Terrestrial Planetary Geology: Moon, Mercury, Venus & Mars

Small, rocky and no longer active. MOON and MERCURY Small, rocky and no longer active. So similar, it’s worth studying them together (even though one’s a moon and one’s a planet).

Key Similarities Small masses: Moon: 0.012; Mercury: 0.055 MEarth Small sizes: 0.27 and 0.38 REarth Therefore, low escape speeds: 2.4 and 4.2 km/s Highly cratered surfaces No atmospheres (other than temporarily attracted solar wind), SO there are Big temperature swings (100--400 K for Moon, 100--700 K for Mercury: the largest variation of all planets) Rotation and revolution locked in RESONANCES (1:1 and 3:2) No (Moon) or extremely weak (Mercury) magnetic field

Surfaces of Moon and Mercury

Key Differences Density: 3.3 g cm-3 for Moon (lower than any terrestrial planet) vs. 5.4 g cm-3 for Mercury (similar to Earth and Venus). Formation: Mercury probably similar to Earth, Venus and Mars. Moon was probably formed by a collision of a large planetesimal (maybe Mars size) with the proto-earth after much of the metals had differentiated and sunk. Somewhat higher crater density on the Moon: more impacts further out in Solar System. Maria --- dark lava flows: clearly seen on the Moon; Mercury has intercrater plains that are not clearly volcanic in origin, but it probably did have extensive volcanism early in its history.

SPIN-ORBIT RESONANCES or TIDAL LOCKING THE MOON The BODY TIDES the EARTH raises in the MOON have forced it into SYNCHRONOUS ROTATION: Its rotational period = its orbital period OR the Moon’s “day” = it’s “year” = 1 month We see only one face of the Moon (approximately) The Moon's bulge is larger than that the Earth could currently induce: it probably `froze in' billions of years ago when the Moon was only ~250,000 km away and just solidifying.

The Synchronized Moon There is a wobbling, mostly due to the Sun's differential force, called LIBRATION, which means that over the entire year we see 59% of the Moon's surface. Most moons in the solar system are similarly TIDALLY LOCKED to their planets. Pluto and Charon are mutually SYNCHRONIZED.

Tidally Locked Mercury MERCURY has been TRAPPED into a slightly more complicated 3:2 SPIN--ORBIT RESONANCE due to body tides raised by the Sun Pspin = 58.6 days; Porbit = 88.0 days; 1 Mercury Day (noon-noon) = 2 Mercury Yrs = 176.0 days Mercury's highly eccentric orbit means that it can never be synchronized at all times, but tidal forces are strongest near PERIHELION and the 3 spins = 2 orbits closely correspond to synchronicity at perihelion. Similar RESONANCES explain the gaps in Saturn's rings, and gaps in the ASTEROID BELT.

Mercury’s 3:2 Spin-Orbit Resonance

Mercury Is Hard to See: Tight Orbit Means it’s w/in 28 deg (< 2 hr) of Sun

Determining Planetary Rotation via Radar

Craters and Surface Dating No atmosphere means that the Moon and Mercury are continually bombarded with meteoroids of all sizes. The most heavily cratered areas are the oldest parts of the surface. Less heavily cratered areas (MARIA on Moon) solidified later.

Lunar History

LUNAR MISSIONS RACE TO THE MOON: more political than scientific. JFK pledged to get there by the end of the 60's. The Soviet LUNA's were the first to pass, crash-land and photograph the backside in 1959. US: Unmanned RANGER missions -- crash landed in 1963/64. LUNAR ORBITERS sent detailed photos: 1966/67. SURVEYORs made soft landings and didn't sink into a REGOLITH (pulverized surface layers) in '66--'68 Manned MERCURY and GEMINI series. Debate on how to best get people there and BACK.

Apollo Missions APOLLO 11: Landed on 20 July 1969; N. Armstrong, B. Aldrin in LUNAR MODULE; M. Collins in ORBITER. Apollos 12, 14, 15, 16 and 17 also landed successfully, with the last man on the moon, H. Schmitt leaving on 14 December 1972.

Some More Recent Unmanned Missions CLEMENTINE (1994): wonderfully detailed photos in 11 bands, from IR through UV. It also had extremely accurate altimeters. LUNAR PROSPECTOR (1998) probed layers with radars & confirmed the presence of ICE CRYSTALS mixed with REGOLITH in craters in the POLAR REGIONS where they are permanently shaded from sunlight. Huge Aitken basin: 2000 km across; 10 km deep! Near lunar S pole.

SURFACES AND INTERIORS OF THE DEAD ONES MOON LUNAR ROCK compositions: same ELEMENTS and similar types as Earth. Key test that the universe is not too weird. But, fewer VOLATILE (more easily evaporated, like Aluminum) compounds and more REFRACTORY (hard to evaporate, like Titanium) ones are found on the Moon. MARIA are LAVA FLOWS, mainly basaltic. TERRAE or highlands comprise most of the CRUST.

Cratered and Lava Covered Lunar Surface

Lunar Interior SEISMIC monitors revealed minute moonquakes (or moonquivers) CRUST (60 km thick on Earth side, 150 km on back  fewer maria on the rear) RIGID LITHOSPHERE --- extends another 900 km or so. SOFT ATHENOSPHERE --- inner ~700 km The "moonquakes" are produced at the lithosphere/athenosphere boundary. Central ~300 km is a partially metallic CORE, perhaps still partially molten.

Cutaway View of Lunar Interior

MERCURY Only MARINER 10 took close-up images of Mercury -- and these were only good on one side. So our knowledge of Mercury was poor until current Messenger Mission: flybys turning to orbiter. Large cliffs (a.k.a. scarps) up to 3 km high, due to compression after cooling. Caloris basin --- 1400 km across, seismic shocks from impact produced "weird terrain" on the opposite side of the planet. Many fewer MARIA than on the Moon: thicker crust. Its craters are typically smaller than those on the Moon: its stronger gravity didn't let ejecta fly as far.

Impact Craters & Mercury’s Scarps

Caloris Basin and Weird Terrain: Huge Impact and Focused Seismic Waves

Mercury’s Interior Mercury's density is almost as high as the Earth's yet its mass is much less. Therefore it’s less compressed by gravity. Together these facts imply its CORE is larger: about 75% of its radius, or 1800 km.

Magnetic Fields The Moon has no large scale magnetic field. Mercury's very small MAGNETIC FIELD may be a remnant of a DYNAMO when it was FASTER SPINNING and MORE LIQUID inside. Tidally locked so it spins SLOWLY and since it’s small the liquid solidified quickly. BUT BOTH MERCURY AND THE MOON HAVE LOST ALMOST ALL THEIR HEAT OF FORMATION AND ARE NOW TECTONICALLY DEAD.