Life in the Universe

MoonLecture 19

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The Moon as Seen from EarthThe key features of the lunar surface can be seen with binoculars, a small telescope, or even the naked eye.

CratersMariaHighlands3Maria : about 15%, 85% are highlands

We can see about 59% of Moons surface : Libration

Notice that the Moon appears to nod up and down and wobble back and forth. This apparent motion, called libration, has two causes. First, because its orbit around the Earth is slightly elliptical, the Moon appears to rock back and forth around its north-south axis. Second, because its rotation axis is not exactly perpendicular to the plane of its orbit, the Moon appears to nod up and down in a north-south directionLunar spin axis is about 6.7 degrees tilted against its orbit.4Lots of CratersThe largest crater on the Moon.

Clavius Crater144 miles wide16,000 ft deep

about 3000 craters larger than 1kmseveral millions total

Impact craters on Earth: ~200

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Mare Imbrium6Figure 10-5 Mare Imbrium from Earth Mare Imbrium is the largest of 14 dark plainsthat dominate the Earth-facing side of the Moon. Its diameter of 1100 km(700 mi) is about the same as the distance from Chicago to Philadelphiaor from London to Rome. The maria formed after the surroundinglight-colored terrain, so they have not been exposed to meteoriticbombardment for as long and hence have fewer craters. (CarnegieObservatories)

7Figure 10-5 Mare Imbrium from Earth Mare Imbrium is the largest of 14 dark plainsthat dominate the Earth-facing side of the Moon. Its diameter of 1100 km(700 mi) is about the same as the distance from Chicago to Philadelphiaor from London to Rome. The maria formed after the surroundinglight-colored terrain, so they have not been exposed to meteoriticbombardment for as long and hence have fewer craters. (CarnegieObservatories)

8The physical features of lunar craters and maria show that they wereformed by meteoroids impacting the Moon's surface at high speed.The crater labeled in the photograph, called Aristillus after a Greekastronomer who lived around 300 B.C., is 55 km (34 mi) across and3600 m (12,000 ft) deep. (Photograph: Lunar and PlanetaryInstitute/Universities Space Research Association)

Footprint on the regolith

9Figure 10-12 The Regolith Billions of years of bombardment by space debris havepulverized the uppermost layer of the Moons surface into powdered rock.This layer, called the regolith, is utterly bone-dry. It nonetheless stickstogether like wet sand, as shown by the sharp outline of an astronautsbootprint. (Apollo 11, NASA)

BasaltAnorthositeBreccias10Figure 10-5 Mare Imbrium from Earth Mare Imbrium is the largest of 14 dark plainsthat dominate the Earth-facing side of the Moon. Its diameter of 1100 km(700 mi) is about the same as the distance from Chicago to Philadelphiaor from London to Rome. The maria formed after the surroundinglight-colored terrain, so they have not been exposed to meteoriticbombardment for as long and hence have fewer craters. (CarnegieObservatories)

11Figure 10-7 The Near and Far Sides of the Moon About 50,000 individual infrared images from the Clementine spacecraftwere combined to create these views of the near (Earth-facing) side andfar side of the Moon. (NASA)

12Figure 10-7 The Near and Far Sides of the Moon About 50,000 individual infrared images from the Clementine spacecraftwere combined to create these views of the near (Earth-facing) side andfar side of the Moon. (NASA)We could see the far-side of the Moon first in 1959!

13Figure 10-10 Iron on the Moon Images at different wavelengths from the Clementinespacecraft were used to make this map of the concentration of iron onthe lunar surface. The areas of highest concentration (red) coincideclosely with the maria on the near side (compare Figure 10-7), confirmingthat the maria formed from iron-rich lavas. The lowest iron concentration(blue) is found in the lunar highlands. (No data were collected in the grayareas.) The large green region of intermediate iron concentration on thefar side is the South PoleAitken Basin (see Figure 10-7). The impact thatcreated this basin may have excavated all the way to the Moons mantle,in which iron is more abundant than in the crust. (JSC/NASA)

Seismometers were set up by Apollo about 3000 moonquakes per year. These quakes are weak (Richter scale 0.5 to 1.5) and deep (600-800km).

Moon is an one-plate world.

Thick crust.

A small (2-3% of total mass), partially liquid iron-core. No magnetic field.No tectonics

14Figure 10-11 The Internal Structure of the Moon Like the Earth, the Moon has a crust, a mantle, and a core. The lunar crust has an average thickness of about 60 km on the Earth-facing side but about 100 km on the far side.The crust and solid upper mantle form a lithosphere about 800 km thick. The plastic (nonrigid) asthenosphere extends all the way to the base of the mantle. The iron-rich core is roughly 700 km in diameter. (CompareFigure 9-9.)

Earths core has 32% of the total mass.Sample ReturnsApollo MissionSix Apollo missions : 382 kg.Three Luna missions : < 0.5kg.

The moon has been visited by 12 Apollo astronauts during missions in the 1960's and 1970's, and by numerous robot spacecraft. A total of ~382 kg of samples were returned by these Apollo missions, and and additional 321 g of samples were returned by three Russian Luna robot spacecraft. There are also several meteorites on earth that are of known lunar origin. Below is a summary of lunar geology gleaned from several references, cited below.15Moon Rocks

Thong, Shovel, Hammer, Drill 16Late Heavy BombardmentLate Heavy Bombardment = lunar cataclysm = terminal cataclysm

Proposed in 1973 by Tera et al. who noted a peak in radiometric ages of lunar samples ~4.0 - 3.8 GaSharply declining basin-formation rate between Imbrium (3.85 Ga) and final basin, Orientale (3.82 Ga)Few rock ages, and no impact melt ages prior to 3.9 Ga (Nectaris age)

Clearing of Remnants Late Heavy Bombardment

Gomes et al. (2005, Nature)Red = Jupiter, White=Saturn, Green (cyan) = Netune, Purple=Uranus18Rate of Crater Formation on the Moon

Origin of the Moon

Fission theory : predict that Earth and Moon should have the same chemical composition (moon rocks lack volatile material)Capture theory : can nicely explain the different composition. But very hard to capture an object (b/c has to be just at the right speed!)Co-creation theory : hard to explain the different chemical composition.20Collisional Ejection Theory

21Figure 10-18The Formation of the Moon This figure shows how the Moon could have formed in the aftermath of a collisionbetween a Mars-sized protoplanet and the proto-Earth. (Adapted fromT. Grotzinger, T. H. Jordan, F. Press, and R. Siever, Understanding Earth, 5th ed.,W. H. Freeman, 2007)

22Figure 10-17 The Moons Tidal Recession The Earths rapid rotation drags the tidal bulge of the oceans about 10 ahead of adirect alignment with the Moon. The bulge on the side nearest the Moonexerts more gravitational force than the other, more distant bulge. Thenet effect is a small forward force on the Moon that makes it spiral slowlyaway from the Earth.

In summaryImportant ConceptsSurface structure of the MoonMariaImpact cratersHighlandsLunar Cataclysm or Late-heavy bombardmentFormation of the MoonOrigin of the MoonChanging lunar orbit due to tidal frictionImportant TermsLibrationLHB, Lunar cataclysm, terminal cataclysmMare/Maria

Chapter/sections covered in this lecture : sections 10-1 through 10-523