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Venusian Channel Formation As A Subsurface Process
Guest_AlexBlackwell_*
post Jan 27 2006, 04:38 PM
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There's a new and interesting preprint at JGR-Planets in Press:

Lang, Nicholas P.; Hansen, Vicki L. — January 2006
Venusian channel formation as a subsurface process
(2005JE002629)
PDF [8.7 Mb]
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elakdawalla
post Jan 27 2006, 05:04 PM
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Cool -- a paper that contradicts the assumption central to my Master's thesis tongue.gif

The observation central to this work is: there are lots of channels on Venus but if you look at the topography along them, they don't go from uphill to downhill; they undulate a lot. The assumption I used in my thesis was one first suggested by Goro Komatsu and Vic Baker: that the channels could be used as a marker of paleo-flatness, so you could learn something about what's happened in Venus tectonics since the channels formed by examining how topography along channels differs from flatness. This paper says, no, the channels formed that way, because they didn't form on the surface, they formed below the surface at the interface between the plains lavas and some other layer.

I'll be very curious to see if they present this at this year's Lunar and Planetary Science Conference and what the other people in the Venus community think about it. But it may be hard to get an unbiased opinion. Vicki Hansen (second author on this paper, the first author is very likely a Ph.D. student of hers) has long represented one polar extreme in an extremely, nastily polarized debate about how to interpret geology from Magellan SAR images; the other pole was occupied by my graduate advisor, Jim Head, and his coworker Alexander ("Sasha") Basilevsky. After witnessing one particularly ugly and non-constructive debate between these two poles at an LPSC meeting, one of my fellow students (Geoff Collins, now a professor at Wheaton) summed up their debate: "I feel like Sasha is arguing 'Can't you see the forests around you?' while Vicki is shouting 'No, no! There are only trees!'"

--Emily


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ljk4-1
post Jan 27 2006, 05:07 PM
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QUOTE (elakdawalla @ Jan 27 2006, 12:04 PM)
Cool -- a paper that contradicts the assumption central to my Master's thesis  tongue.gif

--Emily
*


Emily, how did you decide to choose this topic for your thesis? Is it available online? Thank you.

This does explain your image symbol, at least. smile.gif


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elakdawalla
post Jan 27 2006, 05:38 PM
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QUOTE (ljk4-1 @ Jan 27 2006, 09:07 AM)
Emily, how did you decide to choose this topic for your thesis?  Is it available online?  Thank you.

I went to my archives, blew the dust off the CDs, and uploaded the text and figures to a site where you can download them if you really care. It wasn't ever finished far enough to be ready to be submitted for publication, though it was being formatted as a JGR paper, so it's very much a draft, but have at it! It's not doing anybody any good not being read. It's questionable though whether it will do anybody any good being read -- it's only been "peer reviewed" by my advisors.

The topic was suggested by my advisor. I did the mapping work with Jim and Sasha (though I had to teach myself how to do the GIS stuff with lots of help from various tech support people), then there's a whole section on geophysics that I did with Marc Parmentier.

The paper: Understanding the tectonic history of the Baltis Vallis region, Venus, from observations of canali topography (PDF, 60 k)
The figures (PDF) (WARNING! 14 MB!!)
QUOTE (ljk4-1 @ Jan 27 2006, 09:07 AM)
This does explain your image symbol, at least. smile.gif
*

smile.gif I figured nobody would understand what the canale was if I tried to stick one in the space allotted for an icon; those gorgeous Venus craters are second best!

--Emily


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Guest_AlexBlackwell_*
post Jan 27 2006, 08:28 PM
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QUOTE (elakdawalla @ Jan 27 2006, 05:04 PM)
The observation central to this work is: there are lots of channels on Venus but if you look at the topography along them, they don't go from uphill to downhill; they undulate a lot.  The assumption I used in my thesis was one first suggested by Goro Komatsu and Vic Baker: that the channels could be used as a marker of paleo-flatness, so you could learn something about what's happened in Venus tectonics since the channels formed by examining how topography along channels differs from flatness.  This paper says, no, the channels formed that way, because they didn't form on the surface, they formed below the surface at the interface between the plains lavas and some other layer.

Although different mechanisms and processes are involved, there are interesting parallels with similar debates regarding Mars' ancient valley networks.
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elakdawalla
post Jan 27 2006, 09:36 PM
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QUOTE (AlexBlackwell @ Jan 27 2006, 12:28 PM)
Although different mechanisms and processes are involved, there are interesting parallels with similar debates regarding Mars' ancient valley networks.
*

Huh. I didn't know that -- do you mean that people are talking about a subsurface (or maybe subglacial) formation mechanism for the valley networks? I'd be shocked and amazed if Komatsu and Baker weren't also involved in these studies -- they've cornered the market on planetary channels.

--Emily


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Guest_AlexBlackwell_*
post Jan 27 2006, 09:55 PM
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QUOTE (elakdawalla @ Jan 27 2006, 09:36 PM)
Huh.  I didn't know that -- do you mean that people are talking about a subsurface (or maybe subglacial) formation mechanism for the valley networks?

Sure. For example, there are numerous papers on a groundwater sapping origin for Mars' ancient valley networks. Just off the top of my head: Goldspiel and Squyres [2000].

EDIT: You might be aware of a special issue of the journal Geomorphology entitled, appropriately enough, "Extraterrestrial geomorphology." The papers, published in 2001 but undoubtedly prepared before that, are a bit dated, though.

This post has been edited by AlexBlackwell: Jan 27 2006, 10:39 PM
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Bob Shaw
post Jan 27 2006, 10:15 PM
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QUOTE (elakdawalla @ Jan 27 2006, 10:36 PM)
Huh.  I didn't know that -- do you mean that people are talking about a subsurface (or maybe subglacial) formation mechanism for the valley networks?  I'd be shocked and amazed if Komatsu and Baker weren't also involved in these studies -- they've cornered the market on planetary channels.

--Emily
*



Emily:

Well, Baker likes floods. Big floods. Reallllllly big floods!

So maybe sub-glacial channels wouldn't have the same, er, flush of enthusiasm...

Bob Shaw

(who *must* one day visit the scablands!)


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Guest_BruceMoomaw_*
post Jan 28 2006, 09:45 PM
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This is another question that could be answered by Bruce Campbell's proposed "VISTA" Discovery Venus subsurface radar sounder -- a mission with which I'm starting to be rather taken.
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hendric
post Jan 29 2006, 12:48 AM
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Emily,
Are there any lakes, or lake shorelines, along the valley? I would assume for such a long valley that there was one point somewhere where it hit a local minimum and created a lake of "whatever".


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Guest_BruceMoomaw_*
post Jan 29 2006, 04:59 PM
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Actually, Mars scientists seem to have always preferred groundwater sapping to surface runoff as a formation mechanism for the valley networks -- although some of them in turn believe that precipitation (either rain or snowmelt) in high regions may have trickled locally underground to start the sapping in the first place, and some believe that there is evidence for BOTH mechanisms, perhaps at different times in Martian history.

The thing about sapping is that it can work using a much smaller total amount of liquid water. It's very easy to visualize an ancient Mars which gradually chilled down and developed a steadily thickening surface layer of permafrost, replacing surface runoff with underground liquid-water sapping at greater and greater depths until the process shut off completely save in the few remaining regions of geothermal activity.
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Guest_BruceMoomaw_*
post Jan 29 2006, 05:00 PM
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As for the Venusian channels, I understand that there's nothing anywhere on any of them that remotely resembles a lakebed.
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Guest_BruceMoomaw_*
post Jan 29 2006, 05:06 PM
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Michael Carr has quite a bit on this subject in his extremely useful 1995 book "Water on Mars" (which I recommend to non-scientists as well as scientists -- it's thorough but remarkably easy to understand). He was a major proponent of sapping at the time, and I believe still is. The question is whether the extreme shortage of visible tributaries in the Martian valley networks is due to an actual original lack of them -- as would be the case from sapping -- or whether the erosion and aeolian soil movement that's gone on in the billions of years since then has just covered up a lot of the initial little tributaries that you'd get from surface runoff. (This is one reason why a Martian SAR orbiter would still be useful scientifically to punch through the upper few meters of loose soil.)
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