Ceres Bright Spots Continue to Mystify
On 23 April, the Dawn probe settled into its first operational orbit of the dwarf planet Ceres, termed the "RC3" orbit, and began imaging and science operations a day later. At an altitude of 13,500 kilometers, Dawn collected data that will help refine our estimates of Ceres' mass, continued the search for the tenuous indications of water vapor detected by the Herschel space observatory early last year, and imaged the surface in nighttime and in daylight at a resolution of 1.3 kilometers per pixel, in multiple spectra.
Recall that the higher one's orbit is from the surface of the body being orbited, the longer it takes to make one orbit around it. At 13,500 kilometers above Ceres, Dawn orbited once every 15 days (this is its orbital "period"). Dawn spent only about two weeks in RC3, enough time for just one trip around the dwarf planet. RC3, like all of Dawn's planned orbits, is a polar orbit, meaning that it's inclined 90 degrees from Ceres' equator. In this orbit, the spacecraft passes over both the north and south poles of Ceres, and can collect images as the dwarf planet rotates beneath it. A polar orbit is ideal for survey and mapping missions like Dawn's, since it ensures that the entire surface can eventually be observed by the spacecraft.
This also ensured us a fresh batch of images of the now-famous bright spots, for which the attendant mystery has only mounted as Dawn draws closer. Imaged from closer than ever before in the RC3 orbit, what was once one bright dot and then two bright dots is now revealed to be a multitude of spots peppering one of Ceres' largest impact craters.
Ceres imaged from the RC3 orbit, with multiple bright spots visible near the center. Credit: NASA
Over at UnmannedSpaceflight, a forum for discussion of, well, unmanned spaceflight, user ZLD posted an animation that realigns this sequence of imagery to create a "flyover" perspective of the bright spots:
Credit: ZLD from UnmannedSpaceflight forums, NASA
As with each round of images before it, this doesn't answer any of the questions about the origins of the bright spots, and only heightens our anticipation for the next batch. As we witness the spots continue to multiply in closer imagery, it's worth going a bit more in depth about how spacecraft typically collect imagery, and what that may tell us about what we're observing.
Images of Ceres in the visible spectrum are acquired by Dawn's Framing Camera (FC) instrument, which, like its counterpart device on New Horizons, Cassini, and other robotic spacecraft, translates the incoming light into a digitized image using a Charge-Coupled Device (CCD). A Charge-Coupled Device is grid of tiny photoelectric cells, or "wells." Each well corresponds to one pixel in an image. The lens of Dawn's Framing Camera collects trillions of tiny packets of light called photons and sends them to the CCD. The device is designed so that when a photon enters a well, it increases that well's negative charge. Electronics can measure the charge in each well and then create a corresponding pixel in an image. The more light received by a well, the greater the well's charge will be. The greater the well's charge, the brighter the corresponding pixel will be. The result is a grid of pixels representing one image from the Framing Camera which can be transmitted back to Earth as binary zeroes and ones.
A Charge-Coupled Device. The innermost rectangular area is a grid of thousands of "wells" which collect packets of light and converts them to values of brightness. Credit: Dyxum.com
But there is an important catch: each of the tiny wells in Dawn's CCD has a finite capacity for light. If Dawn is imaging something particularly bright, too much light flowing into a well can cause that well to "spill over," with excess light falling into adjacent wells. In the resulting image, this is represented as light "bleeding" into adjacent pixels of the image.
From the Hubble image that showed the bright spots as only one bright smear on the face of the planet, to the images acquired by Dawn before it entered into Ceres orbit in March, one pixel could have covered anywhere from 4 to 30 kilometers of Ceres' surface. This means that the adjacent well, or pixel, that the bright spot's light was spilling over into could have covered a huge swath of terrain that had no bright features at all. At these low resolutions the "bleed" of light was enormous. Now, in the RC3 orbit, one pixel in an image covers only 1.3 kilometers of the surface, and we can resolve many individual bright spots, as the adjacent pixels they bleed into represent much less Ceresian surface area. We can now resolve the non-bright regions in between the spots.
We don't know how large the bright spots actually are yet. The more light they reflect, the smaller they need to be to produce this level of overexposure. In future orbits, one pixel from an image will cover 410 meters of Ceres' surface, then 140 meters, and finally 35 meters. It may be that the spots are the size and shape that they appear now, reflecting a substantial amount of light. Or, in the final orbit, at maximum resolution, it may turn out that the bright spots are a great many smaller but extremely bright features, "multiplying" again as Dawn draws closer to Ceres.
In either case, the most prominent speculation is that some configuration of ice reflecting sunlight is causing the brightness. Most simply, it could be that a very recent impact caused the exposure of ice from below Ceres' surface. More exotically, cryovulcanism, where pressurized pockets of liquid water in the interior of a moon or planet shoot to the surface through subsurface vents, has also been suggested. Dawn mission scientists have downplayed this possibility, and it's not clear where Ceres would get the energy necessary to maintain the level of geological activity needed to produce cryovolcanoes. Diapirism, where subsurface ice pushes upwards through structural faults in lower density material above, may also be a culprit. This effect may also be responsible for the raised "mound" feature imaged by Dawn in the RC3 orbit, which ZLD from the UnmannedSpaceflight forums also produced a "flyover" animation for:
Credit: ZLD from UnmannedSpaceflight forums, NASA
Or ice may have nothing at all to do with it. While the bright spots are the most intriguing features, there is plenty more already visible on the surface that will be of great scientific interest: raised protrusions like the mound above, surface cracks that are possible faults, and notable deviations in albedo (brightness) other than the famous spots themselves. All of these may be a part of the geological story that the spots, bright though they are, cannot fully illuminate for us on their own. We'll know more as Dawn investigates surface features at lower and lower altitudes, which it is on its way to do right now. On 9 May, Dawn began firing its ion thruster to start the transition to its next orbit, termed the "Survey" orbit, which will place Dawn at an altitude of 4,400 kilometers above the surface.
As you may recall from a previous post on Dawn, its ion propulsion trades power for extreme efficiency. Unlike spacecraft with chemical propulsion, which add or subtract great amounts of velocity in large one-off engine burns to substantially raise or lower their orbits, Dawn's ion propulsion burns continuously for long durations, slowly changing the craft's velocity and hence its orbit. Rather than quickly stretching its orbit like a rubber band, Dawn nudges its orbit into a new shape very gradually, like kneading dough. The process of changing its orbit while traveling significant distances along that orbit produces a spiral trajectory, as illustrated in this diagram showing the transition from the RC3 orbit to the Survey orbit:
Dawn's spiral descent from RC3 orbit to Survey orbit. I've added some additional annotations for clarity. Credit: Dawn Blog/NASA
As Dawn's thruster continually fires and the spacecraft's velocity decreases, the arc of its orbit bends lower and lower towards Ceres. The constant minute adjustment of its orbit requires the spacecraft to be pointed very precisely, so in these transition periods Dawn is not doing much in the way of imaging or communicating with Earth, though it takes occasional breaks from thruster firing for check-ups.
Final insertion into the Survey orbit should occur around 6 June. At that point, Dawn will resume mapping, imaging, and investigating Ceres at a higher level of resolution, and some of our questions about Dawn's mesmerizing bright spots and other fascinating surface features may be answered.