The MOON : Part 3

The Lunar Surface

Galileo Lunar Drawings
Fig. 1. Galileo’s Lunar Drawings.
Various aspects of the Moon at different phases, as drawn by Galileo during 1610.

Any telescopes when pointed directly at the Moon will find almost immediately that it is a very different place from Earth. Firstly, the Moon is an airless world that is exposed to the harshness of the Sun and the vacuum of space. Temperatures range from well above the boiling point of water to well below freezing, depending on exposure to direct sunlight or being cast in shadow. Surface temperatures, receptively, may reach up to 105°C at midday or can plunge below −160°C in shadow or at night. Lack of any significant lunar atmosphere, mean astronaut spacesuits have to be heaters and refrigeration units, and acting as good efficient air conditioners. This explains the necessary bulk of these suits. By simply walking from the shadows causes an instant temperature drop of about 265°C!

In 1610, it was Galileo Galilei (1564-1642) was first to telescopically closely observe the Moon. He was simply astonished by its surprising complexity. The whole lunar surface was seen to be littered with thousands upon thousands of craters in random distribution, but also intermixed with high mountains. These observations, were accompanied with several drawings that appeared in Galileos significant work, the Sidereus Nuncius or Sidereal Messenger

Galileo Lunar Drawings
Fig. 2. Frontispiece of Galileo’s Book Sidereus Nuncius (1610).

Although to the naked-eye the Moon has the appearance of a fairly even selenium-grey coloured surface, others once theorised it to be something very similar to that of the Earth. Early astronomers oddly once imagined that that the Moon had several seas and these were usually termed the Latin name singularly mare and as the genitive, maria. Continued usage of this is certainly misconceived, as all mare contain no water. The name, however, has stayed. The creation of the maria are now thought to be ancient lava flows that occurred on the lunar surface 3.5 or 4.0 billion years ago. We now know that the main face-parts in the Man in the Moon are all examples of some of the luna maria.

There are many other types of terrain seen on the Moon, and to avoid confusion with the lunar (and planetary) features, the neutral language of Latin always used. However, most of the these terms are not highly descriptive nor accurate, Ie. Lakes, marshes and oceans — even though the Moon has no known water. Common examples of these appear placed on lunar maps.

Common Lunar Terrain
MareSingular for Maria
MonsSingular for Mountains
PateraShallow, disk-like depression
PlanitiaPlains or Basins
RilleNarrow, linear valleys
Valles Valleys

Features such as the rugged mountains, mountain chains or mountain ranges, do tower well above the sea plains of the mare. Highest of these may top more than six-and-a-half kilometres in skywards — being some 0.2% of the lunar diameter. On the near side face of the Moon, highest peak from base to top is Mons Huygens, at 4.7 km.

Lunar mountains are often termed either as Mons, Latin for mountain. I.e. Mons Hadley, or as Montes for the mountains in chains or ranges. I.e. Montes Apenninus (the Apennine Mountains) or Montes Caucasus (the Caucasus Mountains.) Largest of the mountain chains is Montes Rook, named after the English astronomer Lawrence Rook (1622-1662).

Heights of the mountain peaks can be determined by measuring the length of shadows being cast across the much flatter lunar surface below them. Other features such as rilles can also be seen that look as if running water has moved across the surface, cutting deep ravines or channels. The Clementine lunar satellite from February to March 2004, measured the entire topography of the Moon, and found the difference between the lowest and highest place being 18.1 km. (0.5% the lunar diameter.)

The lunar surface is also covered by layers of tiny micrometeorites and dust to the depth of between two and eight metres. In other places this may be twenty metres thick on the Moon. This loose material layer is called the regolith, whose general composition is heterogeneous mixture of glass fragments from the impacts and some from the fragmentation of the underlying lunar rock. The footprints, for example, left on the Moon by the Apollo astronauts will not last for all eternity as material from the continued bombardment will erase it — taking period of at least five hundred million years!

Names of the Lunar Craters

First introduced in 1651 by Giovanni Riccioli (1598-1671), all the largest craters have been named after famous or historical astronomers, or people of note. I.e. Tycho, Kepler, Hipparchus, Messier, Copernicus, etc. Most of the brightest or biggest craters are named after astronomers or legendary characters from either the ancient and in the Renaissance. Others are named after people in science, the arts and even politicians. Over the years, especially since the human exploration of the Moon, the number of known craters has significantly increased. Often they are the smaller raters that surround the larger ones, being labelled by single designated Roman capital letter. I.e. Copernicus A, Kepler D, etc. Names have also been recognised for the invisible portion or far side of the Moon, which cannot be seen from the Earth. Naming of lunar features is now exclusively controlled by the International Astronomical Union (IAU).

Names and places on the Moon can be found on lunar maps. Some books also have maps of the Moon within them. All maps are often reversed, so that identification of features similar to our telescopic view.

Formation of Craters

Lunar craters can range from 350 kilometres down to only several centimetres. In general, most craters have high walls sometimes with a central dimple or are internally littered with debris. Most craters were originally formed by meteorite impacts and are usually clean and smooth inside the surface of the crater, mostly with tiny dimples in their centres. Each dimple has been produced by meteorites hitting the surface, liquefying the rock in slurries, then setting into the familiar crater shape.

Moon Craters
Fig. 3. The Lunar Craters.
Image taken of the Moon near First Quarter, showing the many craters along the terminator in the southern portion of the lunar surface.
Image courtesy of Nick Loveday (2009)

These central dimples are indisputably indications of this type of impact. The surface after this collision melts the surround rock, minus the impact material that was either not thrown back into space nor splashed across the lunar surface. Liquified rock comes from the friction and huge kinetic energies that are released by the impact. In time, the crater literally sets in the familiar shapes we see today. Most of these kinds of lunar craters are very old and formed perhaps between 3.5 and 4.5 billion years ago, and it is likely groups of craters was formed in the several impact areas during the initial formation of the Solar System.

As the Moon has little atmosphere to speak of, so the lunar features cannot suffer the weathering effects of water or wind, enduring for periods exceeding many times that of the age of the Solar System. For this reason the Moon is really just some quick snapshot of its own early history.

Some craters have been likely made by volcanism, where heat sources inside the Moon pushed up the surface into volcanic cones. As the lunar interior cooled the underlying rocks contracted, leaving exposed domes with the large volume of space underneath it. A local moonquake, or perhaps some nearby meteorite impact hits the dome, shattering the shell so that material crashes down into the remaining void. This produces the many rough and rocky crater interiors, which are seemingly commonplace on the Moon.

Another important lunar feature is that the newer meteor impact craters have radiating white streaks or rays extending huge distances across the lunar surface from the impact crater. This is probably made from hot ejected material splattered linearly across the lunar surface. They mostly appear only to radiate from the big impact craters like Tycho and Copernicus. Most of these radial features are estimated to age between one and three-and-a-half billion years.

The Origin of the Moon

The true origin of the Moon still puzzles many planetary astronomers. The Apollo astronauts who landed on the lunar surface, main goal was to solve this particular problem, which was expected to be achieved by taking surface samples of rock and dust. Prior to any of the landings two prominent theories existed. Some believed the Moon was once an independent body from the Earth, captured sometime in the distant past by its stronger gravity, via the so called the capture theory. Others thought that the Moon was actual formed from the Earth. This was by either by hitting our Earth with some large Mars-like object or by some main separation by the past high rotation of the Earth. This latter theory presumes Earth fracturing into two distinct parts, the second or smaller portion became our Moon.

To prove if these three theories were correct only the lunar rocks could tell. If the rocks were dissimilar in composition, then the first conclusion would likely to be correct. If similar in makeup, then the Moon would prove to have the same geology as the Earth. The only direct visual evidence of the presumed Earth-Moon connection was the Pacific ocean basin. This ocean covers mare than half the Earths surface and it was assumed that the Moon could have been carved out of this huge expanse.

All the rock samples provided by the twelve Apollo astronauts and three Soviet lunar explorers, has proved to be about the same age as the Earth. The major difference in composition of variations was from lunar meteorite impacts. This seems to have happened in the first billion years of the Solar System. These observed differences have still kept this debate opened.

Observations of the Moon

Amateur lunar observations are best made when the Moon is high in altitude when the conditions of seeing and transparency are generally better. Sometimes this cannot be avoided, especially two to four days before and after new moon. The best time to observe the Moon is usually around first and last quarter.

At the time of new moon the earthshine can be seen on the lunar face of the darkened moon is illuminated by the light from the Earths surface. From the moon, the observed Earth phase is always opposite, so at this time, the near Full Earth would be an incredibly bright −14 or −15 magnitude! When the moon is at low altitudes to the horizon also discloses its yellow colour, caused simply by the moonlight refracting through more of the Earths atmosphere.

Observation made during full moon becomes limited, as sunlight shines from lunar zenith. Having no shadows, the lunar disk appear almost featureless. Perhaps the most interesting features at this time are the radiating ray structure strewn from the brighter craters like Tycho and Copernicus. Sometimes using a green filter will vastly improve contrast of these features. Daylight observations of the Moon may be achieved using polarising filtered. This helps by partly increases the contrast of the lunar features, but still leaves many craters normally washed-out by the sky brightness.

Last Update : 10th October 2012

Southern Astronomical Delights © (2012)

For any problems with this Website or Document please e-mail me.