Mars is only about half the size of Earth, and its year lasts 23 Earth months. In other respects, though, it has several similarities with Earth: its day is only 41 minutes longer than ours; the tilt of its axis is 24 degrees (Earth's is 23.4 degrees); it experiences seasons parallel to ours; and it has polar ice caps which contain water ice and also carbon dioxide ice.
The small size of Mars means its gravity is weaker than ours, and thus it retains only a thin atmosphere (less than 1/100th the density of ours). Mars "air" is 95% carbon dioxide, with traces of oxygen, nitrogen, argon and water vapour. Liquid water once flowed on the surface of Mars, creating vast networks of riverbeds. Even after most of the water had escaped into space & the remainder was frozen out, flow patterns are evident around volcanoes which indicate that sub-surface water may have re-melted during volcanic eruptions.
The surface shows numerous volcanic and impact craters. Volcanic activity is apparently dormant, but these are the largest dormant volcanoes in the Solar System. Mount Olympus towers 25 kilometres above the surrounding plain, three times higher than Mt. Everest!
The largest valleys were created not by water erosion but by rifts or faults in the crust. The 1965 visit by Mariner 4 put to rest the suppositions that vegetation caused the varying dark areas on the surface and that canals had been constucted by intelligent life. "Mariner Valley" is a seismic fracture zone 4800 kilometres long and four times deeper than the Grand Canyon.
So if we were to visit Mars, what would it be like? The air would appear reddened by suspended dust. The weather would be "breathtaking" with over 30 dust storms per year (some of them global). Mars weather is generated in the "summer" hemisphere, with warm currents crossing the equator to the low pressure system at the opposite pole. We'd be sheltered from the Sun's radiation to some extent by the dust, and also the thin clouds of water and carbon dioxide vapour. Light snowfalls might be seen near the poles, but some of it would be dry ice! We would prefer to land near the equator, where the temperature is a moderate 27°C (the poles get down to -120°C). The thin atmosphere has a temperature gradient, decreasing with altitude, just as Earth does, but the variation is a little more drastic. Your feet would be a comfy 25°C on a summer day, but your eyes would be as much as 10°C cooler.
The two moons overhead would appear relatively small. From the surface of Mars, nearby Phobos would have about 1/3rd the diameter of our Moon as viewed from Earth. Tiny Deimos would measure less than 1/10th the Moon's diameter.
And would we find any Martians? The 1976 Viking lander experiments to look for microscopic life in the soil produced results which could not come from any known biological reactions. However, they did not correspond to any predicted chemical processes either. The results were labelled inconclusive. Experts are still in disagreement over the infamous "Mars Meteorite", ALH84001, which probably does not contain remnants of organisms, but maybe it does. In other words - keep your helmet on if you go there to visit!
We have collected samples from the surface of the Moon, from the US manned expeditions, and Soviet unmanned probes. Going to the Moon is a 3 day trip, whereas getting to Mars (using existing technology) takes about 10 months. A NASA sample/return mission to Mars is planned for 2005, but in the meantime we have to be content with the information we receive from orbiting and landing probes.
Some of the craft we have sent to Mars managed to get there in one piece. Notable in the early exploration of the Solar System are the Mariner and Viking missions by the US and Mars probes sent by the Russians.
Five Mariner probes were sent to Mars. Mariner 3, launched 26.8.62, lost communications because a shroud failed to jettison. Mariners 4, 6 and 7 all made successful fly-bys (1964 - 69). Mariner 4 was the first probe to send results from Mars, revealing craters on the surface. Mariner 9 became the first object to orbit another planet on 13.11.71. It continued its mission for a year, photographing the surface.
Three Russian Mars probes flew by the planet, and three went into orbit (1971 - 73). Only one of the four landers reached the surface intact, but its transmission failed after 20 seconds - the picture showed nothing.
The two Viking missions were entirely successful, each comprising an orbiter and a lander. Most of the pictures you'll see of Mars in books are attributable to a Viking. The landers carried panoramic cameras which took colour photographs. Instruments analysed the atmosphere and sensed ground tremors.
The surface of Mars was shown to be a red, stony desert with no visible signs of life. Sand dunes and porous-looking rocks were visible in some pictures. Miniature biological labs carried on the landers tested for the presence of microorganisms in the soil, but results were attributed to chemical reactions in the soil rather than organic life.
The entire surface was explored by the orbiters. Images clearly showed its red surface with craters, faults, volcanic peaks, lava flows and canyons. White carbon dioxide frost and clouds were also visible.
In recent years, NASA has sent two new vehicles to the red planet, Mars Pathfinder and Mars Global Surveyor. The Pathfinder mission was a landing craft which sampled soil and air. It used a unique bounce-landing capsule fitted on the outside with balloons to protect its contents from the impact with the surface. It carried a cat-sized rover which explored within 100 metres of the lander, at a site which was once a run-off channel. The rover, unfortunately, was not equipped to detect life.
Mars Global Surveyor remains in a polar orbit, to image the entire surface with high resolution. It will circle the planet for an entire Martian year (about 2 Earth years), monitoring weather changes. As the first in a decade-long series of "Surveyors", it will also serve as a data relay station for a further three years.
|Mission Statistics:||Mars Pathfinder|
|Launch:||2nd December 1996|
|Arrival:||4th July 1997|
|Roll-off of Sojourner rover:||5th July 1997|
|Sojourner:||Weighed 25 pound (on Earth), six wheels, named by Valerie Ambrose, 12, of Bridgeport, CT Observations made at the surface include: temperatures range from -80°C to 0°C; atmospheric pressure rises and falls daily in response to warming by the Sun; wind directions change throughout the day, from the south at night, from the west in the morning (when the winds are strongest), from the north in the early afternoon (weakening during the afternoon), and from the east in the evening; the rocks are more varied in appearance than the ones seen at the Viking lander sites, with three main types identified (large, rounded rocks with weathered coatings; small, grey, angular unweathered rocks; and flat white rocks); rock types include andesites ("Barnacle Bill"), and basalts ("Yogi"), the iron / manganese ratio in Yogi making it clearly dissimilar to Earth's rocks; soil particles are extremely fine - less than 50 microns in diameter (finer than talcum powder); dust is settling continuously out of the atmosphere, transported by dust devils.|
|Mission Statistics:||Mars Global Surveyor|
|Launch:||Cape Canaveral, 7th November 1996 using Delta II expendable rocket.|
|Arrival:||11th September 1997 using aerobraking to establish orbit.|
|Final Orbit:||2 hours circular at 378km altitude, sun-synchronous to maintain sun at standard angle above horizon in each image.|
|Mapping Mission:||to begin early 1999.|
|Instruments:||orbital camera, thermal emission spectrometer, ultra-stable oscillator, laser altimeter, magnetometer / electron reflectometer, Mars relay system.|
|Observations made from orbit:||first discovery that Mars has a planet-wide magnetic field (detected 15th Sept 1997), about 1/800th the strength of the field at Earth's surface.|