[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

[marsbug]

Forgive me for asking what may be a dumb question, but can the evidence for a northern ocean be squared with the view of mars as an ice ball? The evidence in question may not actually be of an ocean, but if it is then mars must have been warm and wet for at least part of its history, and if its not what is the simpler explanation?
Edit: I should make it clear that by 'evidence of a northern ocean' I'm referring to the traces of coastline around the northern basin, not any more subtle evidence for an ocean which I'm not aware of.

[dvandorn]

Forgive me for asking what may be a dumb question, but can the evidence for a northern ocean be squared with the view of mars as an ice ball? The evidence in question may not actually be of an ocean, but if it is then mars must have been warm and wet for at least part of its history, and if its not what is the simpler explanation?
Edit: I should make it clear that by 'evidence of a northern ocean' I'm referring to the traces of coastline around the northern basin, not any more subtle evidence for an ocean which I'm not aware of.

If Mars had developed salty oceans before the late heavy bombardment nearly 4 billion years ago, that might explain the amount of salt that is distributed all across the planet. Impact cratering is generally not thought to be a great horizontal mixer of material, mixing more vertically than horizontally (at least based on the lessons learned from the Moon), but the very magnitude of the LHB could have distributed salts from the bottoms of obliterated southern hemisphere salty seas all across the planet (especially considering how far-flung the LHB must have thrown salty water from these putative seas into the early Martian atmosphere).

I can also picture a Mars which has quickly lost much of its atmosphere after the magnetic field died, cooling drastically, losing much of its liquid water to evaporation and ice sublimation and exposing tens of thousands of square kilometers of salty seabed; winds may then have eroded these salt flats and distributed the salts across the planet. So you don't necessarily need the LHB to explain the ubiquity of salts on Mars without requiring the entire planet to have been covered with salty seas, but it remains one plausible transport mechanism.

If there was a Great Northern Ocean, I'm thinking it must have post-dated the LHB, since there is no reason to believe that the impact flux would have limited itself to the southern hemisphere. I truly believe that, at the end of the LHB, Mars looked pretty much like the southern hemisphere looks today, but all over. The smoothed terrains we see in the north must have been overlain over a rough lunar-highlands-type of terrain, unless you want to try and explain how such a heavy bombardment could have completely missed one whole hemisphere of the planet... (Just trying to apply Occam's Razor, here.)

Nonetheless, it's important to remember that Mars with a thicker atmosphere and any kind of greenhouse effect would have been considerably warmer than it is today -- if you moved the Earth to Mars' orbit, it would be somewhat cooler but generally habitable (our seas and oceans wouldn't all freeze over, etc.). It's a touch disingenuous to suggest that Mars could never have been warm and wet because of its distance from the Sun and the related lower insolation than that received here on Earth. Mars is cold and dry today primarily because it lacks a magnetic field and thus the solar wind has sputtered a major percentage of its original volatiles right off of it. Had Mars not lost its magnetic field early on, it might still be warm enough and have enough atmosphere to support liquid water on the surface.

-the other Doug