Unraveling the History of Unraveling the History of the Moonthe Moon

The Lunar Surface

• Highlands • low density rocky slag formed on top of molten Moon• heavily cratered

Maria• 17% of Lunar surface

• Huge lava flows from volcanoes

• Basalts - similar to volcanic rock on Earth

• but no water

• fewer volatile (= easily melted) elements.

• Much less iron.

Crater BullialdusCrater Bullialdus

Craters as ChronometersCraters as Chronometers

The number of craters on a surface The number of craters on a surface can be used to estimate its age: can be used to estimate its age: older surfaces have been exposed to older surfaces have been exposed to impacts for longer and show more impacts for longer and show more craterscraters

ComplicationsComplications Any changes in the impactor population over timeAny changes in the impactor population over time Secondary impacts formed by ejecta from a single large Secondary impacts formed by ejecta from a single large

impactimpact Saturation: when a surface is so heavily cratered that a new Saturation: when a surface is so heavily cratered that a new

crater can form only at the expense of an older onecrater can form only at the expense of an older one There is a need for measured surface ages in order to There is a need for measured surface ages in order to

calibrate the crater countingcalibrate the crater counting

Solid line: trend for southern highlandsSolid line: trend for southern highlandsDotted points: average for MariaDotted points: average for Maria

Calibrating the rate of lunar Calibrating the rate of lunar crateringcratering

Moon rocks (mostly Moon rocks (mostly recovered during recovered during the Apollo missions the Apollo missions but a few by the but a few by the Soviet Luna Soviet Luna probes)probes)

Dating RocksDating Rocks

Radioactive elementsRadioactive elements• U-235 half-life = 710 million yearsU-235 half-life = 710 million years• U-238 half-life = 4.5 billion yearsU-238 half-life = 4.5 billion years• Th-232 half-life = 13.9 billion yearsTh-232 half-life = 13.9 billion years

Ages of lunar featuresAges of lunar features

Heavily cratered highlands: 4.1-4.5 billion years Heavily cratered highlands: 4.1-4.5 billion years oldold

Large basins: 3.9 – 4.1 billion years oldLarge basins: 3.9 – 4.1 billion years old Maria flooding: 3.0 – 3.9 billion years oldMaria flooding: 3.0 – 3.9 billion years old

Compared to the Earth, the typical lunar surface Compared to the Earth, the typical lunar surface is very ancientis very ancient

High early cratering rateHigh early cratering rate

Late heavy bombardment?Late heavy bombardment?

A proposed interval about 3.8 to 4 A proposed interval about 3.8 to 4 billion years ago in which the moon billion years ago in which the moon and the inner solar system were and the inner solar system were subjected to heavy asteroid subjected to heavy asteroid bombardmentbombardment

Time of the formation of impact Time of the formation of impact basins?basins?

The crust facing the earth may be The crust facing the earth may be thinner than on the farsidethinner than on the farside

The Interior of the Moon

• Geologically inactive.• No magnetic field no molten core.

From Modern Astrophysics, by Carroll & Ostlie.

Crust is 107 km thick on far side.

Crust ~ 0 km thick on near side.

Most maria are on near side.

Earth

Center of mass offset 2 km towards Earth.

3476 km