Earth-rise on Moon

The Moon

A12 A14

A15A17

A11

A16

L24

L20L16

Apollo and Luna Landing Sites

Lunar Surface – Sandy and Rocky

The Moon: Some Factoids• Lunar exploration: Luna series (USSR),

Surveyor (US) unmanned missions

• Apollo manned missions explored geology

• Density = 3.34 g/cc ; composition?

• D (Earth, Mercury,Venus) =

5.52, 5.43, 5.24 g/cc Composition?

• Basic surface and interior similar to rocky part of the Earth (little or no metallic core or magnetic field)

Highlands and Marias

• Highlands were heavily cratered during the early history, from formation to about 4 billion years ago

• Marias (dried up lava oceans) formed later, about 3 billion years ago

Cratered Highlands and Marias

Highlands are much older than marias. Why?

Evolution of highlands and maria

Craters and chronology (age)

• Highlands are highly cratered regions that must be older than smoother regions of surface since most cratering activity was at the time of the formation of the solar system

• Marias are much younger, and evolved later at places where large crater impact basins were formed and filled with lava from the interior

Atmospheres and Surfaces

• Lack of atmosphere - results in extreme temperatures - exposes surface to meteoroid impact

craters - lack of current geological activity and

evolution (ceased long time ago)• Moon and Mercury lack atmosphere (too small

and too close to the Sun)• Both have similar surface geology craters and marias

Formation of a lunar sea: Impact of meteoroid

Impact Basin Filled with Lava Maria

A Mare

Maria – Lightly cratered smooth surface

Lava River

Lunar Geology

Formation of Moon

• No generally accepted theory: three scenarios

- Accretion

- Fission (breakup)

- Capture

Possibly a combination

Formation of Moon

Earth Tides on Moon

Tidal forces due to the Earth forced the Moon to rotate at the same rate as Its revolution; therefore it keeps the same face towards the Earth

Another view of Moon ?And, Mercury !

Cratered highlands of Mercury

Mercury – Basic Statistics

• Mass ~ 1/20 Earth’s

• Diameter ~ 1/3 “

• Density ~ 5.5 g/cc

• Year ~ 88 Earth days

• Day ~ 59 “

• 0.4 AU from the Sun

• Surface T: Day 700 F, Night -300F

Mariner Fly-By’sSeveral orbital reconnaissance satellites, but no space probes landed yet

Mariner Fly-By Picture Mosaic

Best Mercury Sighting at MaximumWestern Elongation

Eccentric orbit implies different maximum angles

Greatest Elongations of MercuryOptimum Mercury Sighting at Maximum Elongation,

and/or just after sunset or before sunrise

NOT easily seen, like Venus as “evening star”: Mercury is too small, too close to the Sun, and lacks atmosphere and hence has small albedo

Mercury – Basic Features• Metal ball (mostly iron) surrounded by heavily

cratered rocky crust like Moon

• But weak magnetic field, why?

• No molten, moving core!

• Only refractory elements; volatile elements evaporated

• Geology like the Moon, e.g. Caloris basin resembles lunar maria – large crater impact basin flooded later with lava

• No Plate Tectonics, why?

Interiors of Earth and Mercury

Dopper Radar measurement of rotation speed of Mercury

Aracebo Radio Telescope(Puerto Rico)

A “Day” on Mercury• A year is the period of revolution (orbit) around

the Sun• Two kinds of “day” on a planet: Sidereal and

Solar• Sidereal day is the actual period of rotation on

its axis (viewed with respect to the stars, not the Sun)

• Solar day is the period of rotation with respect to the Sun

• On the earth: solar day = 24 hrs, longer than sidereal day by 4 mins.

• Mercury’s sidereal day is 2/3 of a Mercury year

Rotation and Revolution of Mercury:Day and Year

• Rotation period ~ 2/3 revolution period

• A solar day is twice as long as a year !!

• Solar Day (176 d) = 2 x Sidereal year (88 d)

• Wait two years to see the sun in the same place in the sky (on the earth it is just 1 solar day, or 1 sidereal day + 4 mins)

• What’s going on?

Rotation and Revolution of Mercury

Strong Tides due to Sun Periods of Mercury

A solar day on Mercury is twice as long as the sidereal year

Like the Moon

Moon’s rotation periodequals orbital period: see only one side or face of Moon

Hot Poles on the Equator !

• Perihelion of Mercury is 1.5 times closer than aphelion

• Mercury receives 2.25 times more solar energy at perihelion than at aphelion

• Two extremities of the equator pointing towards the sun constitute two hot ‘thermal’ poles

• Largest variation in temperature between day/night, but maximum temperature ~700 F is still less hot than Venus (> 800 F)