Dawn's Survey Orbit at Ceres |
Dawn's Survey Orbit at Ceres |
Jun 15 2015, 05:47 PM
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#1
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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 24 2015, 05:28 PM
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#2
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Junior Member Group: Members Posts: 71 Joined: 3-February 11 Member No.: 5800 |
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Jun 24 2015, 05:58 PM
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#3
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Member Group: Members Posts: 716 Joined: 3-January 08 Member No.: 3995 |
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Jun 24 2015, 08:26 PM
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#4
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
Both may be where impacts exposed subsurface intrusions of the white stuff. I'm looking at photos of Ganymede, which has a lot of rayed craters that exposed bright, subsurface ice. The phenomenon on Ceres is definitely something different. Related, perhaps, but every example on Ganymede has common characteristics that none of the examples on Ceres match. If this is a mere "ray" event (also visible on the Moon, Mercury, etc.), then there's something very different on Ceres about how the material is surfaced or how it ages. I'd guess we're either seeing the remnants of a ray system that sputters away quite differently than the rays on Ganymede, et al, do, or these are eruptions. If it were simply subsurface salt, I'd expect them to look like Ganymede/Moon rays and to be less selective in their occurrence. Perhaps icy ray material on Ceres is coarse near the point of origin and sparse far from it and therefore sputters away rapidly except where it's most coarse. Eg, a 20 kg lump of ice may stick around much, much longer than lots of microgram particles. Alternately, I'd stick with the idea of eruptions: Which may be triggered when the impact occurs, or possibly much later. |
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Jun 24 2015, 09:06 PM
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#5
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Senior Member Group: Members Posts: 3516 Joined: 4-November 05 From: North Wales Member No.: 542 |
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Jun 24 2015, 09:42 PM
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#6
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Senior Member Group: Members Posts: 2530 Joined: 20-April 05 Member No.: 321 |
I see a citation from Lanzerotti, et al, 1978 to the effect that:
Solar wind erosion for pure water ice is about 10 angstroms per year at 1 AU and falls off with the inverse square of the distance. So, at Ceres, about 3 angstroms per year. At that rate, a 10 cm ice layer would vanish entirely in about 300 million years. That's longer than the age of Tycho on the Moon, and about as old as Aristarchus. So, chunks about 30 cm across would survive near younger craters, but not near older ones, and snowflakes would disappear almost instantly in geological terms. |
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