The radiometric environment for Mars limb observations by the Mars Sample Return Earth Return Orbiter

Kurzfassung

After launching from the Martian surface via the Mars Ascent Vehicle (MAV), the Orbital Sample (OS) containing the samples collected on Mars must be identified by the Narrow Angle Camera (NAC) on the Earth Return Orbiter (ERO) spacecraft to determine the exact orbit of the capsule before rendezvous. The NAC will observe above the limb of Mars and must find the small OS from a large distance, so noise contributions from straylight must be well characterized to ensure detection of the OS. Initial studies conducted at JPL suggested that Martian clouds could be a major straylight source. In response, ESA has established a Tiger Team (Cloud Tiger Team, CTT) with members from the science community, industry and space agencies. The CTT has assessed the radiometric environment at Mars likely to be encountered by the NAC---from the surface, to atmospheric aerosols, to high-altitude clouds---using the High Resolution Stereo Camera (HRSC) onboard Mars Express (MEx) and the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter (MRO). HRSC is a pushbroom scanning instrument onboard MEx that obtains multi-spectral high-resolution and stereoscopic imagery of the Martian surface and has been in operation since 2004. It has four color channels (blue, green, red, and near-IR) as well as broader panchromatic channels. From high-altitude broom observations and limb observations, vertical profiles of atmospheric radiance from the surface to 50-80 km can be extracted. For this analysis, we have selected more than 60 vertical radiance profiles from the panchromatic nadir channel (585-765 nm), taken from observations between 2004 and 2021. MCS is a limb and on-planet viewing radiometer onboard MRO with two telescopes and nine spectral channels (5 mid-IR, 3 far IR, and one visible/near-IR). Each channel has a linear array of 21 detectors with which MCS instantaneously measures radiance profiles of the limb evert two seconds from the surface to about 80 km altitude at 5 km vertical resolution. MCS typically observes in the forward limb ("in-track") direction of the spacecraft's motion (3AM/PM). In MY33, in addition to standard in-track viewing, MCS performed cross-track observations -- viewing the limb 90° to the left or right of the spacecraft's motion -- nearly continuously, enabling measurements at two additional local times (3 AM/PM +/- 1.5 hours) and covering a wider range of phase angles than other MYs. Here, we analyze radiance profiles from MY33 and MY34 from the visible/near-IR A6 channel (0.3-3 micron). Given the expected launch window of the MAV of Ls=60-180° and OS detection within 30 days, we focused on four periods, Ls=60-120°, Ls=120-180°, Ls=180-230°, and Ls=180-230° during the MY34 global dust storm. We have restricted the observations from both HRSC and MCS to those between latitudes 45°S-45°N due to the equatorial orbit of the ERO. Additionally, we ignore observations with high phase angles (>150°) considering a NAC Sun exclusion angle of 30° and poor OS illumination conditions at high phase angles. Both the HRSC and MCS radiances are converted to a common 400-900 nm bandpass representative of the NAC. For each Ls and phase angle bin we compare radiance profiles from HRSC and MCS and find good agreement between the two instruments. We construct a bounding profile from the maximum radiance at 5 km intervals from 0-90 km. Radiances are typically higher in HRSC below 30 km and in MCS above 30 km. At most altitudes, we find higher radiances as Ls progresses toward the dusty season and at the highest phase angles. At specific latitudes, high altitude aerosols are present in 1-5% of observations and significantly increase the worst-case radiance above 50 km. During the MY34 global dust storm, radiances above 40 km are larger than during the same season in MY33. The bounding profiles provide important input for straylight calculations of the NAC and for the validation of models that may be used as input for straylight calculations.