[meteorite-list] MESSENGER Finds Unusual Groups of Ridges and Troughs on Mercury

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 15 Nov 2012 12:21:38 -0800 (PST)
Message-ID: <201211152021.qAFKLcuo026275_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=231

MESSENGER Mission News
November 15, 2012

MESSENGER Finds Unusual Groups of Ridges and Troughs on Mercury

MESSENGER has discovered assemblages of tectonic landforms unlike any
previously found on Mercury or elsewhere in the Solar System. The
findings are reported in a paper led by Smithsonian scientist Thomas
Watters, "Extension and contraction within volcanically buried impact
craters and basins on Mercury," published in the December issue of the
journal Geology and available online at

http://geology.gsapubs.org/content/40/12/1123.full .

The surface of Mercury is covered with deformational landforms that
formed by faulting in response to horizontal contraction or shortening
as the planet's interior cooled and surface area shrank, causing blocks
of crustal material to be pushed together. Contraction from cooling of
Mercury's interior has been so dominant that extensional landforms
caused by fault formation in response to horizontal stretching and
pulling apart of crustal material had not been previously documented
outside of the interiors of a few large impact basins.

The MESSENGER spacecraft, in orbit around Mercury since March of last
year, has revealed families of extensional troughs, or graben, that are
encircled by contractional wrinkle ridges arranged in circular rings.
The troughs can form complex patterns varying from the outlines of
polygons inside the ridge rings to arcs that parallel the bounding ridges.

"The pattern of winkle ridges and graben resembles the raised edge and
cracks in a pie crust," said Watters of the Center for Earth and
Planetary Studies at the National Air and Space Museum. The "pie crust"
analogy also fits another notable aspect of these collections of
tectonic landforms ???????? their association with "ghost" craters. Ghost
craters are impact craters that have been flooded and buried by lava
flows. The thin volcanic deposits overlying the rim of a fully buried
impact crater serve to concentrate contractional forces, leading to the
formation of a ridge ring that reveals the outline of the buried crater.

"The special arrangement of the wrinkle ridges and graben in many of the
ghost craters on Mercury is due to a combination of extensional forces
from cooling and contraction of unusually thick lava flow units and
contractional forces from cooling and contraction of the planet's
interior," says Sean Solomon of the Columbia University's Lamont-Doherty
Earth Observatory, coauthor and principal investigator of the MESSENGER
mission. The eruption and rapid accumulation of very fluid lava flows
into thick cooling units on a planet undergoing a high rate of global
contraction may be why these systems of tectonic landforms in ghost
craters on Mercury have not been seen elsewhere in the Solar System.

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MESSENGER's Flight Traffic Controller

As the MESSENGER mission's Deep Space Network (DSN) scheduler, Jessica
Call vies for time on the network of large antennas around the globe for
scientists and engineers to talk to the spacecraft. "Since each antenna
can point only to one spacecraft at a time, schedulers like me negotiate
on behalf of the various deep space projects to develop a plan so that
each spacecraft can share this valuable resource," she explains. Read
more about her critical role on the team here
<http://messenger/who_we_are/member_focus.html>.
------------------------------------------------------------------------
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and
Ranging) is a NASA-sponsored scientific investigation of the planet
Mercury and the first space mission designed to orbit the planet closest
to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and
after flybys of Earth, Venus, and Mercury will start a yearlong study of
its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie
Institution of Washington, leads the mission as Principal Investigator.
The Johns Hopkins University Applied Physics Laboratory built and operates
the MESSENGER spacecraft and manages this Discovery-class mission for NASA.
Received on Thu 15 Nov 2012 03:21:38 PM PST