PYTS/ASTR 206 – Mercury 1 l Announcements n Homework #2 late submissions due now w50% credit n Mid-term #1 on Thursday wBased on the first 10 lectures (incl. this one) wMultiple choice - bring #2 pencils wLasts 1 hour and starts on time wYou won’t need a calculator wAny questions on formulas will include the formula wPractice exam available on class website wDon’t memorize numbers… …but understand quantities relative to each other e.g. IR wavelength longer than UV wavelength 1AU much bigger than the Earth-Moon distance

PYTS/ASTR 206 – Mercury 2 PTYS/ASTR 206 – The Golden Age of Planetary Exploration Shane Byrne – Mercury

PYTS/ASTR 206 – Mercury 3 In this lecture… l Mercury’s strange orbit n Extreme temperatures n Hot and cold longitudes l Mercury’s even stranger interior n Giant core n Magnetic field l Mercury’s surface n Like the Moon – but not quite n A planet that shrunk n Newly found Volcanoes n Caloris basin and the ‘weird’ terrain

PYTS/ASTR 206 – Mercury 4 l Closest to the Sun n Average distance 0.39 AU l Mercury’s orbit is odd compared to other planets n Both eccentric and inclined Mercury’s Orbit PlanetInclinationEccentricity Mercury7°0.21 Venus3.4°0.01 Earth0°0.02 Mars1.9°0.09 Jupiter1.3°0.05 Saturn2.5°0.06 Uranus0.8°0.05 Neptune1.8°0.01

PYTS/ASTR 206 – Mercury 5 l Mercury’s surface gets extremely hot n Perihelion - closest to the sun n Mercury is 0.31 AU from the sun n Aphelion – Furthest from the sun n Mercury is 0.47 AU from the sun l Remember this? n R is the solar distance in AU n Mercury at perihelion 14,225 W m -2 wTemperatures as high at 700K n Mercury at aphelion 6188 W m -2 wTemperatures as high as 570K n Night-time temperatures on Mercury as low as 100K Solar power = 1367 W m -2 / R 2

PYTS/ASTR 206 – Mercury 6 l Mercury is hard to observe from the Earth with telescopes n Always visible for less than 2 hours n Either just before dawn or just after Sunset n Always close to the horizon

PYTS/ASTR 206 – Mercury 7 l We have other tools available (I) n Radar observations - Arecibo n With radar we supply the ‘light’ and wait for the reflection

PYTS/ASTR 206 – Mercury 8 l Radar observations tell us about Mercury’s spin period n Doppler shift in wavelength – caused by motion n Wavelength is shorter when source and observer approach n Wavelength is shorter when source and observer recede l The amount of wavelength shift of the radar tells us the rotation speed

PYTS/ASTR 206 – Mercury 9 l Another example of the Doppler shift n Convection cells on the Sun have.. wPortions that rise towards the surface – approach observers on the Earth wPortions that sink away from the surface – recede from observers on the Earth n Wavelength of the emitted radiation is shifted slightly

PYTS/ASTR 206 – Mercury 10 l Back to Mercury… n Doppler shift of the radar reflections n Mercury rotates every 59 days n Orbital period was 88 days n Ratio is 3:2 n i.e. three rotations every 2 orbits l This was a surprise n People expected a ratio of 1:1 wSynchronous rotation wLike Earth’s Moon NOT THE CASE FOR MERCURY

PYTS/ASTR 206 – Mercury 11 l Mercury does 1½ rotations every orbit l At perihelion position n Sub-solar longitude is 0°, then 180°, then 0°, then 180° then 0° etc… n These are the ‘hot’ parts of Mercury l At aphelion position n Sub-solar longitude is then 90°, then 270°, then 90°, then 270° then 90° etc… n These are the ‘cold’ parts of Mercury Radio emission from Mercury Senses temperature from the upper few meters of the surface Shows the ‘hot’ longitudes 180° apart 100 K difference

PYTS/ASTR 206 – Mercury 12 l We have other tools available (II) n Spacecraft - Mariner 10 w3 flybys in 1974 & 1975 wOnly mapped 50% of the planet n Messenger w3 flybys in 2008 & 2009 wEnters orbit around Mercury wAlready confirmed all the Mariner 10 results wFilled in the gaps in image coverage

PYTS/ASTR 206 – Mercury 13 l Mariner 10 imagery n Most of our Mercury results still come from this mission

PYTS/ASTR 206 – Mercury 14 l Mercury is an anomaly among the terrestrial planets n Usually larger planets are denser n High gravity compresses material l Mercury’s density 5430 kg m -3 n Almost as dense as the Earth n Rock density ~3000 Kg m -3 l Mercury’s radius is 2440 km n Only 40% of Earth’s radius n Only 6% of Earth’s volume Mercury’s Interior

PYTS/ASTR 206 – Mercury 15 l What’s going on? n Mercury’s iron core is enormous compared to the planet n Core radius wEarth 54% of planet wMercury 75% of Planet n Core Volume wEarth 16% of the planet wMercury 42% of the Planet n Mercury has a thin mantle n Probably a very thin (~100km) crust

PYTS/ASTR 206 – Mercury 16 l How did this happen? n Maybe Mercury formed that way n Maybe rock boils away in hot early phase n Maybe a giant impact occurred like on the Earth l Giant impact theory is most popular n Large object hits an already differentiated Mercury n Iron core is protected n Mantle of rocks is stripped off

PYTS/ASTR 206 – Mercury 17 l Another variation of the giant impact theory n Mercury is completely destroyed n Reforms from iron rich debris n Rocky debris lost l Giant impacts in the very early solar system could be commonplace n Earth – large Moon n Mercury – large iron core n Venus – retrograde spin n Mars – topographic dichotomy n Uranus – spin axis in orbital plane n Probably not all of these were caused by giant impacts

PYTS/ASTR 206 – Mercury 18 l Mariner 10 also discovered a dipole magnetic field n Weaker version of Earth’s field, ~1% as strong n Implies part of Mercury’s core is still molten n A surprise considering that Mercury is small and so should cool off fast n Liquid core confirmed with terrestrial radar recently

PYTS/ASTR 206 – Mercury 19 l Much like the Moon n Radar returns indicate regolith-like surface i.e. rough terrain composed of unconsolidated fragments n Spectrally very similar to the lunar highlands n Lots of craters …but… n Smooth plains and cratered regions are mixed on Mercury n Smooth plains look volcanic but aren’t dark like the lunar Maria wAll the iron is in the core Mercury’s Surface

PYTS/ASTR 206 – Mercury 20 l Intercrater plains n More cratered than lunar Maria n Not dark – no iron n Origin argued about for years: n They look volcanic n But ejecta from large impacts can also form plains wE.g. Cayley plains – Apollo 16

PYTS/ASTR 206 – Mercury 21 Lunar Mare formation period Known from Apollo samples Lunar Maria Lunar Highlands Cratering rate low after these lava flows form Mercury’s Plains formation period Mercury’s Plains

PYTS/ASTR 206 – Mercury 22 l Smooth plains n Erupted as flood volcanics n Lightly cratered n Still little iron

PYTS/ASTR 206 – Mercury 23 l Messenger uncovers volcanic features n One mystery solved but many remain

PYTS/ASTR 206 – Mercury 24 l Tectonics on Mercury l Mercury likely started with a faster spin. n 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

PYTS/ASTR 206 – Mercury 25 l Core cools with time n Inner core of frozen iron grows n Planet shrinks a little n Usually not a problem… w…but Mercury has a huge core n Mercury shrunk several km in radius l Shrinkage compresses the rigid lithosphere n Causes thrust faults to form n Extensive set of lobate scarps n No preferred azimuth n Global distribution Discovery Rupes

PYTS/ASTR 206 – Mercury 26 l Mercury has many impact basins n Much like the Moon l One stands out – The Caloris impact n Impact structure is 1550 Km across n Six concentric rings Km across wMountain chains up to 2km high n Dated at Gyr ago wLate heavy bombardment

PYTS/ASTR 206 – Mercury 27 l Caloris was flooded with volcanic material soon after forming n Causes subsidence of basin and compress features – wrinkle ridges l Much later volcanism around the edges Extensional Fractures Compressional Ridges

PYTS/ASTR 206 – Mercury 28 l Rebound of the lithosphere comes later n Causes extensional cracks Extensional Fractures at basin edge “The Spider” Extensional Fractures at basin center

PYTS/ASTR 206 – Mercury 29 l The “Weird” terrain n Properly “Hilly and Lineated” terrain n Seismic waves from the Caloris impact all meet at the antipode at the same time. wModeling suggests vertical motions of up to 1km wTerrain broken up into 1km sized blocks

PYTS/ASTR 206 – Mercury 30 l Mercury forms, perhaps with a large core or suffers a giant impact l Lithosphere forms l De-spinning 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 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 e.g. Kuiper n Named after our founder Kuiperian Pre-Tolstojan Tolstojan Calorian Mansurian Mercury’s Timeline

PYTS/ASTR 206 – Mercury 31 l The Moon and Mercury have a lot in common n Both origins connected to giant impact n Dominated by impacts with regolith surfaces n Similar surface materials n Both in a some form of spin orbit resonance n Both have been partly resurfaced by flood volcanism n Both geologically dead for Gyr l But their histories and internal structure are different MoonMercury Perhaps no core?Dominated by a huge iron core Basaltic volcanismPlains volcanism not compositionally distinct No significant tectonics Global liniment system from spin-down Global thrust-fault system from contraction Very iron poorMost iron-rich planet No current magnetism Earth-like dipole field LRO Messenger

PYTS/ASTR 206 – Mercury 32 l Teaser for next lecture n Mercury has a thin atmosphere… n …and water ice deposits in its polar craters

PYTS/ASTR 206 – Mercury 33 In this lecture… Next: Craters l Reading l Chapter 11-1, 11-2, 11-3 to revise this lecture l Chapter 11-6 & 11-7 for next lecture l Mercury’s strange orbit n Extreme temperatures n Hot and cold longitudes l Mercury’s even stranger interior n Giant core n Magnetic field l Mercury’s surface n Like the Moon – but not quite n A planet that shrunk n Newly found Volcanoes n Caloris basin and the ‘weird’ terrain