Why Ceres might be an invader from space

15 December 2015

Interview with

Professor David Rothery, Open University

What's black with white spots and half made of water? The rather surprising white spots on Ceresanswer is the solar system's largest asteroid, Ceres, scientists had thought that this spherical body, which measures about 1000km across and orbits out beyond Mars, was made of rocky rubble left over when the planets finished forming. Now, closer inspection by NASA's "Dawn" probe, which is currently orbiting Ceres, has shown that the dark black surface of the asteroid is punctuated with bright white spots, which appear to be made of salts, ammonia and water ice. But finding these means that Ceres might not originally be from the asteroid belt at all but is instead a deep space interloper! Chris Smith spoke to David Rothery about the finding...

David - Ceres is the largest asteroid; it's the largest body in the asteroid belt; it was the first asteroid to be discovered very early in the 19th century. 

The white spots were actually suggested from the best Hubble space telescope pictures you could get that they were just areas that looked brighter than elsewhere.  As the Dawn spacecraft approached, the most obvious one came into view. 

In the middle of this crater that's tens of kilometres across, we have areas kilometres in size which are highly reflective.  People were wondering what they were and people are still not sure.  I went on the "Dawn" mission website just now, and there's a popular pole on the go, you can vote for what you think the white spots are. 

10% of people think it's some sort of volcano - presumably an ice volcano - nobody thinks it's a hot lava volcano; 6% said they're geysers; 6% said they're made of rock of some kind; 28% said ice; 11% said salt and 39% said other, so I don't know what they were thinking.

Chris - But now we do have this week some new data, in the form of these two papers in Nature, which are using data and measurements made by the Dawn spacecraft.  So this gives us a much clearer idea as to what these things are.  So what are those papers saying?

David - Well we do have a better idea.  One paper, which is based on data from the framing camera, they're showing that the central parts of the white spots, probably, are some kind of salt - a hydrated version of magnesium sulphate, and you can see a haze rising about the white spot...

Chris - So that suggest something coming off of the white spot which is scattering light around it?

David - Absolutely, something is coming off the surface and it seems to be water vapour, but sublimed off some ice, carrying dust particles with it.  So the source of the water or the ice available to sublime when sunlight hits it is being replenished each day on Ceres because the available life sublimes when the sunshine hits, then it ceases to the sublime, the haze disappears, you don't see it at dusk.  By the next dawn there's enough water ice arrived at the surface to sublime to produce a fresh haze, so there's some active internal transport going on and that's very exciting.

Chris - So is this water coming from inside somewhere then?

David - Yes, I think the water, which is feeding the haze, must be coming from inside Ceres. 

Now there's no evidence, unlike many of the larger icy moons, that Ceres has an internal ocean of liquid water - that would be very surprising, we can't imagine a heat source to keep it warm enough for there to be liquid water. So it's just ice, not too far below freezing point, that's convecting and moving around maybe. 

To be honest we're flummoxed, we don't really know, but the evidence is stacking up to suggest that the places on the surface where ice is being replenished and able to sublime a way to space, and you get left behind these hydrated salts.

Chris - And why is this important, or how does this remodel, or refashion or view and our understanding of the solar system?  How everything we have and we see today got where it is now?

David - Well, to have so much water inside Ceres is a problem.  Ceres orbits not too far beyond Mars, much closer than Jupiter, and it's this side of the snow line.  Get to about Jupiter's distance from the Sun, it's cold enough for lots of ice to condense directly from the solar nebular when the solar system is forming. 

Where Ceres is now, for most of the time at least, has been too warm for a lot of water ice to condense so it's possible Ceres form further out from the Sun and has migrated inward.  And that probably brings us onto the second paper in the same issue of Nature which is providing us that Ceres is from even further away because it is suggesting that the surface contains what they are describing as ammoniated phyllosilicates - that means clay minerals with ammonia trapped within them. 

But nitrogen or ammonia trapped inside Ceres is a very big problem if Ceres grew where we now see it, and the suggestion from this work is well maybe Ceres started off in the very outer solar system - Pluto or beyond - and somehow found itself way inwards.  So it doesn't look like the archetypal biggest asteroid any more, it is a guest in the asteroid belt, and it's a guest that's the mystery we now need to try and solve.

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