Dawn's first orbit, including RC3, March 6, 2015- June 15, 2015 Options

[JRehling]

Excellent analysis (as ususal) but I'd be surprised if the bright material on the surface of Phobos was ice, so is it salt? If so that gets us somewhere.

Thank you, ngunn.

Indeed, Phobos shows no signs of H2O, and the various colors on Phobos are interpreted as different kinds of rock, with the details unknown, but which kinds of rock may not matter so much for this discussion. (The brighter rock is considered as possibly mafic.)

http://onlinelibrary.wiley.com/doi/10.1029...B02354/abstract

Of course, ice can participate in different dynamics that rock cannot, but if this is simply a matter of impacts and mass movement, that may not matter.

Other worlds where we've seen the results of downslope movement of fine material on crater slopes include Eros and possibly Hyperion, but those patches on Phobos seem to look the most like Ceres'... for now.

[ngunn]

I was mulling over the question of how compact clusters of impact craters could form in the absence of an atmosphere to cause disintegration of the incoming object. I found this source helpful:

Having trouble posting the link but it's abstract 1759 from LPSC44, 2013
http://www.google.co.uk/url?sa=t&rct=j....93756505,d.ZGU
Ah that one seems to work . .

[Gerald]

Yes, thanks, that's a plausible approach! Once a loosely bound "rubble pile" approaches a celestial body closer than the Roche limit, it will disintegrate.
Then it's straightforward, that the impact will expose fresh / bright subsurface material.

[Ken2]

It is a interesting article and hypothesis - however it did seem to prefer a 2 body system ( earth moon ) to break up an asteroid - though a single impact's delayed secondaries was also mentioned which would probably be the best for Ceres

I would think the main multi-body impact swarm mechanisms for Ceres would be

1) Direct secondaries from large impacts,
2) Medium/long duration secondaries from large impacts orbiting debris (BTW where are all the little moons that could be formed from this? - Cere's orbit must not be stable very long (relatively speaking))
3) Does Ceres have enough gravity to break up a rubble pile into a chain / cluster before impact? - if so then maybe just a Ceres impact of a very loosely bound body. (see previous post!)
4) Jovian / Martian perturbed comet/asteroids that get lucky and impact Ceres as chains.
5) Maybe a close approach to Ceres causing breakup and a lucky return later - but I would think the dispersal would be too much by the return date.

I don't think any of these look good for the spot 5 cluster, other than 1),3) - 1) an impact and direct secondaries. (maybe an impact into a preexisting central peak and it oblong blasted part of the peak out and caused shrapnel at the local peaks of the second spot cluster terrain. (loose analogy - mt saint helens sideways explosion)
2) Medium/long duration secondaries - would probably hit a crater rim (though if spot 's host crater initially had a central peak - maybe it could hit it first),
3) would likely be most of a long chain (visible elsewhere on Ceres.) - though maybe Ceres' gravity could break it up as a cluster shortly before impact.
4,5) I doubt the chain would be so localized for a distant breakup - probably just a few separate craters.

[ngunn]

3) Does Ceres have enough gravity to break up a rubble pile into a chain / cluster before impact?

I was looking for an answer to that very question. A bit of googling comes up with a Roche limit for a rubble pile approaching Ceres at a distance of approximately 1500 km. so I'd offer a tentative "Yes"' (though I'd like to hear from someone more knowledgeable on that point). The resulting crater cluster would not necessarily be much elongated if the impact was near vertical.

[Gerald]

It depends on the density, shape, and rigidity of the rubble pile. The upper density limit for a non-rotating spherical rigid rubble pile would be twice the density of Ceres to disintegrate. In this limiting case the rubble pile would disintegrate just at Ceres' surface.
In the fluid case the limiting density would be about the 14.5-fold of Ceres' density, beyond any reasonable assumption, i.e. any reasonable rubble pile of the fluid type would disintegrate.
The truth for just gravitationally bound non-rotating rubble piles should be somewhere between these two extreme models.
For a non-rotating liquid-model rubble pile the density of Ceres, disintegration would be near 476 km x 2.44 = 1161 km above center, or 1161 km - 476 km = 685 km above the surface.
Disintegration near 1500 km from Ceres' center would apply e.g. to the liquid model of density (476 /1500)³ x 14.5 = 0.032 x 14.5 = 0.464 the density of Ceres, or about 0.032 x 14.5 x 2.1 g/cm³ = 14.5 x 0.067 g/cm³ = 0.97 g/cm³.
(Edit: added the missing factor 14.5 for the liquid model in the previous sentence)

Note:
- 1.26³ = 2, in the formula of the Roche limit for the rigid case,
- 2.44³ = 14.5, in the simplified formula of the Roche limit for the fluid case.

[Blue Sky]

I would think a rubble pile approaching on a glancing blow could get broken up.
Comet Shoemaker-Levy was disrupted on a close pass by Jupiter years before
it impacted.

[ngunn]

Well I used a density ratio of 3 in the formula for the liquid case to get 2.455x450x1.44(cube root of 3) = 1590 km.

[Gladstoner]

There appears to be a dark linear feature extending between the bright areas:



Contrast enhanced and marked:



This could be a valley, a northward-facing slope, or an albedo feature. It persists in most images, but seems to disappear when near the limb, probably due to the unfavorable angles (edit: it may still be visible in the right image):



For comparison, a narrow valley on the floor of a nearby crater:

[elakdawalla]

Hmm, I think you may be on to something, Gladstoner. It's right at the limit of resolution.

I just asked someone on the Dawn team whether the JPEG artifacts in the Ops Nav 8 happened before transmission to Earth or after, and she said after. So we have better quality to look forward to upon PDS release.

As part of a blog entry I'm working on, I mapped all the schmutz on the detector; I find 8 or 9 spots that show up after stacking all the animation frames (I'm not completely sure that the upper rightmost one is schmutz; there are fewer frames in that area). It is very difficult to find these artifacts in the Ops Nav 8, but eventually I found them; they are almost completely eradicated by JPEG artifacts. Which isn't necessarily a bad thing!

[Bjorn Jonsson]

...As part of a blog entry I'm working on, I mapped all the schmutz on the detector; I find 8 or 9 spots that show up after stacking all the animation frames

To speed things up, when processing the PDS Vesta images I made a Photoshop file containing a selection which includes just the schmutz that I had to remove semi-manually. IIRC I had six of these in that file but discovered that there were a few more that are more subtle and that I will be adding to the Photoshop selection in the future.

But now there is this: http://www.jpl.nasa.gov/news/news.php?feature=4594

I'm not exactly happy if I turn out to be right but the Dawn team now seems to think the bright stuff might be ice as I have been suspecting (comparisons to Callisto played a role there in my case). Something else causing the bright spots would have been more exciting.

[Gerald]

There appears to be a dark linear feature extending between the bright areas...

This could be a valley, a northward-facing slope, or an albedo feature.

Since there seems to be no parallel brighter line in any of the images, a valley seems to be the least likely option.
A northward-facing slope would be more consistent. So it could be the southern border of a depression.

---

Regarding the bright material: I would be really surprised, if it would actually turn out to be water ice. My favorite at the moment is some kind of reworked evaporite, with most of its humidity already sublimated.

[fredk]

the Dawn team now seems to think the bright stuff might be ice

That release only says "possibly ice" and doesn't elaborate. Perhaps you're referring to other released statements?

[Gladstoner]

I'm not exactly happy if I turn out to be right but the Dawn team now seems to think the bright stuff might be ice as I have been suspecting (comparisons to Callisto played a role there in my case). Something else causing the bright spots would have been more exciting.

It would be relatively 'ho-hum' if the bright areas had a Callisto-like distribution across Ceres, but these are so.... isolated. This could indicate:

1. There are scattered formations of the bright stuff below the surface that occasionally get exposed by impacts or landslides. These could be vein swarms, dikes, diapirs, or otherwise some kind of filled conduits or intrusive structures.

2. The bright stuff is more extensive beneath the surface, but it rapidly fades after being exposed by impacts or landslides. Such events would have to have happened in the (relatively) *very* recent past.

3. Some combination of 1 and 2. Edit: In either case, the original formations/structures could have been disrupted by subsequent impacts, and some of the smaller spots could be from large megabreccia clasts.

4. The bright stuff was deposited on the surface. This could be erupted flows of ice, salt deposits left after sublimation of brine, or who knows. Such deposits could fade over time as well.

(5. Other processes that I can't think of at the moment.)

To me, the fact that the bright area is so isolated, but yet is made up of smaller, scattered spots of varying sizes, seems to indicate that something quite interesting is going on here. 'Mundane' impacts or landslides exposing extensive but rapidly fading material -- the most 'boring' possibility -- seem to be less likely (unless the impact or slide was complex, e.g. disrupted rubble-pile meteoroid, as mentioned in posts above).

A question that will be answered soon: Are the bright spots on a surface that is smooth & flat, rough, or a combination? That should narrow down the possibilites once the slightly higher-res images come in. A (possible) fracture/valley in the midst of the spots could be dramatic indeed.

[John Broughton]

"There appears to be a dark linear feature extending between the bright areas"

It's part of the regional system of parallel grooves. This particular one extends well outside the crater rim on both sides and joins up with the most prominent groove about two crater diameters to the ESE. Some minor spots in the crater are not aligned, so I expect we'll see more cracks show up there as the resolution improves.