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Ultraviolet spectral reflectance of asteroids

Presentation #101.05D in the session “Asteroid Surveys: Gotta Catch 'em All”.

Published onOct 03, 2021
Ultraviolet spectral reflectance of asteroids

Remote spectroscopy plays an important role in studying the surface physics of asteroids. Compared with longer wavelengths, the ultraviolet (UV) waveband has been demonstrated to be more sensitive to some mineral properties by laboratory measurements, space missions, and remote observations [Butterworth et al. 1985, Cloutis et al. 2008, Becker et al. 2020]. There are also evidences that near-UV wavelengths can be sensitive to space weathering effects [Hendrix et al. 2006]. However, although it has been shown that UV spectra can be extremely useful in analyses of planetary surfaces, only the IUE dataset from more than a decade ago and a few scattered UV observations exist for asteroids’ UV dataset. Instrumental noises of IUE make the spectra only reliable between 240nm and 320nm, which leaves observational gaps in near-UV waveband.

Besides HST, the Neil Gehrels-Swift observatory is currently the only in-orbit facility that has the UV capability to measure asteroids’ UV spectra. Swift’s UV grism has an effective wavelength range from 170nm to 500nm for the first order [Kuin et al., 2015], which is optimized around 260nm. Since 2010, we have observed 18 asteroids with Swift’s UV grism. To more widely compare with general observations such as optical and infrared spectra, the 18 asteroids were selected to variously cover the broadly used taxonomy (cf. Bus 2002). To study rotation effects, every asteroid was exposed for five or six times with an interval from about 20min to 100min.

We have developed a program with the UVOTPY software [Kuin et al. 2015] to analyze the data. The extracted UV flux spectra and derived reflectance spectra reliably covered the waveband between 200nm and 500nm, which well fill the observational gaps, link near UV observations to visible observations, and therefore will benefit comparative study between different wavebands and compositional analysis. For example, In the filled gap of 300-400nm, there are significantly more laboratory studies of meteorites, and planetary analog materials in the RELAB spectral database that can be used to better constrain asteroids’ surface conditions.

[1] Becker, T. M. et al., 2020, PSJ, 1, 53 [2] Bus, S. J., & Binzel, R. P., 2002, Icar, 158, 146 [3] Butterworth, P. S., & Meadows, A. J. 1985, Icar, 62, 305 [4] Cloutis, E. A., McCormack, K. A., Bell, J. F., et al. 2008, Icar, 197, 321 [5] Hendrix, A. R., & Vilas, F. 2006, AJ, 132, 1396 [6] Kuin, N. P. M. et al., 2015, MNRAS, 449, 2514

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