Near-IR Spectroscopy of Amalthea (JV) and Thebe (JXIV)

D. Bustani, J.F. Bell III, J. Veverka (Cornell), R.H. Brown (LPL, U. Arizona), D.P. Cruikshank (NASA Ames)

We used the NASA IRTF, the NSFCAM 256 tex2html_wrap_inline15 256 InSb array camera, and the JPL Cooled Coronograph (CoCo) to obtain H and K spectra of the faint inner Jovian satellites Amalthea (JV, tex2html_wrap_inline17 =14.1) and Thebe (JXIV, tex2html_wrap_inline17 =15.7). The observations were obtained on 25 and 26 July 1996 UT, near Jupiter opposition, when the satellites had a maximum limb elongation of 36" (JV) and 49" (JXIV). CoCo was designed for imaging of faint sources in the vicinity of much brighter sources (e.g., stellar companions), but in order to further supress scattered light from Jupiter (an extended source tex2html_wrap_inline21 tex2html_wrap_inline23 times brighter than these faint satellites), we inserted a special hard mask aperture ahead of the NSFCAM/CoCo optical path. In order to obtain spectra with this setup we attached a crude tex2html_wrap_inline21 2" wide razor-edge slit to the top of the hard mask. The resulting spectral resolution is moderate (R tex2html_wrap_inline21 440 in H and R tex2html_wrap_inline21 870 in K) and is adequate for initial near-IR survey spectra of these objects.

We obtained 4 spectra each of JV and JXIV in H (1.44 to 1.98 tex2html_wrap_inline31 m) and 4 spectra each of JV and JXIV in K (2.0 to 2.35 tex2html_wrap_inline31 m), as well as comparison spectra of Titan, Mimas, two solar-type stars, and Ne and Kr wavelength calibration lamps. Our calibrated Titan and Mimas CoCo spectra are consistent with previous H and K measurements of these objects.

Our preliminary spectra of Amalthea reveal a relatively featureless spectrum in the 1.44 to 1.98 tex2html_wrap_inline31 m and 2.06 to 2.17 tex2html_wrap_inline31 m regions, but show evidence for an as-yet unidentified absorption from 2.20 to 2.30 tex2html_wrap_inline31 m, apparently centered near 2.27 tex2html_wrap_inline31 m. Our preliminary spectra of Thebe reveal a relatively featureless spectrum from 1.44 to 1.65 tex2html_wrap_inline31 m and from 2.00 to 2.27 tex2html_wrap_inline31 m, but show evidence for an as-yet unidentified weak absorption from tex2html_wrap_inline21 1.70 to tex2html_wrap_inline21 1.80 tex2html_wrap_inline31 m, centered near 1.75 tex2html_wrap_inline31 m. Continued refinement of the reflectance and wavelength calibrations is proceeding, and possible origins and band assignments for these near-IR absorption features are being investigated.