Mars water cycle: an 11 Mars year climatology of water vapor by SPICAM on Mars Express

Water vapor has long been a key target in Martian exploration, as its detection confirmed the existence of an active water cycle driven by dynamic exchanges between surface ice reservoirs and the atmosphere. Since its first spectroscopic identification in 1963, ongoing observations—most notably from orbiting spacecraft—have significantly advanced our understanding of the spatial and temporal behavior of water on Mars.

In this study, we present a comprehensive climatology of water vapor column abundances spanning 11 Martian years (MY), derived from observations by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument aboard the European Space Agency’s Mars Express mission [1]. Operating in nadir, SPICAM measures the near-infrared sunlight reflected from the Martian surface and atmosphere, providing daytime water vapor data with broad seasonal and latitudinal coverage. However, due to its reliance on solar illumination, SPICAM is unable to probe the polar night, where water vapor is predicted to be extremely scarce and mostly below the instrument’s detection threshold.

Despite the limitations imposed by the orbital configuration of Mars Express—which results in uneven spatial and temporal coverage—SPICAM has successfully monitored the Martian atmosphere across nearly all seasons and latitudes during local daytime conditions. This long-term dataset offers a unique opportunity to study interannual variability in the water cycle, including responses to major atmospheric perturbations.

Our climatology includes two Martian years that experienced Global Dust Events (GDEs), allowing us to conduct a preliminary assessment of how such planet-encircling storms impact water vapor distribution. We also perform cross-comparisons with water vapor datasets from other past and ongoing missions, addressing a long-standing challenge in reconciling inter-mission measurements.

To enhance the completeness of the climatology, we apply the kriging method—a well-established geostatistical interpolation method based on Gaussian process regression—to estimate water vapor values in regions and seasons with sparse coverage. This gap-filling enables a more continuous picture of the Martian water cycle and facilitates the analysis of year-to-year variability.

Finally, by averaging over the full 11-MY dataset, we construct a reference annual cycle of water vapor on Mars, which serves as a baseline for future comparisons, model validation, and the identification of anomalous behavior.

To further improve the accuracy and vertical sensitivity of water vapor retrievals, we also incorporate results from a synergistic retrieval approach developed by [2] and [3] This method combines simultaneous nadir-pointing observations from SPICAM (in the near-infrared) and the Planetary Fourier Spectrometer (PFS, in the thermal infrared), both aboard Mars Express. Individually, each instrument is sensitive to different portions of the atmospheric column—SPICAM to the lower atmosphere under illuminated conditions, and PFS to higher altitudes through thermal emission. When used together in a joint retrieval framework, they offer a more complete and vertically constrained view of water vapor distribution than either instrument alone.

The synergy method thus yields more accurate water vapor column abundances and enables the first nadir-based estimates of vertical partitioning of water vapor—an aspect traditionally inaccessible to single-instrument nadir retrievals. The resulting composite dataset, which spans almost the entire SPICAM survey, has proven to be highly robust and serves as an important reference for climatological studies. Notably, the synergy also reveals significant discrepancies with predictions from the Mars Climate Database, especially in the northern hemisphere during summer, highlighting potential limitations in current models of water vapor transport and vertical confinement.

Finally, in addition to nadir observations, SPICAM also conducted measurements in solar occultation mode [4], which allowed the retrieval of vertical profiles of water vapor at high vertical resolution (~1–2 km), primarily during the twilight terminator. These observations complement the nadir dataset by providing a window into the vertical structure of water vapor in the lower and middle atmosphere (typically from 10 to 70 km), including its diurnal variations and seasonal evolution. Solar occultation data are especially valuable in characterizing the hygropause altitude, tracking the seasonal ascent and descent of water vapor, and capturing sharp vertical gradients during northern summer, when water transport to high altitudes is most active.

This work not only contributes to a more detailed understanding of Mars’ water cycle dynamics but also provides critical observational constraints for atmospheric models and climate evolution studies.

[1] Montmessin, F., Korablev, O., Lefèvre, F., Bertaux, J.-L., Fedorova, A., Trokhimovskiy, A., et al. (2017). SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere. Icarus, 297, 195–216. https://doi.org/10.1016/j.icarus.2017.06.022 

[2] Montmessin, F., & Ferron, S. (2019). A spectral synergy method to retrieve martian water vapor column-abundance and vertical distribution applied to Mars Express SPICAM and PFS nadir measurements. Icarus, 317, 549–569. https://doi.org/10.1016/j.icarus.2018.07.022

[3] Knutsen, E. W., Montmessin, F., Verdier, L., Lacombe, G., Lefèvre, F., Ferron, S., et al. (2022). Water Vapor on Mars: A Refined Climatology and Constraints on the Near‐Surface Concentration Enabled by Synergistic Retrievals. Journal of Geophysical Research: Planets, 127(5). https://doi.org/10.1029/2022JE007252

[4] Fedorova, A., Montmessin, F., Korablev, O., Lefèvre, F., Trokhimovskiy, A., & Bertaux, J. (2021). Multi‐Annual Monitoring of the Water Vapor Vertical Distribution on Mars by SPICAM on Mars Express. Journal of Geophysical Research: Planets, 126(1). https://doi.org/10.1029/2020JE006616