Light and Shadow on Mars

Photo credit Mars Reconnaissance Orbiter (MRO): On Mars, surface features are heavily shaped by wind. This photo shows wind-carved features called yardangs, which are common on the planet. The wind also forms ripples in the sand and builds small dunes. Note the extremely dark and sharp shadows cast by the yardangs.

FYI, the sand in that valley is not really blue — it’s the typical “Mars orange” you would expect. MRO images of Mars are digitally processed to better reveal details. Principal investigator Alfred McEwen, a planetary geologist at the University of Arizona, explained how the images are given a min-max stretch to enhance contrast. This process sometimes generates false colors.

In Mars’ thin atmosphere light is not scattered much, so the shadows cast by the yardangs are sharp and dark. The highest atmospheric density on Mars is ≈ 0.020 kg/m3, similar to the density of air 35 km (22 mi) above the Earth’s surface. At Earth’s surface the density of air is 1.29 kg/m3 —  that’s 65X the density on Mars. So with 56X fewer particles in a given volume of air, 56X less light is scattered.

Next time you’re outside on a sunny day and standing on a hard flat surface, take note of two features of your shadow. First, the shadow is not totally black. There’s still plenty of light being scattered (or reflected from nearby surfaces) to allow objects in the shadow to be seen. Second, the edges of the shadow are fuzzy and indistinct. This is because sunlight not blocked by your body scatters on its way to the ground and infiltrates the boundary of your geometrically defined shadow.

For comparison, look at the shadow cast by NASA’s Curiosity Mars rover. Click on it to see a larger version:

Note how sharp the shadow edges are, and how dark the shadow’s interior is. Both are consequences of Mars’ thin atmosphere. FYI, if you’re wondering who took the photo, it was actually a “selfie” digitally assembled from 81 individual images. This allowed the rover’s camera arm to be removed from the photo. You could do that with your own selfies too, but it would require extensive editing.

Humankind will some day walk on Mars. One of the first things they’ll notice is Mars’ weaker gravity — their weight will be 38% of what it was on Earth. Then they’ll notice how the sky is “salmon” colored instead of blue, a consequence of both atmospheric density and composition (95% CO2 with traces of other gasses). But at some point, they’ll also notice their unearthly shadows.

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