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The frozen oceans of Mars

All right, I don't mean to gloat, but I've got to say "I told you so."

Last year I wrote a column about the possibility of large quantities of water trapped below the Martian soil, and wouldn't you know it, those clever scientists working with the Mars Odyssey satellite have gone ahead and found just that. In a discovery that is sure to influence the future of Mars exploration, NASA announced the detection of vast amounts of water ice only a few meters below the surface of Mars.

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This discovery will unquestionably change our assumptions about the sort of land forms and geological features we see down on the surface, as well as modify our theories about how the Martian climate may have changed in the past. And it's certainly perked up the ears of hopeful astro-biologists (scientists who search for microbial life elsewhere in our solar system), as on Earth, sub-surface ice is known to support entire ecosystems.

But something that may come as a surprise is the technique NASA used to find the buried ice. As we all know, no human has yet set foot on Mars with rock-hammer in hand, nor have we even sent a robotic lander to drill down a few feet into the Martian dirt (although such a mission may soon appear in NASA's plans). No indeed. The ice, even under several meters of bone-dry, rusty soil, was detected by a satellite orbiting hundreds of miles above Mars. So why now? We've had satellites orbiting Mars and imaging its surface for the last few decades, and we've never seen water before. What was different this time?

The answer lies in the incredible power of modern remote sensing technology. It was impossible to detect the Martian ice with images alone. To be sure, some of the high-resolution pictures of Mars returned by the Mars Global Surveyor were tantalizing. At the top of some high ridges and crater walls there seemed to be small gullies that looked an awful lot like meltwater runoff from underground ice. But they might also have been formed by wind erosion or shifting sand. Ambiguous results may be frustrating to scientists, but they're also a challenge. If regular satellite images can't confirm or deny the presence of water, then what sort of measurement would?

In the case of the Mars Odyssey satellite, it was gamma rays. The Mars Odyssey has an instrument called a gamma ray spectrometer, which measures the energy of gamma rays coming from the surface of Mars. Gamma rays are the most energetic form of light known; they're dangerous beasties, able to kill living cells on contact. In nature, gamma rays are only created during intense explosions (like the death of a star), or through radioactive decay. Neither of these processes would seem to have much do to with water. But with remote sensing, you have to think of ways to detect things from large distances, and often you can do that indirectly. Water doesn't emit gamma rays, but hydrogen does under some special circumstances, and hydrogen is an integral component of water.

So how did scientists use gamma rays to detect the water? It goes something like this: everything in the universe gets bombarded regularly by cosmic rays. Cosmic rays are high-energy particles that shoot through space, and are produced by everything from supernova explosions to our very own Sun. From laboratory experiments, we know that each chemical has a distinct response to being hit by a cosmic ray, which is a powerful enough collision to dislodge neutrons from the nucleus of an atom.

In the case of a hydrogen atom, a very specific energy gamma ray is released as the atom settles down after being smacked by a cosmic ray. And gamma rays are so powerful that they can easily sail through a couple measly meters of soil, straight on up to the collector in Mars Odyssey. Since each chemical's gamma rays are of slightly different energies, it's no mystery which chemical you're looking at. You know the gamma rays had to be emitted by hydrogen.

Is it certain that the hydrogen beneath the surface of Mars is contained in water ice? Pretty much yes. There really isn't a lot of hydrogen sitting around in rocks, and we have excellent estimates of how much hydrogen to expect in Martian soil. Similar techniques were used by the Lunar Prospector mission, which mapped hydrogen concentration on the Moon's surface and, surprisingly, found a reasonable amount of water ice sitting around in a few cold, dark craters (we think the water is left over from comets that collided with the Moon long ago).

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NASA has actually gotten quite good at finding hidden water recently, using a number of remote sensing techniques. In the outer solar system, water was found by measuring tiny fluctuations in the magnetic fields of planets and their moons. Two moons of Jupiter, Europa and Ganymede, are proving to be very interesting in this respect. Both moons possess a layer of liquid salt water trapped beneath many kilometers of ice, with Europa's ocean being maybe only five to 10 miles under the surface.

In the case of the magnetic field technique, scientists were not only able to detect the chemical presence of water, but were also able to tell that it was in liquid form. We still haven't detected liquid water on Mars, but some planetary scientist suspect there may be aquifers (liquid water stored in soil) in some locations. That discovery may require a more extensive in-site investigation.

Encouragingly, the gamma rays measured by Mars Odyssey signaled the presence of quite a lot of water, especially around the polar regions. Exactly how much water exists in the icy soil is not yet known, but preliminary measurements suggest it might be comparable to the volume of water in the Great Lakes. This may turn out to be turning point in our plans to explore Mars, as the presence of so much water bodes well for a human mission to Mars someday in the future.

And it's not drinking water that they're after, but the ability to make rocket fuel. Mars, unlike our Moon, is a real planet unto itself and has a substantial gravitational field (about one third that of Earth). During the Apollo missions, all that was needed to escape the Moon's gravity was the boost of a rocket pack on the back of the lunar lander. When humans land on Mars, they'll need a more substantial launch pad and a good amount of fuel in order to return to Earth. The water we just found on Mars may just provide that fuel.

In a way, you could say the Space Shuttle uses water for rocket fuel. In actuality, the water has been separated into two tanks of liquid hydrogen and liquid oxygen. When the liquids are combined and sparked, it creates the titanic explosion that lifts the Shuttle into orbit. It might make sense for future Mars astronauts to bring a small rocket fuel processing plant with them, then feed it with melted ice dug from underneath the soil. Not having to bring ready-made rocket fuel with them would make the mission a lot easier.

Before we start mining the frozen oceans of Mars, there are still a whole lot of questions we need to answer. This discovery may well direct NASA to make a robotic sample return mission a high priority. And if that's the case, it won't be long until we have the first chunk of the frozen seas of Mars in our hands.

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