[X] My Assistant

[ngunn]

On the convective updraught point - have you calculated the terminal velocity of these things in Martian air? My gut feeling is that it would be uncomfortably high in this context.

[centsworth_II]

I have a hard time imagining the beads/berries forming high in a mushroom cloud
and then falling back to join the outward rushing ground surge. I would think that
the rapid evolution of the ground surge would be over by the time the more slowly
evolved beads had a chance to reach the ground. What am I missing?

[ngunn]

Centsworth - since nobody else has come back on that point, yes, the relative timescales of the various processes that have to happen to emplace the berries in the Burt scenario is one aspect that is giving me problems when I try to 'run the simulation' in my head. Emotionally I like his ideas. I've always been uneasy about 'follow the water'. I'd prefer the science to be conducted under a directive like 'go and explore the universe and report what you find'. As a physicist I'm happier with bangs and things flying through the air than I am with subterranean chemical wizardry. Nevertheless this one is getting harder to believe the more I think about the details - and that's not good for any hypothesis.

[MarsIsImportant]

Another major problem concerning an impact scenario is the geographic perspective. Is this one surge or many? If it is one, then the crater of origin should be traced. If that were the case, then we wouldn't be having this argument.

Regardless, the distribution of the hematite over Meridiani is a major problem. The signs suggest it is a more localized origin.

[dburt]

Centsworth - since nobody else has come back on that point, yes, the relative timescales of the various processes that have to happen to emplace the berries in the Burt scenario is one aspect that is giving me problems when I try to 'run the simulation' in my head. Emotionally I like his ideas. .... As a physicist I'm happier with bangs and things flying through the air than I am with subterranean chemical wizardry. Nevertheless this one is getting harder to believe the more I think about the details - and that's not good for any hypothesis.

ngunn and centsworth - Thanks for your comments. Unfortunately, I am not a physicist, nor an aerodynamic modeler, so if you find problems in the details of my made-up explanation, improve on them please. My starting point all along has been two facts:

1) Those berries, other than being round, just don't look like any actual concretions I've ever seen, not even the Utah-Arizona "Moqui marbles" that so much as been made of. Their near-perfect sphericity and size limitation makes them look much more like, say, hailstones.

2) My co-author Paul Knauth, when I was studying volcanic surge deposits in the 1970s, was himself studying some of the oldest rocks preserved on Earth (Archean Era), in South Africa. A puzzling feature of these rocks, in some sections, is 100's of square km of several-meters-thick spherule beds, preserved in marine sediments (meaning they have been water reworked, chemically altered, and flattened somewhat by the weight of overlying rock). These were initially thought to be volcanic accretionary lapilli, but their huge aerial extent and details of geochemistry (including Ni-enrichment) eventually convinced him and most other people who study them that they were related to meteorite impacts. Here is the link to the abstract of a 2003 summary article by Don Lowe in Astrobiology (full article accessible via a link), as cited in our 2005 Nature article (which has a photo of the spherules):

http://www.liebertonline.com/doi/abs/10.10...110703321632408

The full article reviews the historical controversies involved.

We're both Mars nuts from our earlier subsurface brine studies, and when the first web images came back from Opportunity in Eagle Crater showing the intricate pattern of cross-beds in sandy layers, I thought "surge" as a possiblity. At the same time, looking at the images on his own computer, Knauth thought "impact spherules" upon seeing the berries. We met for lunch that day and thus the hypothesis of "impact surge" was born - athough we initially called it "brine splat" to account for the salts. Shortly afterwards we invited Ken Wohletz of Los Alamos to join us, inasmuch as he knows far more about the nuts and bolts than we do. Naturally, we were both astounded by the complex "son of a beach" story, described as a scientific discovery, not merely a working hypothesis, that the MER team came up with a few weeks later - their story seemed so unlike the cold, dry Mars we knew, especially for the highlands, and ignored the impact craters all over the place.

Anyway, make up your own details, but the fact is that large terrestrial impacts (including the one that is presumed to have killed the dinosaurs, Chicxulub) produce, via vapor condensation processes, billions of spherules, comparable in sphericity and diameter to the berries at Meridiani, and scatter them at least as widely. Although terrestrial target rocks are much poorer in Fe than any on Mars, meaning the spherules are not particularly Fe-rich, the Fe in the spherules is invariably fully oxidized, and they are characteristically enriched in Ni (plus a whole suite of other elements that cannot be analyzed by the rovers). The Ni is usually concentrated in high temperature ferrite spinels, an end-member of which is the mineral trevorite, NiFe2O4 (Ni is 2+, Fe is 3+). No one yet knows how Ni occurs in the Ni-enriched Meridiani berries (Oppy's instrumentation is too primitive).

The MER team has argued that Oppy's berries can't be impact spherules, because terrestrial examples tend to be concentrated in distinct beds, but this argument ignores the obvious fact of concentration by faster settling velocities through seawater plus later current reworking (analogous the the obvious fact of wind-reworking for the lag concentration of berries on the surface of Meridiani).

So there you go - I really don't know exactly where in the vapor cloud they formed, or how long it took, or the details of how they were distributed, but the inescapable fact is that impact spherules characteristically happen, at least for ancient impacts on Earth (like Mars, a planet with atmosphere and subsurface volatiles).

Hope this admission of ignorance helps.

HDP Don

P.S. Please note that for purposes of simplification I am using "impact spherules" in a very general and inclusive way that not all scientists would agree with (especially those who like to sub-classify everything). I am using it for all varieties of spherules resulting from vapor condensation after an impact (and excluding the splash droplets called tektites). Such condensates include direct condensates such as metals, glasses, minerals presumably including hematite, salts, brines, and liquid water or ice and snow. Some direct condensates, particularly brines, form sticky coatings on surfaces of other particles and tend to stick them together into spherical aggregates called accretionary lapilli. Accretionary lapilli and rare glassy microspherules can be produced by volcanism; the other varieties seem to require impacts (much more vaporization at a larger scale). Most impact spherules (direct condensates) are tiny and on Mars probably would be lost among the sand grains. Accretionary lapilli tend to be the largest impact spherules (or impact-related spherules, for the purist) and, as indicated in previous posts, that is what we hypothesize the hematitic spherules at Meridiani to be. Evidence, in addition to their relatively large size, includes the concentric structure seen in some broken ones, their sticky nature as indicated by occasional doublets and far rarer triplets, and doubts about their hematite content (somewhere in the range of 50-100%). The terrestrial impact spherules referred to above are mainly relatively large accretionary lapilli and other spherules believed to represent altered glass. Accretionary lapilli formed solely by terrestrial particulate matter plus condensing steam would not be expected to display an extraterrestrial trace element signature and, indeed, many don't (which is one reason why their impact origin has been controversial). Sorry for the technical lecture - I tend to oversimplify for this audience, and my co-author Knauth objected when he read the above post.

[dburt]

Geothite with hematite...it looks fairly Martian to me.

http://ocw.mit.edu/ans7870/12/12.108/f04/i...b1/lab1-16.html

...very much like Meridiani, at least in a general sense.

MarsIsImportant - So which is the hematite, and which the goethite? Shiny, but where's the blue-gray color? I agree with you that those mineral surfaces are rounded, like those of the spheroids at Meridiani (and like those of concretions). There the resemblance ends. That is a typical photo of what a mineralogist calls "botryoidal growth" (sometimes called "mamillary growth" for a reason I won't specify) - basically meaning smooth rounded surfaces growing on a big lump. Many minerals, including both hematite and goethite, can grow that way. Concretions, including chert nodules in limestone, commonly grow that way, in nodular masses. Despite millions of spherules imaged, over 3 1/2 years, no such nodular lumps, or even hints of them, have ever been imaged by Oppy - that's only one of my criteria for stating they almost certainly aren't concretions. Helpful of you to provide the image. Thanks.

--HDP Don

[dburt]

Another major problem concerning an impact scenario is the geographic perspective. Is this one surge or many? If it is one, then the crater of origin should be traced. If that were the case, then we wouldn't be having this argument.

Regardless, the distribution of the hematite over Meridiani is a major problem. The signs suggest it is a more localized origin.

MarsIsImportant - Excellent suggestion. Three problems: 1) There are far too many candidate craters to choose from, 2) Oppy is nowhere near mobile enough to map a particular layer across all of Meridiani, to see if it got coarser in any particular direction, towards a particular parent crater. 3) If the parent impact crater or craters were in Meridiani itself, the exercise might be doomed from the start, because you would have only fine material to work with until you got right next to the impact target. Our present suspicion is that the surface of Meridiani Planum is probably covered my many relatively thin surge deposits resulting from late impacts within Meridiani itself. This process has possibly helped to homogenize the distribution of the hematitic spherules.

BTW, when you say "the signs suggest it is a more localized origin" what exactly do you mean? (Sorry to be so dense.) Thanks.

--HDP Don

[helvick]

Ngunn - you asked about the terminal velocity of blueberries on Mars. For the current martian atmosphere (~12g/m^3) it would range from ~30 to ~50m/sec for fairly smooth spheres (Cd=0.5) composed of a material with a density of 5g/cc and diameter ranging for 2 to 6mm.

A surge cloud may be quite a bit denser than the current atmosphere and the atmosphere at the time probably bears little relation to the current one. In any case if you take a could density of 24g/m^3 those terminal velocities drop to ~20-35m/sec and a cloud density of 100g/m^3 yields velocities of ~10-17m/sec.

[MarsIsImportant]

...Solution to many observational problems--HiRISE. Also, Triangulation is fairly easy.

[MarsIsImportant]

MarsIsImportant - So which is the hematite, and which the goethite? Shiny, but where's the blue-gray color? I agree with you that those mineral surfaces are rounded, like those of the spheroids at Meridiani (and like those of concretions). There the resemblance ends. That is a typical photo of what a mineralogist calls "botryoidal growth" (sometimes called "mamillary growth" for a reason I won't specify) - basically meaning smooth rounded surfaces growing on a big lump. Many minerals, including both hematite and goethite, can grow that way. Concretions, including chert nodules in limestone, commonly grow that way, in nodular masses. Despite millions of spherules imaged, over 3 1/2 years, no such nodular lumps, or even hints of them, have ever been imaged by Oppy - that's only one of my criteria for stating they almost certainly aren't concretions. Helpful of you to provide the image. Thanks.

--HDP Don

I know all this already. You don't need to lecture me.

I simply brought this up because the article that you shot down suggested geothite could be a source for the hematite spherules...thinking outside the box.

[Shaka]

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.

[Bill Harris]

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

[ngunn]

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.

[Kye Goodwin]

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?

[tty]

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.