Spacecraft missions have provided abundant evidence of water on Mars, dating back to the Mariner 9 mission, which arrived at Mars in 1971. This article provides a mission-by-mission breakdown of the discoveries they have made.
Mariner 9 Mariner 9 arrived at Mars in November 1971 and became the first spacecraft to orbit another planet. Its imaging campaign, which covered 85% of the Martian surface, revealed large fluvial channels originating in patches of chaotic terrain, possibly formed by the melting of permafrost, that appear to flow northward into the Chryse Planitia region. These were the first strong geomorphological pieces of evidence that liquid water had once flowed on the Martian surface. The findings from the Mariner 9 mission underpinned the later Viking program, which examined many of these features in greater detail. The Valles Marineris canyon system, a primarily tectonic rift structure stretching over , is named after the spacecraft.
Viking program
Viking program discovered many geological forms that are typically formed from large amounts of water. Huge river valleys were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Large areas in the southern hemisphere contained branched valley networks, suggesting that rain once fell. The flanks of some volcanoes are believed to have been exposed to rainfall because they resemble those occurring on Hawaiian volcanoes. Many craters look as if the impactor fell into mud. When they were formed, ice in the soil may have melted, turned the ground into mud, and then the mud flowed across the surface. Normally, material from an impact rises, then falls. It does not flow across the surface, going around obstacles, as it does on some Martian craters.Raeburn, P. 1998. Uncovering the Secrets of the Red Planet Mars. National Geographic Society. Washington D.C.Moore, P. et al. 1990. The Atlas of the Solar System. Mitchell Beazley Publishers NY, NY. Regions, called "chaotic terrain", seemed to have quickly lost great volumes of water, which caused large channels to form downstream.Morton, O. 2002. Mapping Mars. Picador, NY, NY Underground volcanism may have melted frozen ice; the water then flowed away, and the ground collapsed, leaving chaotic terrain.
Results from Viking lander experiments strongly suggest the presence of water in the present and in the past of Mars. All samples heated in the gas chromatograph-mass spectrometer (GSMS) gave off water. However, the way the samples were handled prohibited an exact measurement of the amount of water. But it was around 1%. General chemical analysis suggested the surface had been exposed to water in the past. Some chemicals in the soil contained sulfur and chlorine that were like those remaining after seawater evaporates. Sulfur was more concentrated in the crust on top of the soil than in the bulk soil beneath. So it was concluded that the upper crust was cemented together with sulfates that were transported to the surface dissolved in water. This process is common in Earth's deserts. The sulfur may be present as sulfates of sodium, magnesium, calcium, or iron. A sulfide of iron is also possible. Using results from the chemical measurements, mineral models suggest that the soil could be a mixture of "about 80% iron-rich clay, about 10% magnesium sulfate (kieserite?), about 5% carbonate (calcite), and about 5% iron oxides (hematite, magnetite, goethite?)". These minerals are typical weathering products of mafic igneous rocks. The presence of clay, magnesium sulfate, kieserite, calcite, hematite, and goethite strongly suggests that water was once in the area. Sulfate contains chemically bound water; its presence suggests water was around in the past. Viking 2 found a similar group of minerals. Because Viking 2 was much farther north, pictures it took in the winter showed frost.
Mars Global Surveyor
The Mars Global Surveyor's Thermal Emission Spectrometer (TES) is an instrument able to detect mineral composition on Mars. Mineral composition gives information on the presence or absence of water in ancient times. TES identified a large (30,000 square-kilometer) area (in the Nili Fossae formation) that contained the mineral olivine. It is thought that the ancient impact that created the Isidis basin resulted in faults that exposed the olivine. Olivine is present in many mafic volcanic rocks; in the presence of water, it weathers into minerals such as goethite, chlorite, smectite, maghemite, and hematite. The discovery of olivine is strong evidence that parts of Mars have been extremely dry for a long time. Olivine was also discovered in many other small outcrops within 60 degrees north and south of the equator. Olivine has been found in the SNC (shergottite, nakhlite, and chassigny) meteorites that are generally accepted to have come from Mars.
On December 6, 2006, NASA released photos of two regions called Terra Sirenum and Centauri Montes, which appear to show the presence of liquid water on Mars at some point between 1999 and 2001.Mars photo evidence shows recently running water. The Christian Science Monitor. Retrieved March 17, 2007
Hundreds of gullies have been discovered that were formed from liquid water, possibly in recent times. These gullies occur on steep slopes and mostly in certain bands of latitude.
Below are some examples of gullies photographed by Mars Global Surveyor.
A few channels on Mars displayed inner channels that suggest sustained fluid flows. The best-known is the one in Nanedi Valles. Another was found in Nirgal Vallis.
Many places on Mars show dark streaks on steep slopes, such as crater walls. Dark slope streaks have been studied since the Mariner and Viking missions. It seems that streaks start out being dark, then they become lighter with age. Often, they originate with a small, narrow spot, then widen and extend downhill for hundreds of meters. Streaks do not seem to be associated with any particular layer of material because they do not always start at a common level along a slope. Although many of the streaks appear very dark, they are only 10% or less darker than the surrounding surface. Mars Global Surveyor found that new streaks have formed in less than one year on Mars.
The generally accepted explanation for the streaks is that they are formed from the avalanching of a thin layer of bright dust that is covering a darker surface. Bright dust settles on all Martian surfaces after a period of time.
Dark streaks can be seen in the images below, as seen from Mars Global Surveyor.
Some parts of Mars show inverted relief. This occurs when materials are deposited on the floor of a stream and then become resistant to erosion, perhaps by cementation. Later, the area may be buried. Eventually, erosion removes the covering layer. The former streams become visible since they are resistant to erosion. Mars Global Surveyor found several examples of this process. Many inverted streams have been discovered in various regions of Mars, especially in the Medusae Fossae Formation, Miyamoto Crater, and the Juventae Plateau.
The image below shows one example.
Mars Pathfinder
Pathfinder found temperatures varied on a diurnal cycle. It was coldest just before sunrise (about −78 Celsius) and warmest just after Mars noon (about −8 Celsius). These extremes occurred near the ground, which both warmed up and cooled down fastest. At this location, the highest temperature never reached the freezing point of water (0 °C), so Mars Pathfinder confirmed that where it landed, it is too cold for liquid water to exist. However, water could exist as a liquid if it were mixed with various salts.
Surface pressures varied diurnally over a 0.2 millibar range, but showed 2 daily minima and two daily maxima. The average daily pressure decreased from about 6.75 millibars to a low of just under 6.7 millibars, corresponding to when the maximum amount of carbon dioxide had condensed on the south pole. The pressure on Earth is generally close to 1000 millibars, so the pressure on Mars is very low. The pressures measured by Pathfinder would not permit water or ice to exist on the surface. However, if ice were insulated with a layer of soil, it could last a long time.Atmospheric and Meteorological Properties , NASA
Other observations were consistent with water being present in the past. Some of the rocks at the Mars Pathfinder site leaned against each other in a manner geologists term imbricated. It is believed that strong flood waters in the past pushed the rocks around until they faced away from the flow. Some pebbles were rounded, perhaps from being tumbled in a stream. Parts of the ground are crusty, maybe due to cementing by a fluid containing minerals.
There was evidence of clouds and maybe fog.
Mars Odyssey
Mars Odyssey's Gamma Ray Spectrometer (GRS) had discovered huge amounts of water over vast areas of Mars. In both hemispheres, from 55 degrees latitude to the poles, Mars has a high density of ice just beneath the surface; one kilogram of soil contains about 500 g of water ice. But, close to the equator, there is only 2 to 10% of water in the soil. Scientists believe that much of this water is locked up in the chemical structure of minerals, such as clay and sulfates. Previous studies with infrared spectroscopes have provided evidence of small amounts of chemically or physically bound water. The Viking landers detected low levels of chemically bound water in the Martian soil. It is believed that although the upper surface only contains a percent or so of water, ice may lie just a few feet deeper. Some areas, Arabia Terra, Amazonis quadrangle, and Elysium quadrangle, contain large amounts of water. Analysis of the data suggests that the southern hemisphere may have a layered structure. Both poles showed buried ice, but the North Pole had none close to it because it was covered by seasonal carbon dioxide (dry ice). When the measurements were gathered, it was winter at the North Pole, so carbon dioxide had frozen on top of the water ice. There may be much more water further below the surface; the instruments aboard the Mars Odyssey are only able to study the top meter or so of soil. If all holes in the soil were filled by water, this would correspond to a global layer of water 0.5 to 1.5 km deep.
The Phoenix lander confirmed the initial findings of the Mars Odyssey. It found ice a few inches below the surface, and the ice is at least 8 inches deep. When the ice is exposed to the Martian atmosphere, it slowly sublimates. In fact, some of the ice was exposed by the landing rockets of the craft.
Thousands of images returned from Mars Odyssey support the idea that Mars once had great amounts of water flowing across its surface. Some pictures show patterns of branching valleys. Others show layers that may have formed under lakes. Deltas have been identified.
For many years, researchers believed that glaciers existed under a layer of insulating rocks. Lineated deposits are one example of these probable rock-covered glaciers. They are found on the floors of some channels. Their surfaces have ridged and grooved materials that deflect around obstacles. Some glaciers on Earth show such features. Lineated floor deposits may be related to lobate debris aprons, which have been proven to contain large amounts of ice by orbiting radar.
The pictures below, taken with the THEMIS instrument on board the Mars Odyssey, show examples of features associated with water present in the present or past.
Much of the surface of Mars is covered by a thick, smooth mantle that is thought to be a mixture of ice and dust.