August 4, 2011
Over the years, spacecraft have seen plenty of dried-up riverbeds on Mars, along with rocks that formed in watery environments eons ago. No question about it, the Red Planet used to be wet. NASA can stop sending press announcements about water in the Martian past. We got it.
Now scientists are reporting something much more exciting: the first strong evidence of liquid water currently on Mars.
Pictures taken by the HiRISE camera on the Mars Reconnaissance Orbiter show dark streaks at the foot of steep slopes, which scientists say are probably the traces of salt water that reaches the surface. We’re not talking about rushing streams. In their paper in Science magazine reporting the findings, Alfred McEwen of the University of Arizona and his co-authors write: “We assume [the streaks] are usually dry at the surface, perhaps wet only in the subsurface and perhaps in small surface areas while moving.” So maybe it’s like damp beach sand after a wave has just retreated. The streaks, which range from 2 to 15 feet in width, appear by the hundreds in seven different places (20 more locations are under investigation) during the Martian summertime, when temperatures can reach as high as 80° F. The seasonal flows, if that’s what they are, have been traced in the HiRISE images for as long as three Martian years.
As often happens, the scientists can’t be 100 percent sure. McEwen says the best way to nail down the case for liquid water is through laboratory studies on Earth. Part of the problem is that water on Mars doesn’t behave like water on Earth, and is unlikely to last very long on the surface due to the planet’s cold temperatures and the weak atmospheric pressure. That makes it difficult to observe from afar unless you happen to be looking at just the right time.
Best of all would be to visit those locations with a lander. Unfortunately, the Curiosity rover planned for launch to Mars in November—the most sophisticated lander ever sent to the planet—won’t go anywhere near the newly found streaks. And even if it could, NASA wouldn’t send it there. Watery environments may be the best places to look for signs of life, but Mars scientists face an ethical Catch-22: By the rules of “planetary protection,” you can’t send an unsterilized rover to a place where it might contaminate native Martian life. And sterilizing a rover like Curiosity is time-consuming and expensive.
It’s a dilemma.