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The Moon and Mercury
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Distance between Earth and Moon measured accurately using lasers
Viewed from Earth, Mercury is never far from the Sun
Orbital Properties
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Moon Mercury Earth
Radius 1700 km 1440 km 6380 km
Mass 7.3 × 1022 kg 3.3 × 1023 kg 6.0 × 1024 kg
Density 3300 kg/m3 5400 kg/m3 5500 kg/m3
Escape Speed
2.4 km/s 4.3 km/s 11.2 km/s
Physical Properties
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•Maria - large dark flat areas, due to lava flow
•Early observers thought they were oceans
Surface Features on the Moon and Mercury
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Many craters (from meteorite impacts)
Surface Features on the Moon and Mercury
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Far side of Moon has some craters but no maria
Surface Features on the Moon and Mercury
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Mercury cannot be imaged well from Earth; best pictures are from Mariner 10
Surface Features on the Moon and Mercury
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Cratering on Mercury is similar to that on Moon
Surface Features on the Moon and Mercury
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•Tidally locked to Earth
•Rotation rate is the same as the time it takes to make one revolution
•Same side of the Moon always faces Earth
Moon’s Rotation Rates
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•Thought to be tidally locked to the Sun
•Disproved in 1965
•Mercury’s day and year are in a 3:2 resonance
•Rotates three times while going around the Sun twice.
Mercury’s Rotation Rates
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Lunar Exploration
Soviets had first contact with Moon:
First spacecraft to fly past Moon: January 1959
First spacecraft to (crash) land on Moon: September 1959 (sigh)
First pictures of far side of Moon: October 1959
United States the only country to send people to the Moon:
First person on Moon: July 1969
Last person on Moon: December 1972
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• Craters about 10 times as wide as the meteoroid creating them, and twice as deep
• Rock is pulverized to a much greater depth
• Most lunar craters date to at least 3.9 billion years ago; much less bombardment since then
Lunar Cratering and Surface Composition
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Meteoroid strikes Moon, ejecting material; explosion ejects more material, leaving crater
Lunar Cratering and Surface Composition
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Lunar Cratering and Surface Composition
Craters come in all sizes, from the very large…
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Regolith: Thick layer of dust left by meteorite impacts
Moon is still being bombarded, especially by very small “micrometeoroids”; softens features
Lunar Cratering and Surface Composition
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Meteorites also hit Earth; this crater is in Arizona: Most famous in US (hint for exam)
Lunar Cratering and Surface Composition
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More than 3 billion years ago, the moon was volcanically active; the rille here was formed then:
Lunar Cratering and Surface Composition
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Mercury is less heavily cratered than the Moon
Some distinctive features:Scarp (cliff), several hundred kilometers long and up to 3 km high
The Surface of Mercury
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Caloris Basin, very large impact feature; weird terrain on opposite side of planet
The Surface of Mercury
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The Surface of Mercury
“Weird terrain” is thought to result from focusing of seismic waves
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Moon’s density is relatively low, and it has no magnetic field—cannot have sizable iron/nickel core
Crust is much thicker than Earth’s
Interiors
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Mercury is much denser than the Moon and has a weak magnetic field—not well understood!
Interiors
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Current theory of Moon’s origin:
•Glancing impact (Big Rock) of Mars-sized body on the still-liquid Earth
•Causing enough material, mostly from the mantle, to be ejected to form the Moon
Computer model:
The Origin of the Moon
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Time before present:
Event:
4.6 billion yr Formation of Moon; heavy bombardment liquefies surface
3.9 billion yr Bombardment much less intense; lunar volcanism fills maria
3.2 billion yr Volcanic activity ceases
Evolutionary History of the Moon
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Mercury much less well understood:
• Formed about 4.6 billion years ago
• Melted due to bombardment, cooled slowly
• Shrank, crumpling crust
Evolutionary History of Mercury
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Assignment
• R&D Questions– 1,2,3,5,7,10,19