[meteorite-list] Mercury's Contraction Much Greater Than Thought (MESSENGER)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 17 Mar 2014 12:13:44 -0700 (PDT)
Message-ID: <201403171913.s2HJDikK029797_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/news_room/details.php?id=254

MESSENGER Mission News
March 16, 2014

Mercury's Contraction Much Greater Than Thought

New global imaging and topographic data from MESSENGER show that the
innermost planet has contracted far more than previous estimates. The
results are based on a global study of more than 5,900 geological
landforms, such as curving cliff-like scarps and wrinkle ridges, that
have resulted from the planet's contraction as Mercury cooled. The
findings, published online today in Nature Geoscience
<http://www.nature.com/ngeo/journal/vaop/ncurrent/index.html>, are key
to understanding the planet's thermal, tectonic, and volcanic history,
and the structure of its unusually large metallic core.

Unlike Earth, with its numerous tectonic plates, Mercury has a single
rigid, top rocky layer. Prior to the MESSENGER mission only about 45% of
Mercury's surface had been imaged by spacecraft. Old estimates, based on
this non-global coverage, suggested that the planet had contracted
radially by about 1/2 to 2 miles (0.8 to 3 kilometers), substantially
less than that indicated by models of the planet's thermal history.
Those models predicted a radial contraction of about 3 to 6 miles (5 to
10 kilometers) starting from the late heavy bombardment of the Solar
System, which ended about 3.8 billion years ago.

The new results, which are based on the first comprehensive survey of
the planet's surface, show that Mercury contracted radially by as much
as 4.4 miles (7 kilometers) -- substantially more than the old
estimates, but in agreement with the thermal models. Mercury's modern
radius is 1,516 miles (2,440 kilometers).

"These new results resolved a decades-old paradox between thermal
history models and estimates of Mercury's contraction," remarked Paul
Byrne, a planetary geologist and MESSENGER Visiting Investigator at the
Carnegie Institution of Washington. "Now the history of heat production
and loss and global contraction are consistent. Interestingly, our
findings are also reminiscent of now-obsolete models for how large-scale
geological deformation occurred on Earth when the scientific community
thought that the Earth only had one tectonic plate. Those models were
developed to explain mountain building and tectonic activity in the
nineteenth century, before plate tectonics theory."

Byrne and his co-authors identified a much greater number and variety of
geological structures on the planet than had been recognized in previous
research. They identified 5,934 ridges and scarps attributed to global
contraction, which ranged from 5 to 560 miles (9 to 900 kilometers) in
length.

The researchers used two complementary techniques to estimate the
contraction from their global survey of structures. Although the two
estimates of radius change differed by 0.6 to 1 mile (1 to 1.6
kilometers), both were substantially greater than old estimates.

"I became interested in the thermal evolution of Mercury's interior when
the Mariner 10 spacecraft sent back images of the planet's great scarps
in 1974-75, but the thermal history models predicted much more global
contraction than the geologists inferred from the scarps then observed,
even correcting for the fact that Mariner 10 imaged less than half of
Mercury's surface," noted MESSENGER Principal Investigator Sean Solomon,
of Columbia University's Lamont-Doherty Earth Observatory. "This
discrepancy between theory and observation, a major puzzle for four
decades, has finally been resolved. It is wonderfully affirming to see
that our theoretical understanding is at last matched by geological
evidence."

This image
<http://carnegiescience.edu/httpwwwcarnegiescienceeduprmessengerbyrnecontractionimage>
shows a long collection of ridges and scarps on the planet Mercury
called a fold-and-thrust belt. The belt stretches over 336 miles (540
km). The colors correspond to elevation -- yellow-green is high and blue
is low. Contact Paul Byrne at pbyrne at carnegiescience.edu, or (281)
486-2140.
------------------------------------------------------------------------

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
entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to
begin a yearlong study of its target planet. MESSENGER's first extended
mission began on March 18, 2012, and ended one year later. MESSENGER is
now in a second extended mission, which is scheduled to conclude in
March 2015. Dr. Sean C. Solomon, the Director of Columbia University's
Lamont-Doherty Earth Observatory, 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 Mon 17 Mar 2014 03:13:44 PM PDT