Unless otherwise noted, all the images on this page were obtained from a variety of NASA web sites.

Mercury is a small planet (not greatly different in size than the moon) and the closest planet to the sun. The surface is heavily cratered. Surface temperatures range from a high of 870^oF to a low of -300^oF. This, together with a relatively low escape velocity (9600 mph) means that the planet has lost most of whatever initial atmosphere it might have had, retaining slight traces of hydrogen and helium, perhaps trapped from the solar wind. We can safely rule out this planet as a suitable site for life as we understand it.

Venus is just slightly smaller than the Earth and has an escape velocity of 23,300 mph. It has a dense covering of white clouds (largely sulfuric acid) that give the planet a blinding, featureless white appearance in a telescope. Here, in an ultra-violet photo, the cloud structure can be seen.
 

Everything significant about the surface features of the planet we know from either Earth-based radar or radar imaging from orbiting spacecraft, as shown here using Magellan imaging data. The black streaks in this global composite represent missing radar data.

The geological processes suggested by such imaging indicate a geological history quite different from the Earth. Venus appears to lack active plate tectonics and the entire surface has been reconfigured by volcanic outpourings. The atmosphere of the planet is dense and composed of 96% carbon dioxide with 3% nitrogen and a trace of water vapor. Venus has apparently experience a run-away greenhouse effect, with an average surface temperature of 850^oF - the hottest planet in the solar system!

The Earth as photographed from Lunar orbit (Apollo 8). You live here so I will make the assumption that you know something about the place! Keep in mind that this is the only real estate in the solar system that provides the environment we require. We really should take better care of the place!

Of all the terrestrial planets, Mars has always been a target of speculation with respect to the presence of life. Earlier in this century, the dark surface makings seemed to represent vegetation, while the polar caps (the southern polar cap is just visible here) would wax and wane with the seasons, suggesting the possibility of seasonal melt-water.
 

Unfortunately, both high resolution spacecraft imagery (above) and pictures from surface landers (left) show a much-less benign environment. The planet demonstrates both impact and volcanic cratering. The present surface is very dry with a tenuous atmosphere of 95% carbon dioxide, 3% nitrogen, and almost 2% argon.

Mars is only about half the size of the Earth, with an escape velocity of 11,200 mph. Surface temperatures range from a high of 96^oF to a low of -190^oF.

While surface water is now rare, high resolution imagery from the Mars Polar Orbiter suggests that water may be frozen in the near sub-surface as permafrost. Here are gullies on steep slopes below a cliff face (located at about 70^o south latitude, suggest the presence of water flowing out from subsurface permafrost layers in the relatively recent past. In the remote past, major surface features suggest the presence of flowing surface water and even small oceans. Mars certainly had a more significant atmosphere and was probably warmer very early in its history.

The rim of this crater shows "seepage features" like those in the previous image and some at NASA have interpreted the smooth material on the crater floor as "ponded" material.  Other researchers suggest that these interior deposits may be lava flows.

Many of the valley systems on Mars have features similar to those on Earth that are the result of water drainage, although the Martian exampls always appear less developed than comparable terrestrial drainage systems.

An artist's rendition of what Mar's might have looked like at an earlier and wetter stage in its history. The key issues are the duration of such "wet" intervals and the chemical characteristics of the aqeous system. Recent geochemical data from the Spirit and Opportunity rovers suggests that the free water was quite acidic, which does not bode well for chemical mechanisms that might have produced martian life forms.

While few people expect to encounter any life forms on Mars today, it may well be that life was once present on the planet in simple form. This speculation has been fueled by controversial data from meteorites that appear to have originated on Mars. While unmanned missions to the planet may unravel some aspects of any past presence of life on Mars, the definitive answer must probably await as yet unplanned manned missions to the planet.