Lunar Geology ASTR 2120 Sarazin

Interior of the Moon Density low (3.3 gm/cc), very little iron No iron core Very small heat flow out of interior Little radioactive heating No magnetic field No molten iron core Only weak earthquakes Generated near core Monthly due to tides from Earth

Interior of the Moon Core (partially molten?) Semi-solid athenosphere Solid lithosphere Crust

Time in past (billion years) 4.6 Moon forms Lunar History Event 4.5-4.3 Moon melted, some differentiation 4.4-4.3 Crust forms 4.3-3.9 Highlands cratered 3.8-3.2 Cratering of mare, lava flows fill 3.2 Moon s interior cools, no more lava flows 3.2- present No geological activity, only a few craters

Formation of the Moon: Differences with Earth All lunar rocks igneous No sedimentary rocks Lunar rocks little water Lunar rocks lack volatile materials (easily evaporated)

Formation Theories: Fission Pros: Moon s average density is similar to outer layers of Earth (Daughter Theory) Cons: Moon has no water; no known mechanism to eject the Moon Fission

Formation Theories: Co-creation Pro: Moon s orbit near plane of Earth s equator; coevolution consistent with ideas of solar system formation Cons: No iron in Moon compared with Earth (Sister Theory) Co-creation

Formation Theories: Capture (Girl You Met in a Bar Theory) Pro: Moon has different composition formed elsewhere Con: Dynamically very difficult to get capture without collision Capture

Impact Hypothesis Large object hits Earth obliquely Vaporization of ejected rock drives out water and volatiles Resulting debris has density typical of Earth s outer layer Iron core sinks to center of Earth Combines best aspects of previous hypotheses

Large protoplanet strikes proto-earth

Size Matters Moon is much less geologically active than Earth Smaller less radioactive heating

General Theory: Geology of Terrestrial Planets and Moons Early bombardment era Later geology is driven by internal heating Radioactivity Residual heat from formation Tidal friction heating For radioactive heating, the bigger the planet, the more geology More mass more radioactive atoms

Geology of Terrestrial Planets and Moons

The Terrestrial Planets ASTR 2120 Visible Sarazin Venus Radar

Mercury: Geology and Rotation Spider crater on Mercury from MESSENGER

Mercury Orbit a = 0.39 AU Orbital eccentricity 0.206, most elliptical planet orbit Mass 0.055 M Earth Density 5.43 gm/cm 3, very dense interior

Earth-Based Observation of Mercury Orbit around Sun has phases like Moon Period = orbital period 88 days Small, distant, close to Sun few good Earth-based images

Rotation Period? Earth-based observations (before 1965) P rot = P orb = 88 days? Confused phases with surface features Synchronous rotation? Elliptical orbit odd?

Radar Measurement of Rotation (1965) Mercury Δt = 2 R/c Mercury

Rotation Radar measurements Time delay at edge radius of planet Doppler effect = rotation speed P rot = 59 days P orb Not Synchronous P rot = (2/3) P orb exactly! Spin-Orbit Resonance

Mercury s Interior New View from Mercury Messenger Spacecraft

Mercury s Interior High density large iron core, ¾ of planet s diameter Weak magnetic field 0.5% of Earth field Offset from center of planet Remnant permanent magnet?? Molten outer core? Due to slow rotation? Iron-nickel meteorite

Mercury s Surface Very similar to Moon

Caloris Basin

Jumbled Terrain Opposite Caloris Basin

Caloris Basin One of last major impacts? Source of asymmetry (elongation) in Mercury, spin-orbit resonance Location is correct Mariner 10 spacecraft confirmed

Mercury Radar Image

Ice on Mercury? Largely confirmed by Mercury Messenger spacecrart

Mariner 10

Mercury from Mariner 10 3 useful passes by Mercury Always saw same side of Mercury illuminated Only about half of planet imaged Resolution rather low

Mercury Messenger Inserted into orbit around Mercury March 2011

What a long strange trip its been First Flyby of Mercury on Monday, January 14 Launched August 3, 2004 Gravity breaking in fly-by of Earth (August 8, 2005), and two flybys of Venus (October 24, 2006; June 5, 2007) Three flybys of Mercury with gravity breaking January 14, 2008 October 6, 2008 September 29,2009 Insertion into orbit around Mercury, March 18. 2011 Crashed into Mercury, April 30, 2015

Science Questions Why is Mercury so dense? (almost pure iron) What s Mercury s life story? (I looked on FaceBook, but it was ambiguous). Image hidden side of Mercury. Was there a big smash in the past? Why does Mercury have a magnetic field? Despite being very hot most places, could Mercury have ice at the poles?

Hidden Side of Mercury

Venus

Often brightest star in sky Earth s twin? No moons Covered with clouds; albedo 0.76 Venus

Space Probes to Venus Mariner 2, 1962 Venera 7 (1970), 9, 10, 13, 14, 17, 18 Pioneer Venus (1978) Magellan (1989-94) Venus Express (ESA; 2005-2014)

Venera 13

Surface of Venus Barren landscape Angular edges low erosion Basaltic (lava) type rocks Low surface wind speeds

Visible Radar

Crater Counts on Earth and Venus

Venusian Surface History Large (>35 kilometer) impact craters pepper the surface of Venus. Venus' thick atmosphere stops smaller impactors. The uniformity points to a global resurfacing event ½ billion years ago.

Surface of Venus Two main continents Ishtar Terra Aphrodite Terra No clear evidence of plate tectonics

Topography of Venus

Shield Volcanoes Over 1600 major volcanoes Some actually observed to erupt?

Volcano Sapas Mons Vertical scale exaggerated by 15x

Volcano Maat Mons Vertical scale exaggerated by 15x

Shield Volcanoes Shield volcanoes form above lava hot spot On Earth, most volcanoes at plate edges (subduction)

Shield Volcanoes on Venus Earth: moving plates chain of smaller volcanoes Venus: no moving plates single larger volcanoes Mars: same as Venus

Venus Geology Continents, large shield volcanoes Relatively young surface (500 million yr) Active geology No sign of plate tectonics Lack of rigid crustal plates; surface too hot and soft to break?