Characterization of aerosols during the dusty season in the Tharsis region on Mars

Introduction and Methods

We characterized the seasonal variations of water ice clouds and dust in the Tharsis region during the dusty season using observations from the Planetary Fourier Spectrometer (PFS) aboard the Mars Express (MEx) mission, the Mars Climate Sounder (MCS) and images from Mars Color Imager (MARCI) aboard the Mars Reconnaissance Orbiter (MRO) spacecraft. We selected four Martian years (MYs): 28, 31, 34 and 35 for our investigation of years with and without global dust storms (GDSs). We analyzed thermal fields with total dust and water ice optical depths obtained from the PFS and MCS instruments, and we compared them with MARCI images during the dusty season. We defined quasi-simultaneous measurements with images as observations taken within a Ls difference of 5° and an LT of 3 hours for the Tharsis region. We investigated the altitudes of hygropause based on MARCI images and the thermal fields retrieved from PFS observations. We defined the hygropause as the layer in which the difference between condensation and atmospheric temperatures is approximately 0 ± 0.5 K.

Results

We identified three Ls intervals of cloud occurrence during all studied MYs: the first (180° – 210°), the second (240° – 270°) and the third (330° – 360°). The northeastern and east clouds of Olympus are observed in all Ls intervals. Clouds are found around Pavonis, as well as filament of clouds connected to Arsia during three seasons. During the first period, clouds appear on the eastern side of Ascraeus. Clouds as a southern filament, as well as clouds over the caldera of Ascraeus characterize the second period. In the third period, clouds are mostly visible over the caldera and the northeast side of Ascraeus. Clouds were observed over the southwestern region of Arsia, with some occurring as a northeastern filament to Pavonis during three periods.

Unlike clouds, dust occurs everywhere. Dust is suspended in the atmosphere close to the surface, especially in valleys around volcanoes. Decreases in dust content are observed above volcanoes in PFS observations and in MARCI images. MARCI images clearly show the features of volcanoes’ calderas. In MY 28, the GDS in the Tharsis region began around 20° of Ls later than the global onset, except for Arsia. For Arsia, the GDS began around 5° later than the global onset. This is probably associated with the fact that the GDS began as a series of dust storms mainly over Noachis Terra [1,2,3]. This delay was not observed at the onset of the GDS in MY 34. This is likely because the GDS began in the northern regions, such as Chryse Planitia [4].   

During the decline of GDSs, we observed decreases in atmospheric dust content between PFS observations and MARCI images. PFS observations were usually taken at LT = 9 or at LT = 21, whereas MARCI images were always taken between LTs = 13 – 16. We observed a significant difference in atmospheric dust amounts between the afternoon and evening LTs.

Many MCS and PFS coincidence measurements show consistent results in thermal fields as well as in aerosol optical depths. In some cases, the lack of agreement between the two instruments was due to the differences in the days or locations of the measurements.

Conclusions

The clouds over the four Tharsis volcanoes evolve with the seasons and change location according to the wind direction. Trade winds could be responsible for the origin of clouds near volcanoes [5]. Indeed, the locations of these clouds - northeast for Olympus and Ascraeus and southwest for Arsia - align with the directions of the trade winds. During the third period, most of the clouds over Ascraeus and Pavonis likely originated from local mountain circulation. The clouds over Pavonis during the first period may also be due to local mountain circulation because of weak global winds. The upward winds along the slopes could explain the origin of these clouds. However, [6] suggested that the water vapor pumping mechanism by upslope winds during the dusty season is unlikely due to the high altitude of the hygropause and the low amount of water vapor in the lower atmosphere.

On the other hand, we demonstrated that minimal hygropause altitudes could be as low as 18 km and maximal ones as high as 50 km. Therefore, we cannot rule out the possibility of the mechanism proposed by [7] and [8], at least for Pavonis Mons during the first period. A second explanation for the origin of the clouds, especially for Pavonis Mons, could be the “fountain” model proposed by [9]. In this case, the sun heats the volcano slopes sufficiently to drive this mechanism. As a result, hot air rises by strong upward motions, reaching altitudes of around 30-40 km. This causes a cold pocket due to the adiabatic cooling of the uplifted air. Thus, water vapor transported from the surface by these motions could condense in regions of the cold pocket. The altitudes of the cold pockets are similar to the altitudes of the hygropause during the dusty season in the Tharsis region. [6] proposed another explanation in which gravity waves played a primary role by perturbing hygropause. They stated that gravity waves caused orographic cloud formations like AMEC during the perihelion season in the early morning.

Acknowledgments

This work was funded by an internal project ‘mini-grant’ received by PW from INAF. The PFS experiment was built at the Institute for Space Astrophysics and Planetology (IAPS) of the National Institute for Astrophysics (INAF) and is currently funded by the Italian Space Agency (ASI; agreement number 2026-6-HH.0).

References

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