[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]

I'm no geologist or chemist but the impact surge argument seems very straight foward and logical, possibly more so than any other hypothesis I've heard. I understand that the chemistry of home plate is very suggestive of the presence of water. Could the chemistry of home plate be accounted for by the impact surge hypothesis alone, or would the occasional presence of water still be required?

[dburt]

I'm no geologist or chemist but the impact surge argument seems very straight foward and logical, possibly more so than any other hypothesis I've heard. I understand that the chemistry of home plate is very suggestive of the presence of water. Could the chemistry of home plate be accounted for by the impact surge hypothesis alone, or would the occasional presence of water still be required?

Superb question! Really sets me off (the flattery doesn't hurt either ). In principle, what impacts do is vaporize, excavate, and scatter what was already in the target volume, as modified by vapor condensation and generally minor chemical input from the impactor (a comet or meteorite). Turbulent surges also scour the surface across which they flow, possibly modifying the cloud chemistry. Impact doesn't create anything new, except high pressure minerals and impact melt (and vapor and condensates, including spherules) - it's main effect is to homogenize and scatter what was already there.

Most salts at Meridiani (except jarosite which, following Roger Burns, we attribute to iron sulfide weathering - impact acid steam condensation has alternatively been suggested by a colleague, Misha Zolotov) probably formed in liquid water (brine) long before impact may have scattered across Meridiani. If early Mars worked like early Earth, the salts mainly date back to shortly after the planet formed, when acid steam, released by planet-wide degassing, condensed into the first warm seas. The acids rapidly reacted with basic rocks like basalt to form neutral salts containing Mg, Ca, and Na.

To concentrate the dissolved salts into a dense brine or crystals, we like freezing more than evaporation. That is, given the choice between hot vs. cold for early Mars, we choose cold, because Mars has always been much farther from the Sun and much smaller than Earth. As the seas froze down and the ice sublimated, the brines beneath got more and more concentrated (denser), and gradually sank into the subsurface, where they were trapped beneath ice or permafrost.

Whether cold or hot (relatively), early Mars was certainly bashed by impacts, probably most of them just before 3.8 billion years ago (the so-called late heavy bombardment). These impacts obliterated the early surface history of Mars and scattered salts and, we hypothesize, sulfides across the surface. Afterwards, the surface of Mars was about as it is today, extremely cold and dry, with an atmosphere close to a vacuum, so that liquid water (and even ice in most places) was unstable or extremely ephemeral. Large amounts of water apparently survived in the subsurface, however, as both ice and (probably) deep brine (as evidenced by occasional catastrophic releases to outflow channels that possibly formed ephemeral seas in the northern lowlands).

We tentatively date the Meridiani surface rocks (= youngest geologically) to the tail end of the late heavy bombardment period, when the surface of Mars already could not support liquid water as streams or lakes (given that there is no geological evidence of any at Meridiani - in this our interpretation differs completely from that of the MER team). The surface climate matters very little for the impact process itself, but does affect the preservation potential (cold and dry = very slow wind erosion only, allowing weak Meridiani sediments, cemented only by soluble salts, to survive until the next distant impact buried and preserved them). So to answer your question (you say, at last!), no liquid water is indicated by the salts, other than minor quantities resulting from condensation of steam in the original surge cloud, or whatever the salts themselves could attract from the atmosphere (i.e., water in surface brine films and occasional drips). This minor water was enough to rust sulfides and dissolve (leach) the most soluble salt (probably a chloride), leaving hollow crystal cavities, but was not enough to crystallize clays, destroy jarosite, recrystallize the soluble salts in bulk (reducing rock permeability to zero), or form the extremely coarse (giant in places) salt crystals that characterize actual evaporites or other water-soaked salt deposits. Our impact surge hypothesis resembles the extremely complex one of the MER team only in that we agree that the salts must have been transported from somewhere else (a realization they apparently came to only after we had pointed out to them, in our initial "brine splat" presentations of 2004, conceptual problems with having the most soluble and least soluble salts intimately mixed together in an alleged evaporite). That "somewhere else" could be any large salt concentration in the subsurface for us, or a hypothetical giant vanished playa lake (for which there is absolutely no surface or shallow subsurface evidence at or near Meridiani) for them. Our hypothesis can be generalized to any sandy, salty, layered sequence on Mars (including Home Plate in Gusev Crater), theirs cannot. Our allows for the actual appearance and extremely broad distribution of "blueberries" as impact-derived spherules (with similar-appearing ones occurring at Home Plate and probably many other places); theirs does not. And so on and so on, but I hope you are getting the general idea. I emphasize that the above impact story is merely a working hypothesis that undoubtedly is wrong in some details, but in its present form it appears consistent with all available evidence (I trust everyone will feel free to disagree vociferously).

Wow, ask a simple question and get a simplified geological history of Mars! What a deal! Oh well, you know me by now. As Arne said in T2, "Of course, I'm a terminator", I say "Of course, I'm a herr doktor professor" (who kills grandmothers by his tests, and every else by his long lectures...).

[centsworth_II]

...no liquid water is indicated by the salts, other than minor quantities...

And besides the large amounts of liquid water involved in the original
formation of the salts, right?

If early Mars worked like early Earth, the salts mainly date back to shortly
after the planet formed, when acid steam, released by planet-wide degassing,
condensed into the first warm seas. The acids rapidly reacted with basic rocks
like basalt to form neutral salts containing Mg, Ca, and Na.

[dburt]

And besides the large amounts of liquid water involved in the original
formation of the salts, right?

You got it. Lots of water somewhere, somewhen, but not at Meridiani then (or since).

--Don

[centsworth_II]

Lots of water somewhere, somewhen, but not at Meridiani then (or since).

Why assume that the salts did not form at Meridiani billions of years ago.
Is it only because they had to form somewhere else in order to be moved
to Meridiani by base surge?

[dburt]

Why assume that the salts did not form at Meridiani billions of years ago.
Is it only because they had to form somewhere else in order to be moved
to Meridiani by base surge?

Good question. If I understand it, your argument is they had to form somewhere, so why not at Meridiani, where we find them? (Great example of Occam's razor at work, by the way.) I don't want to write another long post, because I'm about to go home, but maybe I can practice with a short one. The basic arguments are several: 1) We see no evidence of a geological environment that would permit salts to grow at the surface - no shales, meaning no standing water for an evaporite basin or even puddle - not even in deep material excavated from Victoria. The playa lake, if any, has "vanished". 2) The Meridiani salts appear to be an intimate mixture of highly soluble and highly insoluble salts (a mechanical mixture, in other words) plus poorly characterized fine debris (not crystalline clays). Salts crystallize out of an evaporite basin in inverse order of solubility - least soluble first, in a "bathtub ring" around the outside fringe (usually gypsum), and then more and more soluble salts in zones towards the center. Meridiani is nothing like this. The MER team agrees with us that the salts had to come from somewhere else - they invoke wind transport and mixing (reasonable - plenty of wind on Mars), we invoke impact transport and mixing (plenty of evidence of impacts, of all ages, but especially for early Mars). 3) The salts are very fine grained and the rocks appear uniformly porous and permeable. Growth from water, or soaking in water, would have caused permeability decrease and crystal size increase. This argues strongly against the MER team interpretation that the salty rocks were soaked in brine many times after they were deposited, and that brine mixing in a huge, uniformly permeable volume produced concretions. (The US Government permanently stores military radwaste in evaporitic salt beds, because they are so impermeable.) There are other more subtle geochemical arguments, but those 3 are the easiest to understand, I think. Let me know if you need clarification.

Well, out to play in the real desert (i.e., go home) . Soon the daily summer afternoon dust storms (haboobs) will begin in Phoenix (sort of a very dilute, relatively slow-moving, cool, dry analog for a ground-hugging surge cloud - but something that a martian surge cloud could easily decay into with distance, perhaps). Thank y'all very for your comments (even you, Bill Harris - I really admire your succinctness ).

--Don

[centsworth_II]

Let me know if you need clarification.

Ok, everyone agrees that the sulfates were moved to their present location from
elsewhere. I'm wondering about the very fine layering seen at Meridiani. I have
a hard time imagining such fine layers being formed by such a violent, sudden
activity like base surge. Do you have any feeling for what depth of layering
would be attributable to a single base surge event?

Apart from the fine scale festoons that the MER team attributes to water, there
are the massive chunks that really do look like fossil dunes (or large wind ripples).
Do you also attribute those to base surge?

Even assuming that the layers and berries could be the result of base surge, I
have a hard time understanding the formation of the large crystals whose
dissolution formed the vugs. You've said:

"No more moisture is required than would be present in the original surge cloud
(mainly condensing steam) or could later be removed from the atmosphere by
water-attracting (hygroscopic or deliquescent) salts."

And:

"Only minor water, such as liquid condensed from the steamy surge cloud or water
attracted from the air by the salts itself, is needed to leave the Meridiani crystal cavities"

Is the formation of such large in-ground crystals by trace water (on Earth) a geological
observed fact? Or is it a "fringe" hypothesis? I have no real geological background.

[dburt]

I'm wondering about the very fine layering seen at Meridiani. I have
a hard time imagining such fine layers being formed by such a violent, sudden
activity like base surge. Do you have any feeling for what depth of layering
would be attributable to a single base surge event?

Apart from the fine scale festoons that the MER team attributes to water, there
are the massive chunks that really do look like fossil dunes (or large wind ripples).
Do you also attribute those to base surge?

Even assuming that the layers and berries could be the result of base surge, I
have a hard time understanding the formation of the large crystals whose
dissolution formed the vugs. You've said:

"No more moisture is required than would be present in the original surge cloud
(mainly condensing steam) or could later be removed from the atmosphere by
water-attracting (hygroscopic or deliquescent) salts."

And:

"Only minor water, such as liquid condensed from the steamy surge cloud or water
attracted from the air by the salts itself, is needed to leave the Meridiani crystal cavities"

Is the formation of such large in-ground crystals by trace water (on Earth) a geological
observed fact? Or is it a "fringe" hypothesis? I have no real geological background.

Excellent questions. I'll start with the fine layering. That fine layering and cross-bedding was what most astonished me the first time I was exposed to volcanic surge deposits in Utah, while studying volcanic rocks as possible sources for uranium (under a government-university contract) in the late 1970's. The deposits had been misidentified as "water laid tuff" for about 15 years by a few Geological Survey employees, who couldn't believe it either (they ignored the lava flows on top, which might have suggested volcanism). Of course, they were mineral deposit types and sedimentologists who had never studied volcanoes before, whereas I cheated by having a leading expert on explosive volcanism (former ASU Prof. Mike Sheridan) with me at the time, which unfairly helped the diagnosis. Co-author, Ken Wohletz, a doctoral student at the time, was along and did his ASU Ph.D. thesis on surge deposits, before he was hired at Los Alamos. (BTW, a 1983 paper by Wohletz and Sheridan first suggested that Mars rampart crater ejecta should be examples of impact surge deposits.)

While I'm recounting history, in the mid-1950's Gene Shoemaker (who later became world famous for his studies of impact cratering) started out studying volcanic surge surge deposits in New Mexico and Arizona (looking for uranium too) before returning to school to complete a Ph.D. dissertation on Meteor Crater, AZ. He couldn't believe the giant sandy cross beds (dune forms) he saw were caused by explosive volcanism either, and initially attributed them to wind action. (Even most volcanologists were making similar mistakes in that era.) Since then studies by numerous volcanologists (clued in by nuclear bomb tests, where surges and their cross-bedded deposits were first described) have documented that thin bedding, low-angle cross-bedding, high angle cross-bedding, dune forms, and ripple forms can occur in surge deposits, all features that excellently mimic wind or water deposition. Both air and water are fluids that carry solids. Evidently, a hot, turbulent suspension of divided solids in a rapidly-moving gas cloud behaves as a fluid too. The stickiness that condensing steam gives to particles apparently can help them be deposited more rapidly than normal sediments (truly wet surge deposits can be plastered onto the side of trees). Some terrestrial surge deposits appear so ambiguous that non-volcanologists are still arguing with volcanologists over volcanic vs. wind-caused deposition - and those are the deposits that occur next to a crater (information from Wohletz).

The extremely poor sorting (mixture of large and small pieces), together with the cross-bedding and fine layering, is fairly diagnostic of surge deposits close to the explosion site, but sorting increases with distance, so that a mainly sandy deposit can be hard to diagnose. Bomb sags, caused by ballistic ejecta of rocks by the explosion, have been used to diagnose volcanic surges (e.g., at Home Plate), but ballistic ejecta are also typical of impacts. Disseminated accretionary lapilli resembling the Meridiani "blueberries" (except in composition) are common in some volcanic surge deposits, and would never be expected in a wind- or water-deposited sediment. (Post depositional concretions tend to be much more irregularly sized, shaped, and distributed than accretionary lapilli.)

The thickness of the deposits depends on the size (magnitude) and distance of the explosion - but several meters to several tens of meters are typical of volcanic deposits, where the explosions involved much less energy release than a decent-sized impact releases. For very small volcanic surge deposits (such as those protected from erosion by overlying basalt flows at Peridot Mesa, AZ) you can hike out and follow the gradual transition from several 10's of meters of coarse, cross-bedded ejecta (containing blocks 1 m or more across) to a thin deposit only a few cm thick, consisting of well-sorted fine sand - still finely layered, with shrinkage cracks resembling those at Meridiani.

As for large, in-ground crystals: I invite you to go to an Arizona rock shop and buy a "desert rose". These are fairly large crystals (up to many cm across) that form in damp desert soil from the little moisture available. They are made of gypsum, hydrated calcium sulfate, a salt mineral reported from Meridiani (the least soluble one). Or take some wall-board, which is made of fine gypsum, moisten it in a jar for a few weeks, and watch it "rot" as it recrystallizes (as wall-board in a home will do if made damp for too long). I haven't carried out this experiment: let me know the results if you try it.

We hypothesized that the Meridiani crystal cavities were caused by some type chloride salt crystals that grew right after surge deposition, when things were still warm and very damp - early diagenesis, a sedimentologist might call it. Leaching could have occurred later, by frost.

That's all for now - got to go to a meeting. Hope this answers your questions.

--Don