Sometimes to get the best map of Mars, you need to take your photos at night.

Admittedly, those photos are infrared, and the map isn’t your typical planetary atlas. Instead, researchers assembled more than 20,000 photos taken from orbit during the Martian night to find out how different parts of the planet surface retain heat after sunset. That conveys a lot of information about the make-up of the surface: whether it’s sand, dust, or rock, and something about the chemistry of each. My newest column at The Daily Beast has the story:

The color of a material that we see depends on the kinds of visible light it absorbs: a green shirt absorbs red and blue light, for example. A similar principle holds for infrared, but with the added benefit that the absorbed light can be given off long after the source of that light is gone. Think of an asphalt parking lot, which during summer can still be warm hours after the Sun sets. Infrared light is heat, so a material that holds onto heat for a long time will “glow” in the new map, while other materials will appear cool and dark.

That measure is known as thermal inertia, and it provides information far beyond what we can get from visible light alone. [Read more….]