Mars, on the other side of Earth from Venus, is Venus's direct opposite in terms of physical properties. Its atmosphere is cold, thin, and transparent, and readily permits observation of the planet's features. We know more about Mars than any other planet except Earth. Mars is a forbidding, rugged planet with huge volcanoes and deep chasms. The largest volcano, Olympus Mons (Olympic Mountain), rises 78,000 ft above the surface, higher than Mount Everest. The plains of Mars are pockmarked by the hits of thousands of meteors over the years.
Until the arrival of Mars Pathfinder and Mars Global Surveyor in 1997, most of our information about Mars came from the Mariner and Viking spacecrafts. Mariner 9 orbited the planet in 1971 and photographed 100% of the planet, uncovering spectacular geological formations, including a Martian ?Grand Canyon? that dwarfs the one on Earth. Called Valles Marineris (Mariner Valley), it stretches more than 3,000 mi along the equatorial region of Mars and is over 2.5 mi (4 km) deep in places and 50 to 62 mi (80 to 100 km) wide. The spacecraft's cameras also recorded what appeared to be dried riverbeds, suggesting the one-time presence of water on the planet. The latter idea gave encouragement to scientists looking for life on Mars, for where there is water, there may be life. However, to date, no evidence of life has been found. Temperatures range from 80F at the equator during the day to ?199F at the poles at night.
Mars rotates upon its axis in nearly the same period as Earth?24 hours, 37 minutes?so that a Mars day is almost identical to an Earth day. Mars takes 687 days to make one trip around the Sun. Because of its eccentric orbit, Mars's distance from the Sun can vary by about 36 million miles. Its distance from Earth can vary by as much as 200 million miles. The atmosphere of Mars is much thinner than Earth's; atmospheric pressure is about 1% that of our planet. Its gravity is one-third of Earth's. Major constituents are carbon dioxide and nitrogen. Water vapor and oxygen are minor constituents. Mars's polar caps, composed mostly of frozen carbon dioxide (dry ice), recede and advance according to the Martian seasons.
Mars has four seasons like Earth, but they are much longer. For example, in the northern hemisphere, the Martian spring is 198 days, and the winter season lasts 158 days.
The Mars Pathfinder lander and its rover, Sojourner, set down on the edge of a boulder-strewn outflow channel known as Ares Vallis on July 4, 1997, and provided scientists with a wealth of information on the rocks, soils, and atmosphere of Mars. The lander sent back the first live pictures of the planet's topography, and its tiny rover explored a variety of rocks and analyzed their mineral composition with its cameras and onboard X-ray spectrometer.
In its three months of operation, the mission returned more than 16,000 images of the Martian landscape from the lander's camera and 550 images from the rover.
Analysis of the reddish surface soil pointed to the presence of oxidized iron, indicating that the planet's surface is rusting. Sojourner samples of soil taken from several sites found their composition similar to those analyzed by the two Viking landers in 1976, indicating that the Martian winds have distributed the soil evenly over the planet.
Scientists were surprised to learn how rapidly the Martian temperature fluctuates due to atmospheric turbulence. It can change by as much as 30?40F (17?22C) in a matter of minutes, possibly due to strong, gusty winds bringing warm air from one region or cold air from another.
Scientists have inferred from the variety of rocks and sediments found in the Ares basin that the spacecraft landed in a channel that was once awash with torrential floods greater than any known on Earth. The diversity of rocks deposited there suggests their different origins, and it appears that they were washed down from the highlands at a time when great floods moved over the surface of Mars.
Before Pathfinder, knowledge of the kinds of rocks present on Mars was based mostly on the Martian meteorites found on Earth. Chemical analysis of the Martian rocks and soil found at Ares Vallis confirmed that these rocks have compositions distinct from those of the Martian meteorites found on Earth.
The Sojourner rover traveled a total of about 328 ft (100 m), performed more than 16 chemical analyses of rocks and soil, and explored 820 sq ft (250 sq m) of the planet's surface. Communications were lost with the lander on Sept. 27, 1997, after 83 days of relaying data.
NASA launched the Mars Global Surveyor spacecraft on Nov. 7, 1996, to provide detailed maps of the planet's surface, to track its distribution of minerals, and to monitor its weather. The spacecraft entered Mars's orbit on Sept. 11, 1997, and began mapping operations in mid-March 1999.
Surveyor discovered the first clear evidence of an ancient hydrothermal system near the equator. This implies that water was stable at or near the surface and that a thicker atmosphere existed in Mars's early history.
Most surprising to mission scientists was finding that the planet's northern hemisphere was exceptionally flat, with slopes and surface roughness increasing towards the equator.
Surveyor's three-dimensional views of the planet's northern polar ice cap showed often striking canyons and spiral troughs in the water and carbon dioxide ice that can reach depths as great as 3,600 ft below the surface. Its data also showed that large areas of the ice cap were extremely smooth, with elevations varying only a few feet over many miles.
In 1999, the Mars Global Surveyor discovered magnetic stripes about 100 mi (160 km) wide, the remnants of the planet's ancient magnetism, running east and west on the surface of Mars. At their longest, the band-patterns of magnetic fields extend as far as 1,240 mi (1,996 km) along the surface.
NASA's Mars exploration program suffered a setback with the loss of the Climate Orbiter as it entered the Martian atmosphere in Sept. 1999. The following December scientists also failed to establish contact with the Polar Lander after it reached Mars.
In 2000, NASA announced that the Mars Global Surveyor had observed features that looked like gullies carved out by flowing water and deposits of soil and rocks that were transported by the flow. Because gullies had never been seen before on Mars, the Surveyor images suggested that there might be current sources of liquid water at or near the surface.
In the spring of 2002, NASA released exciting news that large quantities of water ice had been found just below the surface of Mars. The discovery was made by NASA's Mars Odyssey spacecraft, which was launched in April 2001 and has been collecting data since late 2001.
On June 10, 2003, NASA launched Spirit, the first of two Mars Exploration Rovers (MER), followed on July 7 by the takeoff of the second rover, Opportunity. Spirit landed on Mars in the Gusev Crater on Jan. 3, 2004, and within hours was transmitting the first panoramic images to Earth. Opportunity landed successfully on Jan. 25 in the Meridiani Planum region on the opposite side of the planet.
Spirit spent 12 days on its landing platform sending pictures to Earth before rolling onto the surface to study the soil, rocks, and atmosphere. Several days before Opportunity landed, Spirit started having computer-memory problems, a glitch remedied after a week of sporadic communication with the rover. Both Spirit and Opportunity found chemical and rock layering evidence that water once flowed freely on Mars.
The main instruments on board the rovers are: a panoramic camera, a miniature thermal emission spectrometer for identifying rocks and soils and taking the temperature of the atmosphere, a Mossbauer spectrometer for studying the mineralogy of rocks and soil, an alpha particle X-ray spectrometer for chemical analysis of rocks and soil, magnets for collecting magnetic dust particles, a microscopic imager, and a rock abrasion tool to scrape rocks for a fresh surface to study.
Opportunity uncovered water evidence in the Meridiana Planum region in early March 2004. The rover discovered salts usually found in rocks either formed in water or subjected to long exposure to water. The physical characteristics of the rocks bolstered the water evidence with surfaces that appeared to have been pocked from salt deposits; round, BB-sized particles formed from mineral accumulation inside a porous, water-soaked rock (called ?blueberries?); and a pattern or layering usually associated with the action of water. A few days later on the other side of Mars, Spirit found water evidence in a rock that appeared to have minerals crystallized out of water.
The rovers were designed for a 90-day mission but performed so well that NASA decided to extend their duties for as long as they keep working. As of the middle of June 2006, Spirit had travelled 4.27 mi (6,876.18 m) and Opportunity had travelled 5.02 mi (8,080.38 m).
The Mars Reconnaissance Orbiter was launched in Aug. 2005 and went into orbit around the planet in March 2006. From Nov. 2006 to Nov. 2008, the orbiter will gather information about Mars. After that, it will communicate with various Mars landers until at least the end of 2010 and, if needed, until 2015.
Mars was named for the Roman god of war because when seen from Earth its distinct red color reminded the ancient people of blood. We know now that the reddish hue reflects the oxidized (rusted) iron in the surface material.