8 February 2005 AST 2010: Chapter 8 1

The Moon & Mercury

8 February 2005 AST 2010: Chapter 8 2

Cratered Worlds

The Moon is our nearest cosmic neighbor the only other world humans have ever visited

Mercury is in many ways similar to the MoonBoth are

relatively smalllacking in atmospheredeficient in geological activitydominated by the effects of impact cratering

8 February 2005 AST 2010: Chapter 8 3

Properties of the Moon and Mercury

Moon Mercury

Mass (Earth=1) 1/80 1/18

Diameter (km) 3,476 4,878

Diameter (Earth=1) 1/4 1/3

Density (g/cm2) 3.3 5.4

Surface gravity (Earth=1) 0.17 ~ 1/6 0.38 ~ 1/3

Escape velocity (km/s) 2.4 4.3

Rotation period (days) 27.3 58.65

Surface area (Earth=1) 0.27 0.38

8 February 2005 AST 2010: Chapter 8 4

General Properties of the Moon

The Moon has 1/80 of the mass of the Earth1/6 of the surface gravity of the Earth

The surface gravity of the Moon is not strong enough for it to hold on to gases to form an atmosphereWithout an atmosphere, the Moon’s surface is not altered by weather erosionThe Moon is geologically dead Craters on the Moon are a record of impacts over its history

8 February 2005 AST 2010: Chapter 8 5

Unmanned Exploration of the Moon

1959: Soviet spacecraft Luna 3 flew to the Moon and photographed its far side1962: President Kennedy set the goal of landing men on the Moon1966: Luna 9 landed on the Moon and transmitted pictures to Earth2000: Lunar Prospector spacecraft detected frozen water on the Moon

8 February 2005 AST 2010: Chapter 8 6

Manned Exploration of the Moon9 Moon flights and 6 landings between 1968 and 1972Apollo 8 to 17Apollo 11

Neil Armstrong and Buzz Aldrin stepped onto the Moon

Astronauts performed experiments and brought back samples of rock and soil

8 February 2005 AST 2010: Chapter 8 7

The Moon’s Composition and Structure

The Moon’s composition is not the same as that of the Earth

The Moon’s average density is 3.3 g/cm3 compared to 5.5 g/cm3 for EarthThe Moon’s material is like that of the Earth’s mantle and crustThe Moon is depleted in iron and other metals, and it lacks a large metal core

Water ice has been found in craters near the Moon’s poles

8 February 2005 AST 2010: Chapter 8 8

Lunar SurfaceTwo main features of the lunar surface:

Dark, large flat “seas” or mariaHeavily cratered highlands

The maria (singular of mare, Latin for sea) are areas of ancient lava flowsLunar mountains are all the result of impacts

The mountains are impact debris accumulated around the lips of craters

8 February 2005 AST 2010: Chapter 8 9

Lunar HistoryRadioactive dating of lunar samples yields ages of 3.3 to 4.4 billion years

This is consistent with the theory that the Earth and Moon formed about 4.5 billion years ago

The highlands are believed to be the older surface areas, formed early in lunar historyThe maria are thought to be younger surfaces

Lava flowed from volcanoes present shortly after the Moon formed, when it still had molten material

8 February 2005 AST 2010: Chapter 8 10

Evidence of Volcanic Activity

The gas bubbles are characteristic of rock formed from lava

Mare Orientale

8 February 2005 AST 2010: Chapter 8 11

On the Lunar SurfaceThe surface of the Moon is covered with fine powdery material, a few inches thickThis “dust” is the product of impacts

Astronaut’s bootprint in the lunar soil

8 February 2005 AST 2010: Chapter 8 12

Impact CratersCraters on the surface of the Moon are a record of its historyThe craters were created by impacts, NOT volcanic activity (the maria were produced by lava flows, not violent eruptions)It is important to understand the craters on the Moon and apply the results to other planets and moons

8 February 2005 AST 2010: Chapter 8 13

Volcanic Versus Impact Origin of Craters

Volcanoes and impact craters have different shapes

8 February 2005 AST 2010: Chapter 8 14

The Cratering Process

8 February 2005 AST 2010: Chapter 8 15

8 February 2005 AST 2010: Chapter 8 16

8 February 2005 AST 2010: Chapter 8 17

Using Crater CountsThe maria are believed to exhibit a slow rate of cratering over the last few billion years or soTo fit the cratering of the highlands with the age of the surface, we must assume that the rate of cratering was higher before then

8 February 2005 AST 2010: Chapter 8 18

The Origin of the Moon

Hypotheses for the origin:1. Fission theory2. Sister theory3. Capture theory4. Giant impact theory

8 February 2005 AST 2010: Chapter 8 19

Impact Computer Models

8 February 2005 AST 2010: Chapter 8 20

Origin of the Moon

The theory must explain:Why the Moon’s composition is similar to the Earth’s mantle and crust (sister theory)Why the Moon and Earth are nearly the same age (capture theory)How the Moon came to be Earth’s satellite (fission and capture theory)Similarities and differences in chemistry of rocksSimilarities in isotopic abundances of oxygen

8 February 2005 AST 2010: Chapter 8 21

MercuryNearest planet to the SunNamed for the messenger god of Roman mythologyOutwardly similar to our Moon in size and appearance

Heavily crateredNo mountains or valleys

Except for Pluto, Mercury hasThe largest eccentricityLargest angle to the eclipticSmallest size

8 February 2005 AST 2010: Chapter 8 22

Composition and StructureMercury’s density is high for a planet with no atmosphereMost likely model predicts that Mercury has a large metallic core surrounded by a thin (compared to the Earth) mantle

8 February 2005 AST 2010: Chapter 8 23

Mercury’s Strange Rotation

Difficult to determine rotation from surface markingsMercury’s rotation measured with doppler radar

8 February 2005 AST 2010: Chapter 8 24

Doppler RadarOne side of the planet is rotating toward Earth, while the other side is rotating awayPart of the signal is reflected with higher frequency and part with lower frequencyThe amount of frequency spreading tells us the amount of rotation

8 February 2005 AST 2010: Chapter 8 25

Mercury’s Rotation

Mercury rotates with respect to the stars in 59 Earth-days

This is Mercury’s sidereal day

Mercury orbits the Sun in 88 Earth-daysIt’s sidereal day is 2/3 of its orbital period, a situation astronomers predict is stable for a planetA solar day on Mercury is the length of 2 orbits, or 176 days!

8 February 2005 AST 2010: Chapter 8 26

Surface of Mercury

8 February 2005 AST 2010: Chapter 8 27

Origin of Mercury

How to explain the large fraction of metal in Mercury?The giant impact hypothesis:

A giant impact during the early period of the solar system could have ripped much of Mercury’s original mantle freeThe mantle and impactor then disappeared, perhaps into the Sun