Was there ever life on Mars? In fact, could there still be microbes living on Mars now? It's still a distinct possibility. But given the difficulties involved in sending people and specialized equipment to Mars to look for samples, we could be waiting decades to find out. So it's a good thing there's a ready alternative: according to scientists, any life that exists on Mars may well also exist on its moons, especially Phobos.
According to Jay Melosh, of Purdue University, "A sample from the moon Phobos, which is much easier to reach than the Red Planet itself, would almost surely contain Martian material blasted off from large asteroid impacts." Added Melosh, in a press release: "If life on Mars exists or existed within the last 10 million years, a mission to Phobos could yield our first evidence of life beyond Earth."
Melosh and a team from NASA's Planetary Protection Office tried to figure out if a sample from Phobos might contain enough recent material from Mars to include viable Martian organisms. The idea was that if asteroid impacts on Mars could launch material later found on earth, it would be even more likely that similar material would be found on the Martian moons... particularly Phobos, the one nearest the planet.
Melosh and his team concluded that a seven-ounce sample scooped from the surface of Phobos could contain, on average, about 0.1 milligrams of Mars surface material blasted from Mars over the past 10 million years and as much as 50 milligrams of material from the past 3.5 billion years. They presented their findings at a joint NASA-European Space Agency meeting in Austria.
"The time frames are important," Kathleen Howell, Hsu Lo Professor of Aeronautical and Astronautical Engineering, emphasized, "because it is thought that after 10 million years of exposure to the high levels of radiation on Phobos, any biologically active material would be destroyed."
When an asteroid hits the surface of a planet it blasts a spray of material into space. The result of such a blast on Mars would be particles about one-thousandth of a millimeter in diameter, or 100 times smaller than a grain of sand — about the size of terrestrial bacteria.
By plotting more than 10 million possible paths such particles could take — including possible speeds, angles of departure and orbital forces — Melosh's team figured out which trajectories would be most likely to intercept Phobos, and where they might land on the moon during its eight-hour orbit around Mars.
The probability of a particle landing on Phobos depends primarily on the power of the blast that launched it from the surface. "It is estimated," said graduate student Loic Chappaz, "that during the past 10 million years there have been at least four large impact events powerful enough to launch material into space, and we focused on several large craters as possible points of origin. It turns out that no matter where Phobos is in its orbit, it would have captured material from these powerful impact events."
Shortly after Melosh and his team submitted their report, a 37-mile-diameter crater was found on Mars. Dubbed Mojave, it is estimated to be less than 5 million years old, which means there might be an even greater amount of Martian material on Phobos containing viable organisms than they'd estimated.
"It is not outside the realm of possibility," Melosh suggested, "that a sample could contain a dormant organism that might wake up when exposed to more favorable conditions on Earth." Melosh added that there would be no reason to worry about an "Andromeda Strain"-style epidemic. "Approximately one ton of Martian material lands on Earth every year," he explained. "There is a lot more swapping back and forth of material within our solar system than people realize. In fact, we may owe our existence to life on Mars."
"It's difficult to believe there hasn't been life somewhere out there in the vast expanse of space," Howell added. "The question is if the timeline overlaps with ours enough for us to recognize it. Even if we found no evidence of life in a sample from Phobos, it would not be a definitive answer to the question of whether or not there was life on Mars. There still may have been life that existed too long ago for us to detect it."