[meteorite-list] MESSENGER Provides First Optical Images of Ice Near Mercury's North Pole
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 15 Oct 2014 11:03:22 -0700 (PDT)
MESSENGER Mission News
October 15, 2014
MESSENGER Provides First Optical Images of Ice Near Mercury's North Pole
NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging
(MESSENGER) spacecraft has provided the first optical images of ice and
other frozen volatile materials within permanently shadowed craters near
Mercury's north pole. The images not only reveal the morphology of the
frozen volatiles, but they also provide insight into when the ices were
trapped and how they've evolved, according to an article published today
in the journal, /Geology/.
Two decades ago, Earth-based radar images of Mercury revealed the polar
deposits, postulated to consist of water ice. That hypothesis was later
confirmed by MESSENGER through a combination of neutron spectrometry,
thermal modeling, and infrared reflectometry. "But along with confirming
the earlier idea, there is a lot new to be learned by seeing the
deposits," said lead author Nancy Chabot, the Instrument Scientist for
MESSENGER's Mercury Dual Imaging System (MDIS) and a planetary scientist
at the Johns Hopkins University Applied Physics Laboratory in Laurel,
Beginning with MESSENGER's first extended mission in 2012, scientists
launched an imaging campaign with the broadband clear filter of MDIS's
wide-angle camera (WAC). Although the polar deposits are in permanent
shadow, through many refinements in the imaging, the WAC was able to
obtain images of the surfaces
of the deposits by leveraging very low levels of light scattered from
illuminated crater walls. "It worked in spectacular fashion," said Chabot.
The team zeroed in on Prokofiev
the largest crater in Mercury's north polar region found to host
radar-bright material. "Those images show extensive regions with
distinctive reflectance properties," Chabot said. "A location
interpreted as hosting widespread surface water ice exhibits a cratered
texture indicating that the ice was emplaced more recently than any of
the underlying craters."
In other areas, water ice is present, she said, "but it is covered by a
thin layer of dark material inferred to consist of frozen organic-rich
compounds." In the images of those areas
the dark deposits display sharp boundaries. "This result was a little
surprising, because sharp boundaries indicate that the volatile deposits
at Mercury's poles are geologically young, relative to the time scale
for lateral mixing by impacts," said Chabot.
"One of the big questions we've been grappling with is 'When did
Mercury's water ice deposits show up?' Are they billions of years old,
or were they emplaced only recently?" Chabot said. "Understanding the
age of these deposits has implications for understanding the delivery of
water to all the terrestrial planets, including Earth."
Overall, the images indicate that Mercury's polar deposits either were
delivered to the planet recently or are regularly restored at the
surface through an ongoing process.
The images also reveal a noteworthy distinction between the Moon and
Mercury, one that may shed additional light on the age of the frozen
deposits. "The polar regions of Mercury show extensive areas that host
water ice, but the Moon's polar regions -- which also have areas of
permanent shadows and are actually colder -- look different," Chabot said.
"One explanation for differences between the Moon and Mercury could be
that the volatile polar deposits on Mercury were recently emplaced,"
according to the paper. "If Mercury's currently substantial polar
volatile inventory is the product of the most recent portion of a longer
process, then a considerable mass of volatiles may have been delivered
to the inner Solar System throughout its history."
"That's a key question," Chabot said. "Because if you can understand why
one body looks one way and another looks different, you gain insight
into the process that's behind it, which in turn is tied to the age and
distribution of water ice in the Solar System. This will be a very
interesting line of inquiry going forward."
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 was 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 Wed 15 Oct 2014 02:03:22 PM PDT