One of the most intriguing, unanswered scientific questions is why do Earth and Mars appear different today? At the time of their formation several billion years ago, Mars and Earth shared similar conditions. Both planets harbored vast quantities of surface water, thick atmospheres, and climates warmer than at present.

Today, Earth is a lush world filled with a countless number of animal and plant species. In contrast, data gathered from Mars over the last 30 years shows that the planet lies trapped in conditions reminiscent of a global ice age. The dry and seemingly lifeless Martian surface makes the Sahara look like an ocean in comparison, and average daily temperatures make Antarctica seen balmy. Comparing the history and evolution of the two planets will yield clues into Earth's past and possibly its future.

Despite its forbidding climate, surface temperatures on Mars resemble the Earth's more than any other planet. These similarities in temperature result in part from the fact that Mars orbits the Sun only slightly farther out than the Earth as compared to other planets. For example, the ground at some locations near the equator may warm up to as high as 25 deg. C (77 deg. F) at noontime. However, daytime temperatures still average well below freezing, and night temperatures dip much lower.

The Air Up There. Martian temperatures may seem almost inviting to the seasoned outdoors explorer, but the composition of the atmosphere leaves much to be desired from a human perspective. Most of the Martian air consists of carbon dioxide (CO2), similar to conditions on Venus. If breathing carbon dioxide seems uninviting, the density of the air will appear worse. Average barometric pressures on Mars measure lower than that found at Earth's sea level by a factor of more than 125. In other words, the air at the surface of Mars is thinner than that found on Earth at an altitude 19 times higher than Denver, Colorado.

The extremely thin Martian air directly impacts the mystery of potential life on Mars, either in the past or present. The reason is that almost all of the water lies trapped in the Martian polar ice caps or frozen beneath the surface. Liquid water cannot exist on the surface because the thin atmosphere will cause melting ice to evaporate directly into water vapor. Scientists call this process "sublimation."

Water World? Although liquid water on Mars will quickly evaporate, photographs transmitted back to Earth by previous NASA missions to the planet reveal giant flood channels, dry river beds, and flood plains on the surface. This evidence of past water on Mars leads some scientists to consider Mars as the prime location in the Solar System to search for extraterrestrial life. The speculation is that because Mars once possessed a thicker atmosphere and vast quantities of surface water billions of years ago, then the planet may have harbored conditions favorable to the formation of life despite its present forbidding climate.

Sensors aboard various NASA spacecraft launched to Mars over the last 30 years have shown that advanced life forms almost certainly do not exist on the planet today. However, many feel that the planet may hide bacterial forms of life or their fossil remains. Although Mars Global Surveyor will not conduct a search for life on Mars, it will gather detailed data that will help in understanding the mystery of the missing water. This type of study will provide important background data that will help scientists in their search for Martian life on future missions.

Natural Wonders. Geologically, Mars is also one of the most interesting planets in the Solar System. This small, red-colored, rocky planet ranks in size as twice as large as the Moon, but only half as large as Earth. Despite the planet's small size, it contains large water and CO2 ice caps at the poles, a canyon much deeper than the Grand Canyon and longer than the United States, crater valleys as large as the western United States, and a handful of monstrous volcanoes that make Mount Everest appear tiny in comparison.

A study of Martian geology is crucial toward revealing clues into the history of the Earth. The reason is that Mars is the only planet in the solar system that both has an atmosphere, and contains surface features that cover almost the entire range of history. On Earth, pristine rocks and other surface features from the first billion years of our planet's existence do not exist because geological events, weather, and life have caused drastic alterations. Because Earth and Mars shared similar conditions near the time of their formation, an exploration of Mars will allow us to take a peek into Earth's past in a way not possible by studying the Earth by itself.