Transcript Document 7217203

Terrestrial Planetary Geology:
Moon, Mercury, Venus & Mars
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no longer active.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
Huge Aitken basin: 2000 km
across; 10 km deep!
Near lunar S pole.
• 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.
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.