LAUSR

The high-resolution echelle mode of the Imaging Ultraviolet Spectrograph (IUVS) instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) mission has been designed to measure D and H Lyman-α emissions from the martian atmosphere to obtain key information about the physical processes by which water escapes into space. Toward this goal, the absolute calibration of the instrument is critical for determining the D and H densities, the D/H ratio, and the escape flux of water. The instrument made observations of interplanetary hydrogen (IPH) along multiple look directions and conducted several post-launch calibration campaigns during cruise as well as during orbit around Mars. The calibration efforts monitored instrument degradation and produced a consistent calibration factor at the hydrogen Lyman-α wavelength (121.567 nm). The instrument was calibrated with the diffuse emission of interplanetary hydrogen (IPH) as a standard candle using measurements and model results from the Solar Wind Anisotropies (SWAN) instrument. Validation of the calibrated instrument was made by (1) comparisons to simultaneous observations of the IPH made with the lower resolution FUV-mode of the IUVS instrument that were independently calibrated using standard stars as well as by (2) comparisons to same-day observations of Mars at hydrogen Lyman-α made with the Hubble Space Telescope that were calculated with a radiative transfer model. The FUV-mode stellar calibrated values and HST-based model results agreed with the echelle SWAN calibrated values to within 6% and 4%, respectively. The calibrated IUVS instrument can be used to interpret emissions of atmospheric species at Mars for insights into water evolution at the planet, as well as observed IPH measurements made during cruise for further insights into dynamics of the inner heliosphere.