[X] My Assistant

[dvandorn]

As we all know, Martian meteorite ALH84001 has interesting structures that have now been debated endlessly as to their origins. The more interesting point, however, is that these structures occur within carbonate inclusions in the rock.

Carbonate Martian rocks have generally not been found from orbit by remote sensing equipment. And in ALH84001, the carbonate "nuggets" are rather tiny inclusions.

If there *are* carbonate rocks on Mars, how the heck do we find them? And if they tend to exist merely as tiny inclusions in other rocks, how do we analyze them (or even see that they're there) in situ?

-the other Doug

[dburt]

...If there *are* carbonate rocks on Mars, how the heck do we find them? And if they tend to exist merely as tiny inclusions in other rocks, how do we analyze them (or even see that they're there) in situ?

Not with presently available tools, apparently. If they were at all abundant, they should have been detectable via their distinctive Thermal IR emission spectra. Inasmuch as they seem to be extremely rare at best, other tools will be needed. If they were coarsely crystalline (as via crystallization in liquid water), their crystal form or cleavage should have been visible to the MI on the rovers, or similar tools on future rovers (e.g., MSL). If sufficiently abundant (more than a few per cent), they might be detectable via X-ray diffraction of powders. Chemcam on MSL might be able to spot measure carbonate compositions in, e.g., veins. If they contain the right trace elements, they might fluoresce under ultraviolet or black light (after dark). Or their detection via more sophisticated tools might have to await sample return to Earth.

-- HDP Don

[centsworth_II]

...their crystal form or cleavage should have been visible to the MI on the rovers, or similar tools on future rovers (e.g., MSL)...

Don't forget the AFM and optical microscope on Phoenix. In just a few months we should be seeing
truly microscopic images of mineral grains on Mars. I wonder how the high ice content of the area will
affect any microscopic carbonate crystals that may be present.

Edit:
And from the "optical microscope" link above:
"The Optical Microscope has 4 types of light sources: Red, green, blue and UV Light Emitting Diodes
(3 LEDs of each type). Acquiring images, while either red, green or blue LEDs are switched on, provides
color information of the soil samples and allows for generation of true-color images of these samples.
The CCD detector is not sensitive in the spectral region (λ ~ 350 nm), where the UV LED emits. Sending
UV light onto the target can thus reveal luminescence of that target. The amount of luminescence can then
be assessed by comparison with the one from a UV calibration target that is also available on the SWTS."

I never thought of that, "The CCD detector is not sensitive in the spectral region (λ ~ 350 nm), where the UV LED emits."
So the detector does not see the UV light illuminating the crystal grains, but will see any light emitted from the grains in
response to the UV light. Cool!

This post has been edited by centsworth_II: Nov 27 2007, 09:49 PM

[edstrick]

For some reason, "au natural" CCD's have a sharp response cutoff at short wavelengths and do not respond to UV at all beyond that cutoff. They have to have a fluorescent coating applied in a thin uniform film for extended UV response.

[dburt]

But they are more sensitive to near-IR (just above 700 nm) than they are to visible light. On consumer digicams, near-IR wavelengths are blocked by a filter; on the Rover cams (except for the MI camera) they are not, meaning that unfiltered navcam images are dominantly near IR-images (which would tend to render red hematite, or vegetation, if there were any, white) and pancam images represent whatever filter was placed in front of the CCD (with the most sensitive light response presumably for the near IR filter).

-- HDP Don

[edstrick]

Yep. It's an odd coincindence... CCD's specifically have a long-wave limit roughly the same as infrared film's extreme limit.

Infrared terminology tends to vary from sub-field to subfield, with everybody having different ideas of what short, middle and long-wave IR are. I tend to think of wavelengths from beyond far red to about 1.1 micrometers as Photographic Infrared.

I think Near-IR tends to be a hopelessly confused term at times, including photo-IR and wavelenfths up to 1.8 or 2-point-someting.... say 2.5, 2.8 micrometers <seems to depend on whoever's detectors, optics transmission, etc.)

I call anything beyond 1.1 up to around 5 micrometers as middle IR. You need special detectors, maybe special optics. Solar energy declines with wavelength and thermal emission increases till they cross over somewhere near the 5 micrometer window.

I call 5 to 20 micrometers thermal IR, where everything from room temperature to dry ice emits heat. There's two atmosphere windows, short and long, divided by the 15 micron CO2 band which is opaque on venus, earth and mars.

Everything beyond 20 mm is far infrared out to some ill-defined start of sub-millimeter waves, maybe 100 to 200 microns.. atmosphere's opaque except for short distances or at ultra-extreme altitudes for most of that band.

[marsbug]

Can't say how important this is but it's connected to searching for martian carbonates so I'll post it here. I trust doug to remove it if it owes more to good PR than science!

[dvandorn]

Well.

We seem to have, if not an answer, at least more information. Carbonates have been detected via evolved gas analysis by TEGA and by their distinctive crystalline structures in the only useful AFM image I've seen. They seem to be tiny grains in the soil (at least in the soils at the Phoenix site), and make up (if I heard the quote correctly) something like 6% of the samples analyzed.

So, we now have direct evidence of both the presence of carbonates and of their current structural state in the regolith. I understand the perils of globally generalizing to multiple soils based on what is seen at this admittedly non-representative location... but we're seeing enough carbonates here to at least start asking questions like:

A. How much carbonate rock is ground up into the soils of Mars?

B. For a given range of estimates of (A.) above, how much carbonate rock would have to have been emplaced and subsequently eroded into dust to account for the total mass? (i.e., are we talking about massive deposits from large ocean beds, or small emplacements in scattered lakes and small seas? Or just a few scattered crater lakes here and there?)

C. For a given range of estimates of (B.) above, and with a given range of atmospheric models, how much standing water had to have been available to form the estimated mass of carbonates, and for how long did the standing water have to persist to form that mass?

D. To what degree would a competing sulfur dioxide cycle have modified the known terrestrial example of carbonate formation?

I, for one, would feel comfortable designing a lander (or series of landers) designed to give us enough information to start answering those questions. And I think these are important questions in understanding the climate history of Mars.

-the other Doug

[Julius]

Full inline quote removed - seriously - the quote was 4x the length of the reply!!!! - Admin



In fact that was going to be my question regarding how representative Phoenix soil samples are of Mars?

Also should we expect to find large deposits of calcium carbonate buried somewhere on Mars which we still havent detected?

[marsbug]

B. For a given range of estimates of (A.) above, how much carbonate rock would have to have been emplaced and subsequently eroded into dust to account for the total mass? (i.e., are we talking about massive deposits from large ocean beds, or small emplacements in scattered lakes and small seas? Or just a few scattered crater lakes here and there?)

Well if the six percent figure is typical of martian soils (although I suspect it's not or we would have seen it before?) I imagine there must have been a lot. If the impactor that made heimdall just happened to bullseye, or near bullseye, a carbonate deposit from ancient times would that produce carbonate particles in the soil like phoenix sees? Or would the effects of the impact alter the carbonates?
I wonder if there are any impact craters on earth that are known to have hit carbonate deposits for comparison.

[dvandorn]

We don't see this admixture of carbonates from orbit in this area. That tells me that mixing the carbonates into the regolith at this kind of level (five to six percent) effectively hides it from orbital sensing. We needed something like TEGA and the AFM, working together, to make a positive identification of carbonates in this soil. So, that answers why we don't see it elsewhere -- it can't be seen easily from orbit, and nowhere else on Mars have we landed the kind of instruments that can actually detect this level of finely ground carbonate "flakes" in the soil.

So, indeed, it's possible that all of the soils on Mars contain a similar amount of carbonate material. We simply don't have the ability to detect it with the instruments currently deployed.

On the question of how representative this soil is, and whether or not local impact events have controlled its composition, there are a lot of ways to go with that. For one thing, Mars has for some time been spoken of as having a "ubiquitous dust layer" that is pretty much homogenous in composition and character everywhere on the planet, spread over the top of every landform by the global air circulation. It's very difficult to separate the soil components at any given location that are primarily derived locally, and the components that have been blown in on the wind.

Also, the polar regions build up layer after layer of accreted dust every year; Phoenix is only a few hundred kilometers from places where alternating layers of water ice and dust are being laid down, year after Martian year. We don't have a theory that even begins to address how long these polar soils at the Phoenix site have been in situ (though the lack of smaller impact craters hints that this terrain is being renewed pretty regularly). And if these soils are being actively renewed, how much of the material we see in place right now can have been locally derived?

There are a lot of questions to answer. I think it shows that my original set of four questions has to have been a pretty good starter set, since even setting limits on the estimate ranges seems to be bringing up other really good questions.

-the other Doug

[tty]

I wonder if there are any impact craters on earth that are known to have hit carbonate deposits for comparison.

There is, Chicxulub for example. However it is thought that most of the carbonates involved in that impact were dissociated in CO2 and CaO. The CaO ultimately is converted back into CaCO3. I should think this mechanism would work on Mars too.
As a matter of fact carbonate grains are not that common on Earth. They tend to either dissolve, or to be cemented into limestone

[ngunn]

We are well within the recently revived putative shoreline of the 'Mare Boreale' here. Perhaps the Heimdall meteorite had a rather large carbonate bullseye to aim for.

[dvandorn]

Well, see, that's one of the things I'm talking about. Does the observed amount of carbonate in this soil, if applied globally, require massive ocean beds of limestone as the source of the now-pulverized carbonate remnants? What are the upper and lower limits of globally distributed soil carbonates of the range that absolutely requires such a large source? How many locations do we have to visit and test to determine just how ubiquitous this admixture of carbonates in the soil really is?

Also... I know there are people who are applying the observation of perchlorates in the Phoenix soils to the life detection experiments on both Viking landers. If you're going to speculate that the Phoenix soils are ubiquitous in composition to the extent of the distribution of perchlorates, you sort of have to admit the possibility that carbonates are similarly ubiquitous.

I guess one of the things I'm thinking is that this line of inquiry leads to definition of the type of sensors you need to deploy, and the results you would expect to see from them, so that you can begin to actually establish useful estimate ranges for the total amount of carbonates mixed into the soils.

-the other Doug

[ngunn]

All your questions are excellent oDoug. I was just pointing out that the carbonates need not be evenly distributed globally, but perhaps will be found commonly where impacts have punched through to a regionally widespread post-oceanic evaporite layer beneath the northern plains.

At this point I need a chemistry lesson - not the first time I've asked for one here and I've never been disappointed!

In the absence of shellfish, how do you get from a CO2 saturated ocean to carbonate rocks? Can straightforward evaporation (or freezing and sublimation) do the trick?