[X] My Assistant

[dburt]

Dburt
If I understand correctly you are proposing that that the spherules are hematite microkrystites that condensed out of the impact plume and then distributed through an extremely thick surge deposit rather than as a boundary layer. This stretches my imagination to a degree and there does not seem to be any evidence of tektites coincident with the microkrystites, or any evidence of splash forms or other melt products. The apparent thickness of the spherule rich layer does not fit a single impact layer scenario and the fact that the hematite rich area of Mars is limited to Meridiani indicates that hematite microkrystites are not a feature of impacts on Mars, or indeed to the best of my knowledge do they have a hematite analogue in Earth impact surge deposits. Or am I missing something?

Aussie - A warm welcome to a new face with new questions. As stated in previous posts, once you've made the high-temperature hematite spherules somewhere in Meridiani, by whatever mechanism, and deposited them however and wherever you like, you can depend on as many later impact episodes as you like to distribute them uniformly, spread out over as long a period of time as you like. Look at all the berries distributed by the Victoria impact. How many tectites or microkrystites do you see related to the Victoria impact, although I have heard no one suggest it was other than an impact (and you only see the heavily wind-eroded impact breccia exposed right at the edge of the crater)? As we have hypothesized from the beginning, clearly there was something unusual about Meridiani to initially form the hematitic spherules (in this we fully agree with the MER team) - we just don't know what. Was it the nature of the impactor (possibly a relatively rare metallic meteorite), the nature of the target (possibly containing a large Fe,Ni sulfide deposit, as proposed for Mars by Roger Burns), the size of the impactor (implying scaling up of small impact-related spherules that would normally be lost amongst the sand grains) or something else? We don't claim to know.

--HDP Don

[dburt]

Well, time to quit again, and here are a couple more photos (now that I've laboriously taught myself how to embed them). These were taken on 3 Jan 2006 in the volcanic explosion surge deposits at Coronado Mesa, about an hour's drive out of Phoenix (part of the Superstition Mts. caldera complex). Knauth used ones like them to illustrate his LPSC talk that March - these were taken by me on the same occasion. The first shows a large cross-bed similar in scale (about a 3 m cliff) to that in Burns Cliff, with flat beds on top. Lots of violence (listening Shaka), but no paleo water table. The composition is pure rhyolite (silicic igneous rock) - you'd have been shredded and roasted in seconds.



The second shows polygonal shrinkage cracks in the surface of a lower-down surge bed - very similar to those so common in Meridiani (and very common in other surge beds). NOT mud cracks, not even close. Pocket knife gives scale.



Enjoy. No dust storm yet tonight.

--HDP Don

[ngunn]

hematite hail has a nice ring to it.

You didn't tell me you had a tin roof.

Keep in mind that the impact makes its own atmosphere

Yes, I am not forgetting this. For very large impacts the pre-existing martian atmosphere would probably have played only a minor role in shaping events. It is at this end of the scale, it seems to me, that you have most chance of producing relatively large haematite hailstones.

My reasons for reposting the link to that hydrology paper were twofold. First, because it seemed relevant to Nprev's geology question. I fully agree that it offers an explanation for the 'wet Meridiani' interpretation rather than for the observations directly. That leads me to the other reason - it is an example of a 'choreography' paper. It sets out a plausible cause, a plausible sequence of events and a plausible timeframe for that version of events. The equivalent for the surge hypothesis would surely have to involve a quantitative dynamic model of the impact and its aftermath.

I am proposing this as a way forward, not as an obstacle. In your position I would seek to proceed as follows:

1/ Take this question and separate it entirely from the Meridiani debate:- "Can a large meteorite impact with a rocky planet result in the production of abundant 5mm haematite accretion lapilli, and if so under what range of initial parameters?"

2/ Find an independent team of dynamic modellers willing to take it on, preferably people with no stake in the Meridiani question.

The outcome would be valuable whatever it was. If for example the answer was "Yes, but you need a planet with an atmosphere as dense as that of Venus and containing free oxygen" that wouldn't help you at Meridiani but it would be an interesting result in its own right and could be chalked up as a significant gain for the impact surge idea. Running a model would also firm up everybody's notions on what to expect in a Martian surge deposit and so help with recognising them in future. The excercise would also be sure to raise the profile of the whole subject of impact surges and maybe attract youngsters to the field.

[MarsIsImportant]

Dr. Burt,

That crossbedding on that cliff you showed us has clear signs of high speed winds. No such signs are indicated within Burn's Cliff on Mars. You might suggest St. Mary's cliff; but, that face has fallen apart. Until we get a much closer look, any such evidence is easily disputed.

An impactor on Mars as big as the one you suggest would create high speed winds--like you said, it would create its own atmosphere. Such a large event should reveal itself fairly easily through topographic evidence. Where is the topographic evidence?

If your model is anywhere near the truth, then Meridiani should not be so unique.

[don]

HDP don - Nice pictures, but more of terrestrial volcanic deposits (where are the impact deposits)? And you say my comments are biased with terrestrial blinders. I don’t think anyone doubts that high angle cross bedding occurs in surge deposits associated with nuclear bomb or rhyolitic volcanic explosions (btw, wrong composition for meridiani). The cross beds in your photo appear poorly sorted and pyroclastic debris is quite evident along flow surfaces (at least with the resolution of my computer screen). Yep, that’s a miocene volcanic surge. Unless I missed something, we don’t see that at meridiani. Other then a cross bed, where is the connection?

I think Grotzinger (2005) calls the polygonal features “recent” dessiciation or dehydration features not contemporaneous with deposition. I tend to think the cracks you show are contemporaneous with deposition.

other don

[dburt]

HDP don - Nice pictures, but more of terrestrial volcanic deposits (where are the impact deposits)? And you say my comments are biased with terrestrial blinders. I don’t think anyone doubts that high angle cross bedding occurs in surge deposits associated with nuclear bomb or rhyolitic volcanic explosions (btw, wrong composition for meridiani). The cross beds in your photo appear poorly sorted and pyroclastic debris is quite evident along flow surfaces (at least with the resolution of my computer screen). Yep, that’s a miocene volcanic surge. Unless I missed something, we don’t see that at meridiani. Other then a cross bed, where is the connection?

I think Grotzinger (2005) calls the polygonal features “recent” dessiciation or dehydration features not contemporaneous with deposition. I tend to think the cracks you show are contemporaneous with deposition.

Other Don - Did I ever claim that those weren't volcanic deposits? See my post #278 for a statement on where the terrestrial impact deposits went, and earlier posts for why we have to use volcanic deposits as analogs. And how is the exact rock composition relevant to the internal morphology and structure of a violent explosion deposit? For the coarser pyroclastic debris in the photo, substitute "surge-transported blueberries" if it makes you feel better about what the photo is telling us.

The connection with Burns Cliff is that a purely impact or other surge process can produce a large eolian-appearing cross-bed cut off on the top by flat beds, such as we see exposed on Burns Cliff, without requiring an old water table (a.k.a. "Stokes surface" or "supersurface") for which there is absolutely no other evidence. I repeat, no evidence whatsoever (no shales, no mud cracks, no berry concentrations, no salt concentrations, nothing). Also, the Burns Cliff exposure has an unexplained and unaddressed channel-like gouge taken out of the left side of it, which is perfectly well explained by the surge hypothesis (as a vortex, such as those discussed by Sue Kieffer and several others for surge deposits) but is highly unlikely in a water-table controlled planar "supersurface". Grotzinger et al. (2005, EPSL, p. 48, Fig. 6a) refer to this channel in a figure caption as "scour and infill" as though that were perfectly explainable and expectable in terms of wind erosion and and an erosion-controlling water table. I'll give you a hint - it's not - not at all. Can you explain that scour in terms of their model? They appear to have hypothesized a "supersurface" ("Wellington contact") of regional extent on the basis of a single large cross-bed with a large channel-like scour taken out of the top of it - which scour, on the face of it, makes their hypothesis untenable. Do you disagree? If so, why?

If HDP Grotzinger inferred that the shrinkage cracks at Meridiani were recent and you inferred that those at Coronado Mesa were not, how is this relevant to their external morphology, which is the data? One of the biggest mistakes you can make in science, as I have emphasized repeatedly in this thread, and probably the biggest barrier to new discovery, is to confuse the actual data (observations) with what someone or other has inferred from that that data. Learn to make your own inferences if you want to make discoveries. The true beauty of the way the MER missions have been run (and I cannot praise NASA, Steve Squyres, Jim Bell, and the rest of the MER team enough for this) is that now we can all do that with photos and other data on the web.

BTW, I'm about to add a postscript to my post #278 on why terrestrial (and lunar) impact craters are probably a poor guide to Mars impact craters - so refer back to that if you're curious. In my initial reply last night I completely forgot to mention the unique subsurface cryosphere of Mars and the abundance of easily-scoured sand and dust on the surface.

I enjoy learning from you.

--HDP Don

[dburt]

Don, do you have a link to that image, please? Very interested...

nprev - Here's the image my always rose-colored memory recalled seeing recently:
http://qt.exploratorium.edu/mars/opportuni...R9P2956M2M1.JPG

However, on looking at the bottom center berry again, it's certainly not very convincing (the apparent darker zone is off center, and doesn't carry across the edge, plus a similar discoloration is seen on other, unbroken berries, presumably owing to their having been pressed or scraped). MI images of some of the broken ones from earlier in the mission are far more convincing in terms of zoning (including the photos reproduced in our 2005 Nature paper - Fig. 5d from sol 28 and Fig. 5e from sol 142). In any case, as mentioned in my original post, zoning is not diagnostic of either accretionary lapilli or concretions - they both can display it.

Too bad Oppy's MI has not been able to "stop and smell the spherules" more as it has circumnavigated Victoria Crater. We might have learned something about their response to impacts. At first glance, they certainly look no different for having been shocked by impact and excavated from the crater. As mentioned in previous posts, this presumably reflects the friability (poor cementation) of the Meridiani rocks. The Victoria cratering event probably was somewhat analogous, at a far larger scale, to shooting a bullet into a pile of sand. That is, most of the energy was absorbed by internal heat and vapor generation rather than being transmitted as shock waves - very unlike impacts into hard, solid bedrock.

--HDP Don

[dburt]

....
My reasons for reposting the link to that hydrology paper were twofold. First, because it seemed relevant to Nprev's geology question. I fully agree that it offers an explanation for the 'wet Meridiani' interpretation rather than for the observations directly. That leads me to the other reason - it is an example of a 'choreography' paper. It sets out a plausible cause, a plausible sequence of events and a plausible timeframe for that version of events. The equivalent for the surge hypothesis would surely have to involve a quantitative dynamic model of the impact and its aftermath.
....
1/ Take this question and separate it entirely from the Meridiani debate:- "Can a large meteorite impact with a rocky planet result in the production of abundant 5mm haematite accretion lapilli, and if so under what range of initial parameters?"

2/ Find an independent team of dynamic modellers willing to take it on, preferably people with no stake in the Meridiani question.
...

ngunn - Thanks for your reply. I regard that hydrology hypothesis as merely an extension of the original hypothesis (artesian spring = desert oasis). Without the original hypothesis, it has no reason to exist, and there is no independent evidence in favor of it. If you're sufficiently clever, and the authors certainly are, you can model practically anything plausibly if someone else has specified the desired result in advance. A recent summary of that hydrology hypothesis is provided in this abstract from last week's Mars meeting:
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3173.pdf

Note that on the third page, the abstract recognizes that permanent high groundwater acidity is highly implausible (something the original MER team hypothesis failed to recognize), then accounts for temporary high groundwater acidity at Meridiani by misquoting MIT's late Roger Burns (who as a brilliant geochemist never proposed what they claim he did). The authors state: "Following the model of Burns [16-17], the mildly oxidizing fluids would have reacted with pyrrhotite in the basaltic aquifers, producing substantial acidity and liberating dissolved ferrous sulfate." This way of producing the putative acid groundwaters at Meridiani sounds plausible only if you know nothing about the relation of the water table to sulfide oxidation in mineral deposits. The reasons it sounds highly implausible to me are 1) after travelling underground through hundreds of km of finely divided, FeO-rich basalt, the groundwaters would be highly reducing, not oxidizing, and 2) sulfide oxidation in mineral deposits can only happen ABOVE the water table, in the so-called zone of aeration (a.k.a. vadose zone). In fact, new sulfides are deposited just below the water table (so-called supergene enrichment, which typically produces the highest grade ores). Roger Burns knew all this and his gossan model for Mars therefore depended on sulfides being oxidized only ABOVE the water table.

Inasmuch as the MER team hypothesis has a rising water table being continuously at or above the ground surface, subsurface sulfide oxidation to produce the alleged groundwater acidity seems impossible on the face of it. An obvious internal contradiction. (Our impact model proposed post-depositional jarosite formation via Burns-style oxidation of finely divided sulfides above the water table, although it does not depend on it - jarosite could alternatively be formed by reaction with acid vapors in the steamy surge cloud.) The bottom line to me (temporarily wearing my geochemist hat) is that the second hypothesis seems no more plausible than the one on which it depends, and likewise contains several internal contradictions, from a geochemical if not hydrological perspective.

Regarding your numbered suggestions, we regard the answer to the first question as "yes" solely because we can see no other even mildly plausible way of making the hematitic spherules and accounting for all of their properties. If you can suggest a better way, you can be first author of the resulting paper (and can then have the huge warm, wet Mars crowd abuse you instead of me ). Our current starting parameters are only 1) that Mars seems to be an exceptionally Fe-rich planet (its basalts, sampled and driven into space by impacts and then landed on Earth, are 2X to 3X as Fe-rich as terrestrial basalts), 2) that it seems to be an exceptionally salty planet, possibly owing to global loss and/or freezing of its originally huge hydrosphere (salty oceans) and 3) salty, steamy vapors produce blue-gray hematite flakes as a very common insoluble mineral in terrestrial fumaroles, even in extremely Fe-poor volcanic systems. Steamy salty surge clouds on Mars should be analogous, and should produce blue-gray hematite too.

We regard the second goal as highly laudable, but currently unattainable. Any dynamic modellers interested in Mars currently "have a stake in" Meridiani, one way or another.

Thanks for your comments and recommendations.

--HDP Don

[dburt]

Well, that seems to about cover Mars impacts for tonight. Attached find a couple of photos of fumarolic blue-gray (specular) hematite flakes that I took several years ago for a different purpose (no scale available, but the field of view is about 1 cm). The locality, studied in 1983, is Tepetates, San Luis Potosi, Mexico, an extremely Fe-poor volcanic system (a group of large rhyolite domes), yet hematite flakes like these are still very common in the fumarolic deposits. The larger pinkish crystals that the blue-gray hematite flakes are growing on is the fluorosilicate topaz and it's presence indicates that the steamy, salty, acidic fumarolic vapors were rich in fluorine as well as chlorine and iron.





These photos possibly demonstrate that a giant steamy, salty impact system on iron-rich Mars might well have precipitated many trillions of nano-scale hematite flakes in a turbulent, condensing, dark vaporous cloud, that these in turn could have accreted into billions of tiny hematitic spherules that eventually ended up in the rocks and on the surface at Meridiani. Sure sounds like Burt's Believe It or Not, doesn't it? Oh well, if you can come up with a better explanation for those blue-gray spherules, my hat's sure off to you (concretions just won't cut it, I'm afraid, for all the reasons I've already enumerated at length).

--HDP Don

[ngunn]

I'll be here less often for a bit - it's time to appreciate landscapes closer to home. Thanks to all, most recently to dburt for that long patient nickel explanation. (Yes I do understand it a bit better now but not really well enough to comment.)

I hope you find an objective, hard-science quantitative test of your ideas somewhere, even if it's not the one I suggested. I think that's what it would take to make people less dismissive. Purely descriptive arguments (however good) just won't do the trick, I fear, once positions are this polarised.

Of course there's always the possibility that the other party will construct a numerical model to try to prove the hailstones are impossible - and end up knee-deep in virtual hailstones!

[nprev]

Thanks for the link, esteemed HDP!

Gotta say, though, I really don't see any concentric features on them berries. What does strike me is that many of the 'early' berries back near Eagle that were embedded in matrix or exhumed via wheel trenching had odd, bumpy surfaces, seams, etc. while this particular set (in fact, the majority of those examined at other locales) seem much more homogeneous in terms of surface texture.

I suppose that weathering of exposed berries is the most likely explanation, but the dichotomy is a bit puzzling and perhaps perceptually misleading. The smooth ones do look a lot like condensates that would be congruent with your general hypothesis within your scenario(s), but what of the bumpy berries? Fine features like that would not be expected (in my opinion, at least) to occur on rapidly forming gaseous or liquid condensates due to the flow and consequent erosive action of the surrounding medium.

Okay, here's my alternative hypothesis for berry formation: [EDIT: several hours pass as I stare at my screen...] Okay. There are two types of blueberries. Type I berries are hematite concretions with distinctive, odd surface textures formed by repeated H2O saturation as evidenced by the extensive sedimentary deposits in Meridiani. Type II berries are artifacts of many, many meteoritic impacts in the region during wet periods characterized by smooth surfaces due to their relatively rapid formation and cooling. Both types are chemically similar due to the fact that they both precipitated out of the same matrix and very similar aqueous solutions; the significant variable that produces morphological differences is duration of favorable conditions for formation.

Send me my Nobel Prize or a dunce cap now, whichever is most appropriate... Also, just for fun, here's a pic of the biggest Type I berry-analog in the whole Solar System...

[Bill Harris]

...many trillions of nano-scale hematite flakes in a turbulent, condensing, dark vaporous cloud, that these in turn could have accreted into billions of tiny hematitic spherules...

About as likely as frogs spontaneously generating from mud. Interesting belief, but where is your proof?

From the many MOC and MRO images taken of the Meridiani Plains where is the indication of the brine splat/base surge?

--Bill

[Aussie]

Dburt,
You make the case that Mars basalt is Fe rich, the planet is exceptionally salty and salty, steamy vapours produce blue-grey hematite flakes as a very common insoluble mineral in terrestrial fumaroles. So when an impact occurs steamy salty surge clouds on Mars should be analogous, and should produce blue-grey hematite nano-scale hematite flakes that would condense and accrete into the spherules. But this scenario requires almost instantaneous oxidisation of the basalt Fe content. Michelle Minitti et al found that it took 3 days at 700 C to oxidise a 0.1 to 0.6 um hematite coating on a mars meteor composition in a CO2 environment. http://minitti.asu.edu/publications/abstracts/hem_ab.pdf
I appreciate that you have argued that the surge cloud will create its own atmosphere, and vapourisation as well as melt is involved, but I still have reservations over the speed at which this process must occur. I find the scenario hard to accept in the absence of any rigorous modelling or terrestrial analogue, and in the light of Minitti’s results. Not impossible, but the sequence of events necessary to form hematite spherules seems far more complex and problematic than the formation of terrestrial impact microkristites.

Also, if this combination of impact energy and martian basalt can produce the spherules, why has this process not occurred in a number of impacts rather than being isolated to a few comparatively small regions? And the regions where grey hematite has been identified (Aram Chaos, and Ophir and Candor Chasma in Valles Marineris ) all have indicators for aqueous activity in the distant past. This seems to point to an aqueous rather than impact cause.

I understand that Geothite can transition to hematite in temperatures as low as 70 C in saturated water vapour given that in an aqueous system, crystal growth effects lower the transition temperature from that required in the dry state. (Catling and Moore Icarus 165 (2003) 277–300). So there is potential that low levels of hydrothermal energy could have created the appropriate conditions for hematite conversion from a goethite spherule precursor in Mars’ early life. A scenario possibly as tenable as impact accretion.

[centsworth_II]

Also, if this combination of impact energy and martian basalt can produce the spherules, why has this process
not occurred in a number of impacts rather than being isolated to a few comparatively small regions?

In connection with this question, my impression is that the hematite signature of the Meridiani region forms a confined shape with a fairly defined edge. I would expect that if the hematite was formed by large impacts that it's signature would be more widespread, with less shape and definition. Each large, hematite producing impact would shoot a surge out radially. In my mind, I have to imagine the hematite from a series of impacts being shot toward what would become the Meridiani region and not shot outward in other directions. From space, it looks like the hematite of Meridiani collected there rather than shot there by impact. Why was hematite not shot in directions away from Meridiani as well?

[ElkGroveDan]

Why was hematite not shot in directions away from Meridiani as well?

Great question. And I bet Don has an answer.