Analyses of data from the MErcury Surface, Space ENvironment,
GEochemistry, and Ranging (MESSENGER) spacecraft’s second
flyby of Mercury in October 2008 show that the planet’s
atmosphere, magnetosphere, and geological past are all
characterized by much greater levels of activity than scientists
first suspected.
On October 6, 2008, the probe flew by Mercury for the second
time, capturing more than 1,200 high-resolution and color images of
the planet unveiling another 30 percent of Mercury’s surface
that had never before been seen by spacecraft and gathering
essential data for planning the remainder of the mission.
“MESSENGER’s second Mercury flyby provided a number
of new findings,” says MESSENGER Principal Investigator Sean
Solomon at the Carnegie Institution of Washington. “One of
the biggest surprises was how strongly the planet’s
magnetospheric dynamics changed from what we saw during the first
Mercury flyby in January 2008. Another was the discovery of a large
and unusually well preserved impact basin that was the focus for
concentrated volcanic and deformational activity. The first
detection of magnesium in Mercury’s exosphere and neutral
tail provides confirmation that magnesium is an important
constituent of Mercury’s surface materials. And our nearly
global imaging coverage of the surface after this flyby has given
us fresh insight into how the planet's crust was formed.”
These findings are reported in four papers published in the May
1 issue of Science magazine.
Just after MESSENGER's closest approach to Mercury (200 km from
the surface), high-resolution color images of Mercury were obtained
(500 meters per pixel). MESSENGER images the surface through 11
filters that span visible and infrared wavelengths. Credit:
NASA/Johns Hopkins University Applied Physics Laboratory/Arizona
State University/Carnegie Institution of Washington. An Abundance
of Magnesium
The probe’s Mercury Atmospheric and Surface Composition
Spectrometer, or MASCS, detected significant amounts of magnesium
in the planet’s atmosphere, reports William McClintock of the
University of Colorado at Boulder’s Laboratory for
Atmospheric and Space Physics. “Detecting magnesium was not
too surprising, but seeing it in the amounts and distribution we
recorded was unexpected,” said McClintock, a MESSENGER
co-investigator and lead author of one of the four papers.
“This is an example of the kind of individual discoveries
that the MESSENGER team will piece together to give us a new
picture of how the planet formed and evolved.”
The instrument also measured other exospheric constituents
during the October 6 flyby, including calcium and sodium, and he
suspects that additional metallic elements from the surface
including aluminum, iron, and silicon also contribute to the
exosphere.
Radically Different Magnetosphere
MESSENGER observed a radically different magnetosphere at
Mercury during its second flyby, compared with its earlier January
14 encounter, writes MESSENGER co-investigator James Slavin, of the
NASA Goddard Space Flight Center, lead author of another paper.
“During the first flyby, MESSENGER entered through the dusk
side of the magnetic tail, measuring relatively calm dipole-like
magnetic fields closer to the planet, and then exited the
magnetosphere near dawn,” Slavin says. “Important
discoveries were made, but scientists didn’t detect any
dynamic features, other than some Kelvin-Helmholtz waves along its
outer boundary, the magnetopause.”
But the second flyby was a totally different situation, he says.
“ MESSENGER measured large magnetic flux leakage through the
dayside magnetopause, about a factor of 10 greater than even what
is observed at the Earth during its most active intervals. The high
rate of solar wind energy input was evident in the great amplitude
of the plasma waves and the large magnetic structures measured by
the Magnetometer throughout the encounter.”
The magnetospheric variability observed thus far by MESSENGER
supports the hypothesis that the great day-to-day changes in
Mercury’s atmosphere may be due to changes in the shielding
provided by the magnetosphere.
The Rembrandt Basin
One of the most exciting results of MESSENGER’s second
flyby of Mercury is the discovery of a previously unknown large
impact basin. The Rembrandt basin is more than 430 miles in
diameter and if formed on the east coast of the United States would
span the distance between Washington, D.C., and Boston.
The Rembrandt basin formed about 3.9 billion years ago, near the
end of the period of heavy bombardment of the inner Solar System,
suggests MESSENGER Participating Scientist Thomas Watters, lead
author of another of the papers. Although ancient, the Rembrandt
basin is younger than most other known impact basins on
Mercury.
“This is the first time we’ve seen terrain exposed
on the floor of an impact basin on Mercury that is preserved from
when it formed” says Watters. “Landforms such as those
revealed on the floor of Rembrandt are usually completely buried by
volcanic flows.”
Mercury’s Crustal Evolution
Just over a year ago, half of Mercury was unknown. Globes of the
planet were blank on one side. With image data from MESSENGER,
scientists have now seen 90 percent of the planet’s surface
at high resolution and can start to assess what this global picture
is telling us about the history of the planet's crustal evolution,
says Brett Denevi, a MESSENGER team member at Arizona State
University and lead author of one of the papers.
“After mapping the surface, we see that approximately 40
percent is covered by smooth plains,” she says. “Many
of these smooth plains are interpreted to be of volcanic origin,
and they are globally distributed (in contrast with the Moon, which
has a nearside/farside asymmetry in the abundance of volcanic
plains). But we haven’t yet seen evidence for a feldspar-rich
crust, which makes up the majority of the lunar highlands and is
thought to have formed by flotation during the cooling of an early
lunar magma ocean. Instead, much of Mercury's crust may have formed
through repeated volcanic eruptions in a manner more similar to the
crust of Mars than to that of the Moon.”
Scientists continue to examine data from the first two flybys
and are preparing to gather even more information from a third
flyby of the planet on September 29, 2009.
“The third Mercury flyby is our final ‘dress
rehearsal’ for the main performance of our mission: insertion
of our probe into orbit around Mercury in March 2011 and the
continuous collection of information about the planet and its
environment for one year,” adds Solomon. “The orbital
phase of our mission will be like staging two flybys per day.
We’ll be drinking from a fire hose of new data, but at least
we’ll never be thirsty. Mercury has been coy in revealing its
secrets slowly so far, but in less than two years the innermost
planet will become a close friend.”
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. 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.