[X] My Assistant

[dburt]

Relevant to Emily's boulder observation, the "Gullies and layers" HiRISE image was not the first to show layers with abundant boulders, indicating poor sediment sorting in layered slopes. Previous images included, e.g., PSP_001691_1320 "Gullied trough in Noachis Terra, released on 28 Feb., and PSP_001942_2310 "Signs of fluids and ice in Acidalia Planitia" released on 9 May. That these bouldery layers might represent ancient ballistic impact ejecta seems a reasonable suggestion, because much of the present martian surface is littered with boulders presumed to be ballistic impact ejecta. Other possibilities for boulder deposits might include, e.g., ancient talus or landslide deposits at the foot of slopes, stream boulders in channels, volcanic ejecta near vents, glacial moraines, or iceberg dropstones.

As an aside, the related suggestion that at least some of the fine-grained layers above or below any boulder deposits (or elsewhere on Mars) could likewise represent ancient impact deposits (non-ballistic fine-grained sand and dust distributed over vast areas by fast-moving, turbulent, erosive gaseous density currents - a.k.a. impact surge clouds - or by the winds as later fallout) already seems to have aroused considerable controversy on this forum, but again that's peripheral to Emily's boulder comment.

--Don

[MOD EDIT: "Brine Splat Burt" discussion moved here -> http://www.unmannedspaceflight.com/index.p...ic=4308&hl= -EGD]

[centsworth_II]

As an aside, the related suggestion that at least some of the fine-grained layers above or below any boulder
deposits (or elsewhere on Mars) could likewise represent ancient impact deposits (non-ballistic fine-grained
sand and dust distributed over vast areas by fast-moving, turbulent, erosive gaseous density currents - a.k.a.
impact surge clouds - or by the winds as later fallout) already seems to have aroused considerable controversy
on this forum, but again that's peripheral to Emily's boulder comment.

So you're the dburt of Basal Surge fame?

"ASU geologists L. Paul Knauth and Donald Burt, who along with Kenneth Wohletz of Los Alamos National
Laboratory, say that base surges resulting from massive explosions caused by meteorite strikes offer a simpler
and more consistent explanation for the rock formations and sediment layers found at the Opportunity site."
http://www.asu.edu/news/stories/200512/200..._meteorites.htm

I haven't followed the situation closely enough to ask any good questions, but I wonder if anyone else here
would like to ask about your current views.

for reference, the basal surge thread is here:
http://www.unmannedspaceflight.com/index.p...surge&st=30

[silylene]

Don,

Thanks for taking the time for a detailed reply.

I was thinking that deposition of suspended dust clouds from impacts could give uniform thin layers, as we saw in Meridiani. However, I do not have a good mechanism for glueing these settling dusts down into a layer unless the substrate had a moisture content.

Or perhaps, the impact dusts settled down onto a very shallow lake, and left layers underwater as they settled below? And the festoons are ripples from wind driven flow? Sorry, I don't know enough geology to differentiate if one possibility is more likely than another.

However, I don't see how surges could have produces all the thin fine bedded layers of similar thickness we have observed at Meridani. If it were surges, I would have expected some layers to be very thick, some thinner, and some containing jumbles of debris of various size. We haven't seen this.

On your criticism of the Fe/Ni ratios on the spherules, you do bring up some good points. However, I don't see how a surge mechanism would create spherules of this composition either.

Same goes for the Br/Cl ratios - the high Br levels always struck me as 'odd'. I wondered if Cl salts are more friable and thus more likely to be blown away, or something like that. Preferential erosion of Cl slats would ultimately cause the Br salts to enrich. All that said, I don't understand how a surge mechanism would account for the Br/Cl ratios either.

[dburt]

Don,

Thanks for taking the time for a detailed reply.

I was thinking that deposition of suspended dust clouds from impacts could give uniform thin layers, as we saw in Meridiani. However, I do not have a good mechanism for glueing these settling dusts down into a layer unless the substrate had a moisture content.

Or perhaps, the impact dusts settled down onto a very shallow lake, and left layers underwater as they settled below? And the festoons are ripples from wind driven flow? Sorry, I don't know enough geology to differentiate if one possibility is more likely than another.

However, I don't see how surges could have produces all the thin fine bedded layers of similar thickness we have observed at Meridani. If it were surges, I would have expected some layers to be very thick, some thinner, and some containing jumbles of debris of various size. We haven't seen this.

On your criticism of the Fe/Ni ratios on the spherules, you do bring up some good points. However, I don't see how a surge mechanism would create spherules of this composition either.

Same goes for the Br/Cl ratios - the high Br levels always struck me as 'odd'. I wondered if Cl salts are more friable and thus more likely to be blown away, or something like that. Preferential erosion of Cl slats would ultimately cause the Br salts to enrich. All that said, I don't understand how a surge mechanism would account for the Br/Cl ratios either.

Silylene,

Perhaps you should read some of the earlier posts - some of this has been discussed. As for dust, I'm not going to worry about as a sedimentary deposit until it is imaged. If there was ever standing water, dust falling into it should have yielded finely laminar shale, likewise not yet imaged (and a strong argument, in my mind, against arguing for standing or flowing water at Meridiani). The "festoons" are allegedly ripples that are utterly unique to water-driven (not wind-driven) flow, according to the claim, although very similar features have been imaged in surge deposits (see above posts), so I am unconvinced, and most of the ones allegedly imaged by Opportunity appear to be topographical artifacts of the downward viewing angle (bedding contours wrapping around small ridges and V-ing up cracks).

The fine laminae and shallow cross-beds typical of Meridiani are also very typical of surge deposits, even quite coarse-grained ones, presumably owing to shear between the very high speed turbulent particulate flow and the substrate. Consistent thickess of laminae may indicate consistent velocity of the passing surge cloud - in any case, it is also typical (as it is of comparable wind and water deposits caused by turbulent flow).

All impact spherules are caused by vapor condensation in a hot turbulent cloud. Specular (blue-gray) hematite typically forms in steamy volcanic fumaroles by condensation and reaction of volatile Fe-chlorides or other volatile Fe species, and this is a very similar environment to that in a steamy surge cloud. The Meridiani difference is that some other sticky condensate must have caused the hematite flakes to preferentially adhere to each other and other particles, and grow as a snowball does, until they got too large, and settled towards the ground, where they were incorporated into the rapidly growing sand deposit, commonly in disseminated form. I don't pretend to understand all the chemistry going on in a dynamic, disequilibrium system like an impact surge cloud. Present were plenty of volatile iron species, at least possibly two sources of Ni species (Fe,Ni impactor or subsurface Fe,Ni sulfides), and abundant volatile salts and steam, and what resulted after condensation and crystallization were "blueberries". I don't know the details. The important point is that their mineralogy (specular hematite), high Ni content, size limitation, perfectly spherical habit, enormous extent, and failure to be distributed along fluid passageways or mixing zones indicates that they cannot be concretions. Therefore they must be condensates, analogous to hailstones. There is no reason to expect their Fe/Ni ratio to match that in meteorites, BTW (contrary to a claim made by the MER team).

As regards Br/Cl, see the above post on salts. By our hypothesis, extreme fractional crystallization of chloride salts, yielding high Br/Cl in residual brines and the last crystallized salts, occurred long before the impact episode, owing either to downward freezing of brines in the regolith (our favored mechanism) or surface evaporation of brine lakes (too cold, by our thinking, although surface freezing followed by sublimation works). These Br-enriched brines or late salts were then available to be incorporated in the impact ejecta, and the brines, at least, could have flowed laterally quite far away from their parent chloride salts. Frost leaching would preferentially remove surface chlorides, leaving surface sulfates, as covered in our 2002 and 2003 publications.

These are very good questions, BTW.

--Don