Planetary Cartography

Astronomers have developed various methods of distinguishing features on the surface of a planet. Each hemisphere is divided into a number of provinces, tropical and non-tropical, with each province generally acquiring a number and a name. Each hemisphere also has a "polar cap".

The planetary surface features are usually classified in the following way:

anguis		meandering fissures		mons	mountains
catena		chains of cratersv		patera	an irregular crater with jagged sides
chasma		canyons or gulfs		planitia	a large flat expanse i.e. a plain
craters		craters		planum	plateau's
dorsa		ridges		terrae	continents
fossa		gorges, or long narrow valleys		tholus	a hill
labyrinthus		a network of valleys cutting though a flat region		vallis	a valley
mensa		flat-topped protuberance ringed by cliffs		vastitas	vast low-lying plains

Very often mountains, valleys, craters and other features are named after famous scientists, astronomers and pioneers who have contributed to the advancement of astronomical knowledge.


The Moon

The average distance of the moon from earth is 384,400 km (238,906 miles). This is an average because the moon's orbit is elliptical. The moon's apparent size varies by as much as 10 percent during the lunar cycle, because of it's distance from the Earth.

The moon is thought to be round, but there is a slight flattening in the visible face, i.e. the lunar hemisphere that faces Earth, while the far side of the moon is slightly raised. Despite being raised, the far hemisphere has at it's center a large depression about 1400 km in diameter and 5 km in depth. Conversely, the near side, despite being flattened, has at it's center the Appenine Mountain chain, which reaches an elevation of 3 km.

The orbit of the moon is mainly determined by the attraction of Earth, but the gravitational forces of the Sun and the other planets introduce perturbations. This makes the precise calculation of the moon's orbit somewhat tricky.

The moon's rotation about it's own axis has one noticeble characteristic - it coincides with that of the earth, so that the same side of the moon always faces the earth. However, because of slight variations in the rotational speed at perigee and apogee, people on Earth get to see about 59% of the surface of the moon at one time or another.

The moon's geographical features were orginally named in the 17th century. The craters visible from Earth bear the names of scholars, poets, artists and philosophers of the past, such as Newton, Plato and Descartes.

Depressions on the lunar surface were originally thought to be seas or oceans, and are called maria (latin for sea). There are two main groups of seas, the "favorable" seas, Sea of Nectar, Sea of Tranquility, Sea of Fertility, etc, and the "dangerous" seas, Sea of Storms, Sea of Cold, Ocean of Tempests, etc. The most pictureseque name on the moon is probably that of the mare, the Lake of Dreams.

The mountain ranges are named after their similarity to their earthly counterparts, Alps, Appenines, Carpathians, Pyrenees, etc.

See the Map of the Near Side of the Moon. (800 KB)

Lunar rock samples have indicated that the moon's surface has less volatile elements, compared to Earth, such as carbon and oxygen, but is richer in other elements, such as titanium, iron, calcium, magnesium and other rare earths. So far, two minerals found on the moon have never been observed before, tranquillite, found near the Sea of Tranquility, and pyroxyferroite, a silicate of iron and calcium.

Tectonically, the moon is relatively calm. Although there are about 3000 earthquakes per year, they are of very low intensity, occur at depths of approximately 1000 km, and are barely detectable on the surface of the moon. The most active epicenter for quakes is under a small mountain range between the Sea of Humors and the Sea of Clouds.

The temperature on the moon ranges from a maximum of 117 degrees Centigrade in daytime to a minimum of -170 degrees Centigrade at night.

Venus

The orbit of Venus around the sun is almost precisely circular, so the planet is always about 108 million kilometers from the sun. Seen from Earth, Venus seems to shift from one side of the sun to the other, sometimes rising before the sun as a "morning star" and sometimes setting after the sun as an "evening star". For this reason, people of ancient times believed that Venus was actually two different celestial bodies, known as Phosphoros and Hesperus to the Greeks, and Lucifer and Vesper to the Romans. Venus is also known as the shepherd star.

Venus is similar in size and mass to earth, but the crust of Venus is thicker and appears to consist of a single basaltic tectonic plate which supports the plains and mountainous regions. The atmosphere of Venus is composed almost entirely of carbon dioxide. It has a permanent cloudy mantle, and layers of mist, and the clouds contain droplets of suphuric acid. There are strong upwards and downwards drafts. Temperatures on the ground are about 450 degrees centigrade, owing to the greenhouse effect of the atmosphere.

Venus has large amounts of heavy metals such as lead and bismuth. It is believed that metals melt in the high temperatures at ground level and condense at higher altitudes, coating the mountain ranges in a bright metallic layer or skin, and contributing to the reflective sheen of Venus.

Metallic mountains of Venus

Sixty percent of the surface of Venus consists of a vast plain with altitudes not exceeding 1000 meters. There are two large mountainous regions on the surface of Venus known as Terra Ishtar and Terra Aphrodite. Terra Ishtar is the size of the United States and it's central region is a large plateau called Lakshmi Planum with an altitude of about 3 kilometers.

See the Map of Venus. (400 KB)


Mars

It is now believed by astronomers that the surface of Mars consisted in ancient times of one-third water, owing to an ocean-like depression in the northern hemisphere of Mars. Scientists have named the two features suspected of being ancient shorelines Arabia and Deuteronilus. The long lines on the Martian surface rise and fall in a way resembling a landscape with a rising elevation etched by sea waves.

This raises the possibility of Mars having once had a civilization. Researchers have recently discovered seven deep caves on the slopes of the Arsia Mons volcano. They have been nicknamed the "seven sisters" and have been given the names: Dena, Chloe, Wendy, Annie, Abbey, Nicki and Jeanne. A manned expedition to Mars is planned for around 2020.

See the Map of Mars 1. (850 KB)

See the Map of Mars 2. (800 KB)

vb-tech.co.za