Postcards from Dawn's Survey Orbit
The Dawn spacecraft, currently orbiting the dwarf planet Ceres in the asteroid belt, set out on this leg of its mission with four planned orbits from which to observe and investigate. All of them were polar orbits, with the spacecraft traveling up one face of and over the north pole, and down the opposite face and over the south pole, to allow for maximum coverage of the surface as Ceres rotates beneath it. Each orbit was carefully designed to accommodate a choreographed set of observations by Dawn's suite of scientific instruments, to provide the fullest possible understanding of Ceres as a resident of our solar system. And each orbit is at a lower altitude than the one before it, so we're treated to increasingly amazing imagery and science from one to the next.
Yesterday, Dawn again fired its extremely efficient ion engines (discussed in more detail in this post) and began the long process of decelerating, reducing its orbital energy and allowing Ceres to pull it closer. It will spend most of July doing this, which will take it to its next science orbit, called High Altitude Mapping Orbit (HAMO), from its current orbit, called Survey.
Dawn had been in Survey orbit since 6 June, investigating all of the surface features that enticed us from its first orbit, RC3, with the benefit of being much closer; Survey orbit had an altitude of 4,400 kilometers compared to RC3's 13,500. The results have been stunning. While the mystery of the most prominent bright spots has not yet been solved, the increased resolution has given us a much more detailed look:
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
At least for the smaller spots toward the right side of the crater, what were once featureless, overexposed white points now appear as smudges that vary in brightness outward from the center. It's suggestive perhaps of dust covering a brighter material, like ice, but Dawn is still too high to determine this with any certainty.
Ceres has a host of other fascinating and mysterious features that have perhaps been overshadowed by the famous bright spots. There are several other regions, particularly craters, that are streaked with patches of bright material:
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
The association of the bright material with craters may point to underlying ice or salts very close to the surface of Ceres that are disrupted when impacts occur. Elsewhere, large fractures or faults that may be evidence of past or present geological activity are visible:
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
One of the most surprising features that has caught the eye of mission scientists is a roughly pyramid-shaped mound in the northern hemisphere:
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
That's an odd shape to find on an airless body that isn't expected to have had much in the way of geological activity. It doesn't quite resemble a volcano, but that doesn't mean that it isn't, or wasn't once. It's quite large -- about five kilometers high and ten kilometers wide. The Curiosity team pointed out its similarity in size to Aeolis Mons, or Mt. Sharp, the Mars peak around which Curiosity is currently investigating. The protrusion also resembles an Earth formation known as a pingo, where subsurface water flow causes ice to push up the earth above it,resulting in a dirt-covered mound of ice:
Pingos in Canada's Northwest Territories. Credit: Emma Pike
Of course, this Ceres feature is orders of magnitude larger than even the largest Earth pingo. But similar processes may be at work. One of the more fanciful speculations about Ceres prior to Dawn's arrival was that subsurface bodies of water, perhaps even a subsurface ocean, could make up the dwarf planet's interior. Water, the great sculptor of our planet's starkest natural features, could be at work in subtler ways on Ceres. Still, the apparent lack of any other feature quite like this mound begs for an explanation, one that we won't have until at least the time when Dawn arrives at its third orbit, HAMO, at 1,470 kilometers altitude.
This will occur on 4 August, meaning that Dawn will spend all of July thrusting with its ion engine to spiral down closer to the Ceresian surface, taking breaks to transmit telemetry to Earth and snap the occasional picture. This is a painful wait for higher resolution photographs and the intensifying of investigations using Dawn's spectrometer and radiation detector, but it will allow us to focus on July's main event -- New Horizon's flyby of the Pluto system on 14 July. The dazzling array of Ceres surface formations will still be there for us to puzzle over in a month.