14N15N detectability in Pluto's atmosphere

Based on the vapor pressure behavior of Pluto's surface ices, Pluto's atmosphere is expected to be predominantly composed of N-2 gas. Measurement of the N-2 isotopologue N-15/N-14 ratio within Pluto's atmosphere would provide important clues to the evolution of Pluto's atmosphere from the time of formation to its present state. The most straightforward way of determining the N-2 isotopologue N-15/N-14 ratio in Pluto's atmosphere is via spectroscopic observation of the (NN)-N-14-N-15 gas species. Recent calculations of the 80-100 nm absorption behavior of the N-14(2) and (NN)-N-14-N-15 isotopologues by Heays et al. (Heays, A.N. et al. [2011]. J. Chem. Phys. 135, 244301), Lewis et al. (Lewis, B.R., Heays, A.N., Gibson, S.T., Lefebvre-Brion, H., Lefebvre, R.[2008]. J. Chem. Phys. 129, 164306); Lewis et al. (Lewis, B.R., Gibson, S.T., Zhang, W., Lefebvre-Brion, H., Robbe, J.-M. [2005].J. Chem. Phys. 122, 144302), and Haverd et al. (Haverd, V.E., Lewis, B.R., Gibson, S.T., Stark, G. [2005]. J. Chem. Phys. 123, 214304) show that the peak magnitudes of the N-14(2) and (NN)-N-41-N-15 absorption bandhead cross-sections are similar, but the locations of the bandhead peaks are offset in wavelength by similar to 0.05-0.1 nm. These offsets make the segregation of the N-14(2) and (NN)-N-14-N-15 absorption signatures possible. We use the most recent N-2 isotopologue absorption cross-section calculations and the atmospheric.density profiles resulting from photochemical models developed by Krasnopolsky and Cruickshank (Krasnopolsky, V.A., Cruickshank, D.P. 11999]. J. Geophys. Res. 104, 21979-21996) to predict the level of solar light that will be transmitted through Pluto's atmosphere as a function of altitude during a Pluto solar occultation. We characterize the detectability of the isotopic absorption signature per altitude assuming (NN)-N-14-N-15 concentrations ranging from 0.1% to 2% of the N-14(2) density and instrumental spectral resolutions ranging from 0.01 to 0.3 nm. Our simulations indicate that optical depth of unity is attained in the key (NN)-N-14-N-15 absorption bands located between 85 and 90 nm at altitudes similar to 1100-1600 km above Pluto's surface. Additionally, an (NN)-N-14-N-15 isotope absorption depth similar to 4-15% is predicted for observations obtained at these altitudes at a spectral resolution of similar to 0.2-0.3 nm, if the N-2 isotopologue N-15/N-14 percent ratio is comparable to the 0.37-0.6% ratio observed at Earth, Titan and Mars. If we presume that the predicted absorption depth must be at least 25% greater than the expected observational uncertainty, then it follows that a statistically significant detection of these signatures and constraint of the N-2 isotopologue N-14/N-15 ratio within Pluto's atmosphere will be possible if the attainable observational signal-to noise (S/N) ratio is >= 9. The New Horizons (NH) Mission will be able to obtain high S/N, 0.27-0.35 nm full-width half-max 80-100 nm spectral observations of Pluto using the Alice spectrograph. Based on the NH/Alice specifications we have simulated 0.3 nm spectral resolution solar occultation spectra for the 1100-1600 km altitude range, assuming 30 s integration times. These simulations indicate that NH/Alice will obtain spectral observations within this altitude range with a SIN ratio similar to 25-50, and should be able to reliably detect the (NN)-N-14-N-15 gas absorption signature between 85 and 90 nm if the (NN)-N-14-N-15 concentration is similar to 0.3% or greater. This, additionally, implies that the non-detection of the (NN)-N-14-N-15 species in the 1100-1600 km range by NH/Alice may be used to reliably establish an upper limit to the N-2 isotopologue N-15/N-14 ratio within Pluto's atmosphere. Similar results may be derived from 0.2 to 03 nm spectral resolution observations of any other N-2-rich Solar System or exoplanet atmosphere, provided the observations are attained with similar S/N levels. (C) 2013 Elsevier Inc. All rights reserved.