THE SOLAR SYSTEM
the Alps, Apennines and Carpathians. Isolated peaks
and hills abound, and there are also domes, which are
low swellings often crowned by summit craterlets. One
feature of special interest is the Straight Wall, in the
Mare Nubium – which is not straight, and is not a wall!
The land to the west drops by about 300 metres (1000
feet), so that the ‘wall’ is simply a fault in the surface.
Before Full Moon its shadow causes it to appear as a
black line; after full it reappears as a bright line, with
the Sun’s rays shining on its inclined face. It is by no
means sheer, and the gradient seems to be no more than
40 degrees. In the future it will no doubt become a lunar
tourist attraction....
Valleys are found here and there, notably the great
gash cutting through the Alps. The so-called ‘Rheita
Valley’ in the south-eastern uplands is really a chain of
craters which have merged, and crater-chains are very
common on the Moon, sometimes resembling strings of
beads. There are also rills – alternatively known as rilles
or clefts – which are crack-like collapse features. Some of
these, too, prove to be crater-chains either wholly or in
part. The most celebrated rills are those of Hyginus and
Ariadaeus, in the region of the Mare Vaporum, but there
are intricate rill-systems on the floors of some of the large
craters, such as Gassendi, near the northern boundary of
the Mare Humorum, and Alphonsus, the central member
of a chain of great walled plains, near the centre of
the Moon’s disk of which the flat-floored, 148-kilometre
(92-mile) Ptolemaeus is the largest crater.
Many of the maria are crossed by ridges, which are
low, snaking elevations of considerable length. Ridges on
the seas are often the walls of ghost craters which have
been so completely inundated by lava that they are barely
recognizable.
It is now agreed that the craters were produced by a
violent meteoritic bombardment which began at least
4500 million years ago and ended about 3850 million
years ago. There followed widespread vulcanism, with▲ Copernicus, Stadius and
Eratosthenes. Copernicus is
the large crater to the lower
left; it is one of the major
ray-centres of the Moon.
Eratosthenes, smaller but
equally well formed, is to
the upper right, at the end
of the Apennine range.
Stadius, to the right of
Copernicus, is a ‘ghost’
crater whose walls have
been levelled by the mare
lava to such an extent
that they are now barely
traceable. Ptolemaeus group.
A great chain of walled
plains near the centre of
the Moon’s disk. Ptolemaeus
is to the top of the picture,
at the centre; below it comes
Alphonsus, with a reduced
central peak and dark
patches on its floor; below
Alphonsus is Arzachel,
smaller but with higher
walls and a central peak.
The walled plain Albategnius
lies to the left of Ptolemaeus.magma pouring out from below and flooding the basins.
The lava flows ended rather suddenly about 3200 million
years ago, and since then the Moon has shown little
activity apart from the formation of an occasional impact
crater. It has been claimed that the ray-craters Tycho and
Copernicus may be no more than a thousand million years
old, though even this is ancient by terrestrial standards.
Even the youngest craters are very ancient by terrestrial
standards.
There is little activity now; there are occasional local-
ized glows and obscurations, known as Transient Lunar
Phenomena (TLP), thought to be due to gaseous release
from below the crust, but all in all the Moon today is
essentially changeless.B Atl of Univ Phil'03stp 31/3/03 4:05 pm Page 45