Module 9: The Moon in Close-up

Activity 1:

Exploring the LunarSurface

Summary:In this Activity, we will investigate

(a) Moon missions,

(b) the Moon’s vital statistics, and

(c) cratering.

(a) Moon MissionsThe Moon is the only other Solar System body whichhas been visited by a manned mission from Earth.

Grainy images of the Apollo landings on the Moon are part of history now ...

Buzz Aldrin descending fromthe Apollo 11 lunar module

... however in 1969 the firstlanding, and views of Earthrise from the Moon, had a profound effect on the way many people think about theEarth and our place in the Solar System.

As the Earth and Moon share a common orbit around the Sun, sending a spacecraft to the Moon is not particularly difficult. (We will see in later Activities thatsending spacecraft to visit bodies orbiting closer or further from the Sun is significantly more difficult.)

To send a spacecraft plus human occupants from Earth to the Moon and back required a significant payload. Aspacecraft that massive would have been dangerouslycumbersome to maneuver for landing and takeoff on the Moon.

Instead the main spacecraft - the Command Module -was used to travel between the Earth and Moon ...

Command module, Apollo 11,photographed from the LunarModule

… connected to a light Lunar Module built for low weightand high maneuverability in the low lunar gravity andlack of an atmosphere.

Lunar ascent module, Apollo 11,photographed from the Command Module

The six Apollo missions which successfully landed on the Moon brought back 380 kg of soil and rock samples, and visited a range of sites, from flat low-lying plains to craters, the edge of a lunar mountain range, and a huge channel carved in the Moon’s surface by an ancient lava flow.

Apollo 16 lunar rover leavestracks on the Moon

The USSR’s unmanned Luna missions also visited the Moon between 1965 and 1976, with 5 missions achieving soft landings on the Moon.

Luna 9, the first soft lander on the Moon

Luna missions between1972 and 1976 drilled for rock samples and then brought them back to Earth.

Luna 17 & 21 were equipped with robotic lunar rovers, Lunokhod 1 & 2, which were controlled from Earth. Lunokhod 1 toured Mare Imbrium for 11 months; Lunokhod 2 covered 27 km of the Moon’s surface in 4 months.

Artist’s conception ofLunokhod 1

To find out more about the USA and USSR missions to the Moon, visit the History page of the Lunar Prospector site at the following Internet site:

http://lunar.arc.nasa.gov/history/index.html

One advantage of using unmanned missions like the Luna series is the simplicity and cost savings due to the reduced payload because of the absence of a crew or life-support system.

An advantage of using manned missions like Apollo isflexibility - for example, astronauts can assess a landing site and change plans to suit circumstances more easily than can a robotic mission. Although costly, the importance of manned Apollo missions from the public relations point of view for both NASA and the USA should not be underestimated.

For now, we will investigate the main facts of what we know (and are still discovering) about the Moon and its evolution - largely as a result of space missions to the Moon over the last 30 years, both manned and unmanned.

We will revisit these issues when we consider the MarsPathfinder mission, planned to be the first of a new generation of unmanned landers.

In the last Activity we investigated the space missions which have given us a close-up view of Luna, our Moon.

(b) The Moon’s vital statistics

In this Activity we’ll summarize someof the basic information which we have learnt about the Moon.

When we look at the properties of other planets and natural satellites in our Solar System, we will use our Earth - the Solar System we know best - as a reference point.

For this purpose, we use the symbol to refer to Earth,so that, for example, in the following tables M means the mass of the Earth, and D means the diameter of the Earth:

Moon

D

M = M

Earth

core mantle

crust

0.27 D

M = 0.01 M

Core

mantle

crust

The Moon is much less dense than the Earth, which means that its core must be very small.

The different densities also have implications when welook at models for the Earth and Moons’ formation, aswe will see later.

Results from Lunar Prospector experiments confirmed the Moon’s small core in 1999, indicating that the core is less than 4% of the Moon’s total mass. The Earth’s core, for comparison, is about 30% of the Earth’s total mass.

EarthMoon

Av. Distancefrom Sun 1 AU1 AU

Length of “Year”

1 y 1 y

Length of solar day

29.5 d 1 d

Inclinationof axis to ecliptic

1.3° 23.5°

Look back at the previous table, and decide whether you would expect the Moon to have seasons.

Then go to the next slide to see if you are right...

The seasons on Earth are due to the sizeable (23.5°) tilt of

the Earth’s rotation axis.

23.5°

plane of the ecliptic

(To remind yourself of why theaxis tilt causes seasons on Earth,

revisit the Activity on Earth’s Seasons)

The tilt of the Moon’s axis is very small - only 1.3° to the ecliptic - so we would

not expect the Moon to experience significant

seasons, and indeed it does not.

23

1.3°

Note that the plane of the Moon’s orbit around the Earth is tilted 5° to the plane of the ecliptic, but it is the tilt of the rotation axis to the ecliptic that determines whether a planet (or satellite) will experience seasons.

23

Sun-Earth-Moon system

EarthMoon

av. albedo 0.390.10

Note the Moon’s low albedo. The Moon looks bright in our sky not because it is highly reflective, but rather because it is so close to us.The albedo of the Moon (like the Earth) varies depending on the terrain, varying from 5-10% for the maria to between 12-16% for the highlands.

* these will be discussed in the next Activity.

**

EarthMoon

av. albedo 0.390.10

accelerationdue to gravity

g0.17g

The Moon’s low mass and density means that the effectsof gravity are much weaker on the Moon than on Earth.

EarthMoon

av. albedo 0.390.10

accelerationdue to gravity

g0.17g

Because of the low lunar gravity, gases can easily escape.The Moon has only the slightest trace of an atmosphere made up from gases “baked out” (outgassed) from the rocky surface, with a pressure of about 1/100 000 000 000 000 th of p , atmospheric pressure on Earth!

atmosphere Almostnone

78% N2, 21% O2 0.03% CO2, ~2% H2O

EarthMoon

av. albedo 0.390.10

accelerationdue to gravity

g0.17g

The insulating effect of our atmosphere helps to “smooth out” temperature variations here on Earth- with essentially no atmosphere, temperature variations on the Moon are much more extreme.

atmosphere78% N2, 21% O2

0.03% CO2, ~2% H2O

surface temperature

-170 °C +130 °C

- 50 °C + 50 °C

Almostnone

EarthMoon

av. albedo 0.390.10

accelerationdue to gravity

g0.17g

atmosphereAlmostnone

78% N2, 21% O2 0.03% CO2, ~2% H2O

surface temperature

-170 °C +130 °C

- 50 °C + 50 °C

surfacegeology

Heavy cratering, ancient

volcanoes & lava flows

Cratering largely obliteratedby active surface (volcanoes& plate tectonics), weatheringand biological activity

(c) Cratering

Heavy cratering, mostly caused by impacts with Solar System debris over the history of the Moon, forms the most prominent feature of the Moon’s surface.

Surface features on the Moon likecraters stand out best when viewednear the terminator- the boundary between sunlightand shadow on the Moon.

The largest craters are named after philosophers, mathematicians, and scientists.

For example, the crater Copernicus dominates a regioncalled the Ocean of Storms.

Many craters, including Copernicus, exhibit patternscalled rays, caused by debris thrown out by the initial impact. This debris, called ejecta, can in turn create secondary craters when it falls back to the surface.

- some meteorites found on Earth turn out to have beenejected long ago from the Moon.

The low surface gravity of the Moon means that some ejecta can escape entirely

When a meteorite strikes, it penetrates into the lunar surface, deforming the surface layers and vaporizing itself and surrounding rock. The crater starts to form.

Surface material (“ejecta”) is thrown out by the shock wave, and the outer edges of the crater collapse and overturn.

The surface may partly rebound to form a central peak in the crater,

and the walls of the crater may partlyfall back in to create a terraced effect,as can be seenin this crater from the far side of the Moon.

Modelling the formation of a crater by a meteorite impact:

Two of the largest impact craters, Mare Imbrium and Mare Orientale, are the results of impacts so large (it has been estimated that Mare Imbrium was caused by a meteorite 65 km in diameter) that seismic shock wavescaused bythe impacts would have travelled right around the Moon.

This image, centred onMare Orientale, hasbeen imageenhanced tobring out fine details.

In particular, the locations on the Moon directly oppositethese two huge impact craters are regions called jumbled terrain - areas which could have been disturbed by the concentrated shock waves shaking the surface up and down by up to 10 m.

Not all craters are huge: they extend in size all the way down to microcraters in the surface of rocks - tiny crater pits which are less than 1 mm across.

(Why do you think we don’t see microcratering on Earth?)

The regolith absorbs most of the light which falls on it, which explains the Moon’s low albedo (0.10).

The bedrock of the Moon’s surface is covered by the regolith, a layer of loose soil and rocks produced by meteorite impacts over the long history of the Moon.

Footprint left in the Moon’s regolith

Much of the rocks on the Moon’s surface are made upof fragments of older rocks which have been brokenup by meteorite impacts, then fused together by heatand pressure of subsequent impacts. These rocks are called breccias.

Geologists have used radioactive dating to estimate the ages of rocks collected from the Moon’s surface during the Apollo program.

Rate of Cratering

By comparing the ages of the rocks with the amount ofcratering at their original locations, geologists have concluded that the Moon underwent intense bombardmentfrom at least 4.6 billion years ago (which is when its surfacesolidified into a crust), until 3.8 billion years ago.

All are igneous (formed from lava). The oldest rocks collected are 4.4 billion years old - the youngest 3.1 billion years old.

The first 0.8 billion years of the Moon’shistory was dominated by heavy bombardment, including some giant impacts - perhaps by planetesimals.

4 3 2 1 0

Time before the present (billions of years)

Cra

terin

g ra

te

By 3 billion years ago, the crateringrate had dropped to about

1 millionth of its initial value.

Presumably the intense early bombardment was due to planetesimals and smaller debris in the early Solar System as it emerged from the Solar Nebula.

As the debris was gradually swept up into planet making, deflected by gravitational interactions with the large planets like Jupiter and ejected into the outer Solar System, or gradually driven outwards by the young Sun’s strong solar wind, the rate of cratering dropped quickly to its present relatively low value.

In the next Activity we will investigate other prominent features of the lunar surface and its evolution.

NASA: Three-filter color image of the Moon (Galileo)http://nssdc.gsfc.nasa.gov/image/planetary/moon/gal_moon_color.jpg

Apollo 17 astronaut Harrison Schmitt standing next to boulder at Taurus-Littrow during third EVAhttp://nssdc.gsfc.nasa.gov/image/planetary/moon/apollo17_schmitt_boulder.jpg

Apollo 11, Buzz Aldrin descending LM ladderhttp://images.jsc.nasa.gov/images/pao/AS11/10075261.jpg

Apollo 11, Earth rising over Moon’s surfacehttp://images.jsc.nasa.gov/images/pao/AS11/10075248.jpg

Apollo 11, Command/Service modules photographed from Lunar Module in orbit,http://images.jsc.nasa.gov/images/pao/AS11/10075257.jpg

Apollo 11, Lunar Module ascent stage photographed from Command Module,http://images.jsc.nasa.gov/images/pao/AS11/10075285.jpg

Luna 9http://antwrp.gsfc.nasa.gov/apod/image/luna_9_unk.gif

Image Credits

NASA: View of the Mid-Pacific Oceanhttp://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_mid-pacific.jpg

Lunar cratering, Clementine missionhttp://nssdc.gsfc.nasa.gov/image/planetary/moon/clem_strtrk.jpg

Taurus-Littrowhttp://pds.jpl.nasa.gov/planets/welcome/thumb/taurus.gif

Copernicus Craterhttp://images.jsc.nasa.gov/images/pao/AS17/10075987.gif

Mare Imbrium & Orientale Basinhttp://images.jsc.nasa.gov/images/pao/STS34/10063796.gif

Cratering on the Moon's far sidehttp://images.jsc.nasa.gov/images/pao/AS11/10075255.gif

Highland brecciahttp://pds.jpl.nasa.gov/planets/welcome/thumb/breccia.gif

Image Credits

Now return to the Module home page, and read more about exploring the lunar surface in the Textbook Readings.

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