexpected) sulfur Dipole field observed by Mariner 10 spacecraft says

yes… …but that could be a remnant crustal field. New Earth-based radar observations of longitudinal

librations – core is still partly molten

PYTS 395a – Mercury Overview 9

Radar returns indicate regolith-like surface i.e. rough terrain composed of unconsolidated fragments

Spectrally very similar to the lunar highlands

Similar albedo and morphologies i.e. craters and basins everywhere

Old surfaces (craters very degraded) not heavily cratered

Smooth plains that look volcanic but have no basalt signature – no maria

Global sets of tectonic features preserved Global grid of aligned very old faults Global grid of unaligned compressional faults

Mercury’s Surface – Almost Lunar

PYTS 395a – Mercury Overview 10

Mercury likely started with a faster spin.

Solar tides de-spun the planet to its current (59 days) spin rate

Ancient global lineament system observed

Planet bulges less at the equator when spinning slowly

Stresses created when rigid lithosphere readjusts to new shape

Orientations of lineaments are a good match to model predictions

Spindown into a Cassini State

PYTS 395a – Mercury Overview 11

Covers events occurring before the Tolstoj impact basin (~500 km) was formed

Mercury looks very much like the lunar highlands Similar number of large basins (>500 km)

Inter-crater plains are deposited Removes any basins < 500 Km Plains material likely volcanic

although there’s no proof of this.

A handful of other large basins accumulate after plains deposition.

Heavy bombardment

PYTS 395a – Mercury Overview 12

Begins with formation of Tolstoj basin (~500 km)

Smooth plains start to be emplaced Probably volcanic Why not dark ??

Period ends with Caloris impact

Smooth plains Tolstoj impact basin

Smooth plains

PYTS 395a – Mercury Overview 13

Extensive set of lobate scarps exist. No preferred azimuth Global distribution Sinuous or arcuate in plan Interpreted as thrust faults

Evidence for an episode of global compression Planetary shrinkage of 1-2 Km

Discovery Rupes

Global Contraction

PYTS 395a – Mercury Overview 14

Caloris impact was a major event for Mercury ~3.9 Ga

Impact structure is 1300 Km across Six concentric rings 630-3700 Km across

Smooth plains material erupts after some delay Followed by compression (subsidence) Followed by extension (rebound)

Extensional

Fractures

Compressional Ridges

The Caloris Imppact

PYTS 395a – Mercury Overview 15

Seismic waves from the Caloris impact all meet at the antipode at the same time.

Modeling suggests vertical motions of up to 1km

Terrain broken up into 1km sized blocks Official name is ‘Hilly and furrowed’ terrain.

Mariner 10 team called it ‘weird’ terrain.

The Caloris Antipode

PYTS 395a – Mercury Overview 16

Most of the geological action for Mercury is now over

Other geologic periods are relatively quiescent Last lobate scarps form Low cratering rate similar to

today Most recent craters (e.g.

Kuiper) have bright rays

Surface Activity Winds Down

PYTS 395a – Mercury Overview 17

Mercury (and the Moon) possesses a tenuous atmosphere

Calcium now also seen at Mercury

Sodium emission at the Moon and Mercury shows temporal changes

Stirring of regolith by small impacts

PYTS 395a – Mercury Overview 18

Strange material at Mercury’s poles Very bright terrestrial radar returns Ice – from comets Or maybe sulfur from meteorites

Vasavada et al., 1999

PYTS 395a – Mercury Overview 19

Sungrazing comets Kreutz group Source of water?

PYTS 395a – Mercury Overview 20

Taken 2 days ago

PYTS 395a – Mercury Overview 21

Mercury forms, perhaps with a large core or suffers a giant impact Lithosphere forms Despinning results in shape change and global tectonism Heavy bombardment

Homogenizes regolith up to 20 km Large basins form Volcanic flooding – inter-crater plains

Basins <500km removed Core shrinks 1-2 km

Global system of thrust faults forms lobate scarps

Caloris impact structure forms Antipodal ‘weird’ terrain Smooth plains form Subsidence and rebound in Caloris basin

Lighter cratering continues Bright rayed craters

Polar volatiles accumulate

Kuiperian

Pre-Tolstojan

Tolstojan

Calorian

Mansurian

Mercury’s Timeline

85% of Mercury’s

history