PTYS 395a Mercury: Open Questions and New Data Shane Byrne – Background 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 R E 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 l Rocky innermost planet l Half way between the Moon and Mars (radius 2400km) l Massive temperature extremes n -180C at night n +430C during the day

PYTS 395a – Mercury Overview 3 l Orbital period ~88 days is 3/2 times the rotational period l Orbit is eccentric (e=0.21) l Obliquity near 0 n No seasons l Leads to hot and cold poles on the equator l Surface is lunar-like but with important differences l Surface units: n Intercrater plains n Smooth plains n Caloris basin n Global tectonic features Introduction to Mercury

PYTS 395a – Mercury Overview 4 l Mariner 10 had three fly-bys in 1974/5 n Equatorial 700 km (on dark side) n South polar 50,000 km n North polar 400 km l Ironically the mission was not really designed for photogeology Outgoing Incoming l 45% photographic coverage of variable resolution and illumination l Discovery of a dipole magnetic field Mariner 10

PYTS 395a – Mercury Overview 5

6 Themes to cover l Formation and History l Interior – a very abnormal planet…. n Large core n Magnetic field n Still molten? l Surface – like the Moon… but not really… n Tectonics n Volcanics n Cratering n Composition l Atmosphere – exotic ice and metals n Volatiles baked out of rocks n Unusual material (probably water ice) in polar craters

PYTS 395a – Mercury Overview 7 l Mercury’s uncompressed density (5.3 g cm -3 ) is much higher than any other terrestrial planet. l For a fully differentiated core and mantle n Core radius at least ~75% of the planet n Core mass at least ~60% of the planet l 3 possibilities n Differences in aerodynamic drag between metal and silicate particles in the solar nebula. n Differentiation and then boil-off of a silicate mantle from strong disk heating and vapor removal by the solar wind. n Differentiation followed by a giant impact which can strip away most of the mantle. Mercury’s Abnormal Interior

PYTS 395a – Mercury Overview 8 l Core freezes into a solid inner core over time n Slowed by sulfur n Causes planetary contraction l Core still liquid? n Cooling models say probably not wUnless there’s a lot of (unexpected) sulfur n Dipole field observed by Mariner 10 spacecraft says yes… n …but that could be a remnant crustal field. n New Earth-based radar observations of longitudinal librations – core is still partly molten

PYTS 395a – Mercury Overview 9 l Radar returns indicate regolith-like surface i.e. rough terrain composed of unconsolidated fragments l Spectrally very similar to the lunar highlands l Similar albedo and morphologies i.e. craters and basins everywhere l Old surfaces (craters very degraded) not heavily cratered l Smooth plains that look volcanic but have no basalt signature – no maria l Global sets of tectonic features preserved n Global grid of aligned very old faults n Global grid of unaligned compressional faults Mercury’s Surface – Almost Lunar

PYTS 395a – Mercury Overview 10 l Mercury likely started with a faster spin. l Solar tides de-spun the planet to its current (59 days) spin rate l Ancient global lineament system observed n Planet bulges less at the equator when spinning slowly n Stresses created when rigid lithosphere readjusts to new shape n Orientations of lineaments are a good match to model predictions Spindown into a Cassini State

PYTS 395a – Mercury Overview 11 l Covers events occurring before the Tolstoj impact basin (~500 km) was formed l Mercury looks very much like the lunar highlands n Similar number of large basins (>500 km) l Inter-crater plains are deposited n Removes any basins < 500 Km n Plains material likely volcanic although there’s no proof of this. l A handful of other large basins accumulate after plains deposition. Heavy bombardment

PYTS 395a – Mercury Overview 12 l Begins with formation of Tolstoj basin (~500 km) l Smooth plains start to be emplaced n Probably volcanic n Why not dark ?? l Period ends with Caloris impact Smooth plainsTolstoj impact basin Smooth plains

PYTS 395a – Mercury Overview 13 l Extensive set of lobate scarps exist. n No preferred azimuth n Global distribution n Sinuous or arcuate in plan n Interpreted as thrust faults l Evidence for an episode of global compression n Planetary shrinkage of 1-2 Km Discovery Rupes Global Contraction

PYTS 395a – Mercury Overview 14 l Caloris impact was a major event for Mercury n ~3.9 Ga l Impact structure is 1300 Km across n Six concentric rings Km across l Smooth plains material erupts after some delay n Followed by compression (subsidence) n Followed by extension (rebound) Extensional Fractures Compressional Ridges The Caloris Imppact

PYTS 395a – Mercury Overview 15 l Seismic waves from the Caloris impact all meet at the antipode at the same time. l Modeling suggests vertical motions of up to 1km l Terrain broken up into 1km sized blocks l Official name is ‘Hilly and furrowed’ terrain. n Mariner 10 team called it ‘weird’ terrain. The Caloris Antipode

PYTS 395a – Mercury Overview 16 l Most of the geological action for Mercury is now over l Other geologic periods are relatively quiescent n Last lobate scarps form n Low cratering rate similar to today n Most recent craters (e.g. Kuiper) have bright rays Surface Activity Winds Down

PYTS 395a – Mercury Overview 17 l Mercury (and the Moon) possesses a tenuous atmosphere Calcium now also seen at Mercury l Sodium emission at the Moon and Mercury shows temporal changes n Stirring of regolith by small impacts

PYTS 395a – Mercury Overview 18 l Strange material at Mercury’s poles n Very bright terrestrial radar returns n Ice – from comets n Or maybe sulfur from meteorites Vasavada et al., 1999

PYTS 395a – Mercury Overview 19 l Sungrazing comets n Kreutz group n Source of water?

PYTS 395a – Mercury Overview 20 Taken 2 days ago

PYTS 395a – Mercury Overview 21 l Mercury forms, perhaps with a large core or suffers a giant impact l Lithosphere forms l Despinning results in shape change and global tectonism l Heavy bombardment n Homogenizes regolith up to 20 km n Large basins form n Volcanic flooding – inter-crater plains wBasins <500km removed l Core shrinks 1-2 km n Global system of thrust faults forms lobate scarps l Caloris impact structure forms n Antipodal ‘weird’ terrain n Smooth plains form n Subsidence and rebound in Caloris basin l Lighter cratering continues l Bright rayed craters l Polar volatiles accumulate Kuiperian Pre-Tolstojan Tolstojan Calorian Mansurian Mercury’s Timeline 85% of Mercury’s history