Water vapor on Mars: a refined climatology and constraints on the
near-surface concentration enabled by synergistic retrievals
Abstract
With the utilization of a novel synergistic approach, we constrain the
vertical distribution of water vapor on Mars with measurements from
nadir-pointing instruments. Water vapor column abundances were retrieved
simultaneously with PFS (sensing the thermal infrared range) and SPICAM
(sensing the near-infrared range) on Mars Express, yielding distinct yet
complementary sensitivity to different parts of the atmospheric column.
We show that by exploiting a spectral synergy retrieval approach, we
obtain more accurate water vapor column abundances compared to when only
one instrument is used, providing a new and highly robust reference
climatology from Mars Express. We present a composite global dataset
covering all seasons and latitudes, assembled from co-located
observations sampled from seven Martian years. The synergy also offers a
way to study the vertical partitioning of water, which has remained out
of the scope of nadir observations made by single instruments covering a
single spectral interval. Special attention is given to the north polar
region, with extra focus on the sublimation of the seasonal polar cap
during the late spring and summer seasons. Column abundances from the
Mars Climate Database were found to be significantly higher than
synergistically retrieved values, especially in the summer Northern
Hemisphere. Deviances between synergy and model in both magnitude and
meridional variation of the vertical confinement were also discovered,
suggesting that certain aspects of the transport and dynamics of water
vapor are not fully captured by current models.