The Moon and its Evolution

1
Module 9 The Moon in Close-up
Activity 2 The Moonand itsEvolution
2
Summary
In this Activity, we will investigate (a) lunar
mountain ranges (b) lunar maria (c) evolution of
the Moons Surface - the cooling of
planets (d) the far side of the Moon, and (e)
water on the Moon?
3
(a) Lunar Mountain Ranges
83 of the Moons 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 cant talk about heights above sea
level!)
On Earth, mountain ranges are due to tectonic
activity or volcanoes, but not so on the Moon.
4
Even regions which look a little similar to
folded mountain ranges on Earth, turn out to be
regions of overlaidcratering associated debris.
5

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

6

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

7

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

As with all lunar rocks found so far,
anorthosites are igneous - that is, formed from
magma.
As we will see, the anorthosite rock in the
highlands is probably the remnant of the
original lunar crust.
8

• (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.
9
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 largest mare, 1100 km in diameter, is Mare
  Imbrium (the Sea of Showers).

10
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.
11

• 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.
12

• 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.
13
The Moons 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 Moons surface has long been geologically
dead- with essentially no atmosphere, even the
weatheringstage could not progress.
14
(c) Evolution of the Moons Surface
The evolutionary sequence of the Moons surface,
compared to that of the Earth, is modeled as
15

• Earth Moon
• Condensation ?
• Accretion ?
• Differentiation ?
• Cratering ?
• Basin Flooding ?
• (Vulcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

We will look at models for the earliest stages
soon.
16

• 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.
17

• Earth Moon
• Condensation ?
• Accretion ?
• Differentiation ? ?
• Cratering ? ?
• Basin Flooding ?
• (Vulcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

The Moons surface isextensively scarred
bycratering, most of whichhappened in the
earlystages of the Solar System.
18

• Earth Moon
• Condensation ?
• Accretion ?
• Differentiation ? ?
• Cratering ? ?
• Basin Flooding ? ?
• (Vulcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

The cratering weakenedthe Moons crust and
allowed lava from the Moons interior to well
upthrough cracks and floodthe low-lying areas,
cooling to become marebasalt and forming mares.
19

• Earth Moon
• Condensation ?
• Accretion ?
• Differentiation ? ?
• Cratering ? ?
• Basin Flooding ? ?
• (Vulcanism)
• Plate tectonics ?
• Weathering ?
• (Slow decline)

There is no evidence for plate tectonics on the
Moon, ...
20

• 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
21

• Compared to the Earth, why did the Moons
  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 Earths interior is stillquite hot.
The interior of the Moon, a much smaller body
than the Earth, is now too cool to drive
geological activity.
22
Evidence in support of this conclusion comes from

• 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 Moons interior were molten, these
  mascons would sink.

23
The cooling of planets

• In rough terms,

• 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
24

• 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.

25
(d) The Far Side of the Moon

• While this overall model for the Moons evolution
  seems to work well, there are still issues to
  resolve and discoveries to make about 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.
Its 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.
26

• 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?
27
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.
28
The asymmetry of the lunar crust probably
accounts for the off-set in the Moons 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.
29
(e) Water on the Moon?

• After an almost 20 year absence from the Moon,
  NASAs 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.

In 1996 it was announced that Clementine data
taken of permanently shadowed regions near the
Moons south pole indicated the presence of
water ice.
30

• 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 Moons 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.
31

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

32
The Lunar Prospector

• 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.
33
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.
34
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 samesmall comets
claimed to be depositing water in theEarths
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.
35
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/i
ce/ice_moon.html
36
In this Module we have investigated our present
knowledge of the Moon, the conventional model
for the Moons evolution - and one of the
current hot topics in planetary astronomy, the
presence of lunar water ice.

• 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.
37
Image Credits

• NASA
• Highland anorthositehttp//pds.jpl.nasa.gov/plane
  ts/welcome/thumb/igneous.gif
• Mare basalthttp//pds.jpl.nasa.gov/planets/welcom
  e/thumb/basalt.gif
• Galileo image of the Moon, centred on Mare
  Orientalehttp//images.jsc.nasa.gov/images/pao/ST
  S34/10063795.gif
• Apollo 8 image showing mariahttp//images.jsc.nas
  a.gov/images/pao/AS8/10074961.gif
• Hadley rillehttp//pds.jpl.nasa.gov/planets/welco
  me/thumb/hadley.gif
• Lunar South Pole Region, Clementine
  Spacecrafthttp//nssdc.gsfc.nasa.gov/planetary/ba
  nner/moon_south_pole.gif
• Clementine image-enhanced view of the lunar South
  Polehttp//nssdc.gsfc.nasa.gov/planetary/clementi
  ne_sc.gif

38

• Now return to the Module home page, and read more
  about the features and evolution of the Moon in
  the Textbook Readings.

Hit the Esc key (escape) to return to the Module
9 Home Page
39
(No Transcript)