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Chapter 8 The Moon and Mercury Moon Mercury

Mercury from Mariner 10 Mariner 10

Units of Chapter 8 Orbital Properties Physical Properties Surface Features on the Moon and Mercury Rotation Rates Lunar Exploration Why Air Sticks Around Lunar Cratering and Surface Composition The Moon on a Shoestring

Units of Chapter 8, cont. The Surface of Mercury Interiors The Origin of the Moon Evolutionary History of the Moon and Mercury

8.1 Orbital Properties Distance between Earth and Moon has been measured to accuracy of a few cm using lasers Viewed from Earth, Mercury is never far from the Sun: 28°

Laser Ranging of Moon from McDonald Observatory Retroreflectors on lunar suface left by the Apollo astronauts Laser beam to the Moon to measure distance

8.1 Orbital Properties Phases of Mercury can be seen best when Mercury is at its maximum elongation:

8.2 Physical Properties MoonMercuryEarth Radius1700 km2440 km6380 km Mass7.3 × kg3.3 × kg6.0 × kg Density3300 kg/m kg/m kg/m 3 Escape Speed 2.4 km/s4.3 km/s11.2 km/s

8.3 Surface Features on the Moon and Mercury Moon has large dark flat areas, due to lava flow, called maria (early observers thought they were oceans):

8.3 Surface Features on the Moon and Mercury Moon also has many craters, from meteorite impacts:

8.3 Surface Features on the Moon and Mercury Far side of Moon has some craters, but no very large maria: Sea of Moscow Unknown what far side looked like until Russian Luna 3 took first pictures

8.3 Surface Features on the Moon and Mercury Mercury cannot be imaged well from Earth; best pictures were from Mariner 10 until recently. Cratering on Mercury is similar to that on Moon:

8.4 Rotation Rates Moon is tidally locked to Earth – its rotation rate is the same as the time it takes to make one revolution, so the same side of the Moon always faces Earth.

Mercury’s Period of Rotation Determined using Arecibo Radio Telescope Using Doppler Shifted Radio WavesArecibo

8.4 Rotation Rates Mercury was long thought to be tidally locked to the Sun; measurements in 1965 showed this to be false. Rather, Mercury’s day and year are in a 3:2 resonance; Mercury rotates three times while going around the Sun twice.

Why Do Atmospheres Stick Around Planets? Molecules have kinetic speeds due to thermal motion (temperature). If the average molecular speed is well below the escape velocity, few molecules will escape. Escape becomes more probable: for lighter molecules since they have a higher speed for same kinetic energy v molecule = [3 R T /M ] 1/2 ( M is mol. wt.; R is gas const) at higher temperatures for smaller planets escape speed is less v esc = [2GM planet /R planet ] 1/2

8.5 Lunar Cratering and Surface Composition Meteoroid strikes Moon, ejecting material; explosion ejects more material, leaving crater

8.5 Lunar Cratering and Surface Composition Craters are typically 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.

8.5 Lunar Cratering and Surface Composition RegolithRegolith: thick layer of dust left by meteorite impacts Moon is still being bombarded, especially by very small “micrometeoroids”; softens features: “Moonbuggy” tracks

8.5 Lunar Cratering and Surface Composition Despite searches, no water ice has been found on Moon (conflicting evidence on this) Meteorites also hit Earth: BarringerBarringer Crater ManicouaganManicouagan, Quebec ChicxulubChicxulub, Mexico Bye, bye dinos 62 mi. 112 mi. ¾ mile

8.5 Lunar Cratering and Surface Composition More than 3 billion years ago, the moon was volcanically active; the rille here was formed then:

8.6 The Surface of Mercury Mercury is less heavily cratered than the Moon Some distinctive features: Scarps (cliffs), several hundred km long and up to 3 km high Caused by crustal shrinkage as it cooled

8.6 The Surface of Mercury Caloris Basin a very large impact feature Weird terrain found on opposite side of planet caused by seismic shock waves Mariner 10 MESSENGER

Impact Basins in the Solar System Mercury Moon Callisto CalorisMare OrientaleValhalla

Moon’s density is relatively low ( ρ = 3.35 g/cm 3 ) It has no active magnetic field so cannot have a large iron/nickel core There is a “fossil” magnetic field from when interior was more molten Crust is much thicker than Earth’s

Lunar Crust Thickness The far side crust much thicker than the near side This partially explains less maria on far side

Mercury is much denser than the Moon ( ρ = g/cm 3 ) It has the largest relative core size of terrestrial planets It has a magnetic field - discovered by Mariner 10 Big Surprise!

Mercury’s Magnetosphere

Theories for Lunar Origin Fission (mother-daughter) Capture Co-condensation (co-accretion) Catastrophe theories Interaction of planetesimals Ring ejection after large impact impact Evidence against: Angular momentum Statistical improbability Density and elemental abundances Evidence for: Density and elemental abundances Computer modellingmodelling

8.8 The Origin of the Moon Currently accepted theory of Moon’s origin:. glancing impact of a Mars sized body on the still liquid Earth caused enough material, mostly from the mantle, to be ejected to form the Moon Computer model:

Evolutionary History of the Moon 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 crust forms

Lunar Geologic History as Determined from Lunar Missions

Evolutionary History of Mercury Mercury much less well understood Formed about 4.6 billion years ago Melted due to bombardment, cooled slowly Shrank, crumpling crust (creating scarps) Cratering from bombardment during same times as the Moon was being hit

Current/Future MissionsMissions US, Europe, India, Japan, China have active programs sending missions to the MoonEuropeIndiaJapanChina MESSENGER Mission has just arrived at MercuryMESSENGERMissionjust arrived ESA planning BepiColombo missionBepiColombo

MESSENGER PICTURES

Tides Bay of Fundy Differential forcesTidal friction

Tides Earth rotates every 24 hours Moon revolves once in a month Water bulge stays pointing towards Moon So the Earth’s surface rotates under the high water and low water points about every 6 hours (high and low tides) There are also especially higher and lower tides caused by special alignments that include the Sun’s tidal influences (neap and spring tides - next slide) There are also measurable land tides (a few cm – first measured by Michelson with his interferometer)

Spring tides (higher) Earth Sun and Moon in a straight line Neap tides (lower) Earth, Sun and Moon make a right angle The Sun has less effect, but it does modify the lunar tides

Summary of Chapter 8 Main surface features on Moon: maria, highlands (oldest parts-original crust) Both heavily cratered Both have extremely tiny atmospheres, and large day–night temperature excursions Tidal interactions responsible for synchronicity of Moon’s orbit (1:1), and resonance of Mercury’s (3:2) [rev:rot]

Summary of Chapter 8, cont. Moon’s surface has both rocky and dusty material (regolith) Evidence for volcanic activity Mercury has no maria, but does have extensive intercrater plains and scarps