Did water once flow on the surface of Mars ?

—LOREN BUTLER FEFFER

Viewpoint: Yes, evidence from various Mars missions indicates that water once flowed on Mars and helped shape the current landscape.

Sometimes referred to as the Red Planet and named after the God of War, Mars has captured the attention of scientists and storytellers alike for centuries. Many have believed, and hoped, that it could be Earth's twin—a world capable of sustaining life like our own planet. However, recent observation has proven that Mars is quite different from Earth—the key difference being that it seems to lack a significant amount of surface water.

But was Mars always so water-poor in nature? Could this hostile world have once possessed flowing rivers and vast lakes? After much debate, only one conclusion can be reached: Yes, water once flowed freely on Mars's rugged surface.

Water Features on the Martian Landscape

Astronomers have studied the existence of water on Mars for 300 years, watching the seasonal changes in the planet's polar icecaps. The first evidence of flowing water came almost a century before the first person even entered space. Giovanni Schiapparelli, an Italian astronomer, discovered the presence of canali (channels) on Mars's surface. Because some people misunderstood the word "canali" to mean "canals," the belief that the channels were built by intelligent beings grew. While this theory made for wonderful fiction, it did little to further the true understanding of Mars. Later Viking missions would dispel this theory as pure imagination (JPL Mars Program, 2001). However, it did leave a major question: Just where did these channels come from?

As the geography of Mars was documented through tens of thousands of photos during numerous missions, one thing became increasingly clear. Water, and copious amounts of it, had helped shape the Martian landscape. During an earlier period, perhaps several billion years ago, Mars was probably far warmer and wetter than it is today. Lakes, rivers, and maybe even oceans covered the surface and slowly left their mark over the millennia. Schiapparelli's channels were not the result of an alien hand, but erosion. The data obtained from photo mosaics of the Martian surface helped support this hypothesis.

Outflow Channels and Valley Networks

There are two specific types of water-related geological features that appear in several areas on Mars's surface—outflow channels and valley networks. Outflow channels come from chaotic terrain and can be well over 62 miles (100 km) wide and 1,240 miles (1,995 km) in length. Their characteristic flow features, like islands and craters, are typically formed from catastrophic flooding. Valley networks, on the other hand, can be broken up into two categories—long, winding valleys with few tributaries, and smaller valley networks, which can have numerous and complex tributaries. These are shaped through a more peaceful process than outflow channels, and are similar to river valleys found on Earth.

Ken Edgett, a staff scientist at Malin Space Science Systems (MSSS), stated, "… the Mariner 9 spacecraft found evidence—in the form of channels and valleys—that billions of years ago the planet had water flowing across the surface." This hypothesis was also supported by Gerald A. Soffen, Chief Scientist for the Viking missions, who stated that "… the presence of braided channels suggests to many geologists that they are the result of previous periods of flowing water, suggesting a very dynamic planet."

Photos of different sections of Mars during the Viking missions also suggest the earlier existence of flowing water. In the Ravi Vallis, catastrophic outflow channels were revealed. Likewise, evidence was found through NASA's Mariner 9 that rivers and/or flooding most likely formed a complex system of channels in the Chryse Palanitia. And, as discussed by Paul Raeburn in his book entitled Uncovering the Secrets of the Red Planet , cratered terrain with fine channels indicate the former presence of a lake in the Margaritifer Sinus region of Mars.

Data obtained later from the study of sedimentary rock suggest that lakes once existed on the planet surface. One such example exists in the Valles Marineris, a 3,700-mile (5,953-km) long canyon. Images indicate layered terrain, much like that created by shallow seas. Similarly, Jim Head and his colleagues at Brown University found evidence of a former ocean while reviewing elevation data collected by the Mars Global Surveyor ( MGS ).

Carl Allen, a planetary scientist at the Johnson Space Center, discussed one of MGS 's key findings concerning a possible ancient ocean in the Martian northern plains. He indicated that "this part of the planet is incredibly flat and smooth, and it is a set of concentric layers, all exactly the same altitude." An ocean evaporating and/or receding beneath the planet's surface could have left these rings, which are similar to wave-cut shorelines.

The MGS 's instruments also revealed other evidence of the existence of ground water. Allen said, "The exciting result from the spectrometer is several large deposits, some hundreds of miles across, of the iron oxide mineral hematite. Deposits of this material probably require large amounts of water from lakes, oceans, or hot springs." And naturally, liquid water is fluid by nature, which implies movement or flow.

Data also revealed that a great deal of activity occurred in the northern lowlands, suggesting that titanic floods of an unimaginable scale carved outflow channels. To put the power of these outflows into perspective, it would take approximately 10,000 Mississippi rivers to carve out some of these channels. Valley networks discovered in the southern hemisphere resemble those formed by rain, sapping (collapses caused by soil softened by groundwater), or glacial run-off.

On July 4, 1997, the Mars Pathfinder landed on the Red Planet to begin its exploration of the surface. Its landing-zone was in the middle of an ancient flood plain. Photos from the Mars Rover revealed that a Martian rock (named Prince Charming by scientists) may be a conglomerate, created by flowing water that has acted to "round" pebbles, which later are compacted and cemented to form rock.

Mounting New Evidence

Today's technology has allowed us to get a far better look at the surface of Mars than ever before. Because of this technological advance, scientists have been able to discover further evidence of water on Mars, not just from the ancient past, but also in recent times. Andrew Chaikin, the Executive Editor of Space & Science , explained, "The camera that spotted evidence of water gullies on Mars has a set of eyes that boggles the imagination. Even from an orbital height of 235 miles (378 km), the camera can spot objects as small as 3 yards (2.7 m) across."

The camera, built into the MGS spacecraft, captured the images of what appeared to be gullies or washes on the Martian surface. "We see features that look like gullies formed by flowing water and the deposits of soil and rocks transported by these flows," said Michael Malin, Chief Investigator for the Mars Orbital Camera at MSSS. "The features appear to be so young that they might be forming today. We think we are seeing evidence of a ground water supply, similar to an aquifer. These are new landforms that have never been seen before on Mars." Of course, time in space is a relative thing. However, many scientists agree that although it is possible that the gullies were formed long ago, it is equally possible that they were formed quite recently.

These newly discovered gullies may indeed be the evidence required to answer the age-old question scientists have been asking about Mars. Edgett explains, "Ever since [ Mariner 9 ], Mars science has focused on the question, 'Where did the water go?' The new pictures from the Global Surveyor tell us part of the answer—some of that water went underground, and quite possibly is still there."

Edgett also explains why it is difficult for water to currently exist on the surface of Mars. Because of the planet's low atmospheric pressure, almost 100 times less than Earth, liquid water would instantly boil. But Edgett continues, "We've come up with a model to explain [the gullies] and why the water would flow down the gullies instead of boiling off the surface. When water evaporates it cools the ground—that would cause the water behind the initial seepage site to freeze. This would result in pressure building up behind an 'ice dam.' Ultimately, the dam would break and send a flood down the gully."

In a recent study, Laurie Leshin, a geo-chemist at Arizona State University, claims that Mars could be more water-rich than previously suspected. After examining the water-bearing crystals in the 3 million-year-old Martian meteoroid QUE94201 (discovered in Antarctica in 1994), she discovered that the crystals were rich in deuterium, a heavy form of hydrogen. Furthermore, the ratio of the deuterium in the crystals is surprisingly similar to the ratio of the deuterium in the planet's current atmosphere. This suggests that the planet has lost two to three times less water than originally believed, indicating there may be more water beneath the Martian soil than suspected.

In conclusion, it seems evident that water flowed in some quantity on the surface of Mars. How much, if any, still flows there today is up for debate, but there is certainly compelling new evidence to inspire further inquiry. It is clear that water has helped shape the Martian landscape. For proof, one only needs to look at the planet's scarred surface.

—LEE A. PARADISE

Viewpoint: No, the topographical features suggesting that water once flowed on Mars lack crucial elements necessary to support the theory that water flowed on Mars.

Of all the planets, Mars bears the most striking resemblance to Earth. Like Earth, it has a hard rocky surface and experiences changes in seasons due to the tilt of its axis. It has an atmosphere—albeit 1,000 times less dense than Earth's—that further modifies the climate. The temperature, while cold enough for CO 2 to freeze on the surface, is not outside the realm of human experience, with the daily equatorial temperature variation on Mars similar to the seasonal variation in Vostok, Antarctica. Mars even displays seasonal ice and solid CO 2 caps on the poles. But unlike Earth, Mars today lacks large amounts of liquid water on the surface.

The lack of water, coupled with Mars's tenuous CO 2 atmosphere, means that erosion of surface features is much slower than on Earth. Looking at the Martian surface, geologists can see features that date back 3.9 to 4.0 billion years ago, only 500 million years after the formation of the planet. Compared to Earth, where landforms are quite young because of the erosion at Earth's surface, nearly the entire geological history of Mars is available for examination. Interpretation of imagery from remote spacecraft yields several observations supporting the hypothesis that water flowed on the surface in the distant past. Craters in the southern highlands appear to require flowing water to explain the high erosion rates. Valley networks in the older terrain (2.0 to 3.8 billion years old) look like Earth valleys carved by water. Outflow channels, similar to the catastrophic flood features created on Earth during the last ice age, exist on some of the younger terrain. Finally, gully formations with debris fans covering fresh dunes indicate some kind of fluid flow in the very recent past (100 to 10,000 years ago). All of this evidence suggests a rich water history for Mars.

Little Evidence of Water

However, as one looks deeper for supporting observations, very little evidence for water can be found. High erosion rates like those found in the southern highlands would leave behind huge expanses of carbonate rocks, the weathering product between water and the CO 2-rich atmosphere. To date, detailed spectroscopic analysis of the surface reveals no such deposits. In fact, deposits of the mineral olivine, indicating dry conditions, are widespread. The valley networks lack the fine tributary structure seen in similar Earth formations, indicating the source of the flowing liquid came from a few discrete sources rather than flowing over the surface. The outflow channels are of such huge scale that accounting for the shear volume of water released by the floods—enough water to cover the surface of Mars to a depth of 3,300 feet (1,006 m)—becomes difficult. And the gullies, the most compelling geological evidence of recent water modification, are located primarily on the central peaks of craters or on the pole facing slopes of cliffs, making it even more difficult to explain the localized melting of water.

Most researchers in the scientific community agree that the current Mars climate cannot support large amounts of liquid water on the surface. Today, Mars is drier than any desert on Earth. If all the water in the atmosphere condensed on the surface, its depth would be less than the width of a human hair. There is enough water in the polar caps to cover the planet to a depth of 33 feet (10 m), but it is locked in a frozen state. And thanks to the thin CO 2 atmosphere, water that leaves the poles does so through sublimation, turning from a solid directly to gas. Also, due to Mars's distance from the Sun—1.5 times as far away as Earth—the surface temperatures, overall, are far below freezing. Since most of the purported water features occur on older terrain, however, scientists believe that the early climate of Mars was much warmer than it is today.

The average surface temperature of a planet is determined primarily by two factors: its distance from the Sun, and the constituents of the atmosphere. For example, if Earth had no atmosphere, the surface temperature would average-40°F (-18°C), far colder than the freezing point of water. With the addition of

the atmosphere, the global average temperature rises to 59°F (15°C). This additional warming by Earth's atmospheric "blanket" is known as the greenhouse effect. Trace gases in Earth's atmosphere, such as CO 2 and water vapor, cause this warming of the planet by allowing visible radiation from the Sun to pass through the atmosphere, and absorbing infrared radiation emitted from the planet's surface. Currently, the thin CO 2 atmosphere on Mars creates a negligible greenhouse effect, but if the Martian atmosphere were as dense as 10 Earth atmospheres (or 10 bars), the amount of greenhouse warming afforded by the increased CO 2would be enough to warm the planet sufficiently to allow stable liquid water on the surface. This assumes, of course, that the amount of sunlight reaching Mars was the same in the past as it is today.

Since its formation, however, the Sun has been getting steadily brighter as it fuses hydrogen to helium in its core. Detailed astrophysical models of the Sun's interior indicate that the Sun is some 30% brighter today than it was 3.8 billion years ago—when water was supposedly flowing on Mars. In this case, even a 10 bar atmosphere is insufficient to warm the planet under the cooler Sun. The Sun was so faint, in fact, that it was even difficult to support liquid water on Earth at this time. But there is strong evidence for liquid water on Earth 3.8 billion years ago—problem known as the Faint Young Sun paradox.Clearly, through some method or another, Earth managed to get enough early greenhouse gases in the form of CO 2 methane, or water vapor to support liquid water. The geological evidence on Mars, if interpreted as being formed by liquid water, means Mars must have found a way as well. In order to do this, Mars would have required a much more substantial atmosphere than it has today.

Loss of Atmosphere on Mars

Over geological time, Mars has lost much of its atmosphere to space. The losses are due to its small size, and hence its lack of gravity to retain its atmosphere. Noble gases, such as argon and xenon, act as tracers of the history of the atmosphere. Since they are chemically inert, the original inventory stays in the atmosphere rather than reacting with the surface. Any loss of atmosphere results in a loss of noble gases. Since many of these gases have stable isotopes of different masses, measuring the ratio of the lighter isotope to the heavier isotope reveals something about the loss of the atmosphere. If the ratio is enriched in heavy isotopes compared to the original inventory, then a significant part of the atmosphere has been lost. Scientists measure the original inventory from meteorite fragments indicating the conditions in the solar system when Mars was formed, and they measure the current inventory with spacecrafts like Viking and Pathfinder on the surface of the planet. The result: Mars has lost a significant amount of atmosphere in the last 3.8 billion years.

Using the current atmospheric and polar inventories of CO 2 and water on Mars, it is possible to estimate the initial inventory of water and CO 2 on the planet. Unfortunately, even though 95-99% of Mars's atmosphere has been lost to space, less than 1 bar of CO 2 and less than 328 feet (100 m) of water covering the entire planet can be accounted for with atmospheric loss. While this is a great deal of water, it is insufficient to explain the geological features, and it is also an insufficient amount of CO 2 to account for the greenhouse effect needed to warm the planet to the melting point of water under the faint young Sun. Other effects are currently being explored, such as the warming and cooling properties of CO 2 and water clouds. At the present time, the addition of these complex processes make modeling the early climate even more difficult.

Recounting the evidence for an early warm and wet Mars, it seems much of the arguments for flowing water on the surface rest on geological observations from orbit. This being the case, there is nothing to be done but compare them to the only known analogs—the geological features of Earth. Numerous satellite images of Earth indicate that the dominant erosion mechanism in 99% of the images is from water. However, out of 70,000 high-resolution images taken of the surface of Mars, less than 10% show any evidence of water modification. Almost every other image, on the other hand, shows evidence of wind erosion, CO 2 frost, and additional processes never seen on Earth at the same scale. Therefore, a growing number of researchers are attempting to find alternative explanations for the "water" features that are more in sync with the Martian climate.

Alternative Explanations for Mars's "Water" Features

Ample evidence of wind erosion exists on the surface of Mars. The erosion rates in the southern highlands can't be explained by wind erosion in the current climate regime, but just a slight increase in pressure creates a large increase in wind's ability to scour the terrain. Three-and-a-half billion years ago, the atmospheric pressure was perhaps high enough to support the erosion rates seen in the southern highlands. And, since wind erosion doesn't require water and removes rather than deposits material, no special deposits should be found in the area. Wind not only erodes the surface, but transports dust on a global scale. The atmosphere, laden with dust, acts more like a huge ocean carrying sediment over the landscape. In terrestrial oceans, such flows carve channel-like features, and it may be possible that some similar process is acting on Mars. Unlike the ocean, the atmosphere is very thin, but the winds on Mars also move at high speed, which may make up for the lack of density in forming these features.

Since CO 2 is the dominant constituent of the atmosphere, it may play a large role in the formation of the flow features. In particular, the outflow channels may result from a catastrophic release of CO 2 gas from layered deposits beneath the surface. The cold CO 2 gas scours out the terrain much like dense gas flows from volcanoes on Earth remove the sides of mountains. While these hypotheses are still being explored and tested, they represent some new ideas in understanding Mars.

It is clear that Mars, while similar to Earth in general, is radically different in detail. Features formed on the surface are subject to a range of conditions never experienced by similar landscapes on Earth. Liquid water at the surface exists only briefly while wind erosion on a planet-wide scale occurs even today. Therefore, researchers must be careful to remove their "water glasses" and explore Mars in the context of its own environment. Each of the above interpretations—including that of flowing water—requires more information to resolve the debate. To date, researchers have access only to images from the surface. Earth geologists are suspicious of arguments based solely on how something "looks." They prefer to gather more field observations and samples before drawing a conclusion. Mars geologists lack that ability to gather samples from the planet, but until they do so, the true nature of the "water" features will be debated.

—JOHN ARMSTRONG

Further Reading

Boyle, A. "Mars Gullies Hint at Liquid Water." MSNBC June 22, 2001. <http://www.msnbc.com/news/423452.asp> .

Bridges, A. "Mars Hides Much More Water, Study Suggests." 2000. <http://www.space.com/scienceastronomy/solarsystem/marshiding000628.html> .

Caplinger, M. "Channels and Valleys." Malin Space Science Systems. 1995. <http://www.msss.com/http/ps/channels/channels.html> .

Carr, Michael H. Water on Mars. Oxford:Oxford University Press, 1995.

Chaikin, A. "An Eye for Mars: The Camera thatFound the Watery Evidence." 2000. <http://www.space.com/scienceastronomy/camera_technology_000623.html> .

Davis, K. E. Don't Know Much About the Universe: Everything You Need to Know About the Cosmos but Never Learned. New York: Harper Collins Publishers, Inc., 2001.

Hoffman, Nick. "White Mars: A New Model forMars' Surface and Atmosphere Based on CO 2 ." Icarus no. 146 (2000): 326-42.

JPL Mars Program. 2001. "Mars Polar Lander:Searching for Water on Mars." <http://mpfwww.jpl.nasa.gov/msp98/why.html> .

Keiffer, Hugh H., Mildred S. Mathews, andBruce M. Jakosky. Mars. Tucson: University of Arizona Press, 1997.

Maran, S. P. Astronomy for Dummies. Foster City, CA: IDG Books Worldwide, Inc., 1999.

Mars Global Surveyor . <mars.jpl.nasa.gov/mgs> .

Milan, W. "Water on Mars: Back to the Future?"2000. <www.space.com/news/spacehistory/water_mars_history_000626.html> .

NASA. "The Case of the Missing Mars Water."2001. <http://science.nasa.gov/headlines/y2001/ast05jan_1.htm> .

——. "Liquid Water on Mars." 2001. <http://liftoff.msfc.nasa.gov/news/2000/news-MarsWater.asp> .

——. "Mars Surprise." 2000. <http://www.spacescience.com/headlines/y2000/ast22jun_2.htm> .

"New Evidence Suggests Mars Has Been Cold and Dry." <www.spacedaily.com/news/mars-water-science-00l.html> .

Raeburn, P. Uncovering the Secrets of the Red Planet. Washington, DC: National Geographic Society, 1998.

Sagan, C. Cosmos. New York: Random House,1980.

Taylor, M. R. "Water on Mars: Is the Proof OutThere?" 2000. <http://www.discovery.com/news/features/marswater/marswater.html> .

KEY TERMS

CANALS:

An artificial (man-made) waterway, which is usually tubular in shape that allows the passage of water from one place to another. Usually used for irrigation purposes.

CARBONATE ROCKS:

A type of rock formed when water, slightly carbonated by CO 2 gas, flows over surface rock. This material is formed on Earth when rain-water runs over the surface, and the resulting carbonate rocks are deposited as runoff in the oceans.

CHANNELS:

The bed where a natural stream of water runs or the deeper part of a river, or harbor. The term can also be used when referring to a narrow sea between two close land masses.

CONGLOMERATE ROCK:

A rock that contains large amounts of gravel that is rounded. The rounding of the gravel usually takes place in a stream or river.

DEBRIS FAN:

Material at the end of a ravine or gully deposited by the movement of material from the upper slope.

GULLIES:

Trenches in the earth that are formed by running water.

SUBLIMATE:

To cause a solid to change into a gas (or vice versa) without becoming a liquid. CO 2 does this under normal Earth conditions.

TRIBUTARIES:

Streams that feed a larger stream or lake.