[meteorite-list] Mercury's crack habit

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Thu, 31 Jan 2008 08:12:11 -0700 (MST)
Message-ID: <3363.71.226.60.25.1201792331.squirrel_at_timber.lpl.arizona.edu>

Hello Sterling:

Take it one more step. Given the location of the "crater" within Caloris
and given (at least it looks to me) that the channels seem to predate ANY
of the other craters (the channels do not deform any craters, but seem to
have been there when the craters were formed), how about Caloris being a
huge caldera (or at least a subsistance feature)?

Larry

Peace and long life

On Thu, January 31, 2008 1:42 am, Sterling K. Webb wrote:

> Hi,
>
>
> Here in the midst of the Great Tucson Lull, those
> of us not there, get to look at pictures of another place where we're not:
> Mercury! Here's the biggest (stitched)
> picture of the Mysterious Spider at Proctor Crater:
> http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Prockter06.jpg
>
>
> The odd thing in the Messenger photo is not the
> Spider -- it's the Crater. I've been staring at it off and on
> for about an hour and I'm beginning to think the Crater isn't a crater at
> all. At first glance, oh, sure, a crater!
>
> But it appears to me that the illuminated crater wall
> (the sun is from lower right) is completely vertical. No
> impact crater has vertical walls; theoretical models of a crater are
> conical. But then there's slump and rebound, magma flooding, depositation,
> and other processes that fill the crater or change its diameter to depth
> ratio, but none of them, as far as I know, will produce a vertical crater
> wall.
>
> OK, maybe slump, in the extreme case, could produce
> an entirely vertical wall, but shouldn't there be a massive pile of slumped
> debris at the base of that wall? There's no pile of slump there. Then I
> start looking around. It's round; it has a central rebound peak -- it's
> got to be a crater!
>
> But now, even the central peak doesn't really look like a
> peak at all. A peak would be conical; this looks like a vertical fault that
> exposes a stretch of vertical wall (the linear features are at the same
> apparent angle as the crater wall, so presumably they're at the same sun
> angle). It's not a peak; it's a thrust fault.
>
> What we don't know is the side angle, the angle of view
> of the spacecraft. Vertical? Or tilted? And at what angle? But, I don't
> think the geometry of the sunlight changes at any angle. I'm beginning to
> think we're looking at a subsidence caldera here, not a crater at all.
> (Here's a picture of an Earthly one
> from the great vantage point of the International Space Station):
> http://en.wikipedia.org/wiki/Image:La_Cumbre_-_ISS.JPG
>
>
> IF it's a subsidence caldera, the presence of all those odd
> radiating channels is a gimme; explaining them isn't a problem. Well, it is
> for me, but it isn't for a geologist. Oh, I have ideas: a huge magna
> chamber underneath the "crater" and the channel field; expansion of the
> magma that raised and cracked the surface; then the rapid escape of a
> silica-poor and "runny" magma; followed by the collapse of the caldera and
> the channels.
>
> For decades, there was a snarly back-and-forth about the
> craters of the Moon (and other bodies) between the impact guys and the
> volcanic guys. (Americans were mostly impact guys; Brits and Ozzies mostly
> volcanic guys.) Impact guys won the Crater Bowl. So, sticking your neck
> out now and saying "That's no (impact) crater!" is a chancy business no
> matter what planet you do it on.
>
> It would be just like the Universe. You think you've got
> everything figured out -- it's all craters. Then the Universe tosses you a
> planet with a volcano that looks like a crater! And there are a lot of
> other suspiciously volcanic features too. So much new data.
>
> And I want to thank Larry Lebowsky who pointed out the
> last time I posted about "volcanism" on Mercury that I used the term
> "vulcanism" throughout. You will note that this post
> contains no references whatsoever to Spock-like or hard-rubber beings in
> any form.
>
>
>
> Sterling K. Webb
> --------------------------------------------------------------------------
> -
> ----- Original Message -----
> From: "Darren Garrison" <cynapse at charter.net>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Wednesday, January 30, 2008 8:16 PM
> Subject: [meteorite-list] Mercury's crack habit
>
>
>
> http://www.foxnews.com/story/0,2933,326867,00.html
>
>
> NASA Spots Mysterious 'Spider' on Mercury
> Wednesday, January 30, 2008
>
>
> By Clara Moskowitz
>
>
> A whole new side of Mercury has been revealed in pictures taken by NASA's
> MESSENGER probe, which flew by the tiny planet two weeks ago in the
> first mission to Mercury in more than three decades.
>
> MESSENGER skimmed only 124 miles (200 kilometers) over Mercury's surface
> on Jan.
> 14, in the first of three passes it will make before settling into orbit
> March
> 18, 2011.
>
>
> The photos, released today, include one of a feature the scientists
> informally call "the spider," which appears to be an impact crater
> surrounded by more than 50 cracks in the surface radiating from its center.
>
>
> Scientists are perplexed by this structure, which is unlike anything
> observed elsewhere in the solar system.
>
> "It's a real mystery, a very unexpected find," said Louise Prockter, an
> instrument scientist at the Johns Hopkins University Applied Physics
> Laboratory,
> which built the probe for the $446 million NASA mission.
>
> She said whatever event created the spider "is anybody's guess," but
> suggested perhaps a volcanic intrusion beneath the planet's surface led to
> the formation of the troughs.
>
> The last time NASA sent a probe to Mercury was in 1975, when the Mariner
> 10
> spacecraft flew by the planet three times.
>
> MESSENGER'S first flyby gave scientists the first glimpses of Mercury's
> hidden side, the 55 percent of its surface that was left uncharted by
> Mariner 10.
>
>
> MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry,
> and Ranging, also measured another peculiar element of Mercury - its
> magnetic field.
>
> Earth has a magnetic field surrounding it that acts as a protective
> bubble shielding the surface from cosmic rays and solar storms. But
> scientists were shocked when Mariner 10 discovered a magnetic field at
> Mercury, too.
>
>
> "The only other example in our solar system of an Earth-like
> magnetosphere is tiny Mercury," said Sean C. Solomon, MESSENGER Principal
> Investigator from
> the Carnegie Institution of Washington.
>
>
> MESSENGER was able to fly through the magnetic field and take detailed
> measurements that scientists hope to use to discover the origins of the
> inexplicable magnetosphere.
>
> Scientists have been poring over more than 1,200 new images sent by seven
> instruments on the probe, and they are excited to gain new insight into
> the composition of Mercury's surface, the planet's history, and where its
> atmosphere comes from.
>
> "On the eve of the encounter I couldn't sleep at all," said Robert Strom,
> a MESSENGER science team member who also worked on the Mariner 10 mission.
> "I've
> waited 30 years for this. It didn't disappoint at all. I was astounded at
> the quality of these images. It dawned on me that this is a whole new
> planet that we're looking at."
>
> The satellite will further probe Mercury's mysteries in a second pass
> over the planet in October, followed by a third flyby in September 2009.
>
> The probe has traveled 4.9 billion miles (7.9 billion-kilometers) since
> it launched in August 2004. On its journey it soared by Earth once and
> Venus
> twice, offering gorgeous views of these planets as well.
>
> In 2011 MESSENGER will become the first spacecraft to orbit the closest
> planet to the Sun. ______________________________________________
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Received on Thu 31 Jan 2008 10:12:11 AM PST