[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

[David]

What does this "kilometer of sediments" refer to? Is that supposed to be the depth of the stratigraphy? How could we know it actually goes that deep?

Sorry for the ignorant question, I just feel I'm missing something here...

[centsworth_II]

What does this "kilometer of sediments" refer to? Is that supposed to be the
depth of the stratigraphy? How could we know it actually goes that deep?

From satellite images of eroded terrain around the Meridiani
region in which hundreds of meters of layering can be seen.
Below are links to a couple of articles.

http://www.space.com/scienceastronomy/0606..._meridiani.html
http://www.psrd.hawaii.edu/Mar03/Meridiani.html

[David]

From satellite images of eroded terrain around the Meridiani
region in which hundreds of meters of layering can be seen.
Below are links to a couple of articles.

Great, thanks -- but how do we know that these layers are of the same type as the relatively short column we see at Victoria, formed by the same process -- which I gather is what Dr. Burt is claiming?

[centsworth_II]

Great, thanks -- but how do we know that these layers are of the same type as the relatively short
column we see at Victoria, formed by the same process -- which I gather is what Dr. Burt is claiming?

Dr. Burt would probably hope to see layers different from those seen in Victoria. Note the recent
exchange with Shaka (see below). Maybe MRO can get a closer look at those layers.

Shaka: "...We can see scores to hundreds of layers in the lower parts of the Victoria capes.
They are remarkably uniform in scale and appearance. Since a rain of meteorites would distribute
more or less randomly over Mars, it is hard to credit that some would not land closer to Meridiani
and produce much thicker (meter-scale) layers...."

dburt: "Congratulations! You have put your finger right on the weakest aspect of the impact
surge argument.... I can answer you in several possible ways, none completely satisfactory. 1) Oppy
has imaged only a small portion of the Meridiani layers, those at the very top, which, being the
youngest, could have formed when impacting had tailed off, and been distant (its lack of coarse
surface material was, after all, what moved it to the top of possible landing site choices - it's
possibly a biased sample, in other words). Coarse ejecta or surge layers may lie below the layers
exposed, or may even be exposed somewhere deep in Victoria. Such a finding (of coarse pieces)
would still be ambiguous, however, because ballistic ejecta could in theory land anywhere on Mars,
at any time, on top of any type of sediment (and dust could settle, but it wouldn't stick around,
unless the surface were sticky). 2) Coarse surface ejecta has been found at each landing site to
date (and at others abandoned from consideration when too many surface boulders were found).
Also, coarse layers of boulders in the midst of fine layers have been imaged by HiRISE in various
spots - as noted by Emily in the post that inspired me to stop lurking here about a week ago.