From the October 2015 issue

Would the thin atmosphere and absence of light pollution on Mars make for a fantastic night sky, or would martian dust/twilight spoil the view? Why don’t we have broad night-sky pictures?

Dustin Cable, Menlo Park, California
By | Published: October 26, 2015
NASA’s Spirit rover turned its panoramic camera skyward above Gusev Crater in 2005 to capture these views of Orion. The famous constellation appears upside down to Earth’s Northern Hemisphere viewers because of the rover’s southerly path on the Red Planet.
NASA/JPL/Texas A&M/Cornell/SSI
Martian astronomy has some advantages. The surface pressure is near 8 millibars — less than 1 percent of Earth’s surface pressure — meaning that atmospheric refraction is insignificant. One effect of that is the stars would not twinkle. Another is truly dark night skies. Twilight is sometimes very long, such that astronomical imaging needs to be at least two hours away from sunset or sunrise. But the brightest lights on Mars are Curiosity’s Mars Hand Lens Imager’s LED lights and ChemCam laser. And in the skies, the moons are much smaller and fainter than ours.

But the dust is a problem, even in the darkest skies. Typically, stars near the zenith lose nearly 1 magnitude due to dust; that increases to 3 to 4 magnitudes at 15° altitude.

The rover cameras have sensitivities similar to the human eye (Opportunity’s Pancam can just pick out magnitude 6 or 7 red stars on a clear night, and Curiosity’s Mastcam can see magnitude 7 blue stars). That means only the brightest stars are visible at lower altitudes. The cameras’ low sensitivity — and the fact that they cannot move while exposing — means they can see only the brightest deep-sky objects.

The Andromeda Galaxy (M31) is the farthest object to be (barely) imaged; the Orion Nebula (M42) and the Magellanic Clouds are among the few visible deep-sky-objects. A pan-sky image would require about 150 images — over an hour — and one or two Mars days’ worth of the rover’s available bandwidth. So, targeted imaging always wins. In the future, perhaps a wide-angle or zoom camera will be able to accomplish a pan-sky image.

Mark Lemmon
Texas A&M University
College Station