Where we are now Introduction Little things - comets, asteroids, KBOs Slightly larger things - Moon Larger still - Terrestrial planets Really large - Jovian planets Jovian moons + Pluto Extrasolar Planets The Moon Chapters 8.2, 9 Little Erosion Provides an example of the frequency/effects of impacts in the early solar system (note: not all satellites show evidence of cratering) The moon is not primitive; it is more like the planets than the asteroids Outline Topography Topography & Albedo Highland & Maria Lunar Craters Crater Density & Age Modes of Evolution Origin of the Moon Sunlight Topography: precise detailed study of the surface features of a region Topographic detail of the moon is easier to see when the Sun is low in the lunar sky
Albedo Surface Features: Highlands & Maria Sunlight Highlands: bright (15% reflectivity) & rugged regions comprised of craters of varying sizes. These regions tend to be at high elevations Maria: darker (8% reflectivity) & smoother regions with relatively few craters. -mare (singular): Latin for sea Albedo is easier to determine when the Sun is high in the lunar sky (full moon) Maria & Highlands Lunar Surface Highlands Maria Crater: bowl-like depressions found on the surface (note crater: Greek word for bowl) The highlands have many craters The maria don t have many craters (younger surface; not much time for craters to have accumulated)
Craters on the Moon Formation of a Crater Penetrates 2 3 times its own diameter before stopping Ejecta material = several hundred times the impactor mass are due to impacts, not volcanism heat contributes to clearing out the crater Lunar Craters Ice at the Moon s Poles Diameter Of Crater 1/5 diameter of crater Sunlight the tilt of the Moon s rotational axis with respect to the plane of the eclipse is ~ 1.5 degrees. Thus the bottoms of polar craters can continuously be in shadow. Source of ice - meteorites and comets.
Missions to the Moon Astronauts on the Moon (July 20, 1969 December 14, 1972) Scientific benefits 1) Rock samples for composition/age dating 2) Apollo Lunar Surface Experiments Package (ALSEP); for monitoring the solar winds & moonquakes 3) Maps of surface made in orbit Crater density & Age Impact Rate vs. Time High initial impact rate 1) Due to fragmentation of a nearby asteroid, or 2) Due to early stages of planetary accretion 3) Due to planetary migration Note: Number of NEAs ~ lunar impact rates deduced for mare crater counts
Crater Counts: Moon s surface Crater Counts: Moon s surface North Impact Craters North Impact Craters South South Distant past Southern Lava flow! Crater Counts: Moon s surface Crater Counts: Moon s surface North Impact Craters Highlands Impact Craters South Maria Some Later Time Now
Maria: Regions of Recent Maria: Regions of Recent Moon s surface Lava Maria: Regions of Recent Maria: Regions of Recent
Maria: Regions of Recent Maria: Regions of Recent Lava later cools to form Maria Modes of Evolution Impact History Catastrophism: sudden or violent event that substantially modifies the landscape Uniformitarianism: slow, steady modifications occurring over long time scales Highlands - Impacts occurring mostly 3.8 4.5 billions years ago - Rock composition: igneous silicate rocks (mineral oxides of silicon, calcium, magnesium) - Breccias: recemented pieces Basins (Maria) - Last great impacts occurred 3.9 4.1 billion years ago - Areas of recent lava flow
The Moon: Orbital Periods Some other properties (cont) Density: the Moon s density is 3.3 g cm -3 Sidereal month (27.3 days): the period of the Moon s revolution about the Earth with respect to the stars Synodic month (29.5 days): the period of revolution of the Moon with respect to the Sun - By comparison, the Earth s density is 5.5 g cm -3 - The Moon s interior cannot be dense Atmosphere - Negligible: hydrogen & helium from the solar wind - Why doesn t the Moon have an atmosphere? Low gravity & high surface temperature Origin of the Moon The Moon s density is similar to that of the Earth s crust, but the composition is different Formation of the Moon: Possible Explanation Orpheus Daughter Theory: The molten Earth split to form the Moon (Problem: doesn t explain the difference in composition) Sister Theory: The Earth & Moon formed near each other (Problem: doesn t explain the difference in composition) Capture Theory: The Moon formed elsewhere & was captured by the Earth (Problem: doesn t explain how the Moon achieved such a stable orbit) Young Earth Differentiation Moon