[meteorite-list] MESSENGER: Mercury's Surface - The Role of Space Weathering

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 1 Jun 2011 18:03:06 -0700 (PDT)
Message-ID: <201106020103.p52136sj007933_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/soc/highlights.html

MESSENGER: Science Highlights from Mercury's Orbit
May 31, 2011

Mercury's Surface: The Role of Space Weathering

Mercury and its environment constitute a complex system that includes
interactions among the interplanetary medium, the planet's magnetic
field, its tenuous atmosphere (or exosphere), and its surface. Mercury's
environment includes the solar wind and photon flux (flowing charged
particles and light), solar and galactic cosmic rays, and the steady
bombardment by micrometeoroids (solar system dust particles). The
interactions of these elements of the space environment with the surface
generate the observed exosphere <hl_041911.html> and modify the
physical, chemical, and mineralogical properties of the planet's surface
layer of rock and soil, or regolith (Figure 1). This modification of the
surface of a rocky body is commonly termed "space weathering".

Accounting for the effects of space weathering on spectral and
geochemical measurements of Mercury's surface is important in
deciphering the composition of Mercury's crust and its implications for
the planet's bulk composition and geochemical evolution. One of the
major objectives of the MESSENGER mission is to establish what planetary
formational processes gave Mercury its high bulk density, which implies
a much larger fraction of metal than for the other inner planets. The
several competing hypotheses for Mercury's high metal content predict
different crustal compositions. Understanding the nature of Mercury's
crust will therefore provide insight into the processes that formed and
produced the different compositions of the inner planets.

Figure 1. Summary of processes that contribute to the interactions
between Mercury's surface and the planet's environment. These processes
produce Mercury's exosphere and weather the planet's regolith.

Remote sensing measurements can provide information on the elemental and
mineralogical composition of Mercury's surface. MESSENGER's science
payload is measuring Mercury's X-ray, gamma-ray, neutron, and
ultraviolet to near-infrared spectral properties over a large portion of
the planet's surface. A challenge to the interpretation of these
observations is to separate the signatures of planetary geochemical
processes from those of space weathering processes.

Space weathering alters the spectral reflectance signatures of minerals
by causing surfaces to darken, introducing a relative increase in
reflectance with increasing wavelength (also called "reddening"), and
diminishing or removing spectral absorption features. Comparisons of
laboratory spectra of lunar surface samples with remote spectral
observations of the Moon have provided the most extensive information on
the mechanisms of space weathering. Submicroscopic, nanometer-scale
particles of metallic iron are the main contributor to these spectral
alterations (Figure 2). These metal particles, termed "nanophase iron
(npFe^0 )", are found on the rims of regolith grains and within complex,
glass-bonded soil particles called agglutinates.

processes at work in Mercury's Exosphere <highlights/altimetry02.jpg>

Figure 2. Transmission electron microscope image of a lunar
agglutinate grain showing npFe^0 both within the grain rim and within
the interior of the grain. Arrows indicate multiple layers of
finer-grained npFe^0 within the rim. Within the grain interior the
npFe^0 particles are larger (~3 times the size of the rim grains).
Credit: C. Pieters, Brown University.

The npFe^0 particles are formed by multiple processes. On the Moon,
impact of the surface by micrometeoroids and bombardment by solar wind
ions (charged particles) are thought to be the two dominant processes.
Impacting micrometeoroids melt and vaporize regolith grains. The vapors
coat nearby grains, producing glassy, amorphous (atomically
unstructured) rims or patinas. The iron in the vapor is changed by
interactions with solar wind ions and by heating during the impact event
through a process called chemical reduction, in which oxygen is removed.
Reduction leaves behind metallic iron in the form of npFe^0 . The vapor
deposition thus produces glassy, amorphous rims with embedded npFe^0
particles.

Bombardment by solar wind ions chemically reduces the top few atomic
layers of the grain by sputtering away the oxygen molecules and leaving
behind the iron that forms into small metallic particles. Bombardment of
the surface by ultraviolet light can produce similar effects. Mercury
has a global magnetic field, which can limit the ability of solar-wind
ions to reach the surface, whereas ultraviolet light (photons) is not
impeded by a magnetic field. MESSENGER's three Mercury flybys provided
glimpses into the role of the magnetic field in directing solar wind
particles both away from and onto the surface. Simulations of these
interactions indicate that access to the surface by solar wind particles
varies strongly with time and location.

Most of our ideas concerning the effects of space weathering on
Mercury's surface come from extrapolating from what we know about the
Moon. There is some evidence that solar-wind bombardment, rather than
micrometeoroid bombardment, is the dominant agent of space weathering on
the Moon, as well as on asteroids. The micrometeoroid flux is believed
to be at least 5 times greater on Mercury than at the Moon. Because
Mercury is closer to the Sun, however, solar wind and photon fluxes are
also at least 5 times higher than on the Moon. Thus the dominant
processes on the Moon and asteroids both operate at a potentially
increased rate on Mercury, and which process is more important on
Mercury is not yet clear. The observations returned by MESSENGER from
orbit will enable us to quantify rates of these processes on Mercury
relative to lunar and asteroidal environments.
Received on Wed 01 Jun 2011 09:03:06 PM PDT