Mercury is the planet closest to the sun. Given its position near the sun, it is a very difficult planet to observe from the Earth. This is because it is never far from the sun as it is viewed from the Earth. It can be seen only shortly after sunset or shortly before sunrise. Because observation is difficult some information was known incorrectly until recently. It has been long known that Mercury takes almost 88 days to orbit the sun. But a trick of Mercury's rotation made astronomers believe that the same side of Mercury always faced the sun. As astronomers collected data for Mercury, it was discovered that the temperature of Mercury was wrong if one side always faced the sun. In the '60s, astronomers used radar techniques to measure the rotational rate of Mercury and discovered that Mercury took a little over 58 days to spin on its axis.

The surface of Mercury was also very difficult to observe. When the Mariner missions orbited Mercury, astronomers were able to confirm that the surface of Mercury was completely covered by craters. In addition, we found that Mercury had no substantial atmosphere. However, it was discovered that Mercury did have a magnetic field, although a very weak magnetic field. In addition, cracks were discovered on the surface of Mercury. It is believed these cracks are caused by the outer layer of Mercury cooling and solidifying, and then shrinking and cracking as inner layers cool and shrink.

Why does Mercury have no substantial atmosphere? Because Mercury is near the sun its temperature is fairly high, exceeding 400 degrees Celsius. These high temperatures cause any gases to have large velocities. In addition, Mercury has a very small mass and therefore only a weak gravitational pull. Therefore, the high speed gases would easily bounce away from the planets gravitational pull.

Mars is often thought to be the sister planet of the Earth. However, when considering physical characteristics, Venus is actually more like Earth than Mars. The orbit of Venus is closer to the orbit of Earth than is Mars' orbit. The mass and radius of Earth more closely resemble the mass and radius of Venus than the mass and radius of Mars. However, the surface of Venus could not be seen from Earth. This is because the surface is completely obscured by cloud cover. Because of this cloud cover, Venus reflects a large percentage of the sunlight striking it. This makes Venus a bright object in the evening or morning sky.

The atmosphere of Venus is very different from the atmosphere of the Earth. The composition of the atmosphere is mostly carbon dioxide. There is also sulfuric acid in the atmosphere of Venus. The sulfur gives Venus its yellowish appearance. In addition, the atmosphere is much thicker than the atmosphere on Earth. In fact, atmospheric pressure on Venus is about 90 times the atmospheric pressure on the Earth. This would be comparable to water pressure 1000 meters below the surface of the ocean.

The atmosphere is also a major reason for the high temperatures on Venus. The carbon dioxide atmosphere allows the entrance of visible radiation which warms the planets surface. The planet then re-radiates infrared radiation. But this infrared radiation is blocked by the carbon dioxide atmosphere and trapped within. This effect is identical to what happens with your car on a hot sunny summer day. This characteristic is referred to as the greenhouse effect. This is the same greenhouse effect that is often the subject of newspaper and magazine articles. However the greenhouse effect on Venus is much stronger than it would be here on Earth. Studying the greenhouse effect on Venus however, allows us to see the potential problems for a rampant greenhouse effect here on Earth.

Where did this thick atmosphere come from? Radar images of the Venusian surface from Earth based telescopes and, more recently, from the Magellan satellite show us that the surface of Venus is populated with volcanoes. Outgasing by volcanoes is very common here on the Earth. Therefore it is commonly believed that the volcanoes on Venus gave her its thick atmosphere. In fact volcanoes are likely the source of the atmosphere of Earth and Mars also.

This page was last updated on 08/25/04.