Figure 1. This high-resolution image was taken by Viking in 1972. It illustrates some of the valley features seen on Mars. Here, the channel begins in the lower right portion of the image, in the area labeled Hydaspis Chaos. Hydaspis Chaos is a broken and sunken terrain thought to have suffered a catastrophic collapse when all of its groundwater burst forth to form the valleys to the northwest. The water is through to have flowed from Hydaspis Chaos to the northwest portion of the image, carving streamlined islands as it went. (Click to view enlarged image)

Despite the incredible number of water-related features seen on Mars today, little chemical evidence suggests Mars ever harbored liquid water. This paradox has haunted planetary scientists for 30 years, and it was with this problem in mind that NASA sent two satellites to orbit the Red Planet.

The satellites, Mars Global Surveyor and 2001 Mars Odyssey, bear instruments to search for the chemical signatures of ancient water: minerals. Minerals are the small crystals produced by natural processes that make up rocks. They hold all the evidence needed to determine if liquid water ever flowed across the surface of the Red Planet.

Finding Mars Minerals

Minerals reveal much about a planet’s history. Minerals formed on the planet’s surface record environmental conditions at the time of their formation (Was there water around? Was there life?) Those formed below the surface provide clues about the internal structure and workings of a planet. Unable to travel into the past, geologists must rely on these minerals and a few other scant traces of evidence to decipher the world.

On Mars, examining minerals is a complicated business. A few pieces of Mars have been delivered to Earth via meteorite impacts; however, these represent isolated, unknown patches of the planet and are of little use on a global scale. The rovers that have touched down on the Martian surface sent back data on mineralogy; however, they, too, examine only an excruciatingly small area of a very large planet. For comparison, imagine trying to characterize all the ecosystems on Earth by examining an anthill!

Broad, planet-wide surveys of Martian mineralogy are possible only with man-made satellites. Three satellites are currently orbiting Mars: Mars Global Surveyor (MGS), 2001 Mars Odyssey, and the European Space Agency’s Mars Express. Each of these is equipped with specialized instruments for detecting minerals on the Martian surface.

Instruments in orbit

Figure 2. This is a sample of the kinds of spectra Mars Global Surveyor’s Thermal Emission Spectrometer (TES) might send back to Earth. The wavenumbers along the x-axis indicate wavelengths of light being recorded by TES. “Emissivity,” along the y-axis, represents the intensity of the radiation being recorded. The lines here are offset for clarity, but would usually lie right on top of each other. Major dips in the lines indicate wavelengths absorbed by the ground material, and high bumps in the lines indicate wavelengths emitted by the ground material. By analyzing the patterns of bumps and dips, planetary scientists can determine what the surface is made of. (Click to view enlarged image)

Each mineral or combination of minerals absorbs light of certain wavelengths and emits light of other wavelengths. These emissions and absorptions affect the intensity of the radiation that leaves the rock or mineral, creating a “spectral signature” that can be read by satellite instruments. This is the process of spectroscopy, and three instruments are busy performing it now as their satellites orbit Mars.

Mars Global Surveyor carries the Thermal Emission Spectrometer (TES), which specializes in analyzing spectra in the infrared. Mars Odyssey’s Thermal Emission Imaging System (THEMIS) also looks at spectra from infrared radiation, but at a much finer scale. OMEGA, aboard ESA’s Mars Express, analyzes spectra in both the visible and infrared, producing spectacular three-dimensional perspectives of the Martian terrain.

These three instruments map the thermal emissions from the surface of Mars, searching for minerals that will help geologists understand the planet’s past. Most importantly, they are searching for minerals that hold clues about liquid water. Did oceans and lakes of water exist in Mars’ past? Were there rivers, streams, and rain? Did life emerge on a warm, wet Mars as it did on Earth? The instruments are looking for answers in the rocks, and what they have found — or have not found — is surprising, intriguing, and, more often than not, confusing.

The Hematite Bombshell

In May 1998, just eight months after the Mars Global Surveyor spacecraft arrived at Mars, the Thermal Emission Spectrometer sent back startling spectra of the Martian surface. The spectra showed evidence of hematite, an iron oxide that usually forms in the presence of water. Advocates of a warm, wet Martian past celebrated — here was solid evidence of large standing bodies of water on the Martian surface.

TES mapping revealed the hematite deposit extends over 26,300 square km (~10,000 square miles). Investigation of surrounding areas uncovered two more deposits. Both of these areas show other evidence of past liquid water, including outflow channels and valley networks.

“This is really the first evidence we have that water was around long enough for a geological period of time so that potentially life could have had an opportunity to form,” said Dr. Phil Christensen, a professor at Arizona State University, and lead scientist for the TES and THEMIS instruments.

On Earth, hematite forms in a number of ways, most of which involve liquid water, but some of which do not. The primary formation mechanism is precipitation. Iron dissolved in water can precipitate out and deposit itself on the bottom of a stream, lake, or ocean. This produces smooth, layered, crumbly deposits of hematite — which is exactly is seen on Mars.

Though precipitation sounds like a consistent mechanism for producing hematite on Mars, there are problems with the theory. First, there are no obvious lakebed structures nearby — water would not have been able to pool in the region. Second, geologists believe the hematite deposits were likely laid down hundreds of millions of years after the period when Mars may have had liquid water on its surface. Third, on Earth, these types of hematite deposits are usually found with silicates (e.g., quartz), which are not detected in these regions on Mars.

It is also possible that these deposits formed by precipitation out of ground water; however, this idea also has flaws. The specific type of hematite formed from groundwater precipitation is called “red hematite,” for its red color. The hematite found on Mars is “gray hematite.”

In response to the flaws in the water-origin theories, two non-water theories have been proposed to explain the formation of hematite on Mars: volcanic cooling and surface coating. On Earth, hematite can sometimes form during the cooling of iron-rich lava flows in the absence of water, and it has been proposed that the Martian hematite deposits may have formed in the same manner. The major problem with this theory is that there are no volcanoes or sources of lava near the Martian deposits. A second theory suggests that the hematite on Mars does not take the form of actual rocks, but merely a coating over rocks. This happens often on Earth; however, it usually produces red hematite, not gray.

The water/non-water debate began with hematite, but was quickly intensified by more Martian minerals, which TES and THEMIS are hunting even now.

Figure 3. This image shows the original hematite deposit discovered by the Mars Global Surveyor’s Thermal Emission Spectrometer (TES) in 1998. The black rectangular regions represent areas mapped by TES as of May 1998, and the red pixels represent areas with a hematite spectral signature. The deposits found here are very small, and represent only a tiny portion of the entire deposit, mapped later. The background image is a mosaic of the Terra Meridiani region, assembled from images taken by the Viking spacecraft in the 1970s.

Case of the Missing Carbonates

Figure 4. This map, from Christensen et al. (2001), illustrates the final Terra Meridiani hematite deposit mapped by TES. The hematite covers more than 25,000 square km of the equatorial region of Mars.

The TES carbonate findings suggest that Mars never had extensive oceans or lakes, and that liquid water was not a long-lived part of the Martian climate. If one believes the carbonates, Mars has always been a frigid, dry desert.

"Green Planet": Olivine Joins the Anti-Water Ranks

In late 2003, THEMIS cast further doubt on the idea that water once flowed extensively on the Martian surface. The spectrometer detected a thick layer of rock that contains significant amounts of olivine, a fluorescent green mineral commonly found in igneous rocks and the earth’s mantle.

The olivine-rich rock was found at the bottom of a 4.5-km deep canyon called Ganges Chasma. However, olivine decomposes rapidly in the presence of water. So. if water ever flowed through the canyon, the olivine would have decomposed and vanished.

“This gives us an interesting perspective of water on Mars,” Christensen says. “There can’t have been much water — ever — in this place. This is a very dry place, because it’s been exposed for hundreds of millions of years. We know that some places on Mars have water, but here we see that some really don’t.”

Ganges Chasma isn’t the only region where water was scarce. In November 2003, Todd Hoefen and Roger Clark of the U.S. Geological Survey used TES spectra to identify a 30,000 square kilometer (~11,500 square mile) patch of olivine exposed to the surface in Nili Fossae, a region in the northern hemisphere of Mars. They estimated that this patch corresponds to an olivine deposit more than 1 million miles in area. This vast olivine deposit is a serious piece of support for the idea that Mars has always been cold and dry. A small outcrop in Ganges Chasma is one thing — but this is a deposit larger than Oregon.

“The large expanses of olivine, about one million square miles, means chemical weathering on Mars was very low and has been low for most of its geologic history,” said Clark. “This information contradicts a popular view of a past warm, wet period in Mars’ geologic history.”

Figure 5. This mineral map was created by Clark and Hoefen after 500 trillion calculations and spectral analyses. Dark blue regions represent large deposits of the mineral olivine mixed with small amounts of the mineral pyroxene. White regions indicate major olivine deposits, magenta regions represent hematite deposits, and yellow and green regions indicate materials giving off the 7.27 micron spectral feature indicative of sulfates. Though sulfates and pyroxenes are interesting minerals, they contributed little to the water debates, and so are not discussed in this article. On the map, “N” indicates the location of Nili Fossae, where a large olivine deposit is found. “H” represents the location of two hematite deposits: Meridiani Terra on the right, and Aram Chaos on the left. “V1” is Olympus Mons, the largest mountain in the Solar System, and “V2”, “V3”, and “V4” are the three large volcanoes of the Tharsis Montes (note the large amounts of sulfur around the four volcanoes). “VM” represents the Valles Marineris, a Martian version of the Grand Canyon that is as long as the United States is wide. The area labeled “C” is colored lightly magenta, but this is probably due to cloud cover, not hematite. “A” is the large Argyre Crater, which is about the size of Texas, though only a small portion of it is seen here. Note the small patches of white around Argyre — these are olivine deposits concentrated on the crater’s rim.

Looking to the Future

Far from solving the water-on-Mars paradox, the discovery of hematite and olivine, and the complete lack of a discovery of carbonates on Mars, has deepened debate on the subject and summoned more questions than they have answered. Did the hematite form in liquid water? How can the water-intolerant mineral olivine exist in canyons where water must have flowed, if water ever flowed on the Martian surface? If there was water, where are the carbonates? If there wasn’t, how did the Martian valley networks and channels form?

Last month, the new Mars Exploration Rover twins, Spirit and Opportunity, landed on Mars, and began the search for answers. Spirit, roving around in Gusev Crater, is investigating the outflows of major valley systems thought to have once transported Martian rivers. Opportunity, in Terra Meridiani, probes the nature of the hematite deposit discovered by TES. Together, the rover twins will continue to investigate the water-on-Mars paradox from both the landform and geochemical angle.