Perhaps the most fascinating space mystery of the last few months is featured in this gorgeous image taken by NASA’s Dawn orbiter on 6/6/2015: This image shows a crater on Ceres with mysterious bright spots in it. I will probably be overusing the word “mysterious” in this post because I find it absolutely enthralling that nobody knows what these spots are yet. Before I go through a few plausible explanations, some background on Ceres – here is a picture of the whole thing, as taken by Dawn on 5/6/2015:
The bright spots are visible there too, but clearly we’ve gotten a closer look since then. Ceres, like Pluto, is a dwarf planet (meaning it’s big enough to be a planet, it’s not a moon, and it hasn’t cleared its orbital path of other objects), ranking #33 in size among Solar System objects. It is the only recognized dwarf planet in the asteroid belt, between Mars’ and Jupiter’s orbits. The Dawn mission, launched in 2007, visited Vesta, another protoplanet in the asteroid belt, before continuing onward to Ceres, where it is currently orbiting about 2,700 miles above the surface. It will continue to spiral closer and closer to Ceres until November, when its orbit will take it as close as 233 miles from the surface, allowing it to make more and more detailed observations.
Dawn’s main mission is to investigate how water and the size of a protoplanet affect its evolution, which could lead to insights into how the rocky planets in our solar system (including Earth) developed. The unexpected bright spots on Ceres, whatever they turn out to be, will surely help scientists work towards that goal. But what are they?
Sadly, one of the first ideas to be discarded was that the spots were some sort of light-emitting structure on the surface (say, an alien base full of wise little green men). As we can see in the above .gif, the bright spots fade out as they reach the terminator, where the surface sinks into darkness and it becomes night on Ceres. This is strong evidence that the bright spots are, in fact, reflecting light, not creating it.
Early suggestions for the source of the bright spots included cryovolcanism and ice geysers. These ideas came from the fact that, in the words of Andreas Nathues, the scientist in charge of Dawn’s camera, “What is amazing is you can see this feature while the rim is very likely in front of the line of sight.” This would suggest that whatever’s reflecting light on Ceres is actually being spewed from the surface, rather than just sitting calmly in its crater. In that situation, enough pressure would have to build up beneath Ceres’ crust to crack it, pushing ice and water to the surface. If there were cracks in the ice shell, that would give Dawn a serendipitous window to the inside of the dwarf planet. However, recent evidence seems to counteract the idea of cryovolcanism on Ceres. For one thing, there is no mountain, not even evidence of a mound or fissure which could indicate a volcano or geyser. It’s also not clear whether Ceres has enough internet heat or pressure to push much of anything to the surface.
The idea which I favor is that the bright spots on Ceres are ice revealed by some sort of impact. This image of Mars shows a patch of water ice revealed after something crashed into the surface in 2008:
In an event like this, the force of a meteorite falling to the surface excavates a reflective substance (ice, water, salts, or maybe something else) from just below the surface, displaying it in (relatively) clean, fresh patches which reflect light far better than the dusty surface around them. Even if the spots aren’t ice, the fact that they’re brighter than their surroundings would seem to indicate that they are “fresher” and more recent. Over time, dust covers these features again, camouflaging them back into the surface. Even if the bright spots are ice, they’re not necessarily water ice, although there is evidence of water on Ceres, so that would be the intuitive choice.
To me, the hypothesis that these mysterious bright spots are water ice revealed by recent impacts seems the simplest and most intuitive. As much as I look forward to the resolution of this mystery as Dawn gets a better and better look, I relish the wait and the mystery, because right now, THOSE SPOTS COULD BE ANYTHING.