Cratered Worlds: The Moon & Mercury Chapter 7

The Moon Mass 1/80 of Earth’s mass Gravity 1/6 of Earth’s Atmosphere –no real atmosphere –few volatiles (elements that evaporate at relatively low temperatures; e.g. water)

Apollo 17 Dec 1972

Lunar Surface Dominant Features –Craters –Maria = “seas” Dark features Radioactive dating (moon rocks) –age billion yrs –older than rocks on Earth Earth-Moon system about 4.5 billion years old

Geological Features Surface dominated by impacts Highlands –heavily cratered, light colored oldest parts of surface –silicate rocks Maria –17% of surface, mostly on near side –dark material, fewer craters youngest parts of surface –volcanic plains of basalt lava-filled impact basins

Composition & Structure Average density –3.3 g/cm 3 lower than Earth’s mantle (5.5 g/ cm 3 ) Similar to Earth’s crust Composition –mostly lighter silicates –depleted in iron –similar to Earth’s crust Mantle –solid –little seismic activity Core –Small, possibly iron rich –solid and cold Differentiation Moon’s interior molten in past Not geologically active (now)

Impact Craters Not erased by erosion (no atmosphere) –preserved record of impacts –indicator of solar system history Crater Origin –not volcanic –meteor impacts surface “explosions” creates circular craters –typical characteristics bowl-shaped turned up rims central peaks

Crater Counts Number of craters indicates age –many craters = old –few craters = young Ongoing impacts –during last 3.8 billion yrs Compare: highlands vs. maria –maria formed 3.8 billion years ago Based on number of craters –highlands have many more craters highlands older Conclude: –Period of heavy bombardment prior to 3.8 billion years ago

A moon covered with numerous and very old craters created by meteorite impacts likely a)has no ocean to cover the craters. b)orbits a large Jupiter sized planet. c)has a cold, solid interior. d)has no protective magnetic field.

Formation of Moon Giant Impact Theory

Mercury Closest planet to sun Surface similar to Moon –heavily cratered –no atmosphere Only one spacecraft flyby –Mariner 10 flew by 3 times in –New mission Messenger is in planning stages

Mercury - Structure Second smallest of planets by mass and size –Mass 1/18 of Earth’s mass –Radius less than half of Earth’s –Pluto is only planet smaller Density 5.4 g/cm 3 –denser than Earth’s mantle –quite different from Moon Dense iron-nickel core –60% of total mass –almost size of Moon!

Mercury - Surface No evidence of tectonic activity Scarps (cliffs) in middle of craters –crust shrank and cracked (after craters formed)

Earth-Like Planets: Venus and Mars Chapter 7

View from Earth Venus shows phases –otherwise featureless Obscured by clouds Mars appears red –due to iron oxides (rust) –polar ice caps –surface markings Seasonal changes

Martian Canals Schiaparelli (1877) reported canale on Mars –Italian = “channels” –English = “canals” Led to suggestion of intelligent Martian civilization –War of the Worlds (H.G. Wells) Percival Lowell primary American proponent Eventually shown to be optical illusion

Highlights Venus: –seems a “twin” of Earth (but not!) –massive atmosphere large greenhouse effect surface temp about 750 K Mars: –quite cold once much warmer –very thin atmosphere once much thicker –had flowing water in past possibility of life?

Venus: General Properties Mass –82% of Earth’s mass Radius –about same as Earth Density (5.3 g/cm 3 ) –about same as Earth

Surface of Venus Problem: how to see through cloud cover? Spacecraft exploration –Venera 7 (USSR) landed on surface (1970) lasted 23 minutes! –Magellan Orbiter (USA) use radar imaging (from orbit) mapped surface at 100m resolution

Radar Map of Venus

Surface of Venus Surface features –produced by volcanic and tectonic activity –but no plate motion –75% low lava plains produced like lunar maria very “young” surface (few craters) –25% mountains/mountain ranges

Craters on Venus Few small craters –small objects burn up in dense atmosphere Use large (>30 km) craters to estimate age –lava plains million yrs vast geologic activity then not much since

Atmospheric Conditions Extreme atmospheric pressure / density –100x greater than Earth Very high surface temps –around 750 K –little day/night variation due to thick atmosphere Generally hot and dry –no water Occasional “acid rain” –sulfuric acid clouds

Atmosphere of Venus Layers of sulfuric acid clouds –30-60 km above surface Composition –96% carbon dioxide –3% nitrogen –remove CO 2, then atmos. like Earth’s Runaway greenhouse effect –Greenhouse gasses -> high temps –water evaporates –carbon dioxide CO 2 released from rocks –increases greenhouse

Mars: General Properties radius –about 1/2 of Earth’s density 3.9 g/cm 3 –less than Earth; more than Moon –mostly silicates –possible metal core Rotation –period 24 hours, 37 min (like Earth) tilt of axis about 25º; orbital period 1.88 years –seasons similar to Earth’s –duration ~ 6 months (instead of 3)

Surface Conditions Temperatures –Summer: Day 240 K (-33 C) Night 190 K (-83 C) Coldest 173 K (-100 C) Water frost deposits Surface winds mostly moderate –but giant dust storms can occur

MGS Images of Opportunity Landing Site

Mars Atmosphere & Climate Composition: –95% carbon dioxide (CO 2 ) –3% nitrogen (N 2 ) –similar to Venus! Atmospheric pressure –100x smaller than Earth’s –equiv. to 30 km above Earth surf. Clouds –dust clouds –water ice clouds –dry ice (CO 2 ) crystals

Polar Caps At both N and S poles –change with seasons –seasonal ice caps composed of dry ice (frozen CO 2 ) –permanent ice caps composed of water ice

Seasonal Changes

Channels and Flood Plains Evidence liquid water existed on Mars –highlands runoff channels from ancient rainstorms? older than 3.9 million yrs –outflow channels much larger carved by huge floods

Climate Change Evidence indicates –Mars had liquid water (warmer) –much denser atmosphere What happened? “Runaway refrigerator effect” –atmosphere began to escape into space (low gravity) less dense atmosphere -> less greenhouse surface cools water freezes -> less greenhouse more cooling carbon dioxide freezes -> less greenhouse –happened over 3 billion yrs ago

Planetary Evolution Earth, Venus, & Mars: –very different results from planetary evolution –why so different when they started similarly? Runaway greenhouse (Venus) Runaway refrigerator (Mars) Could Earth change drastically? –what triggered changes? Could Mars be made habitable? –terraforming