Global wind map on Mars during and after the global dust storm in 2018 measured by ALMA

The wind velocity field plays a fundamental role in understanding atmospheric physics. Gaining insight into the three-dimensional structure of atmospheric circulation leads to a deeper comprehension of the mechanisms governing the transport of atmospheric constituents. On Mars, however, direct measurements of wind velocities in the middle atmosphere remain scarce due to observational limitations. A few studies have successfully obtained such measurements using high spectral resolution spectroscopy based on the Doppler shift technique (e.g., Moreno et al., 2009; Sonnabend et al., 2012). Comparisons between these observations and global circulation models (GCM) predictions revealed that GCMs tend to underestimate wind speeds and fail to reproduce the observed spatial variability (Moreno et al., 2009). Discrepancies with these earlier measurements, conducted sporadically using ground-based telescopes, highlight the need for additional observational datasets to improve our understanding of Martian middle atmospheric dynamics.

 

One of the most striking phenomena on Mars is the episodic planet-encircling "global dust storm." When such a storm occurs, the optical depth at visible wavelengths can increase by a factor of 10–25 compared to non-storm conditions. The suspended dust particles block sunlight from reaching the surface and absorb solar radiation, resulting in atmospheric heating. Conversely, surface temperatures drop due to the shielding effect of the dust. This dramatic alteration in the thermal structure significantly affects atmospheric dynamics and composition. GCM simulations have predicted that wind velocities in the middle atmosphere can increase by up to 100 m/s during a global dust storm compared to normal conditions (Medvedev et al., 2013). Supporting this, Miyamoto et al. (2021) reported considerably stronger zonal winds (150–200 m/s) during the global dust storm of June–July 2018. Notably, they observed that the intensified winds persisted through late August 2018, marking the end of the storm's decay phase. Further observations are essential to elucidate the evolution of middle atmospheric dynamics during and after global dust storms.

 

This study aims to directly measure wind velocities in the Martian middle atmosphere using data from the Atacama Large Millimeter/submillimeter Array (ALMA). Compared to previous instruments, ALMA offers superior signal-to-noise ratio and enhanced spatial, temporal, and spectral resolution, enabling a global perspective on Martian atmospheric dynamics. ALMA is composed of two arrays: a long-baseline array with 12-m diameter antennas, commonly referred to as the “12-m array”, and a compact-baseline array known as the Atacama Compact Array (ACA). The 12-m array provides higher spatial resolution, while the ACA offers moderate resolution. An intensive monitoring campaign of the Martian atmosphere using the ACA was conducted from June to September 2018, encompassing the peak and decay phases of the 2018 global dust storm.

The observations were conducted on June 21, June 30, July 11, August 12, August 17, August 24, August 28, September 3, September 5, September 10, September 15, September 19, September 23, and September 28, 2018. The rotational transition of carbon monoxide (CO) J = 2 – 1 at 230.538 GHz was observed. The spatial resolution of ACA is ~5.5 arcsec at this frequency, while the Martian apparent angular diameter changed from 16 to 23 arcsec during the observed period. We obtained Doppler wind maps derived from the CO line, which is sensitive to altitudes of approximately 55–75 km. The temporal evolution of these wind maps will be discussed in comparison with predictions from a general circulation model.

 

References:

Medvedev, A. S., Yiğit, E., Kuroda, T., & Hartogh, P. (2013). General circulation modeling of the Martian upper atmosphere during global dust storms. Journal of Geophysical Research: Planets, 118, 2234–2246. https://doi.org/10.1002/2013JE004429

 

Miyamoto, A., Nakagawa, H., Kuroda, T., Takami, K., Murata, I., Medvedev, A. S., et al. (2021). Intense zonal wind in the Martian mesosphere during

the 2018 planet-encircling dust event observed by ground-based infrared heterodyne spectroscopy. Geophysical Research Letters, 48, e2021GL092413. https://doi.org/10.1029/2021GL092413

 

Moreno, R., Lellouch, E., Forget, F., Encrenaz, T., Guilloteau, S., & Millour, E. (2009). Wind measurements in Mars' middle atmosphere: IRAM Plateau de Bure interferometric CO observations. Icarus, 201(2), 549–563. https://doi.org/10.1016/j.icarus.2009.01.027

 

Sonnabend, G., Sornig, M., Kroetz, P. J., & Stupar, D. (2012). Mars mesospheric zonal wind around northern spring equinox from infrared heterodyne observations of CO2. Icarus, 217(1), 315–321. https://doi.org/10.1016/j.icarus.2011.11.009