Mars's water

Knowing how much water there once was on Mars and how much remains would shed light on the history of Mars and the solar system, the possible development of life in Mars’s past, and ways of providing resources for future Mars colonists.

Overview

There is water on Mars. How much water and exactly where it is has become the center of many questions about the planet, including several mounted by the National Aeronautics and Space Administration (NASA). In 1971, the Mariner 9 spacecraft returned photographs of Mars that showed the planet's surface had been extensively scarred sometime in its distant past, probably by liquid water flowing over it. Subsequent detailed investigations by the Viking probes in 1976 and the Mars Pathfinder rover in 1997 confirmed this evidence and added voluminous data to support the idea that liquid water had existed on or close to the planet’s surface at one time.

Most investigations of water on Mars were concerned with ice, as conditions on the planet are rarely favorable to the existence of liquid water at the surface. The highest temperature recorded at the Viking landing sites on the warmest summer days was 244 kelvins, and the average temperature is much colder elsewhere on the planet. It fell to 150 kelvins at the site of the Viking 2 lander during the Martian winter. Even if it were warm enough for liquid water to exist, it was thought that most would quickly vaporize in the low atmospheric pressure of Mars. The atmospheric pressure at the Viking lander sites (the locations where the NASA probes made a soft landing on the planet’s surface) was less than one one-hundredth of the Earth’s atmospheric pressure at sea level. This pressure compares to that found about thirty-five kilometers above the Earth, nearly four times the height of Mount Everest.

Water directly measured by the Viking orbiters was in a vapor state. There was also evidence that water exists in solid form—as ice in subsurface permafrost, in the Martian polar caps, in the surface rocks and soil, in clouds and fog, and as frost. The amount of water vapor discovered in the Martian atmosphere was relatively low compared to Earth's standards. At the Viking lander sites, the atmosphere was 0.03 percent water vapor. This amount compares to the percentage of water vapor at an altitude of nine or ten kilometers on Earth. Mars is so dry that if all the water vapor in its atmosphere could be condensed into a solid block of water ice, it would measure only about 1.3 cubic kilometers—a tiny fraction of the water vapor in the Earth’s atmosphere. The Viking probes measured the part of the Martian atmosphere with the most significant amount of atmospheric water vapor, the area nearest the planet’s north pole.

Even though the Martian air is dry by Earth standards, it is near a saturated state because of the low atmospheric pressure. Daily temperature variations cause the water vapor to condense in the form of ice fogs, clouds, and frost. The fog is quite tenuous. Typical Martian fogs are about half a kilometer in depth, and if they were completely condensed into water, they would form a layer only a single micron thick. Frosts of water vapor, sometimes mixed with frozen carbon dioxide to create an ice called clathrate, form on the planet’s surface seasonally.

The Martian poles are repositories of water ice and frozen carbon dioxide. The depth of water ice at the north and south poles varies. The north polar cap is estimated to be one meter to one kilometer deep. The south cap is less affected by the Martian summers, and its estimated thickness is 0.23 to 0.50 meters. It is primarily solid carbon dioxide. The amount of water ice in the caps is still being determined. Polar water ice directly exposed to the Martian environment can deteriorate by sunlight from solid instantly to vapor by a process called thermal erosion.

Estimates show that Mars’s original water, outgassed from the planet’s interior by volcanism (as on the Earth), may have been sufficient to cover the planet with a layer some forty-six meters deep. Most of it, however, was lost throughout the planet’s history in a process called molecular dissociation. High-energy ultraviolet sunlight split the water molecules into hydrogen and oxygen atoms, which were eventually lost to space. Nearly 270,000 liters of liquid water is lost this way each Martian day. Orbital photographs show, however, that the surface is extensively scarred by what appears to be a channel whose only Earthly analog is caused by running water. Since liquid water requires an atmospheric pressure and temperature much more significant than those on Mars, it is thought that the planet’s conditions must once have been sufficiently different to allow water to flow. The Pathfinder mission supplied evidence of massive flooding.

As the orbital Viking probes circled Mars, they mapped seasonal variations of water vapor over the planet. It was discovered that as the spring and summer temperatures rose, water vapor levels increased also. Most scientists agree that the source of this water vapor was ground deposits of water. Water is encapsulated either in frozen aquifers or within the soil itself. Such frozen ground is called permafrost. Permafrost is located in the regolith of Mars. The depth of this upper layer of ice-laden soil is variable. Permafrost in the polar regions may be up to eight kilometers deep; in the equatorial regions, it may extend to three kilometers.

The existence of large areas, perhaps planetwide areas, of permafrost is supported by orbital images that show four distinct geological formations: meteoritic impacts display a kind of fluid ejecta flowing away from the main crater, not seen on the Moon or the planet Mercury, that indicates that the energy of the meteoritic impact melted the permafrost. Another formation is called polygonal ground. These distinct polygonal shapes, observed from a high altitude, are caused by the repeated freezing and thawing of permafrost on or near the surface. Mass wasting (on Earth, often called landslides or mudslides) consists of distinctive downslope movements of soil, possibly caused by softening from water. Finally, a geological phenomenon called thermokarst was observed on Mars. It is caused by the underground melting of permafrost or underground ice formations, which lead to sinkholes or collapsed surface features.

In addition to permafrost, water is locked molecularly into the crystalline structure of the Martian soil and rocks. The Viking landers discovered that 0.1 to 1 percent of the surface materials consist of water of hydration. This tightly bound water can be released only by heating the materials.

Many of the stream formations may have been caused by spring sapping during a planetary warming cycle. Spring sapping offers evidence of hidden, frozen deposits of ice that may thaw during warm cycles. The Mars exploration rovers Spirit and Opportunity provided evidence of sedimentary processes involving water. BB-sized spherules of gray hematite, referred to by the planetary scientists of the Mars exploration rover program fondly as “blueberries,” were found by the Opportunity rover around the rim of Victoria crater. Formation of these gray hematite spherules occurs in the presence of water. Red hematite is just rust formation without the need for water, but the gray variety is found on Earth, often in connection with hot springs where the oxidation of rust occurs in connection with water. However, it must be pointed out that gray hematite can be formed in connection with some types of volcanic activity, so the connection of the “blueberries” to water is not absolutely confirmed.

The Mars Reconnaissance Orbiter (MRO) provided further evidence that liquid water was still present on Mars, even at or near the surface under certain conditions. In 2011, researchers first noticed dark streaks on the surface of some sloped regions of Mars in images from the MRO's High Resolution Imaging Science Experiment (HiRISE). It became apparent that these streaks, termed recurring slope lineae (RSL), appeared seasonally, leading to speculation that they were caused by flowing liquid water released from the soil as temperatures rose. In September 2015, NASA confirmed that hydrated salts had been detected in the RSL formations, providing solid evidence that they are caused by hydration. Most experts suggested that the briny composition of the water allows it to liquefy around -10 degrees Fahrenheit (-23 degrees Celsius), flowing mainly in the shallow subsurface with some wicking to the surface.

Methods of Study

Most of the information regarding water on Mars came from several NASA spacecraft: Mariner 9, two Viking orbiters, two Viking landing craft, the Mars Pathfinder lander with its small independent rover Sojourner, Mars Global Surveyor, Mars Odyssey, Mars Reconnaissance Orbiter, the Mars Exploration Rovers Spirit and Opportunity, and the Mars Science Lander rover Curiosity. The European Space Agency also began its expanding planetary exploration program by studying the red planet from orbit with its Mars Express spacecraft.

Direct evidence for Martian water came from orbiting instruments that measured atmospheric water vapor over seasonal periods and actually photographed the Martian polar caps, fogs, and cloud formations. Landing craft directly measured water vapor on the surface of Mars. They photographed seasonal frost deposits and clouds while measuring water hydration by heating rock and soil samples. The instrument on the orbiter that measured the water vapor was called the Mars atmospheric water detector. It examined reflected solar radiation from the Martian surface at a spectral band of 1.4 microns.

The Viking lander instrument that analyzed the Martian soil for water was called the gas chromatograph mass spectrometer. Soil samples were taken from the surface of the planet by a robotic arm that directed the sample to a heating chamber. The soil was heated to 773 kelvins, and materials driven off by the heat were analyzed. It was discovered after a series of samples had been analyzed, that between several tenths of a percent and several percent of the surface material was water. Some of the water was believed to be loosely absorbed on the surface, but it was likely that a significant fraction was accounted for by water of hydration.

Indirectly, scientists inferred much about Martian water repositories by comparing orbital photographs with high-altitude photographs of the Earth. Nearly all investigators were convinced that the only known mechanism that could form the clearly defined river- and streambeds was running water. This observation, coupled with simplified dating techniques of counting craters in streambeds to determine approximate ages, enabled speculation about cyclic Martian warming trends. These periodic warming trends could conceivably cause subsurface ice deposits to melt, and subsequent atmospheric pressure increases could allow the water to flow over the Martian landscape, cutting the stream formations in the soil.

Examination of ejecta patterns all over the planet led to speculation that permafrost was a planetwide manifestation. Comparison of earthly geologic formations of polygonal ground, mass wasting, and thermokarst with their Martian counterparts provided evidence for the widespread nature and even depth of the Martian permafrost layer. Photographs of cloud and fog formation over the planet during subsequent orbits enabled the calculation of temperatures, saturation levels, and even the content of cloud and fog banks. After the lander data had verified the orbital photography, a highly accurate picture of Martian water deposits was formulated.

The Mars Polar Lander was sent to touch down in a polar region of Mars and search for direct evidence of subsurface water. Unfortunately, the spacecraft crashed and failed to transmit any data. The Mars Phoenix lander was designed to attempt the same thing. Launched in August 2007, Mars Phoenix landed in the red planet’s northern polar region on May 25, 2008, and began to dig in the soil to search for evidence of water. Mars Phoenix accomplished the first successful powered landing on Mars since the Viking touchdowns in 1976.

Mars Phoenix touched down in the northern polar region on May 25, 2008, at 68.2° north, 234° east, a location within what scientists had named the “Green Valley” of the Vastitas Borealis region. Locally, it was late spring at the time, but the surface temperature was still sufficiently cold that solid ice permafrost was strongly anticipated. After some difficulties with the lander’s robotic arm and a few other critical systems, Mars Phoenix dug a trench in the Martian soil and exposed a white layer just below the surface. In time, that white layer displayed sublimation, the phase change from solid directly to a gas. The rate at which the white layer sublimated strongly suggested that it must be water ice rather than dry ice (frozen carbon dioxide which sublimates at an even greater rate at the local Martian temperature).

Mars Phoenix was outfitted with a Thermal and Evolved Gas Analyzer (TEGA), essentially a combination of an oven and gas spectrometer. With TEGA, project scientists sought to detect water vapor released from heated samples of the white layer delivered to the hardware’s oven chamber by actions of the robot arm and its scoop. Initial problems with clogging a TEGA sample inlet forced project scientists to be extremely careful in preparing for TEGA analysis. This delayed an unambiguous answer regarding whether or not the permafrost was water for several weeks, as Martian winter approached and threatened to shut down the Mars Phoenix lander.

It came as something of a relief when, on July 30, the Mars Phoenix lander’s robotic arm delivered for the first time some viable subsurface material to an open chamber in the Thermal and Evolved Gas Analyzer. Several days of testing with the sticky Martian soil had produced a method whereby the arm’s scoop could drop frozen permafrost into the vent leading down to a TEGA oven. The sample was heated in the oven, and the evolved gases were analyzed. Just as the science team expected, the signature of water was confirmed. University of Arizona scientist William Boynton declared:

"We have water! We’ve seen evidence for this water ice before in observations by the Mars Odyssey orbiter and in disappearing chunks observed by Phoenix last month, but this is the first time Martian water has been touched and tasted."

In the wake of this discovery, NASA announced that funds would be forthcoming to extend the Mars Phoenix mission through at least September 30, a five-week extension.

Meanwhile, NASA’s sequence of orbiters—Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter (MRO)—trained a variety of instruments on the surface of the Red Planet. Their investigations were part of NASA’s continuing program directive to search for water on Mars. MRO carried the largest cameras ever flown to another planet. The MRO data indicated the presence of underground water ice, and its photographs provided circumstantial evidence for changes in the surface where water may have played a role.

A pair of studies using MRO came to the conclusion that Mars once had water to the extent that large lakes and dynamic rivers existed for a long time in the distant past. In the July 17, 2008, issue of Nature, data were presented that showed that the red planet’s ancient highlands, essentially 50 percent of the planet, contain clay minerals that can form only with water. Those clay features were later covered by volcanic lavas. However, the clay was uncovered across the surface by subsequent impact crater events. Features like the crater Jezero once confined a lake, and clay minerals were eroded down from the crater into a delta formation. The presence of these phyllosilicate minerals across the planet added fuel to the possibility that Mars once enjoyed wet environments that might have had the potential for the development of primitive life.

MRO's HiRISE tool provided highly detailed images of Mars' surface that allowed researchers to observe the RSL features, while the craft's Compact Reconnaissance Imaging Spectrometer (CRISM) allowed mineralogy mapping that identified hydrated salts. These methods led to the confirmation of liquid water on the planet. In 2015, the MRO detected hydrated minerals under dark streaks in the mountains called recurring slope lineael (RSL). There seems to be some extent of flowing water beneath the RSLs.

Additionally, by 2018, scientists had used the radar instrument (the Mars Advanced Radar for Subsurface and Ionosphere Sounding) on the Mars Express spacecraft to discover the apparent existence of a subglacial lake approximately one mile below the ice in the Planum Australe region near the planet's south pole. Based on analysis of the feature's composition gleaned from the periodic radar echoes, it was theorized that this reservoir of liquid water would represent the first and largest (approximately twenty kilometers, or twelve miles, wide) known stable body of liquid water on the planet. This discovery, which involved more than three years of dedicated radar surveying in the specific area, has provided continued hope for life on Mars. However, further observation efforts were needed to confirm the findings.

In 2021, the Mars Express discovered bright spots similar to the 2018 discovery on Mars's south pole with temperatures as low as -63 degrees Celsius. This discovery offered support for the confirmation of the 2018 findings. Further support released in 2022 noted that the Martian lake's terrain was similar to Earth's subsurface lakes. Another research study released in 2022 opposed this finding and noted the lake could simply be a specific layering of minerals and carbon dioxide.

Context

MRO studies in 2008 concluded that Mars in the past did have a wet environment, as had been originally suspected after Mariner 9 images changed the scientific assessment of Mars. The newest studies suggested that Mars’s ancient highlands contain clay minerals that form only with water. It appears that Mars had large lakes, vibrant rivers, and smaller wet regions that persisted for thousands and perhaps millions of years. Although subsequent research indicated that liquid water does still flow on Mars, it does not appear that there is the same amount that once shaped the planet's surface.

The question of what happened to Mars’s water is of critical importance to the next generation of space explorers. It is also important to understanding how Earth’s water reserves are balanced on a planetwide scale. Learning how nearly an entire planet’s water resources, consisting of many trillions of liters, could be lost is critical not only to our insights into Mars’s history but also to our understanding of Earth’s future. Although the mechanisms of planetary water loss are understood, it is important to study Mars to learn exactly how and where the planet absorbed its remaining water resources. Scientists use techniques that could be employed to locate Earth’s diminishing freshwater resources by space observations. Scientists may also learn how permafrost water deposits are linked to the contamination of water by soil salts and other impurities and how long-term climatic cycles lead to planetwide weather changes. If the Earth tilted only a few degrees, extreme global changes could be introduced that could have ramifications for the planet’s long-term weather patterns.

Perhaps the most direct knowledge to be gained, however, is whether future colonists will be able to use what water there may be on Mars. Local water will be vital for the establishment of a Mars colony and will ultimately determine its size and usefulness. Water that is obtained from atmospheric distillation, the permafrost, the water of hydration, mining aquifers, or the polar caps will be used for a multitude of purposes, including drinking, agriculture, cooling equipment, washing and cleaning, and breaking down of molecular water into atomic hydrogen, for fuel, and oxygen, for breathing. Water on Mars may become one of the most significant aspects of the desert planet. The confirmation of liquid water on Mars also boosts the prospects of finding evidence of extraterrestrial life, as all the conditions for microbial life in the past or present have been identified.

In June 2013, the Opportunity rover uncovered evidence of a mineral known as montmorillonite near Mars' Endurance Crater. According to analysis and existing scientific research, the mineral is formed in part through the presence of neutral water. NASA officials confirmed that the discovery of the mineral was further strong evidence that Mars had an Earth-like atmosphere during the first billion years of its existence.

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