Dawn's Survey Orbit at Ceres |
Dawn's Survey Orbit at Ceres |
Jun 15 2015, 05:47 PM
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Senior Member Group: Members Posts: 1729 Joined: 3-August 06 From: 43° 35' 53" N 1° 26' 35" E Member No.: 1004 |
daily Ceres picture from the survey orbit
http://dawn.jpl.nasa.gov/multimedia/images...tml?id=PIA19572 I started a new topic, as we are no longer in the first orbit phase |
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Jun 22 2015, 08:49 PM
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Member Group: Members Posts: 153 Joined: 20-December 14 From: Eastbourne, UK Member No.: 7372 |
I would guess the left of the two basins is newer. (Gladstone's previous post.) It has resulted from an impact on the edge of the older basin and is considerably deeper. The crust has been pushed down by the later impact, fracturing the surface and enabling the flow of dust on inclined surfaces.
This is more a conjecture. The left basin has steep terraced sides to the south, but far less well defined rims in its northern half. If the impact was at the edge of the ancient ocean and the totally frozen/rocky Southern polar "continent", the southern crater wall would appear as a normal stepped terrace, the northern crater wall collapsed and less well defined. Also the pressure of the impact would force liquid away from the impact site in the ocean below, allowing the crust to collapse. This collapse would stop at the shore line of the subsurface solid "continent" and indeed such a shore line can be made out and the bowl shape of the collapsed crust made out in an semi circle around the northern crater rim. The centre of the crater appears to be almost dead on the shore line and there is half a central peak, its northern face is missing, either via collapse or sublimation. It is also particularly clear that North of the shore line the surface is way more smooth and shows very few deep impacts, clear signs of resurfacing and linear scarring indicating expansion/contraction or movement of the surface crust. The darker areas of the poles indicates older less recycled surface. There may not be a dust cycle as dramatic as on 67P, the gravity on Ceres prevents sublimating gases moving dust very far, but where volatile ices are near to the surface, dust will move and resurface the comet. I would suggest this is happening on a very small scale now, but in the past when the crust was thinner and in areas where the crust has been thinned or penetrated by impacts, the movement of dust by sublimating gases would have been significant enough to resurface the comet on a local level. It could be that at a low level this is what is keeping some areas of exposed ice/salt exposed, the subsurface sublimating gases escaping at the surface preventing dust settling to recover the exposed areas. Once all the volatiles are exhausted the surface becomes covered in dust. If the volatiles are being replaced by more from below, say in a very deep crater or fissure, the dust could be held at bay for a significant length of time. Following on from that, how many of the craters on Ceres are impact craters and how many sublimation derived features such as those on 67P? An explosion of pressurised, sublimated gases within the crust could leave craters similar to Spot 1 for instance, the ejecta being the unexposed ice of the crust dispersed by the explosion. It might prove interesting to compare the craters with those left by underground atomic bomb tests. The suggestion of Spot 5 being a central mountain that blew off one of its sides, like Mt St. Helens, seems another example. Ceres is dark and absorbs a lot of sunlight, that heat must travel into the crust and have consequences, such as creating pockets of sublimated gas. Even if an impact does not penetrate to the liquid below it exposes these pockets and increases the possibility of the pressure within resulting in the explosive creation of features on the surface. The strange mountain could be another example, a small high energy impact penetrating a large "magma" chamber of volatile gases, perhaps added to by fluid under pressure escaping from the ocean below. The coming week's pictures may suggest a whole different scenario, so this is just little more than a thought experiment to ferment ideas. We are dealing with a surface crust made largely of Water Ice, most especially, I would suggest, in the equatorial and mid latitudes. The ocean may well not ever have developed in polar regions and the surface there maybe largely made of refractory materials, rock and organics. The shrivelled and shrunken appearance of the surface we see today suggests to me that there are no longer large volumes of Water blow the crust, if any, but there may still be lakes and small seas where the energy of past impacts has locally delayed the slow freeze. |
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