ptys 395a mercury: open questions and new data shane byrne – [email protected] background is...
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PTYS 395a Mercury: Open Questions and New Data
Shane Byrne – [email protected] 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