A Sub-pixel Sensitivity Analysis for Atmospheric Retrievals made by the Emirates Mars Infrared Spectrometer (EMIRS) Instrument

Abstract:

In planetary science, remote sensing instruments generally do not make direct observations of desired geophysical and/or atmospheric parameters. Instead, retrieval algorithms that rely on quantifiable light-matter interactions are employed to derive the sought-after parameters. In the case of the Emirates Mars Mission (EMM), dust and water-ice optical depth, surface and lower atmospheric temperatures, and water vapor abundance will all be retrieved from thermal-IR spectra observed by the Emirates Mars Infrared Spectrometer (EMIRS) instrument. While observations made by EMIRS are expected to provide excellent global coverage, specific measurements might not always be possible. In many cases a single field of view (FOV), or pixel, may contain numerous geologic units, emission angles, and surface temperatures, complicating the retrieval process. In order to obtain accurate retrieved quantities related to the dynamics of the lower atmosphere, it is vital we understand how the uncertainty in the retrieved parameters vary with the number of sub-pixels, allowing for the acquisition of reliable results regardless of viewing geometry, latitude/longitude, season, local time, etc.

Synthetic observations of Mars are constructed by populating modeled footprints with various numbers of sub-pixels, representing different requirements for the spatial resolution of the EMIRS forward model. Surface temperatures and vertical profiles of atmospheric temperature and aerosols are modeled at the sub-pixel level with top of atmosphere (TOA) spectra, as observed by EMIRS, computed using the Spectral Mapping Atmospheric Radiative Transfer (SMART) code. Resulting synthetic spectra are compared against a “truth” computation performed at very high spatial resolution (256 sub-pixels) to determine the minimum number of computed sub-pixels to accurately model Mars to within 0.2 x the Noise Equivalent Spectral Radiance (NESR) of EMIRS. A range of scenarios will be investigated to explore the dependence a variety of factors, including emission angle, time of day, and season have on the variation of synthetic spectra within a single EMIRS footprint. It is expected that variations in surface temperature will influence the number of required sub-pixels more significantly than changes in dust loading or aerosol abundance.