Space Weathering: Application to Asteroids and other Bodies

1
Space Weathering Application to Asteroids and
other Bodies Sky Beard, Vincent Chevrier
University of Arkansas Center for Space and
Planetary Sciences, Fayetteville
Arkansas spbeard_at_purdue.edu, vchevrie_at_uark.edu

• Introduction
• Space weathering is the combined affect of mainly
  solar wind ion bombardment and micrometeorite
  impacts 1
• Causes darkening, reddened spectrum, and
  weakened absorption bands 2
• Problems/Questions
• Space weathering occurs on the Moon, but what
  about other objects?
• Example Eros
• NEAR Shoemaker Mission found similar spectrum to
  ordinary chondrites, but significantly less
  sulfur 3. Space weathering affect?
• Objective
• Explore affects of exposure to solar ions on the
  spectrum of different mineral and meteorite
  samples

Before
After
Fig 3 Olivine (A) and Anorthosite (B) spectrums
at different exposure times Blue- 0 hr exposure,
Orange- 1hr exposure, Purple- 2hr exposure,
Green- 4hr exposure. Notice spectrum generally is
flatter, albedo decreases, and shallower bands as
exposure time increases.
A

• Discussion
• Spectrum
• Shallower and broader bands as exposure time
  increased (Fig 4)
• Altering of composition
• Would alter remote sensing results, consistent
  with ideas of space weathering on asteroids

B
C

• Method
• Technics Hummer II Sputter Coater used to
  simulate solar wind with Argon Ions 2 keV
  potential (Fig 1 A)
• Samples (so far) lt 63 um Alumina, Anorthosite,
  and Olivine on aluminum holder
• Put in vacuum, exposed to plasma (Fig 1B)
• Starting pressure kept at 75 mTorr
• Plasma maintained at 20 mAmps for desired time
• Spectrum taken with 6700 Nicolet Fourier
  Transform Infrared (FTIR) spectrometer in the
  Near Infrared range

Fig 2 Sample before and after irradiation. A
Alumina, B Anorthosite, C Olivine
A
Reflectance
Fig 4 Depth of the 4 bands in Olivine spectrum
(Fig 3 A) becoming shallower as exposure time
increases. Blue- 1st band, Pink- 2nd band,
Orange- 3rd band, and Green- 4th band.
A
B

• Future Work
• Meteorite samples
• See if can reproduce known asteroid spectrum
• Run samples through X-Ray Diffraction (XRD) to
  determine alteration of crystal structure

B
Fig. 1 A Hummer II Sputtering Unit B
Anorthosite exposed to plasma
Reflectance

• Results/Observations
• All samples darkened after exposure (Fig 2)
• Generally getting darker with time
• Electrical discharging occurred on sample and on
  Al foil
• Discharging decreased over time
• Spectra shows shallower and broader features
  compared to the unweathered spectrum (Fig 3)

Acknowledgements Special thanks for invaluable
assistance from Walter Graupner, Melissa Jones,
Derek Sears, Dan Ostrowski, and Travis Altheide
References 1 Sears, Derek W.G. and Kracher,
Alfred, Icarus 174, 2005. 2 Wiesli, ReneA.,
Beard, Brian L., Earth and Planetary Science
Letters 216, 457-465, 2003. 3 McCoy, T.J.,
Burbine, T.H. Meteorit. Planet. Sci. 36, 2001.