[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

[dvandorn]

Yes -- I brought up the finely layered nature of the rocks at Meridiani before, and that question was never addressed by the Professor. I can see these rocks being built up by aeolian deposition of millimeter-deep layers over the course of tens of thousands of years, laid down on wet ground (and possibly onto shallow standing water), far more than I can see tens of thousands of base surges, each laying down a very thin, very flat layer of rock of consistent composition to the last base surge, each laying down a very flat, very thin layer with almost no turbulence developing along the surge/ground contact.

Such surges would, I would think, have enough energy in them that we would see scouring and channeling -- the very same types of landforms whose lack that the Professor cites as a disproof of standing water or a playa environment. If these layers were laid down by an energetic base surge, how can they be so overwhelmingly flat, with very little sign of any turbulence? (Remember, the cross-bedding we've seen is the exception, not the rule, in these rocks.) Are you postulating that there were *no* surface features that would have caused turbulence in the surge/ground contact? (We may not be able to watch and observe a surge in detail, but we have a ton of similar surge-emplaced landforms that we have observed in great detail on the Moon, and even with a lack of atmosphere, we see evidence of a fantastic amount of turbulence in the debris flows that generated terrain on Luna.)

I also have a difficult time understanding how these deposits could have been laid down at the end of the Late Heavy Bombardment. There is visual evidence supporting the theory that the rough, cratered terrain generated by the LHB actually underlies the Meridiani deposits. You'd have to assume, based on the range of crater-like landforms, the relative lack of large craters, and the relative flatness of the terrain, that the nearly kilometer of Meridiani deposits were laid *after* the LHB had finished. In other words, if the Meridiani unit was generated by tens of thousands of impacts at the end of the LHB, why would the unit not have been broken up by these impacts as quickly as the base surges laid it down? What makes Meridiani so special that it could be laid down by impacts all around it but not suffer any impacts in the area itself, thus leaving this layered unit (which would have required millions of years of base surges to lay down) mostly intact?

Occam's razor suggests that we're seeing an entire population of dead grandmothers here...

-the other Doug