Module 9: The Moon in Close-up

Activity 2:

The Moonand itsEvolution

Summary:In this Activity, we will investigate

(a) lunar mountain ranges

(b) lunar maria

(c) evolution of the Moon’s Surface - the cooling of planets

(d) the far side of the Moon, and

(e) water on the Moon?

(a) Lunar Mountain Ranges

83% of the Moon’s surface is made up of the lunar highlands - heavily cratered mountainous regionsreaching typical heights of 4 to 5 km above the averagelunar elevation.

(Note that we can’t talk about heights above sea level!)

On Earth, mountain ranges are due to tectonic activity or volcanoes, but not so on the Moon.

Even regions which look a little similar to folded mountain ranges on Earth, turn out to be regions of overlaidcratering & associated debris.

The biggest lunar mountain ranges appear to have been formed by impact from huge planetesimals.

The lunar highlands are made up anorthosite, light gray rock which is made up of relatively light elements like calcium and aluminium.

The anorthosites collected during Apollo missions have been estimated at between 4.0 and 4.3 billion years old by radioactive dating.

As we will see, the anorthosite rock in the highlands is probably the remnant of the original lunar crust.

As with all lunar rocks found so far, anorthosites are igneous - that is, formed from magma.

(b) Lunar Maria

The other 17% of the lunar surface is made up of flat,dark-gray, plains called maria,lying typically 2 to 5 km below the average lunar elevation.

The largest mare, 1100 km in diameter, is Mare Imbrium (the Sea of Showers).

The name maria and its singular form, mare, come fromthe Latin word for “sea”, and reflect the fact that early (seventeenth century) observers of the Moon thought that the maria really were seas of liquid water, and gavethem names like Mare Tranquillitatis (the Sea of Tranquillity) and Mare Nectaris (the Sea of Nectar).

The surface rock of the maria is called mare basalt - solidified lava flows, rich in relatively heavy elements like iron, manganese and titanium - and samples brought back to Earth have been dated at between 3.1 and 3.8 billion years old.

Some of the mare basalt contains holes formed originally by bubbles of gas.

Bubbles of gas can form in lava as it flows out onto the surface, released from being trapped under pressure below the crust.

The maria surface shows some small craters and cracks called rilles.

Hadley Rille, one of the largest, measures 1500m wide, 400m deep, and 100 km long. The walls of the channel are very steep, with slope angles of 25 to 30 degrees. The rilles appear to be channels carved in the surface by the ancient lava flows.

The Moon’s surface is not active now - the samples of mare basalt that have been dated suggest that theflows ceased about 3.1 billion years ago.

The Moon’s surface has long been geologically dead- with essentially no atmosphere, even the weatheringstage could not progress.

The evolutionary sequence of the Moon’s surface, compared to that of the Earth, is modeled as:

(c) Evolution of the Moon’s Surface

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

We will look at models for the earliest stages soon.

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

It appears that the light anorthosite formed first, floating on top as the newly differentiated Moon cooled and formed a crust.

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

The Moon’s surface isextensively scarred bycratering, most of whichhappened in the earlystages of the Solar System.

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

The cratering weakenedthe Moon’s crust and allowed lava from the Moon’s interior to well upthrough cracks and floodthe low-lying areas, cooling to become marebasalt and forming mares.

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

There is no evidence for plate tectonics on the Moon, ...

Earth Moon

Condensation

Accretion

Differentiation

Cratering

Basin Flooding

(Vulcanism)

Plate tectonics

Weathering (Slow decline)

… and the lack of anatmosphere or liquidwater on the Moon means that weathering is essentially absent.

* except for weathering by micrometeorites

*

Compared to the Earth, why did the Moon’s evolution stop so quickly ?

The geological activity of the Earth - volcanic activity, plate tectonics and even atmospheric circulation - isbasically due to the fact that the Earth’s interior is stillquite hot.

The interior of the Moon, a much smaller body than the Earth, is now too cool to drive geological activity.

• seismic studies (which do however suggest that the core may still be molten), • the lack of a general lunar magnetic field (which would probably require a molten interior to set it up), and• the presence of mascons - sizeable mass concentrations - near the surface of the Moon. If the Moon’s interior were molten, these mascons would sink.

Evidence in support of this conclusion comes from

In rough terms,

The cooling of planets

• the amount of thermal energy contained in a approximately spherical planet depends on its volume, and • the rate at which the planet can cool down depends on how much surface area it has to radiate energy away.

Small planets have a relatively large surface area to volume ratio, so cool

quickly

Large planets have a relatively small surface area to volume ratio, so cool slowly

So small planets and natural satellites like the Moon cool relatively quickly, whereas larger planets cool relatively slowly and are likely to stay geologically active for much longer.

While this overall model for the Moon’s evolution seems to work well, there are still issues to resolve and discoveries to make about the Moon.

(d) The Far Side of the Moon

When space missions first visited the far side of the Moon,astronomers were surprised to find that it is considerably more highly cratered than is the near side, and containsalmost no lava flows.

It’s worth a reminder here that the far side of the Moon is not the dark side of the Moon (except around Full Moon) -if you want to check this out, revisit the Activity Lunar Cycles.

The difference seems to be due to the thickness of the crust - the lunar crust is about 60 km thick on the near side of the Moon, but about 100 km thick or more on the far side of the Moon.

So the question now becomes,why is there a marked differencein crust thicknessbetween the nearand far sides of the Moon?

The asymmetry in crust thickness is modelled as being due to the huge impact which formed the Oceanus Procellarum Basin on the near-side Moon, and presumably weakened the near-side crust generally.

The asymmetry of the lunar crust probably accounts for the off-set in the Moon’s center of mass.

The center of mass of the Moon lies about 1.8 km closer to the Earth that the geometric center of the Moon.

Geometric center

Center of mass

Direction of Earth

(e) Water on the Moon?

In 1996 it was announced that Clementine data taken of permanently shadowed regions near the Moon’s south pole indicated the presence of water ice.

After an almost 20 year absence from the Moon, NASA’s lunar exploration recommenced when the Clementine mission, a joint project between the US Strategic Defense Initiative Organization and NASA launched in January 1994, mapped the Moon over a period of roughly 2 months.

This announcement caused considerable controversy! Almost every introductory astronomy textbook points out that lunar rocks, unlike their Earth counterparts, contain no water, and that any water ice on the Moon’s surface would sublime (turn directly to water vapour) and escape.

The Clementine results on water ice were fascinating but inconclusive. Clementine did show for the first time, however, that permanently shadowed areas of the Moon, inside steep polar craters, did exist, and theoretically, it should be cold enough for frozen water to exist in such locations.

Clementine enhanced image of the lunar South Pole, showing deep craters, some of which havepermanently shadowed regions near their rims.

The Lunar Prospector, launched in January 1998, was designed for a low polar orbit investigation of the Moon.

In March 1998 NASA announced that data from theLunar Prospector indicated that water ice was present atboth the north and south lunar poles, in agreement with theClementine results.

The Lunar Prospector

The ice originally appeared to be mixed in with the lunar regolith at low concentrations (less than 1%), but NASA reports that subsequent data taken over a longer period has indicated the possible presence of individual, near-pure water ice deposits buried beneath up to 40 cm of dry regolith, with the water signature being stronger at the Moon's north pole than at the south.

The ice may be concentrated in localized areas (roughly 1850 square km in total, at each pole), with an estimated total volume of ice of 6 trillion kg, but there are large uncertainties associated with this estimate.

The discovery of ice on the Moon, if confirmed, hasimportant consequences for our understanding of planetary evolution, and raises more questions - for example, does the ice date from the early bombardmentstage of the Solar System, or is it partly due to the same“small comets” claimed to be depositing water in theEarth’s atmosphere?

It also has significant implications for those planning long-term missions to the Moon, or even lunar settlements - access to a lunar supply of water, instead of having to take large amounts from Earth, would mean significant savings in the payloads needed on Moon-bound missions.

On July 31, 1999, the Lunar Prospector was deliberately crashed into a deep polar crater at the end of its mission. The aim was to observe the crash area with Earth-based telescopes and see if material thrown up by the crash contained chemical

signatures of water (such as OH- ions), which would indicate that ice was present in the crater. This was a fairly speculative project: for example, it was not known if that particular crater contained ice, and the exact crash trajectory was hard to determine. However no sign of water was discovered - a setback for proponents of the existence of water ice on the Moon, but not a conclusive one.To find out more about Clementine, the Lunar Prospectorand the reports of ice on the Moon, visit the “Water Ice on the Moon” Internet website at:http://nssdc.gsfc.nasa.gov/planetary/ice/ice_moon.html

So far we have not looked at current theories of how the Moon was originally formed.

In the next Module we will investigate a Solar Systembody with strong similarities to the Moon - Mercury. Thedifferences between the two will lead to a discussion ofcurrent theories of the formation of both the Moon and Mercury.

In this Module we have investigated our present knowledge of the Moon, the conventional model for the Moon’s evolution - and one of the current “hot topics” in planetary astronomy, the presence of lunar water ice.

Image CreditsNASA: Highland anorthositehttp://pds.jpl.nasa.gov/planets/welcome/thumb/igneous.gif

Mare basalthttp://pds.jpl.nasa.gov/planets/welcome/thumb/basalt.gif

Galileo image of the Moon, centred on Mare Orientalehttp://images.jsc.nasa.gov/images/pao/STS34/10063795.gif

Apollo 8 image showing mariahttp://images.jsc.nasa.gov/images/pao/AS8/10074961.gif

Hadley rillehttp://pds.jpl.nasa.gov/planets/welcome/thumb/hadley.gif

Lunar South Pole Region, Clementine Spacecrafthttp://nssdc.gsfc.nasa.gov/planetary/banner/moon_south_pole.gif

Clementine image-enhanced view of the lunar South Polehttp://nssdc.gsfc.nasa.gov/planetary/clementine_sc.gif

Now return to the Module home page, and read more about the features and evolution of the

Moon in the Textbook Readings.

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