[meteorite-list] Mercury's crack habit

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 31 Jan 2008 02:42:41 -0600
Message-ID: <03d501c863e5$3e30b820$a12f4842_at_ATARIENGINE>

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 03:42:41 AM PST