[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

[dburt]

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

You raise a lot of excellent points, other Doug, which clearly indicate careful thought. I'll try to take them in order. The fine layering I already addressed in a previous post - it is highly typical of surges and for me was their most surprising feature. Deposits laid down solely on wet ground owing to stickiness probably would not be cross-bedded (as every layer seen to date in Meridiani is). Sedimentts deposited in standing (as opposed to flowing) water would never be cross-bedded and would be likely to be fine mud instead of sand unless they were deposited right at the shore or during a sandstorm. The lack of mud beds (shales) and universality of cross-bedding provide our main basis for stating that Meridiani sediments contain no record of standing water (i.e., no playa is possible, at least not to the depth of exposure/excavation in Victoria or the other craters visited by Oppy).

To make cross-bedding, you need erosion (scouring) followed by deposition. For the wind, this is how sand dunes migrate - they are eroded on one side and the sand is then redeposited on the other side, giving you the giant, high-angle cross beds typical of old dune fields. Dunes (and, for wet and sticky enough particles, antidunes, where the dune grows upwind) are also common in surges. Flowing water making current ripples (so-called "festoons") works the same way, but on a smaller scale. Similar-appearing features can also form in surges, apparently, although they appear to be comparatively rare. Most cross bedding in surge deposits is at very low angles, because the particles are generally moving very fast ("whoosh!"). Low angle cross bedding dominates the Meridiani sediments, consistent with surge deposition, but probably not wind.

As for scouring, inasmuch as every cross bed is a record of scouring, I presume you really meant just to ask about channels. Local channels are highly typical of many areas of surge deposition. They usually are attributed to a swirling vortex (part of the turbulence) moving radially outward from the explosion. Linear, radial channels in volcanic surge deposits (such as those resulting from the 1980 eruption of Mt. St. Helens, WA) have been documented by, e.g, Grant Heiken and Sue Kieffer. By our hypothesis of impact surge deposition, the linear grooves visibly radiating outwards from a great many Mars impact craters (as imaged by both MOC and Themis) would be evidence of just such vortices in an impact surge cloud. Burns Cliff itself appears to contain an example of such a scour - it occurs just beneath what the MER team called the "Wellington Contact" and is near the center of, e.g., this raw image from Sol 287:

http://marsrovers.jpl.nasa.gov/gallery/all...MIP2270L5M1.JPG

Unfortunately Oppy was unable to examine this scour up close, but I saw similar appearing scours while examining various surge deposits in Oregon last summer (described by Grant Heiken and visited by Apollo astronauts, in case some lunar craters turned out to be volcanic). The MER team interpretation is that the "Wellington Contact" is a former water table of regional extent (although it has only been imaged only in Burns Cliff), and the dune-deposited sand was wind-eroded down to this planar water table. How the wind could erode such a localized channel BELOW the water table was never addressed, to my knowledge. (The eroded channel is unlikely to be a stream channel, because it contains no coarse material at the bottom, plus it would be conceptually difficult for water OR wind to erode a stream channel below the water table.) Since they made this suggestion, I have examined several former water tables in exposures of the Navajo and Page Sandstones of Arizona, and I have never seen such a channel feature - the contacts are planar, commonly with shale along them where there was local standing water. That's also where lots of hematitic concretions tend to occur, all clumped together in planar masses. Our impact interpretation would be that the "Wellington Contact" is just a large cross-bed, perhaps marking the contact between two successive surge clouds, with the localized scour marking a turbulent vortex in the younger surge cloud. If so, finding such channels on opposite sides of a given crater exposure might indicate a direction radially towards or away from the parent impact crater (something to look for if Oppy successfully enters Victoria).

Your age question I would answer somewhat speculatively by saying that there has been considerable wind erosion of fines in the past 3.8 billion years. Surge clouds usually are energetic enough to ride up over highlands and settle in lowlands (as notably happened in mountainous areas near Mt. St. Helens in 1980, as today evidenced by the pattern of downed trees). The deposits in highlands tend to be thin and to consist mainly of fines, whereas the lowlands deposits are thick and coarse (more resistant to erosion). In other words, the fluid-like surge clouds tend to "pond" in relatively low areas, despite their energy and high velocity, because they are density currents. The Meridiani lowlands were, in addition, selectively protected from later erosion by their "desert pavement" of dense, unusually large (1-5 mm) hematitic impact-related spherules, which may have been lacking or abraded away in the nearby highlands (pure speculation). BTW, areas of layered deposits in the nearby cratered highlands are reported by Edgett (2005), in his thoroughly-documented article in the first issue of the on-line Mars Journal. As mentioned in a previous post, Edgett also documents that cratering was concurrent with deposition of the main Meridiani sequence (and that some of the lower layers appear to be channeled). With regard to flatness - the wind has done a pretty good job of keeping the terrain flat by erosion (clearly visible at Victoria - look at what happened to the coarse fragments of ejecta); Edgett earlier predicted this from orbital imaging. Also, as mentioned above w.r.t. erosion, although surge clouds can override obstacles (unlike liquids like water or lava), they also tend to fill in lowlands ("pond") and, along with later wind erosion/deposition, could have covered up older or coeval craters.

With regard to comparisons with the Moon: The extremely important difference between Mars and the Moon is that Mars had an atmosphere and lots of suburface brine or ice at the time of late heavy bombardment (and still does today, to a lesser amount). In the dry vacuum of the Moon, impact implies predominantly ballistic processes (like fragments shot from a cannon) no matter how small the particle. The only vapor formed was vaporized impactor and vaporized silicate rock, and these vapors apparently condensed almost instantly into tiny glass spherules. It has been suggested that particle-to particle interactions in a vacuum could have produced a gas-like surge cloud on the Moon, but, to my knowledge, no such finely layered deposit was ever spotted by the Apollo astronauts (who were specifically trained to look for them). Constant "gardening" by micro impacts may have degraded any exposures, however (whereas Mars has enough of an atmosphere to largely protect it from most such "impact gardening" - at the outcrop scale, at least.) On Mars the impact vapors must have contained lots of steam (and vaporized salts), allowing turbulent surge clouds to form that in many ways resembled those formed by smaller explosive volcanic eruptions, where the dominant gas involved is also steam. As each turbulent, scouring, particle-rich surge cloud flowed radially outwards along the ground, it mixed with the atmosphere and decompressed and cooled. Steam (and probably vaporized salts) then condensed onto particle surfaces, making them sticky, and causing them to stick together and agglomerate or acrete into spherules called "accretionary lapilli" (a possible origin for the Meridiani "blueberries" - like a tiny rolled snowball made of sticky particulate rock). In this regard, the so-called rampart craters appear to be unique to Mars, and have always been attributed to either the subsurface volatiles ice or brine or the atmosphere (people still argue - Mars scientists will argue about anything ).

That's rather a long post, but then you asked rather a lot of questions (all great ones, BTW). I agree with you that there are far too many "dead grandmothers" (a.k.a. implausibilities) still associated with the various Meridiani hypotheses (not unusual for Mars). We arrived at the impact hypothesis by the process of elimination - impact seemed to offer fewer implausibilities than either wind/water or volcanism. For example, if Mars wind today can only erode a few meters per billion years, how could it have deposited nearly a kilometer of sediments at Meridiani? And if all those sediments are sulfate-rich, you might have to evaporate a playa lake deeper than Olympus Mons is tall (sulfates aren't all that soluble). I'll stick with the impact hypothesis for now - impact seems like the only process able to do all the work required (until a better idea comes along). Also, as mentioned in an earlier post, we cannot exclude water, wind, or volcanism in any combination as having contributed to the unstudied deeper layers at Meridiani or anywhere else, although we suggest that they might not much be needed.

Impact surge isn't the only plausible depositional process associated with impacts (ignoring the obvious ballistic ejecta, think about all the dust produced! and all the steam! and all the gases other than steam! and all the heat! and all the melt!) - it's just the only one that seems to explain the sandy, salty, cross-bedded, spherule-bearing beds imaged by the first two rovers at both Meridiani and Home Plate, on opposite sides of the planet. It also predicts that similar salty, cross-bedded, spherule-bearing beds could be found at the surface by later rovers (HiRISE imaging, as did earlier MOC imaging, suggests such layers could be common). The most elegant and marvelously detailed Meridiani hypothesis (vanished, highly acid playa lake/wind/soaking acid water/more wind/more soaking acid water/deep flowing - but never standing - concentrated acid brine/magically-diluting-and-mixing water), even if you ignore all of its special assumptions and internal inconsistencies (such as why cold water crystallized the high temperature, blue-gray form of hematite, and why that hematite should be enriched in Ni, and why the soluble salts never recrystallized, and many more I haven't mentioned), at best predicts and explains nothing at all about anywhere else, not even about the almost identical appearing beds at Home Plate.

So when is someone going to give me a hard time about Home Plate? (Although I'm perfectly willing to try to revive more "dead grandmothers" at Meridiani, if I can... )

--Dr. Don