Effects of the global dust storm on the Mars’ upper atmosphere as observed by the Mars Orbiter Mission
Terrestrial planets in the solar system are constantly losing their atmospheres to outer space. The rate at which this loss happens is determined mainly by the size of a planet and temperature of its upper atmosphere. Mars, being a relatively smaller planet compared to Earth, is losing its atmosphere at a faster rate. This loss rate, however, is altered by the changes in the upper atmospheric temperature. Therefore, characterizing the Martian upper atmosphere is extremely important to understand this loss rate and is one of the primary goals of the recent missions to Mars such as NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) and ISRO’s Mars Orbiter Mission (MOM). A global dust storm, more precisely called a “planet-encircling dust event”, is one of the dynamical meteorological phenomena on Mars that can raise its upper atmospheric temperatures by several degrees. As a result, effects of these global dust storms on the Martian upper atmosphere are expected to be multi-fold, some of which are yet to be uncovered. Towards revealing some of these effects, recent measurements by the Indian MOM spacecraft have been playing an important role.
In the first week of June 2018, a global dust storm started growing on Mars and it has grown to its mature phase by the first week of July, 2018. The growth of the global dust storm can be seen in the bottom panel of Figure 1 which shows the 9.3μm infrared absorption column dust optical depth (CDOD) near the Mars surface. Larger values of CDOD indicate more dust on Mars. During this time, MOM spacecraft observed the evening side of Mars by diving down to altitudes as low as 155 km. The Mars Exospheric Neutral Composition Analyser (MENCA) instrument, which is a mass spectrometer on MOM, measured the neutral densities of the Mars’ thermosphere (which lies between 100 and 200 km). By analysing these measurements, scientists at the National Atmospheric Research Laboratory, Gadanki have found that Mars upper atmosphere is undergoing warming and expansion. As the dust storm slowly engulfs Mars over a month period (black dotted curve in the bottom panel of Figure 1), scientists have found that the neutral densities in the Mars thermosphere increased significantly (blue dots in the top panel of Figure 1). Such an increase is also confirmed by the MAVEN mission, which was simultaneously measuring the Martian thermosphere on the morning side (blue dots in the middle panel of Figure 1). Note that, to bring out the dust storm effects clearly, nominal local time variations of the densities are constructed from MAVEN measurements and are removed from the densities measured during the dust storm time. The Ar densities measured by MOM on the evening side are always larger than those measured by MAVEN on the morning side (Figure 1). Such a difference is mainly due to the warming on the evening side and cooling on the morning side, caused by atmospheric circulation.
The dust storm effects shown in Figure 1 are illustrated through a schematic diagram in Figure 2 which shows the state of the Martian thermosphere without a dust storm and its expansion due to a dust storm (dotted blue area). The global dust storm significantly heated and expanded the Martian upper atmosphere so that at any given altitude the densities are higher during the dust storm than without it. From these results it is inferred that the heating and expansion of global dust storm lead to a part of its atmosphere quickly reaches the exobase altitude (which lies at ~ 220 km). Any hot gases above the exobase altitude are more likely to move to further higher altitudes and subsequently escape to outer space. Hence, from the results of the present study it can be inferred that the 2018 global dust storm resulted in enhanced escape of the Martian atmosphere.
These results have been recently published in the Journal of Geophysical Research- Planets, a journal of the American Geophysical Union (AGU) (https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JE006430). This study reiterates the scientific potential of the MOM spacecraft and advantages of multi-spacecraft measurements in exploring the planetary atmospheres.
Figure 1: Effects of the 2018 global dust storm on the Martian thermosphere. First and second panels show the changes in the Ar densities at 170 km observed by the MOM and MAVEN spacecraft, respectively. The densities shown here are after removing the local time variations. Bottom panel shows the 9.3μm infrared absorption column dust optical depth (CDOD) near to Mars surface. CDOD acts as a dust index such that larger values of CDOD indicate more dust on Mars (modified from Venkateswara Rao et al., 2020).
Figure 2: An artistic impression of the Mars upper atmospheric expansion due to the global dust storm. The black arrows in the image indicate the expansion of the upper atmosphere due to the global dust storm (note that the relative sizes of MOM spacecraft, Mars and its upper atmospheric boundaries are not drawn to scale). Image of Mars shown here is taken by the Mars Colour Camera, MCC/MOM (such an image is not available from MCC during the dust storm period).
Venkateswara Rao, N., Gupta, N., & Kadhane, U. R. (2020). Enhanced densities in the Martian thermosphere associated with the 2018 planet‐ encircling dust event: Results from MENCA/MOM and NGIMS/MAVEN. Journal of Geophysical Research: Planets, 125, e2020JE006430. https:// doi.org/10.1029/2020JE006430