NASA announced that geoscientists identified a rare-metal asteroid worth more than total global economy, [there is much hype in these announcements, but the cost and economic impact remains to be seen?]. Becker and 9 co-authors reported in late 2020 that the Main Belt Asteroid called 16 Psyche is the target object of the NASA Discovery Mission Psyche. The authors observed the asteroid at ultraviolet (UV) wavelengths (170–310 nm) using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope during two separate observations. They reported that the spectrum is very red in the UV, with a blue upturn shortward of ∼200 nm. They found that an absorption feature at 250 nm and a weaker absorption feature at 275 nm that may be attributed to a metal-oxide charge transfer band.
The authors also found that the red-sloped, relatively featureless spectrum of (16) Psyche is best matched with the reflectance spectrum of pure iron; however, our intimate mixture models show that small grains of iron may dominate the reflectance spectrum even if iron only comprises up to 10% of the material on the surface. They also stressed that there is a limited database of reflectances for planetary surface analogs at UV wavelengths for comparison with the spectrum of (16) Psyche. The mid- and far-UV spectra (<240 nm) are markedly different for each of the four asteroids observed at these wave-lengths so far, including ones in the same spectral class, indicating that UV observations of asteroids could be used to better understand differences in the composition and processing of the surfaces of these small bodies.
The authors concluded that the UV observations of the asteroid Psyche from 170 to 310 nm is much redder than in the visible spectrum, consistent with previous IUE observations of the asteroid. We did not find significant spectral variations with rotation, though the equatorial region of the asteroid may have a higher overall reflectance than the northern hemisphere. They reported weak absorption features near 250 and 270 nm, which may be attributed to iron-oxide charge transfer bands. The spectrum displays an FUV upturn at wavelengths <200 nm, which may have implications for space weathering.
Due in part to the very limited spectral reflectance database in the UV, they do not find any strong analogs to Psyche in the UV.
The enstatite chondrites for which UV observations exist do not serve as good UV–Vis analogs for Psyche. Pure iron or linear mixture models with materials mixed with significant amounts of iron reproduce the overall shape of Psyche in the UV and visible wavelengths better than any tested material alone. Intimate mixture reflectance models assuming 1 μm iron grains mixed with olivine show that even small amounts of iron (∼10%) can reproduce the overall shape of Psyche’s spectrum, making it difficult to quantify the amount of iron that may be present on the surface of Psyche.
More UV laboratory work is needed to produce a robust library from which the composition of, and space weathering effects on, planetary bodies can be better constrained. Bidirectional reflectance distribution function UV lab measurements will be especially useful for future modeling of the spatial variations along the slit for the different hemispheres viewed by HST here and in future observations. The diversity of spectral features and slopes observed in the four asteroids studied at FUV and mid-UV wavelengths implies that there is still much that can be learned about asteroid surfaces through their examination in the UV spectrum.
[It should be noted that the metal spectral resolutions are not sufficient to identify other metals beyond iron for now. If meteorite compositions are any guide, there will be substantial concentrations of other metals in the various types of asteroids.]
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