Space Weathering Impact on Solar System Surfaces and Mission Science*
John F. Cooper?, Richard E. Hartle, Edward C. Sittler Jr., Rosemary M. Killen
NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA
Steven J. Sturner
CRESST, University of Maryland College Park, College Park, Maryland, USA
Chris Paranicas, Matthew E. Hill, Abigail M. Rymer
Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA
Paul D. Cooper
George Mason University, Fairfax, VA, USA
Dan Pascu
U.S. Naval Observatory, Washington, DC, USA (retired)
Robert E. Johnson, Timothy A. Cassidy
University of Virginia, Charlottesville, VA, USA
Thomas M. Orlando
Georgia Institute of Technology, Atlanta, GA, USA
Kurt D. Retherford
Southwest Research Institute, San Antonio, TX, USA
Nathan A. Schwadron
Boston University, Boston, MA, USA
Ralf I. Kaiser
University of Hawaii at Manoa, Honolulu, HI, USA
François Leblanc
LATMOS, Institut Pierre Simon Laplace, Université Versailles Saint Quentin/CNRS, Verrièresle-
Buisson, France
Louis J. Lanzerotti
New Jersey Institute of Technology, Newark, NJ, USA
Claudia J. Alexander, Henry B. Garrett, Amanda R. Hendrix
NASA Jet Propulsion Laboratory, Pasadena, CA, USA
Wing H. Ip
Institutes of Astronomy and Space Sciences, National Central University, Taiwan
*Full references & other docs at http://nssdcftp.gsfc.nasa.gov/miscellan ... weathering
? John F. Cooper, Code 672, NASA GSFC; Phone: 301-286-1193; John.F.Cooper@nasa.gov.
Submitted to Planetary Science Decadal Survey, National Research Council, Sept. 11, 2009
Abstract
Space weathering is the collection of physical processes acting to erode and chemically modify
planetary surfaces directly exposed to space environments of planetary magnetospheres, the
heliosphere, and the local interstellar environment of the solar system. Space weathering affects
the physical and optical properties of the surfaces of planetary bodies, so understanding its
specifics is critical for interpreting surface data from remote and landed measurements. For full
coverage of space environmental measurements, we recommend expanded interdisciplinary
cooperation between NASA’s Planetary Science and Heliophysics divisions. To grow the field
in the next decade and maximize impact on mission studies, we suggest a balanced mixture of
laboratory measurements, modeling, and theoretical investigations in support of all missions.
Space Environments
Vast expanse of the space weathering environment interacting with solar system bodies is
illustrated in Figure 1 by logarithmic horizontal scale of radial distance from the Sun to α-
Centauri. Principal sources of energy for space weathering of planetary surfaces are UV photons,
solar wind plasma, and energetic particles from the Sun, within a few hundred AU, and external
sources of plasma and energetic particles entering into the solar system from the local interstellar
environment. Beyond the realm of terrestrial planets and the asteroid belts, the solar influence
significantly wanes with the decline in density of the expanding solar wind plasma and
magnitude of the frozen-in magnetic field, while the interstellar influence progressively grows
through interaction of interstellar neutral winds with solar ultraviolet radiation and the solar wind
plasma. Across the heliospheric boundary region near 100 AU, now being explored by the two
Voyager spacecraft, occurs a transition from supersonic (400 – 800 km/s) plasma flows of solar
coronal expansion, the solar wind, to 26 km/s inward flow of the interstellar wind. This plasma
contains both a bulk flow and thermal components associated with typical ion energies up to a
few keV, and energetic components extending to far higher energies, ultimately to the full range
of galactic cosmic ray ions easily penetrating into the heliosphere at GeV energies and higher.
The Sun contributes the innermost source of energetic particles in association with solar flare
and coronal mass ejection (CME) events, the interstellar environment contributes the outermost
sources, and the dynamics of the expanding and variable solar wind provide additional energy to
MeV energies within the heliosphere. The solar wind termination shock, the supersonic-subsonic
flow boundary crossed by both Voyagers (Stone et al., 2005, 2008), and/or the heliosheath region
beyond this shock out to the heliopause, the contact boundary with interstellar plasma, may
further accelerate plasma particles into the energetic range but this is not yet established by the
Voyager measurements. Neither spacecraft detected local particle acceleration at the respective
crossings, although the bulk of acceleration may be occurring elsewhere along the shock
boundary (McComas and Schwadron, 2006). Other possibilities are that the heliosheath ions are
energized by turbulent or reconnecting magnetic fields in the heliosheath, or that these ions
originate instead from penetrating interstellar ions (Cooper et al., 2006; Cooper, 2008). As solar
activity increases and then again declines within the next decade from the current minimum, the
continuing Voyager measurements, supplemented by direct energetic neutral atom measurements
of boundary region emissions by the Interstellar Boundary Explorer (IBEX) in earth orbit, are
expected to resolve origin of the heliosheath ions and to locate the heliopause. What is already
clear is that the termination shock boundary marks the transition from dominance of some space
weathering effects, e.g. erosive sputtering, by the supersonic plasma flow to a broader range of
effects from plasma and energetic particles at higher energies to the cosmic ray regime.
Within these expanding near-solar to heliospheric to local interstellar space environments we
find the objects of primary interest to planetary science: the terrestrial planets, the asteroid belt,
the gas and ice giant planets, comets, the Kuiper Belt, and finally the Oort Cloud. Aside from the
first known member of the Kuiper Belt, Pluto, now officially designated as an ice dwarf planet,
our direct knowledge of Kuiper Belt Objects (KBO) began with the first discovery in 1992, then
followed to date by about a thousand other discoveries of such objects, including a few classified
as members of the inner Oort Cloud. Presumably there are thousands more of similarly
detectable size waiting to be discovered, and far more at smaller sizes. Looking back towards the
Sun, there are also thousands of known asteroidal bodies, including Near Earth Objects
potentially of concern for future Earth impacts, and as a remote possibility the first members of
the fabled Vulcanoid Belt that might be found via increasing sensitivity of near-solar
observations. At the smallest scales there are interplanetary dust grains, the source of the
zodiacal light, extending down in size to nanometers or less (e.g., molecular clusters) and
thought to arise from impact surface weathering of small bodies and from comet outgassing.
The red and white stars of Figure 1 denote the distinctly different space environments of solar
system bodies with and without internally generated magnetic fields. Except for the highest
energy cosmic rays and their atmospheric interaction products, the direct effects of space
weathering do not extend to the solid surfaces of Venus, Earth, and Mars. While the planetary
magnetospheres (red stars) substantially deflect interplanetary plasmas and energetic particles
away from the atmospheres and underlying surfaces, even an ionospheric (white star) interaction
arising from ionization of a thin atmosphere, or surface-bound exosphere, can significantly
impede or totally inhibit access of space plasma to otherwise exposed surfaces. The surface of
Mars is notably oxidized by solar ultraviolet irradiation and to a lesser extent from high energy
(> 100 MeV) cosmic rays and solar energetic particles, while medium-energy (> 1 MeV)
energetic ions can easily penetrate Pluto’s microbar-pressure atmosphere to the surface. On the
other hand, the acceleration of charged particles within the planetary magnetospheres, and
magnetic pickup of exospheric ions, provides additional and potentially more dominant energy
sources for space weathering of surfaces exposed to those environments.
Space Weathering Effects
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Laboratory Measurements
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Recommendations
Support comprehensive specification of space weathering environments through expansion of
cooperation between NASA heliophysics and planetary science divisions on placement of
environmental radiation instrumentation on appropriate missions, compilation of data and
semiempirical models from measurements, and on predictive models for each environment.
Rationale: In the heliophysics community there is the concept of the Heliophysics Global Observatory
(HGO), the collective fleet of operational heliophysics missions, that should be expanded for
interdisciplinary applications to include planetary missions. Heliophysics support for space environment
modeling, e.g. the Earth-Moon-Mars Radiation Environment Module (Schwadron et al., 2007) and earlier
(e.g., NASA GSFC, JPL) models for solar and cosmic ray energetic particle modeling in the terrestrial
planet domain, can be usefully applied to planetary interaction applications. Similarly, missions and data
models for planetary interactions can also support investigation and modeling of interplanetary
environments. HGO data virtual observatory approaches could be applied to planetary missions.
Encourage community-wide and interdisciplinary investigations of universal space weathering
processes through balanced mixture of initiatives on mission instrument data analysis,
laboratory measurements, computational modeling, and relevant theoretical investigations.
Rationale: The space environment is universal in the sense of connecting all the planetary environments,
and so it most efficient to approach space weathering processes from the universal perspective, e.g. that
similar processes act everywhere and the effects differ only in the relative energy deposition rates and
compositional impacts of each process in different locations. Process investigations must be wellgrounded
in measurements for different environments, in broad-spectrum approaches to laboratory
simulations, and best available inputs from theory and high performance computing.
Enable development of plasma ion, energetic particle, and neutral composition spectrometers
for in-situ analyses to characterize elemental and isotopic range of interconnected planetary
surface, atmospheric, ionosopheric, magnetospheric, and heliospheric environments.
Rationale: Our knowledge of composition in these environments beyond the Earth is limited to some
major species with little or no information on the full range of elemental and isotopic composition that is
critical to determination of origins, evolutionary processes, and astrobiological potential. Sample return is
too expensive for general application, advanced in-situ analysis capabilities being required for one-way
missions to most non-terrestrial destinations of the solar system. There is also strong coupling of
composition for these connected environments and this coupling should be considered in weighting the
relative priorities of measurements in each environment.
Provide facilities for more realistic laboratory science and engineering simulations of
planetary surface environments under simultaneous influence of extreme limits on pressure,
temperature, radiation, composition, physical structure, and endogenic or impact activity.
Rationale: There are no truly flat surfaces, particularly when viewed at the microscopic level of most
space weathering processes, and multiple energy sources are typically operating on affected surfaces. The
sensible and accessible surfaces have impact regolith layers extending to meters in depth and likely with
high porosity under conditions of reduced gravity. Multi-phase interactions of ice, grains, and volatiles in
irradiated bulk surface samples need much further investigation with appropriate facilities. Engineering
simulation facilities require development to support realistic and extreme environment testing for future
orbital and landed missions to irradiated icy bodies such as at Europa, Ganymede, Enceladus, and Triton