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A Martian mystery: where the water went

Q: I'm fascinated by all the missions to Mars. One thing I don't understand is all the pictures of dried up rivers and lakes. If Mars was once covered in so much water, where is it now? How did the planet become so dry? (Taylor, Wheeling, West Virginia)

Q: Your article on the Mars rovers talks about evidence of shallow lakes and water once flowing on Mars. What happened to it? How could Mars be so wet in the past and so dry today? (Gerardo, Santa Clarita, California)

A: Mars has many mysteries; we are just beginning to solve them.

Above: Tiny Martian "blueberries" (marble-shaped rocks, less than one-fifth of an inch) found by NASA’s Opportunity rover. Bottom: Larger Utah blueberries (1/25th of an inch to 2 inches) formed about 25 million years ago in groundwater-soaked sandstone, located in what is now southern Utah. The round rocks accumulated after surrounding sandstone dissolved away, and may give clues to how the similar Martian rocks were created. Courtesy of NASA/JPL/Cornell University (top image) and Renda Beitler, University of Utah (bottom image).

The problems are profound. We probe with remote robots millions of miles away, and discover limited data. Then we interpret the findings by extrapolating from our experience on Earth. Extrapolating to an entirely different planet with an entirely different history and environment has many pitfalls. How good is the procedure? We’ll know how good an extrapolation is when we get sufficient evidence that rules out all but a single interpretation. Right now most of our theories are just that — ideas based on data subject to several explanations.

Actually, we are still not sure if Mars really had appreciable amounts of water. Most scientists think so, but some do not, and the doubters’ interpretation of the data is valid.

For example, the intrepid Mars rover, Opportunity, found sandstones. Different scientists have different theories about how the sandstone originally formed, writes space scientist Mark Bullock of Southwest Research Institute in the December 2005 edition of Nature magazine.

The NASA team that interpreted Opportunity’s data says it happened like this: Acidic waters eroded volcanic rocks, and created the fine-grained sand that later became sand dunes. Shallow waters repeatedly flooded the region, eventually creating the sandstone.

Scientists Thomas McCollom and Brian Hynek challenge this interpretation. They say that a volcano exploded, and blew ash all over. The ash settled over the region, and later small amounts of sulphuric acid solution trickled over the ash to produce the sandstone.

Scientist Paul Knauth says, no, a meteor slammed in; the impact produced a surge of rock fragments, salts, sulphides, brines and ice that hugged the ground. Later, water films that worked down among the grains weathered the conglomeration and formed the sandstone. This theory accounts for the rover data without shallow seas, lakes or ground water.

The "sandstone" sediments, however, are not sand in the ordinary sense of the word, says science writer and geologist Alan Longstaff of the Royal Observatory Greenwich in London, England. They are primarily basalt (an igneous rock) and a goodly measure (up to 50%) of sulphates of magnesium and iron, and some iron oxides. "This is important because sand (silicon dioxide, or quartz) on Earth is the end product of water weathering of igneous rocks. Whereas, on Mars we don’t know how much "or little!" water weathering occurred.

If Mars once had lakes of liquid water, what happened to it? Recreating the actual history is complicated. Again we have many theories, each supported by evidence with multiple interpretations.

Figure 2. Martian ice caps, February 1995. A seasonal coating of 1-meter-thick dry ice hides the permanent cap made of 3-meter-thick water ice. Courtesy of NASA, Hubble Telescope, P. James (U. Toledo), S. Lee (U. CO).

Solving part of the liquid-water mystery is easy. The present-day Mars is cold. The average temperature of Mars globally is about -65 degrees Fahrenheit (-55 C). At a winter pole (shown in Figure 2), it can drop to about -200 F (-130 C). So, the planet lacks liquid water because it’s too cold.

Martian permafrost evaporates, but never melts. Ice goes directly to vapor, like clothes drying outside on a frigid winter day — through sublimation. At the equator at noon, the temperature does rise above freezing each day, but the atmospheric pressure is so low that the ice essentially evaporates rather than melts. (Under these conditions, the ice hovers at the triple point where it can exist as ice, water or water vapor.)

The Martian atmosphere has only 0.6% the pressure of Earth’s. And this paltry atmosphere contains little water — only 1/10,000 as much as Earth’s. Consequently, it never rains and rarely, if ever, snows on Mars. Frost forms sometimes and that’s about it.

Like on Earth, two sources of radiation warm Mars: the Sun and the planet’s atmosphere, says meteorologist Craig Bohren, author of What Light through Yonder Window Breaks? But water vapor provides most of the atmosphere infrared (heating) radiation. So, as Mars lost water vapor from its atmosphere, the planet also lost, to a large extent, one of its two furnaces. Mars consequently cooled over the eons; now the temperature stays below freezing. Mars has no liquid water because it has a sparse atmosphere, which caused the temperature to drop too low for liquid water.

Why did Mars lose its atmosphere? That’s the real mystery. A leading theory proposed by physicist Mario Acuna of the Goddard Space Flight Center conjectures that the solar wind gradually blows it away.

Figure 3. Earth's magnetosphere — the region in space shaped by Earth’s global magnetic field — shields Earth from the solar wind. Courtesy of NASA and Wikipedia.

The Sun blasts Mars with charged particles that erode its atmosphere like a desert wind scours the land. Earth’s magnetic field shields us from the solar wind, and protects our atmosphere. Unfortunately, Mars lost its shield. See Figure 3.

Four billion years ago, Mars had a molten core whose electric currents generated a magnetic field that swathed the planet (like ours does today). We know this is true because we have found magnetic Martian rocks, dating older than 4 billion years. See Figure 4.

Figure 4. In January 2006, plucky rover Spirit, still working, took this picture of a group of Martian lava boulders. The frozen lava is still magnetic. Courtesy of NASA.

Then, the planet lost its magnetic field. We sent instruments to Mars (onboard the Mars Global Surveyor satellite) to measure the magnetic field, and discovered it has no overall magnetic field — no magnetism emanating from its core.

So, with no magnetic shield to deflect the Sun’s particles, the solar wind gradually blew the Martian atmosphere into space. Subsequently, Mars lost 70 to 90% of its water — blasted into space and the rest tied up in rocks. Figure 5 shows an ancient martian sky: thick with rain clouds.

Figure 5. An artist’s drawing of ancient carbon dioxide rain clouds building in a pink Martian sky. Courtesy of NASA.

That’s one idea, and it’s a viable, intriguing hypothesis. Venus, however, doesn’t have a global magnetic field, and "look at its 90 bar atmosphere," says planetary scientist Erik Asphaug of the University of California at Santa Cruz. Venus’ atmosphere is extremely dense (90 times Earth’s and 15,000 times Mars) and lacks any protecting global magnetic field. So, how necessary is such a field to preserving an atmosphere?