Surface activity on the Moon and Mercury mostly died off about 3 Ga • Rocky innermost planet • Half way between the Moon and Mars (radius 2400km) • Massive temperature extremes • -180C at night • +430C during the day …as opposed to… Surface history of Venus is only available from ~1.0 Ga onward Surface history of Mars spans its entire existence 0.38 RE 0.39 AU Surface activity and history of Earth destroyed by very active processes
Introduction to Mercury • 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
Outgoing Incoming Mariner 10 • 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 • 45% photographic coverage of variable resolution and illumination • Discovery of a dipole magnetic field
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
Mercury’s Abnormal Interior • 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.
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 • Core freezes into a solid inner core over time • Slowed by sulfur • Causes planetary contraction
Mercury’s Surface – Almost Lunar • 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
Spindown into a Cassini State • 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
Heavy bombardment • 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.
Smooth plains • 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
Global Contraction • 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
The Caloris Imppact • 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 Antipode • 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.
Surface Activity Winds Down • 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
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
Strange material at Mercury’s poles • Very bright terrestrial radar returns • Ice – from comets • Or maybe sulfur from meteorites Vasavada et al., 1999
Sungrazing comets • Kreutz group • Source of water?
Mercury’s Timeline Pre-Tolstojan • 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 Tolstojan Calorian 85% of Mercury’s history Mansurian Kuiperian