Well, Science Marches On!
The Seventh International Conference on Mars, (held in Pasadena, not Mars ) which concluded yesterday, involved a number of presentations directly referring to the "brine splat" hypothesis - not, generally, in a favorable light - Sorry, Prof Don. Perhaps most germane to this discussion is that of J.P. Grotzinger, "Depositional model for the Burns Formation, Meridiani Planum" GrotzingerPDF#3292
Perhaps you should take the paragraphs dealing with your impact hypothesis, and insert your rebuttles, as you did with the Squyres document. Of course some of the points have not changed.

Of course, we could hold the "First International Conference on the Brine Splat Belief" here at our own online Hyde Park... and out do those duners.

--Bill

Helvick - thanks for those velocities. Dburt I'll try to be more specific about my misgivings. You yourself point out that each condensing species requires just the right physical and chemical parameters to form. You also mention that impact surges would be very turbulent. I have difficulty imagining that one of these particles, whatever trajectory it followed, would remain consistently in a suitable environment for superficial haematite accretion for long enough. Helvick's velocities suggest that at least in their later stages they are moving quite fast downward relative to whatever medium they are falling through. Another consequence of the turbulence, and the significant horizontal wind shear I would also expect, would be that particles from many different condensation environments should precipitate out in any given place. That's the haematite monoculture problem that I still don't think you have disposed of satisfactorily. Then there's the point Centsworth raised about any slow-forming spherules trapped (implausibly I feel) in quasi-stasis in a rising convective updraught above the hot crater simply arriving too late on the ground to be incorporated evenly through the the surge deposit.

And is it one or many haematite depositions we are looking at? A couple of us have asked but I don't recall a reply on that point.

You mention spherules associated with terrestrial impacts, which already answers the next question I was going to ask (well done!). I won't try to comment on that till I've looked into it.

You say you're not a physicist - well no-one can be everything - but I wonder if any of your collaborators have really worked through the mechanics (and timing) of the processes - properly, not just in my armchair fashion. You may have all the chemistry and mineralogy in place (or not, I wouldn't know) but the ballet cannot be performed unless the choreography also works.

Nickel: - a quick summary of the issues would be helpful if anyone has time.

nqunn, Dr Burt, I have a question about the nickel enrichment of the spherules. One of the Dec 04 Science articles (R Rieder, R Gellert et al.) summarizes the APXS results from Eagle Crater. Berry Bowl Full was the only spherule target. It had nickel levels about 30% higher than the average of about a dozen rock targets in Eagle Crater and 10% higher than the next highest. The soil targets are a lot more variable in Ni than the rock targets and one Jack Russel (surface) shows Ni about 15% higher than the sole spherule target. If this is the only data it doesn't seem strong enough to hang a lot on, more like a preliminary indication that the spherules might be Ni-enriched. It would be nice to have a few more spherule targets.

Is there any more data on Ni enrichment of spherules from later in the mission?

There are some things that I find problematic with the impact spherule hypothesis

1. Impact spherules while predominantly spherical usually contain a fair proportion of "oddballs", for example dumbbell and teardrop shapes. These are absent in Meridiani.

2. The spherules are all near the extreme maximum size ever seen in terran spherule deposits. On Earth these very large spherules have only been found quite close to impact craters and seem to be rare even there. Also I'm unaware of any deposit on Earth that consists exclusively of such large sperules.

Please forgive the generalism, but this may be a useful context question: How unique is Meridiani in terms of its geology?

I have been visualizing the area as a sort of Yellowstone Park of Mars in terms of its distinction from most of the surrounding terrain (only an analogy, of course, not trying to imply hydrothermal influences here!) IIRC, MGS data revealed very few hematite-rich areas, and none as extensive as those in Meridiani.

What I'm getting at here is that the berries may indeed be a product of unique, site-specific processes. The Gusev lapilli observed do not seem to be comparable in origin or morphology (except in the most gross perspective) to Meridian's berries. Therefore, it may not be useful to consider similar formation mechanisms, save in the most coarse particulars (i.e., presence of water).

This viewpoint weakly supports HDP Don's hypothesis, but any confirmation would, IMHO, require discovery of very similar geology elsewhere on the planet...a real dice-throw when we're talking current UMSF capabilities. We only get precious peeks of ground truth, demonstrably not yet enough to tell a coherent story.

There is a recent paper describing Meridiani as the area of convergence, and emergence, for groundwaters flowing laterally out of raised water tables under the higher ground to the south and west. It's been linked here somewhere - I even printed it out. Unfortunately my filing system is almost as unfathomable as the geological history of Mars.

It had it's own thread, though not many posts surprisingly:

Under Opportunity, 'Meridiani Planum . . .' started by Alex Blackwell.
http://www.unmannedspaceflight.com/index.php?showtopic=3999
http://www.space.com/scienceastronomy/0703..._evaporite.html

I notice the free access to the full paper is no longer available. Pity, it had nice diagrams.

Dburt

From day one the principal shortcoming of the impact surge model IMO was the look of the sediments examined, they just never really looked like impact debris as described for terrestrial impacts. They look just a little too neat and orderly. Granted paper after paper describes a spherule layer associated with Chicxulub impact debris the likeness stops there. It’s difficult to imagine melt globules or tektites going through deceleration and atmospheric compression to have the pristine look of the meridiani spheres. The presence of the spheres within cross bedded units imply formation within an energetic cloud not a gentle distal depositional rain as has been suggested for the surge model.

I guess we were extremely lucky to find distal ejecta layered on top of distal ejecta at meridiani. Odds are we would find proximal ejecta also spread across the plains of meridiani if we are talking about multiple impacts, so where is the diamictite, the coarse ejecta, target rock clasts? Chicxulub impact debris (I’ll ignore Gerta Keller for now) has been well documented and it doesn’t look anything like meridiani sediments. The likelihood of the demolition derby model of impact after impact eventually grinding down any ejecta on mars to a fine-coarse sand is dubious at best.

A comment in a July 15 Space.com article - It's fairly dark rock,'' Jirsa said. "They look like concrete, but in this concrete you would throw pieces of rock of all sizes and shapes and in all possible orientations.'' This was said of a possible discovery of Sudbury meteor impact debris in Minnesota. Now that’s impact debris !!

"the other don"

Shaka - Umm, well, golly, just what did you expect all those HDPs to say? The same question has also been raised with regard to the same abstract 3292, in a general way, in Post 184 from July 10, and answered, likewise in a general way, in my post 194 on the same date (and also in many other posts). As you correctly noted, most of the objections raised by the HDPs in question were already shot down more than a year earlier in our point-by-point rebuttals to their unpublished Nature criticism. Nature declined to publish it after their editor and two external reviewers had evaluated both their claims and our rebuttals. See the attachment to my post 170 of July 9. So if you would like me to evaluate a specific claim, please specify which one, or else I'll just be repeating myself more and more and more...

BTW, I suggest you look closely at Figure 3 of that extended meeting abstract 3292 and compare it with, say, Figure 5 of the following field trip guide:
http://www.gps.caltech.edu/~carltape/perso...136/NavajoB.pdf

You will see that, rather than being an original interpretion based on exposed Burns Cliff geology, it is basically a repeat of what has been the standard story for the Navajo Sandstone for many, many years, applied almost without change to Mars (ignoring the distinctly non-dune aspect of virtually all of the bedding, the 30% sulfate salt content, and most other features, including the unexplained huge gouge taken out of the left side of the single large cross-bed that was alleged to be an old water table). The Navajo and Page Sandstones, type examples for so-called "Stokes surfaces" or old water tables, consist almost entirely of equigranular pure quartz sand (insoluble) and are essentially salt-free. The interdune playas in them were true desert oases with shales containing muddy dinosaur footprints and somewhat palm-like trees called cycadoids growing there too (flowering plants like true palms hadn't evolved yet). I.e., distinctly not salty, with clear geologic indications of standing water. Read the above on-line guidebook for more details. There are no associated signs of flowing water (e.g., no alleged "festoons") at all in the Navajo, as far as I am aware and, as mentioned in several previous posts, the clumped-together to massive-nodular hematitic concretions in it and the overlying Page Sandstone are commonly concentrated at or just below the old water tables - not uniformly in the rock.

BTW, as mentioned in Post 194, I consider it a major advance that HDP Grotzinger has finally admitted that his 2005 "Navajo Sandstone on Mars" hypothesis is actually just a model rather than a discovery, and that he has given our impact ideas some exposure by explicitly attacking them. In science as in life you have to take what little you can get.

--HDP Don

Centsworth - I'm not asking you to imagine anything you don't want to, and you're not missing anything. As mentioned in previous posts, several different surge types conceivably might result from the same large impact - 1) an initial blast surge along the ground, like those imaged related to atom bomb testing in the Nevada desert, 2) a column-collapse type surge cloud containing the berries, analogous to a pyroclastic flow in volcanology (or an Arizona dust storm resulting from thunderhead collapse), 3) local phreatomagmatic type surges resulting from impact melt in the crater reacting explosively with melted ice or other groundwater (analogous to what the MER team has suggested for Home Plate in terms of a vanished volcano). This latter type mechanism is what Wohletz and Sheridan (1983) suggested for the formation of classic "rampart craters" on Mars. By whatever mechanism, the fact is that very similar (except for composition), considerably larger accretionary lapilli have formed in terrestrial impacts, and were more widely distributed in South Africa and Australia than the ones at Meridiani. Also, as mentioned in previous posts, even if you were to dump the hematitic spherules all in the same place next to a crater, it probably wouldn't matter, because later impact surges could scour and scatter them across the entirety of Meridiani Planum, and they would be embedded in the resulting cross-bedded sandy rock. Good question.

--HDP Don

Helvick - So what is the terminal velocity of a golfball- or tennisball-sized hailstone on Earth? They're not very common, but when they do occur, they can be an absolute nightmare for insurance companies, because they can take out the windshields of every ungaraged automobile in a town. And those form in normal thunderstorms, with no dense material (such as hematite nano-flakes of specific gravity of 5.26) contained in the upwelling clouds, and with energies many orders of magnitude less than those implicit in a decent-sized impact.

BTW, I like the way you are taking our argument - that the 5 mm size maximum for the berries might indicate the maximum size that can be supported in an upwelling impact cloud on Mars - and trying to turn it around to bite us. Good debating technique (verbal ju-jitsu). However, keep in mind that the specific gravity of the berries might be considerably less than that of pure hematite (5.26) if they contain other constitutents or were originally accreted loosely, with original porosity. Finally, you might want to share the equations, or at least assumptions and constants, that you are using in your calculations.

--HDP Don

Umm - Goethite as a precursor for hematite was studied by Glotch when Meridiani hematite was thought to result from 300 C metamorphism of a sedimentary iron formation. He was merely trying to find evidence in favor of a now-discarded hypothesis. However laudable that goal, I'd hardly call it "thinking outside the box." Of course, I may be fussy that way.

--HDP Don

ngunn - Good questions. I myself have trouble imagining how golfball-sized hailstones form on Earth, but they do, and for a Mars impact we have many, many orders of magnitude more energy available, with much less gravitational force. For Meridiani spherules, the environment(s) of formation is/are probably not the same as the environment(s) of distribution, and there can be as many periods of surge distribution as there are impacts, if the spherules initially were concentrated on the surface or in easily eroded rocks (mentioned in previous posts). Or several surges are possible per impact, as also mentioned. As to where other types of spherules related to vapor condensation might be, in previous posts I have hypothesized that either 1) they were soluble in late-condensing steam and therefore ephemeral, as most fumarolic condensates related to volcanism are or 2) if insoluble, they are too small to be distinguished from far more abundant sand grains (typical impact spherules, such as the iron condensation spherules that surround Meteor Crater, AZ, are very tiny). Keep in mind that the observational tools of Oppy are incredibly primitive by any terrestrial laboratory standard, however revolutionary they may be in terms of prior Mars exploration.

You can have as many spherule depositional episodes as you want, but one would suffice for Meridiani, I think.

If you want a separate Ni discussion from me - ask for one (although much of it would be a summary of material already posted). My impression is that you'd like one from someone else?

Keep in mind that no matter how defensive or ineffectual I've been in explaining it here, the "impact surge" hypothesis is just one of three out there for Meridiani alone (let alone for Gusev or the rest of Mars). I'm just trying to make this group well-informed consumers of science. You all get to decide whom to believe for yourselves.

And hey, if I could do all the choreography, I wouldn't be sitting here typing this - I'd be a God.

--HDP Don

In early March 2004, about a month after Oppy landed, ASU experienced a severe hail storm around dusk like none it has experienced before or since. I managed to snap these photos before all the ice melted (bricks give scale):

Click to view attachment

Click to view attachment

You might call this a "Eureka moment" regarding the blueberries. Enjoy, and that's all for today.

--HDP Don

P.S. As I write this I'm getting a nice view out of my south-facing 6th floor window of a distant ground-hugging dust storm advancing across the plains south of Phoenix It presumably resulted from column collapse in a thunderstorm in the hills to the east. Such dense, cold air currents commonly knock over construction fences or trailer homes, but are real wimps compared to a volcanic surge, let alone an impact surge.

O'don: That's one of the things that bothers me most about multiple impact scenarios. As for the observed size distributions of various berry populations, all kinds of hypotheses can be erected to match those. I'm not certain any of them lead us to a confident conclusion. It seems that Fe diffusion below a slowly changing water table matches the data as well as other models.

Really, Don, that is the bottom line for me. Ever since Eagle crater I've been looking at this 'rock', and I just cannot believe that it is the result of the incomprehensible violence of impacts.
I stand in awe of impacts. I am convinced that they are the ultimate drivers of the macroevolution of earth's biology, and that all global mass extinctions, as well as many regional catastrophes, are their outcomes.

Where is that violence in the Meridiani evaporites? Where is the chaos? Where is the crushed matter?
You ask what at Meridiani cannot be explained by impact surge. I say "its totality".
Cheers,
Shaka

That's the point, isn't it? A volcanic surge or impact surge is Not the same thing as a hail storm--even though they might have a few similarities.

By the way, I've been through a lot bigger hail storms. Some of them covered the ground completely with 4 to 5 inches of hail. In a couple of instances, it was quite an experience--possibly life threatening if I had not found adequate cover for safety. And I do understand golf ball sized hail. It's the grapefruit sized and larger that are difficult to understand, although somewhat easy to explain.

Thanks again for your reply. I do appreciate that nobody has explanations yet for all the details, but whereas the 'official' model has billions of years to work its tricks your scenario has to do it in, say, half an hour. I think that makes having a convincing model for the choreography vital to the credibility of the case you're making. At the moment you leave a lot of important parameters in soft focus, so it's not surprising that some people express a 'vague sense of unease' in response.

For a while I thought the nickel clue might be a piece of hard evidence one way or another, but I'm beginning to doubt that now. I did read what you said about it in your paper, but it doesn't seem like very strong evidence there. Likewise the rebuttal by Squyers et al., saying that the nickel isn't enriched enough for the iron to be meteoritic simply begs the question of what mix of meteorite and target rock would have gone to form the spherules - another enormous free parameter in your scenario. Yes I was hoping others would chime in but perhaps nobody sees much mileage in it.

CosmicRocker

Fe diffusion in some form makes sense. Why would Grotzinger lead us astray? As I’ve stated before, the proposed surge deposits don’t resemble anything we have here on good ol’ earth. The hallmark or fingerprint of a large impact on earth is chaotic rubble - diamictite beds on a grand scale and shock metamorphism on a micro scale. Obviously the micro-scale isn’t available to us with MER but the large scale evidence should be. Burt and Knauth try to keep the debate to the small scale where Grotzinger et al are vulnerable; raising a red flag is relatively easy at that scale. Look at Chicxulub ejects beds and then meridiani sediments, you won’t see many similarities, yet we should if multiple ejecta beds are ubiquitous at meridiani.

Don’t get me wrong Burt et al have been the greatest thing to happen to the MER team in terms of keeping people on their toes and looking over their shoulder for the latest volley from ASU. The surge model did get some unconstructive recognition at the recent Seventh Int Conf on Mars but I’m sure even that was encouraging for its supporters.

Hmmh you may have found that I made a fairly basic error in my calculations. Seems I worked out terminal velocities for 0.5mm spherules. If so then good catch, if not well it is clearly a good idea to ask for things to be worked out openly. Anyway to answer the questions and rework the hematite spherule calculation.

Some anecdotal web research gives 90mph for 3" diameter hail.

Let's see if that makes sense.

My calculations for this us the following values; Coefficient of drag = 0.4, Density of (terrestrial air) = 1.2kg / m^3 and Density of Hail = 900kg/m^3.
Using the following formulae:
Drag Force : Fd=(1/2)*Cd*rho*A*v^2 , where Cd=Coefficient of Drag, rho=density of the medium (atmosphere), A=cross sectional area of the object and v=velocity
Force due to gravity : Fg= g*M where g = local gravity and M = Mass of the object.

At terminal velocity by definition both of the above must be equal so:

Vt=sqrt ( 2*Mass*g / (Cd*rho*A) ).

This obviously assumes simple fluid dynamics ie relatively slow velocities with no shock waves\subsonic etc. Also the coefficient of drag could vary quite a bit but it's not going to be significantly far from 0.4 for objects that are fairly smooth spheres.

Anyway these terrestrial values yield the following for hailstones:
Tennisball (6.7cm) - 40.5m/sec (91mph)
Golfball (4.27 cm) - 32.3m/sec

For comparison the same items on Mars (12g/m^3 atmospheric density and 3.822m/s^2 gravity) give
Tennisball - 260 m/sec (584mph)
Golfball - 207 m/sec

And so on to my earlier error. Hematite (well something with a density of 3g/cc) spherule. Taking Cd=0.4 because I don't have any better number and using 12g/m^3 for atmospheric density, 3.822 for g.
5mm diameter - 126m/sec (284 mph)
On Earth it would be 20m/sec (45mph)

Apologies again for the errors in the earlier calculations. I don't know whether the change actually skews things in favour of the hypothesis or not. What it certainly would account for is the absence of large concretions - a tennisball sized hematite spherule with a density of 3g/cc has a terminal velocity of about 400m/sec.

That wasn't my intention (Honest!), I think that the hailstone-style formation model in an impact surge column\collapse seems like the easiest way to get the physical attributes the way they are. I'm just trying to see if the numbers that I'm able to hack together make it seem more or less likely. I haven't figured out which way it's leaning yet myself but I've got to admit I'd like to see it be possible - hematite hail has a nice ring to it.

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?

One other interesting thing about the potential terminal velocity of the spherules is that as the size approaches 16mm in diameter the terminal velocity approaches Mach 1 at ground level (~223m/sec). Mach 1 at 50km on mars is around 180m/sec which would correspond to a spherule diameter of 10mm.

I'm not really sure if this is going anywhere but it seems to me that it would be highly unlikely that anything could form smoothly in the transonic regions so that there would be a hard limit somewhere between 5 and 20mm diameter for fairly dense (>3g/cc) accretionary spheres forming in either a vertical plume or laterally travelling surge on mars that approximated the density of the current martian atmosphere. That latter assumption seems unlikely to me though.

I'm wondering how one would go about modelling the dynamics of an impact's plume to try and explore what sort of vertical velocity\pressure\temperature profile would be seen post impact. I can imagine scenarios that would allow accretions to grow to a limit and eventually rain out as they get too big to continue to rise but I've no idea if they can actually happen.

Hydrocode modelling of impact plumes is an ongoing industry in Arizona (Google Scholar the publications of HJ Melosh, E Pierazzo, BA Ivanov, NA Artemieva), but microtektites are mainly seen as the condensation products of vaporized rocks in the distal ejecta of large impacts. I have not seen in the models the scenario of "hailstones" falling out of a horizontal surge cloud as they grow to a certain size. That doesn't say it's impossible, just that it doesn't seem to figure large in the models. The horizontal surges really contribute mostly to the proximal ejecta, forming the chaotic meter-scale clastic deposits such as those that surround Chicxulub in Mexico, Belize, Cuba, Texas etc.

Kye - More data was contained in a July, 2005 Nature paper on soils by Yen et al. - they pointed out (via a graph) that Fe was positively correlated with Ni in the Berry Bowl experiment - suggesting an enrichment in Ni in the berries. That all Meridiani rocks appear to be somewhat enriched in Ni (for unknown reasons) - adding possible support for an Fe,Ni-sulfide target or a Ni-rich impactor - was published by Yen et al. in JGR in 2006.

The most recent Ni discussion, by McLennan et al., is here:
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3231.pdf
This abstract, of course, misquotes our 2005 Nature article as implying that the Fe/Ni ratio in the berries must match that of an Fe,Ni meteorite and seemingly implies that the late Roger Burns (after whom the Burn Formation was named) was a complete fool for inferring that Fe,Ni sulfide deposits should be common on Mars. It compares the Ni/Fe ratio in the spherules with that in the rocks as a whole, clearly comparing apples to oranges, inasmuch as all of the Fe is presumably oxidized (3+) and the Ni is reduced (2+). It should instead have considered the Mg2+ content of the host rocks - that is where the Ni2+ would partition, if the rocks were ever soaked in a brine (really elementary crystal chemistry.)

In this regard, its second page states "At least four distinct groundwater (brine) recharge events...can be documented or inferred..." whereas its last page states "Nevertheless, the general textural integrity of the Burns formation suggests that the amount of fluid that has interacted with these rocks after deposition of diagenetic cements has likely been very small (Fig. 3)." Now if that isn't having your cake (soaking the wind-transported salts in multiple brines) and eating it too (maintaining textural integrity), I don't know what is, as I have brought up in many previous posts. Fig. 3 shows the drastically different solubilities of gypsum vs. some other salts (except jarosite, which was left off). This graph, BTW, provides a strong argument against the extant playa model - if there were a vanished playa with the wind blowing across it, gypsum-only dunes should have been produced, as in ALL terrestrial examples, while the far more soluble salts soaked into the mud or disappeared underground (or at least, stayed too damp for the wind to move).

The assertion that Ni2+ substituted in crystalline hematite (for Fe3+) via addition of a proton (H+) to provide charge balance, seems dubious too, but is technical enough that it probably deserves a separate discussion. (About as probable as a 4-foot tall wanna-be basketball player being allowed to join a championship-bound team of 6-footers because he is standing on the shoulders of a 2-foot tall "little person".)

The bottom line is that I can conceive of no reason why low temperature concretionary hematite, soaked in acid brine in the presence of abundant Mg-phases, should be enriched in Ni, even by adsorption. The Ni2+ should stay with the Mg2+ in the host rocks (as Mg-sulfates, Mg-clays, etc.) which has the same ionic size and charge, and does so in every terrestrial example with which I am familiar.

Incidentally, some adsorption of Ni into hematitic concretions is reported by Beitler et al. (2005) "Fingerprints of fluid flow..." for the Navajo Sandstone here:
http://jsedres.sepmonline.org/cgi/content/abstract/75/4/547
but that was a system without Mg-phases (i.e., the hematite had no competition for Ni) and was possible only because of the alkaline pH (see the calculated adsorption curves in their Fig. 9B). If you are able to look up that article, check out their Fig. 3 for examples of what the spherule and color distribution at Meridiani might look like if fluid flow and brine mixing had indeed been responsible (especially Fig. 3E).

A useful quote (p. 550-551): "Small concretions commonly coalesce to form larger clumped concretions (Fig. 3E, F). Concretionary iron oxides occur in a variety of morphologies including tabular subvertical mineralization filling joints or faults, vertical pipes, subhorizontal planar strata-bound pipes, tubes, sheets, and/or irregular bodies, Liesegang-type banding, and regional zones of organized spherical concretions (Chan et al. 2000, Chan et al., 2004). These range in color from dusky brown to dusky red..." Couldn't have said it better myself.

Speaking of berries, a recent summary of the hematite content of the berries by Joliff et al. (2007) is here:
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3374.pdf
This abstract concludes, after considerably hemming and hawing about coatings, dust, and other factors, that the spherules probably are not pure hematite, but leaves a very wide range of compositions open.

Another spherule abstract by Calvin et al. (2007) is here:
http://www.lpi.usra.edu/meetings/7thmars2007/pdf/3163.pdf
Predictably, it uses the same arguments I use to argue against their being concretions (size and shape, lack of any evident pathway controls, lack of clumping, etc.) to argue that they are concretions. Go figure. The blue-gray color issue is never addressed. At least I was pleased to see that the above two abstracts referred to the berries as "spherules" rather than "concretions". Little by little...

--HDP Don

tty - Thanks for your questions. I may already have answered them in previous posts (especially the postscript to #245), but impact accretionary lapilli are exclusively spherical, as far as I am aware. "Impact spherules" narrowly defined include glass condensates which could take on odd shapes before they congeal. However, even those should not be confused with very irregular to teardrop-shaped tektites - glassy splash droplets of impact melt.

If you think 5 mm is close to the maximum size ever seen in terran spherule deposit, I disagree, unless you are leaving out spherical impact accretionary lapilli, which is what we believe the Meridiani hematite spherules to be (impact spherules, sensu strictu, are direct condensates and are smaller). Also, 5 mm is only the maximum size seen for Meridiani spherules - most are smaller to much smaller.

Hope that helps.

--HDP Don

That paper was not geology as usually understood. It was a hydrological inferrence used solely to justify, completely after the fact, the MER team's "invisible playa" or "lost oasis" or "Navajo Sandstone on Mars" hypothesis for Meridiani. The only unique feature of Meridiani geology as seen from orbit was the huge aerial extent of the specular (blue-gray) hematite signature. Finely layered, probably sulfate-rich sediments occur in many, many other areas around the highlands, as first noted by Malin and Edgett (2000, Science).

--HDP Don

Other Don - Well, got your terrestrial blinders firmly fastened still? This applies to impacts too. This question was first asked by Shaka in his post #56 and answered by me in post #60, as well as in later posts. Two factors you possibly haven't considered: 1) Earth has had solid bedrock for impact targets, owing to plate tectonic processes, shallow marine sedimentology, and so on. On Mars at the end of the Late Heavy Bombardment and prior to most Tharsis volcanism, solid bedrock may have been a commodity in rather short supply. As I said in prior posts (as per Wm. K. Hartmann's "kablooey of dust and steam" quote), beat on Meridiani, and you'll just get more Meridiani. 2) Terrestrial impact studies are highly biased by the immediate removal and alteration of distant fines, such as those we see at Meridiani. They simply aren't available for study on Earth. That doesn't mean they were never there. Volcanic surges tend to be preserved by overlying lava flows or ignimbrites. No such luck for impact surges - you mainly preserve the coarse breccia (suevite) near the crater, or spherules in marine sediments. And I'm getting tired of repeating this - we never claimed that the Meridiani spherules were "melt droplets or tektites" - they appear to be impact accretionary lapilli of unusual composition.

--HDP Don

P.S. (added a day later). Last night I forgot to mention two other reasons, other than possible rarity of bedrock targets and likelihood of erosion/alteration, why impact processes on Mars are probably different from those on Earth (so please remove your terrestrial blinders). The first is that the Martian subsurface is frozen, probably to a depth of several kilometers (the so-called cryosphere), and probably has been since at least the end of the Late Heavy Bombardment (because erosional remnants of very old rampart craters are found, and these are widely accepted as evidence of the martian cryosphere). A cold, brittle, broken impact target, with up to several tens of percent ice (permafrost) cementing it, is probably going to break far more violently as its ice flashes into steam, than any terrestrial bedrock target. Therefore, many more fine particles will be produced. In this regard, so-called rampart craters appear unique to Mars, and are themselves probably preserved only as erosional remants (coarser and/or better cemented, near-crater materials).

The second, as mentioned in previous posts, is that the surface of Mars (unlike that of Earth) has probably always been covered nearly everywhere with a thin to thick veneer of drifting sand and dust (most of it probably impact-derived), because there usually is no liquid water to cement it into a rock (although ice cements it at the poles). An impact surge cloud will scour and transport all that sand and dust and incorporate it into the steam- and salt-cemented surge deposit - which therefore is going to look extremely sandy, compared to most terrestrial surge deposits (volcanic or impact). In an earlier post I already used this feature as an argument for why the sandy surge deposits at Home Plate are probably impact- rather than local volcano-related, despite their single ballistic sag. It may also help explain the sandy nature of the Meridiani deposits (along with simple impact reworking of earlier sandy layered deposits, mentioned in my original post).

Hope that helps remove the blinders. BTW, without mentioning names, some people who study terrestrial and lunar impacts have been EXTREMELY opposed to the idea that the Meridiani deposits could be impact related. Your reaction (and the original one of Shaka) have been comparatively mild. --HDP Don

Apologies, HDP Burt...missed that point in the argument, and I can only concur with that statement.

Your hailstone "eureka" does bring up an interesting aspect of this debate, though. Has anyone seen any evidence for accretion by whatever means in blueberries? Hailstones show layers, but the blueberries that Oppy has bisected within a matrix look pretty uniform. Of course, this may just reflect the behavior of the RAT (plus the fact that we can't blow off the residual dust), but this may also indicate that any accretional layering is very fine in scale--if it's there at all. Finer layers imply more gradual, perhaps even cyclical. formation processes.

Shaka - There are no Meridiani evaporites. There never were, except perhaps in press releases. Each of the first 3 landers (Viking 1 and 2, Pathfinder) detected about 10% sulfates in soil. Duricrust (a moisture-related process resulting from capillarity) was suggested. Oppy detected about 30% sulfates as an improbable mix of highly soluble and nearly insoluble salts (almost no chlorides, which might indicate a true evaporite). To account for this improbable mix the MER team had to back off their initial playa/sabkha claims and hypothesize a totally invisible playa whose wind erosion led to these mixed salts (despite the fact that all terrestrial analogs consist purely of the insoluble salt, gypsum).

I agree with you that impacts are awesome. I don't think that you or most other people here are considering what happens if you impact soft sand, probably with interstitial ice, rather than flinty bedrock. In his 2003 Mars book William K. Hartmann predicted that most energy would be absorbed, and that the only result would be "a kablooey of dust and steam" or what we are calling an impact surge deposit. If you don't think it's "violent" to deposit perhaps several meters of sediment in a few minutes, perhaps you need to expand your definition. Shoot bullets into a sandbank and see what happens. Distant impacts into bedrock are permitted too, of course.

--HDP Don

The ones that have been RATed show no layering, but some of the broken ones do (including at least one imaged in the past few weeks). Our 2005 Nature paper illustrates a broken berry with layering. Of course, layering could be present in either concretions or accretionary lapilli, so we have to look to other indicators for a diagnosis.

--HDP Don

No it doesn't, because then the berries, rather than being randomly distributed in the rock, should be concentrated just below the alleged water table (as they are in the Navajo and Page Sandstones - the putative analogs for Meridiani sedimentology). Diffusion through a stagnant, pore-fluid brine is extremely slow.

--HDP Don

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

ngunn - The "official" model doesn't have billions of years to work its magic - they have to do it while that part of Mars was somehow warm and wet long after all available other evidence indicates that the rest of Mars was cold and dry. Also, you can have as many impact surges as you want, spread out over as may billions of years as you want, so long as Meridiani was the target. Victoria itself probably generated a surge, which Oppy rolled right over without noticing anything amiss (I hope to address this in more detail in a future post).

If you understand, through the medium of the putative brine, likely Ni2+ partioning between good-fit Mg-phases and the misfit Fe3+ phase (hematite), the Ni argument is quite straightforward - there simply shouldn't be any in aqueously-crystallized hematite. You have to know something about about crystal chemistry and partition coefficients though.

--HDP Don

Helvick - Thanks for the clarification. Keep in mind that the impact makes its own atmosphere - it vaporizes everything, even silicate rocks (briefly). Most of the turbulent cloud from which we hypothesize that the blue-gray hematite might have crystallized and been accreted would then have consisted of steam, presumably derived from vaporization of near-surface ice or very deep brine (plus minor contributions from hydrated and hydrous minerals). So the terminal velocity through the much thinner Martian atmosphere would be largely irrelevant. The turbulent could would contain lots of suspended solids too, contributing to its ability to support spherules. We're not particularly happy with that "hematite hailstones" model either (post-depositional oxidation and/or leaching of some other type of accretionary lapilli would be easier conceptually), but it's the only way we can think of to explain the blue-gray color of the spherules (something the concretion model absolutely fails to do - and even forgets to mention).

--HDP Don

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

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.

Click to view attachment

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.

Click to view attachment

Enjoy. No dust storm yet tonight.

--HDP Don

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



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.

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.

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

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

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

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

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.

Click to view attachment

Click to view attachment

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

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!

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...

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

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.

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?

Great question. And I bet Don has an answer.