Examination of the K-band Spectrum of Charon: Possible Evidence for Mul- tiple Ammonia Ices

tiple Ammonia Ices. J. C. Cook1, C. B. Olkin1, S. J. Desch2, R. M. Mastrapa3, T. L. Roush3, A. J. Verbiscer4 Southwest Research Institute, Boulder, CO, 80301 ([email protected]), Arizona State University, Tempe, AZ, 85287, NASA Ames, Moffett Field, CA 94035, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904 Introduction: Remote sensing via near-infrared (NIR) spectroscopy is the only method available to examine the surface composition of Kuiper Belt Objects (KBOs). NIR spectra of Charon, and possibly several other KBOs, have led to the detection of a feature near 2.21 μm, commonly identified as ammonia hydrate [1– 5]. This is somewhat surprising since ammonia ice, which was predicted to be present on icy bodies in the outer solar system, has not been detected. NH2, the photodisassociated product of NH3, is seen in the comae of comets, of which 95% is expected to have originated as NH3 [6]. Based on comets originating from the Kuiper belt and Oort cloud, the NH3/H2O is around 0.5% [7]. Alternatively, infrared observations of protostars [8, 9], the rheology of icy satellites [10], the nitrogen isotopes of Titan’s atmosphere [11] all suggest initial ratios ∼520% which agree well with chemical equilibrium models [12–14]. So the question is: “Where is the ammonia on icy bodies?” Can ammonia take on other forms, such as ammonium (NH+4 )? We present new observations from 2008 combined with previous observations from 2005 of Charon in K-band (1.9-2.4 μm) to reach a resolution. Observations & Data Reduction: We obtained NIR spectra of Charon using NIRI and Altair, the adaptive optics instrument, on the 8-m Gemini North telescope on Mauna Kea. The observations were made over several nights in May and June 2008 when the observed longitude was 300. The goals of the observations were to determine whether or not αN2 ice or hydrocarbon ices were present on Charon, as had been suggested by Verbiscer et al. [15] in previous observations at a similar longitude. The slit width and position of Pluto and Charon was identical to the data collected in 2005 [see 16, for details]. Their position in the slit was dithered in an ABBA pattern. The total integration time was 120 minutes (2008 only), of which about 75% were done under good conditions. During these observations, Charon was 0.8 from Pluto and their spectra were not blended. The spectra were extracted from the 2D images using programs written in IDL following the method of optical extraction [17]. We deviated from this method slightly for the removal of the sky lines using a double subtraction method rather than modeling the sky lines. AB image pairs were first subtracted to remove the sky lines. The five minute individual exposures generally left residual sky lines. We removed the residual sky lines by subtracting the B−A image from the A−B image after aligning the positive spectra of each image. Careful consideration was given to the wavelength calibration before the second step. The sky lines were used to map the distortion seen in the wavelength direction, which was measured up to 40 Å or 9 pix. The IDL bilinear interpolation program was used to make the reconstructed B − A image with a distortion pattern corresponding to the A−B image. This assured proper removal of sky lines and consistent wavelength calibration. This method was applied to the 2005 data for the first time, producing slightly different results than published previously. The spectrum presented in Fig. 1 is a combination of all the observations. We will present at the meeting this spectrum along side the longitudinally resolved spectra and our comparative analysis.