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January 04, 2016

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Du hast eine sehr informative Seite. Gratulation! Ich habe den Link in meinem Blog eingefügt. Grüße aus dem Gebirge. Ernestus.

Vielen Dank!

Ihre Sammlung und Bildersind sind wunderbar - Vielen Dank für die Blog-Link.

Michael

An intriguing post, Michael, thank you! Astroeremology is something I’ve had an armchair interest in for years.

On a separate-but-related subject (and at the risk of not just sounding like a crank, but actually being one), I’m curious if you’re familiar with the magnetic stripes the Mars Global Surveyor detected back in 1997. They’re similar – in appearance, at least – to the ones detected in the Earth's crust that led to the discovery of sea floor spreading. Evidence that greatly strengthened the theory of plate tectonics. As I’m sure you’re well aware, the notion of any similar activity on Mars, even historically, is a controversial one. And given how long ago the Martian planetary dynamo shut down, the stripes out there probably do tell a different geologic story.

I mean, they might be hotspot artifacts (http://phys.org/news/2010-06-theory-magnetic-stripes-mars.html). Personally, though, I think at least some of them are magnetic paleodunes.

I realize that notion is complicated by a number of unanswered questions – e.g., Could highly magnetized ores be formed by eolian deposition? Even if they could, how might that happen in the absence of an atmospheric water cycle like Earth’s? Etc. Back when I first started nosing around astrogeology, I had an idea about this, based on the following (quite possibly bone-headed) assumptions:

1. Eolian deposition is routine on Mars.
2. Some sand dunes lithify.
3. Not all dunes that lithify need to do so in the context of precipitation.
4. Dunes that lithified in a post-precipitation Martian climate might do so if:

• The obliquity of Mars changed – as it is wont to do occasionally.
• As a result, the surface of the planet changed position relative to sun, exposing its massive CO2 ice fields in the southern polar region to increased temperatures, some of which melted.
• Enough CO2 was released to thicken the atmosphere slightly (creating a situation in which both wind patterns changed and small quantities of groundwater were not immediately sublimed away).
• Wind got shut off to particular dune field.
• Subsurface ice melted slightly, owing to elevated global temperatures.
• Moisture wicked up into the dune (because of the new temperature gradient), where it facilitated diagenesis.

But who knows, really? If they are paleodunes, they may have formed in the context of rain in some remote eon of Martian history. By counting the number of impact craters, measuring their size, and looking for evidence of previously observed erosion patterns, we should be able to “date” paleodune fields to known geochronological benchmarks, at the very least. If someone – or something – would take a closer look, all sorts of interesting, better-informed questions might be asked.

HiRISE may have helped sort this out already. But if so, I haven’t read about it. However we went about acquiring the topological data, whether using space-based remote sensing, or a single-use, LiDAR-enabled glider with an uplink to the MRO, or whatever, if we did find something consistent with dunes like those I’m hypothesizing, I’d love to see us drop a robot down to check them for magnetite, limonite, and other hematitic precipitate cements one would expect to see in a lithification process that involved extremely slow, very low T/P redox reactions – which would probably have reinforced the remnant magnetism in the hematite dust.

And, heck, if those magnetic lineations aren’t dunes, that’s probably even more interesting.

Anyhow, thank you again for the stimulating post, and for providing a forum in which to speculate about these mysteries while they remain unanswered. Have a good weekend!

Wayne - many thanks for this provocative comment and for your kind words (apologies for the delay in responding - I've been travelling).

These are intriguing ideas. Since it seems that conventional wisdom on Martian processes is regularly overturned, and since much of Martian atmospheric and fluid behaviour remains mysterious, your suggestions seem perfectly plausible. There's certainly plenty of magnetite around on Mars and there would be no reason why windblown "placer" deposits shouldn't be concentrated.

I do, however, have questions about scale. The rough dimensions of the Martian magnetic stripes are far greater than normal dunes or dune fields - and how much magnetite would have to be accumulated to create detectably signatures of this magnitude?

Oh, and thanks for "Astroeremology"!

wow beautiful the sand dunes especially the Bagnold dunes. how deep can they be from the ground

Images show the dunes piled up on top of the rcok surface - they typically reach a height of around 5 meters or somewhat more.

And, by the way, those "ripple" patterns are strange - see the latest NASA news release http://www.jpl.nasa.gov/news/news.php?feature=6551

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