Welcome Professor "brine splat" Burt, "a chance to ask questions... or raise objections" Options

[Aussie]

The authors do however consider that these rocks are volcanic from a local magma source.

Driebus et al indicated that the composition of Algonquin and Comanche resembled picrite which would seem to make them intrusive candidates? Bit on the Ni conundrum there too.
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1649.pdf

[don]

Don Burt

One of the more thought provoking lines of conjecture for impact surge that you and P.K. mention IMO is the necessity for mechanical mixing of ancestral pre-meridiani sediments to arrive at the measured salt concentrations (as described in your 2004 GSA abstract). The beauty of the idea is its open endedness basically many observations at meridiani can be explained by seemingly predictable acts of fluid flow yet arbitrary, involving eutectic brines and assorted weathering processes - getting by the need for tight solubility calcs that JPL requires. My question, has this idea held up to the post 2004 information coming out of Meridiani (and Gusev)? I’ve included a few lines from that abstract for others:

“In this scenario, the chloride, sulfate, and bromide salts found by Opportunity were deposited mechanically along with the mainly basaltic materials, ice, and brine. Following emplacement, interactions of the fresh ejecta with the martian atmosphere over long times would allow the hygroscopic eutectic chloride and bromide salts to deliquesce and flow downwards, or be washed into the deeper regolith during ice melting to account for the vugs. Differential solubility can preferentially remove chlorides and bromides and leave sulfates. Features at the Opportunity site thus do provide additional evidence of an early hydrosphere on Mars, but it had already disappeared into the megaregolith when impacts produced this remarkable deposit.

"The other Don"

[dburt]

The authors do however consider that these rocks are volcanic from a local magma source.

Driebus et al indicated that the composition of Algonquin and Comanche resembled picrite which would seem to make them intrusive candidates? Bit on the Ni conundrum there too.
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1649.pdf

Aussie - As stated in a previous post, I have no problem with part or all of the floor of Gusev having once been covered with lava, from whatever source (given the lack of any direct indication of the possible location of a volcano or vent, the ambigous phrase "local source" must mean "down below somewhere"). Impacts could have excavated these and/or other rocks to form the Columbia Hills, and impact fines could have formed Home Plate. Impact melts or phreatomagmatic volcanism (explosive interactions with groundwater) might have been involved too - no way to tell at present (although there are multiple indicators of impact cratering and absolutely no indications of volcanic explosion craters such as maars or tuff rings).

The LPSC abstract on Algonquin Class rocks compares them to picrites or komatiites - both of them olivine-rich volcanic rocks officially (although picrite is ambiguous - the term has historically also been used for olivine-rich shallow intrusive rocks). The lavas of Mars have long been assumed to be olivine-rich, based on the high abundance of olivine interpreted from orbital spectra. The high Ni content (actually, high Ni/Mg ratio) is discussed, but not really explained, except to hypothesize vaguely that the martian mantle might be locally heterogeneous in regard to Ni, owing to bulk gravitational separation of olivine as cumulates. They neglect to mention that this process is difficult conceptually when the mantle itself consists mainly of olivine (how do you separate olivine from olivine?). They do mention that Ni enrichment in soils was earlier explained by a meteoritic component. The possibility of Ni-enriched impact melts is not discussed.

Thanks for your observations.

-- HDP Don

[dburt]

Don Burt

One of the more thought provoking lines of conjecture for impact surge that you and P.K. mention IMO is the necessity for mechanical mixing of ancestral pre-meridiani sediments to arrive at the measured salt concentrations (as described in your 2004 GSA abstract). The beauty of the idea is its open endedness basically many observations at meridiani can be explained by seemingly predictable acts of fluid flow yet arbitrary, involving eutectic brines and assorted weathering processes - getting by the need for tight solubility calcs that JPL requires. My question, has this idea held up to the post 2004 information coming out of Meridiani (and Gusev)? ...

"The other Don"

OD - Good question. When that abstract was written, Meridiani was being presented as something resesembling an evaporated sea or lake. The intimate mixture of highly soluble Mg-sulfates (and super-soluble chlorides and especially bromides) with nearly insoluble Ca-sulfates (as opposed to a regular bulls-eye pattern, with the more soluble salts concentrated in the center of the basin) almost demanded some sort of mechanical mixing. For us, impacts could have provided that mixing. A year after we had pointed this out in meeting abstracts, the official story was modified so that the salts were presumed to have been mechanically mixed by wind transport (even though terrestrial examples of such mixing were lacking - in rare terrestrial examples only the nearly insoluble Ca-sulfate gypsum is wind deposited). These granular salts were then presumed to have been soaked multiple times in flowing acid brines (of very special composition - simultaneously saturated with respect to every salt present, including jarosite) without in the least recrystallizing the salts, reducing the permeability, or preferentially leaching the more soluble Mg-sulfates (although some phase had to be leached to produce the cavities). These brines flowing through salts are also supposed to have uniformly deposited uniformly-shaped and sized tiny spherules of the high temperature (specular or blue-gray) form of hematite over an area the size of the state of Oklahoma, although hematite is also abundant in the matrix. (Accretionary lapilli or other spherules related to the impact or impacts was our hypothesis for the spherules, inasmuch as it satisfactorily accounted for their nature and broad distribution.)

Little new data on salts has yet been presented for Victoria Crater, and no comparisons have yet been made with the Endurance analyses, so there has been no reason to modify our original "brine splat" impact idea, allowing for post-impact weathering and diagenesis (including frost leaching of chlorides downwards and wicking of sulfates upwards, analogous to what occurs on desert mine dumps). We thought the argument might be over when nearly identically cross-bedded, salty, spherule-bearing rocks were discovered in the Home Plate area of Gusev, but we were wrong.

-- HDP Don

Edit: BTW, we had earlier (2002 and 2003) mentioned the possibility of impact mixing of salts in papers discussing the nature of martian subsurface brines.

[Doc]

Don Burt

For the sake of knowledge, I would like to know what's ur response on the nature of the festoon marks observed in Erebus class rocks in Meridiani planum;

The impact surge hypothesis claims the festoon marks to be product of aeolian erosion instead of flowing water in the past as concluded by the Athena science team.

On sol 632 and 633, Opportunity used its MImager to image the rock 'Kalavrita'(before and after grinding into it respectively). The rock had festoon marks on it and these marks persisted even after the rover ground into the rock. This shows undeniably that these marks are indeed due to flowing water and are not a product of erosion.

What does this mean to the validity of the Impact Surge hypothesis at Meridiani Planum?
(I hope I am not repeating a discussed issue).

[Kye Goodwin]

Doc, regarding your post 335, and Dr Burt, please help me to understand the rover team model. I think that this is their scenario: Most of the layering in the bright surface layers (the top layers of bedrock at Endurance and Victoria and the surface rock along Oppy's traverse) indicates aeolian deposition, essentially a sand sheet. In a few places festoon cross-bedding indicates that moving surface water has rearranged the aeolian sediments. My question is this: Why aren't the two kinds of layering more obviously different? I am pretty sure that the water flow is not thought to have distorted pre-existing aeolian layering but rather would have eroded away the aeolian layers grain by grain, transported the sand some short distance and then deposited new layers with festoons indicating flow ripples. What I have trouble understanding is why the water deposited sediments have co-incidentally ended up looking so much like the aeolian sediments. The water-mobilized sediments might not have been layered at all or the layers might by clearly thicker or thinner or the grain textures might be clearly different, but the only difference seems to be a slight difference in curvature despite the two kinds of layering having been created supposedly by two completely different processes. Why have we not seen a contact between the two kinds of layering?

I attribute the festoons, if real, to minor variations in the impact-surge process. This explains why all the layering in the bright band shares many qualities and is seamlessly integrated together.

[dburt]

For the sake of knowledge, I would like to know what's ur response on the nature of the festoon marks observed in Erebus class rocks in Meridiani planum;

The impact surge hypothesis claims the festoon marks to be product of aeolian erosion instead of flowing water in the past as concluded by the Athena science team.

On sol 632 and 633, Opportunity used its MImager to image the rock 'Kalavrita'(before and after grinding into it respectively). The rock had festoon marks on it and these marks persisted even after the rover ground into the rock. This shows undeniably that these marks are indeed due to flowing water and are not a product of erosion.

What does this mean to the validity of the Impact Surge hypothesis at Meridiani Planum?
(I hope I am not repeating a discussed issue).

Doc - This HAS already been discussed extrensively above, and Kye just now brought up some relevant points. Also, your statement about "the festoon marks" being the product of eolian erosion is incomplete (see below).

You will not find the slang term "festoon" in many textbooks. The standard term is trough cross-stratification or small-scale trough cross-lamination (for smaller varieties). The "festoon" structure can only be seen in a cross section exposure nearly exactly perpendicular to the direction of flow, viewed looking down (or up) the direction of flow, and not in oblique sections such as you described. Except in vertical cliffs such as parts of the rim of Victoria, oblique sections are the rule in martian outcrops. Most, if not all, of what have been mistakenly called festoons on Mars are simple topographic artifacts of the oblique viewing angle caused by the elevation of the Pancam. As I stated near the beginning of this thread, picture yourself standing behind someone else, with both of you standing in water that comes up to below your waists. The horizontal water line on the other person will have the double trough shape "UU" as you look down, even though it is horizontal (I'm avoiding referring to human anatomy). You would need to have your eyes exactly at water level in order NOT to see this trough effect. The Pancam cannot bend down, and so it always sees this effect in near-horizontal beds, unless it is looking straight at a tall enough vertical cliff. The MI can bend down, but it also will always see this effect unless the surface it is looking at is perfectly vertical, and it is oriented perpendicular to that surface (its depth of field is so low that it can only view perpendicular to any surface; virtually all surfaces it has ever imaged were non-vertical). Even if a cliff is vertical, you still have to be looking exactly down or up the direction of flow to see the structure - random directions won't do.

If, despite this intrinsic problem with rover imaging of outcrops, you believe that some Mars "festoons" are real, consider the following: Although the migrating ripples that form "festoons" are widely believed to indicate flowing water, experts who have spent their professional life looking at volcanic surge deposits have found and photographed essentially identical features in those beds. After we showed some of these photos at LPSC, a non-volcanologist, who had never visited the localities in question, stated that these terrestrial deposits must be water-flow deposits, despite their occurrence in the midst of a pyroclastic flow deposit. If you choose to ignore the volcanologists (not me, but some of my co-authors) and want to believe this person about flowing water, consider this. Wet impact surge, and formation of accretionary lapilli (a.k.a. blueberries or spherules), implies condensation of steam. Condense a lot of steam and the result can be flowing water, either from rain (or snowmelt) or as excess moisture squeezed out very wet sediments. So there is no incompatibility between water flow and wet impacts (water is implicit in our initial "brine splat" term anyway).

That said, I would want some independent evidence of water flow across the horizontal surface at Meridiani, besides the enigmatic (and quite possibly non-existent) festoons. There are too many good arguments against water flow. For example, apparent festoons inexplicably occur in the midst of beds that have been called eolian, with no discontinuities (as pointed out by Kye). Everywhere else on Mars, channels are taken as evidence of water flow. So show me some water channels (keeping in mind that other phenomena, such as glaciers, submarine density currents, and vortices in expanding, decompressing surge clouds, can also scour channels).

Ultimately, you'll have to decide what the evidence supports. Mars, like the Moon and Mercury (and like Earth before the evidence was erased by plate tectonics), apparently was impact craterered to saturation about 3.8 billion years ago. That was THE major event in Mars history, as it was for the Moon and Mercury. That's when Mars lost 99% of its atmosphere and when clays apparently ceased forming by weathering. Since then, except for the wind, localized dying volcanism, local brine breakouts, ice redistribution and movement, and continued impact cratering, Mars has been a pretty cold and desolate place (life among the ruins, as it were). If you choose to ignore the evidence of impact cratering, then you may choose to believe the extremely complex Earth-like scenario that has been derived for Meridiani, one that is not applicable to similar beds elsewhere on Mars (and one that is not well supported by available evidence, IMHO). The impact explanation, whatever its loose ends (and it has a few), accounts for every observation made to date (by both rovers), is very much simpler, and potentially is much more generally applicable to the rest of Mars.

Apologies for getting so technical.

-- HDP Don

[djellison]

Time to close the playground again. It's a subject that has been done to death, into the undead, and now to death again. Everyone has had more than a fair chance to air the opinion, interpretation and respond appropriately. It's not being closed because of the subject, it's being closed because it's going in circles - tens of thousands of words of circles. Posts covering this ground elsewhere will simply get culled, we're done here. Thanks to all who've contributed from all sides - but I don't think anyone could say we're getting anywhere.

Doug