The figure shows an image of Mercury's surface (left; obtained using publicly available mosaic of Mercury from the MESSENGER spacecraft found at http://messenger.jhuapl.edu/) and a color-coded view of the global crater areal density (right), obtained by measuring craters greater than 25 km. The region within the white line corresponds to the heavily cratered terrains analyzed to calculate the age of the oldest surfaces on Mercury.
Boulder, Colo. —
The surface of Mercury is rather different from those of well-known
rocky bodies like the Moon and Mars. Early images from the Mariner 10
spacecraft unveiled a planet covered by smooth plains and cratered
plains of unclear origin. A team led by Dr. Simone Marchi, a Fellow of
the NASA Lunar Science Institute located at the Southwest Research Institute
(SwRI) Boulder, Colo., office, collaborating with the MESSENGER team,
including Dr. Clark Chapman of the SwRI Planetary Science Directorate,
studied the surface to better understand if the plains were formed by
volcanic flows or composed of material ejected from the planet's giant
impact basins.
Recent images from NASA's MESSENGER (MErcury Surface,
Space ENvironment, GEochemistry, and Ranging) spacecraft provided new
insights showing that at least the younger plains resulted from vigorous
volcanic activity. Yet scientists were unable to establish limits on
how far into the past this volcanic activity may have occurred, or how
much of the planet's surface may have been resurfaced by very old
volcanic plains.
Now, a team of scientists has concluded that the oldest
visible terrains on Mercury have an age of 4 billion to 4.1 billion
years, and that the first 400 to 500 million years of the planet's
evolution are not recorded on its surface. To reach its conclusion, the
team measured the sizes and numbers of craters on the most heavily
cratered terrains using images obtained by the MESSENGER spacecraft
during its first year in orbit around Mercury. Team members then
extrapolated to Mercury a model that was originally developed for
comparing the Moon's crater distribution to a chronology based on the
ages of rock samples gathered during the Apollo missions.
The study, "Global Resurfacing of Mercury 4.0-4.1
Billion Years Ago by Heavy Bombardment and Volcanism" by Marchi,
Chapman, Caleb I. Fassett, James W. Head, William F. Bottke and Robert
G. Strom, is in the July 4 issue of the journal Nature.
"By comparing the measured craters to the number and
spatial distribution of large impact basins on Mercury, we found that
they started to accumulate at about the same time, suggesting that the
resetting of Mercury's surface was global and likely due to volcanism,"
said lead author Dr. Simone Marchi, who has a joint appointment between
two of NASA's Lunar Science Institutes, one at the SwRI in Boulder and
another at the Lunar and Planetary Institute in Houston.
Those results set the age boundary for the oldest
terrains on Mercury to be contemporary with the so-called Late Heavy
Bombardment (LHB), a period of intense asteroid and comet impacts
recorded in lunar and asteroidal rocks and by the numerous craters on
the Moon, Earth, and Mars, as well as Mercury.
"Meanwhile, the age of the youngest and broadest
volcanic provinces visible on Mercury was determined to be about 3.6
billion to 3.8 billion years ago, just after the end of the Late Heavy
Bombardment," Marchi said.
Altogether, the results indicate that the time agreement
between the onset of the LHB and the global resurfacing of Mercury
implies not only that the resurfacing was due to volcanism, but also,
according to Chapman, that "the impact of large projectiles hitting
Mercury's thin solid crust during the LHB may have enhanced the observed
global resurfacing."
MESSENGER 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. The Johns
Hopkins University Applied Physics Laboratory built and operates the
MESSENGER spacecraft and manages this Discovery-class mission for NASA.
The figure shows an image of Mercury's surface (left;
obtained using publicly available mosaic of Mercury from the MESSENGER
spacecraft found at http://messenger.jhuapl.edu/)
and a color-coded view of the global crater areal density (right),
obtained by measuring craters greater than 25 km. The region within the
white line corresponds to the heavily cratered terrains analyzed to
calculate the age of the oldest surfaces on Mercury.
Editors: An image is available at http://www.swri.org/press/2013/mercury-crater.htm.
For more information, contact
Joe Fohn, (210) 522-4630, Communications Department, Southwest Research
Institute, PO Drawer 28510, San Antonio, TX 78228-0510.
