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 prior to MGS showed that the planet was 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 yields clues into Earth's past and possibly    its future.     Science objectives for the failed Mars Observer Mission    [ALBEEETAL1992] were essentially identical to those for Mars    Global Surveyor [ALBEEETAL2001].      Basic Measurements and Data Collection    ======================================      Although several spacecraft preceded MGS to Mars, fundamental      measurements remained to be made.  No topographic model of      the planet existed at the 100 meter level (and many areas were      uncertain by kilometers); MOLA provided one with typical      accuracies of 30 m.  Preliminary measurements on the magnetic      field were carried out by early spacecraft; but MGS MAG/ER      was the first instrument to carry out a systematic mapping      effort.  Gravity models had been compiled from Mariner 9 and      Viking data, but MGS RSS provided an order of magnitude      improvement in these -- leading to improved understanding of      the planet's interior.      Atmospheric Processes    =====================      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 Mars' equator may warm up to as high as 25C      at noontime.  However, daytime temperatures still average well      below freezing, and night temperatures dip much lower.       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 are lower than those 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.       Despite the hostile composition, density, and temperature by      today's standards on Earth, the atmosphere of Mars is both      interesting and dynamic.  MGS objectives in this area included      recording global daily images of the planet so that cloud      patterns could be followed and the growth of dust storms could      be monitored over a full martian year.  TES and RSS were both      able to measure vertical structure within the atmosphere,      another key to understanding transport of material within the      atmosphere -- including precipitation of CO2 itself on the      winter polar cap.      Surface Processes    =================      Geologically, Mars is one of the most interesting planets in      the Solar System.  Although only half the diameter of Earth,      Mars maintains large water and CO2 ice caps at the poles, a      canyon much deeper than the Grand Canyon and longer than the      contiguous 48 United States are wide, 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 to deciphering clues      about the history of the Earth.  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, the MGS      exploration of Mars allows us to take a peek into Earth's      past in a way not possible by studying the Earth by itself.       Although liquid water on Mars will quickly evaporate,      photographs transmitted back to Earth by NASA missions prior      to MGS revealed giant flood channels, dry river beds, and      flood plains on the surface.  This evidence of past water on      Mars led some scientists to consider Mars as the prime      location in the Solar System to search for extraterrestrial      life.  The speculation was 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.       Viking and Mars Pathfinder returned information on elemental      composition of some Mars surface materials at specific      landing sites.  But regional and global information was needed      to understand both the current state and history of rocky      surfaces.  MOC provided high-resolution image data; and TES      acquired spectral signatures of rock units so that thermal      inertia, surface rock distributions, and composition could      be inferred.       MOC also revealed contemporary activity on the surface during      the instrument's own lifetime including 20 new impact craters,      numerous boulder trails, secular enlargement of south polar      pits, and fresh channel outflows likely to be water-related.      Search for Life    ===============      Sensors aboard various NASA spacecraft launched to Mars over      the 30 years prior to MGS showed that advanced life forms      almost certainly do not exist on the planet today.  However,      many felt that the planet might hide bacterial forms of life      or their fossil remains.  Although Mars Global Surveyor did      not conduct a search for life on Mars, it gathered detailed      data that will help in understanding the mystery of the      missing water.  This type of study provides important      background data to help scientists in their search for      Martian life on future missions.      Other Studies    =============      In addition to studying Mars, the spacecraft was also      used for experiments of opportunity, such as searching for      gravitational waves during cruise [ESTABROOKETAL1995] and      probing the Sun's corona during solar conjunction [WOO1993].