Dust and clouds on Mars

Dust is present everywhere on Mars, yet its abundance, physical properties, size distribution as well as its impact on the composition, structure and dynamics of the atmosphere has today only barely been addressed and understood. The amount of dust in the atmosphere of Mars has a seasonal cycle, with the first half of the year having less dust than the second half (in the northern hemisphere). During summer in the southern hemisphere, Mars is also at perihelion in its orbit around the Sun, which causes an increase in the amount of dust lifted from the surface and suspended in the atmosphere. There is large inter-annual variability in this dust activity, with some years having smaller regional scale dust storms and some years developing into a global, planet-encircling dust storm. The nature of this variability is still unknown and is an open question in Mars research. The understanding of the mechanisms involved in getting dust from the surface into the atmosphere is of major importance.

Dust is lifted from the surface up into the atmosphere by several processes including transfer of momentum and heat between the atmosphere and the surface. Surface wind stress lifting and dust devils are thought to be the primary mechanisms to lift dust. The detachment threshold, defined as the surface wind shear stress required for grain removal, will depend on the forces induced by the wind shear, the surface adhesion (which inhibits the fine grain removal) and gravitation (which inhibits the larger grain removal). After detachment from the surface, three transport regimes are usually considered: suspension in which the grain remains aloft in the atmosphere, saltation in which the particle will fall back to the surface, and creep in which the particle undergoes rolling and sliding on the surface. When the large particles return to the surface, they impact dust grains which in turn can be injected into the atmosphere.

Dust lifted into the atmosphere will eventually produce clouds and hazes, which can span different spatial scales from local to global, and different timescales from hour to seasons. Dust storms have historically been classified by size as local (long axis <2000 km), regional (long axis > 2000 km), and global storms (Martin and Zurek 1993). However, using MGS/MOC (Mars Global Surveyor / Mars Orbiter Camera) observations, Cantor et al. (2001) refined the definition of local (regional) storms as those covering less (greater) than 1.6×106 km². Sometimes, such events evolve into Global Dust Storms with large and long-lasting storms. The processes that control the creation, expansion, and ultimately the decay of global dust storms are likely critically dependent on the radiative–dynamic feedbacks between dust lifting, the heating of the atmosphere, and the circulation, which result in the intensification and/or the spatial expansion of dust lifting (Kahre et al. 2017).

The dust and water cycles are coupled through cloud condensation processes, but dust also modifies the thermal structure of the atmosphere. Recent studies suggest that global dust storms effectively transport water vapour from the near-surface to the middle atmosphere and increase the escape rate of atmospheric hydrogen (a product of water vapour photolysis) (Vandaele et al. 2019). In 2018, a global dust storm occurred on Mars, providing a unique opportunity to study the latitudinal, longitudinal, and temporal variations of the water vapour vertical profile generated by the global dust storm from the new solar occultation measurements by NOMAD/EMTGO (Nadir and Occultation for MArs Discovery / ExoMars Trace Gas Orbiter) (Aoki et al. 2019).

Mars water ice clouds play an important role in the atmosphere of Mars: they impact the radiative budget, the photochemistry, the transport and circulation of the atmosphere. They have several morphologies and patterns of occurrence, including a seasonal behaviour (Clancy, Montmessin, et al. 2017). On Mars, CO₂ ice clouds also exist, which are important to understand the global CO₂ cycle. Such clouds have been observed in the mesosphere and the polar night (Kieffer et al. 2000; Montmessin et al. 2007).

Answering key open questions on Mars

Even after more than 50 years of space observation of Mars, a lot of questions still remain open. The recent anthology on Mars (Haberle et al., 2017) listed a series of open issues which need answers. For example, concerning the role and impact of dust, the authors mentioned:

What mechanisms are responsible for maintaining the background dust loading and the elevated dust layers during the non-dusty seasons? Could dust devil and local storms explain the background level of dust? How are the elevated dust layers created?

How do interactions between the dust, water and CO₂ cycles affect the dust cycles? What are the exact mechanisms behind dust lifting and transport and how do they affect the dust cycles? How do water ice clouds control the vertical distribution of dust? Do the radiative effects of water ice clouds significantly affect where, when, and how much dust is lifted from the surface? What is the impact on the occurrence of global dust storms?

What mechanisms control the initiation, growth, and decay of dust storms? How do the radiative-dynamic feedbacks between dust lifting, atmospheric heating and circulation impact the creation and expansion of storm? Why do some storms grow while others do not? Why and how do storms stop?

What mechanisms control the inter-annual variability of global dust storms? Could the intertwined dust, water and CO₂ cycles explain the inter-annual variability? Could the depletion of surface dust or change in the surface wind stress threshold lead to the observed variability?

What is the current global dust budget and how has it evolved over time? Is there a transfer of dust from one hemisphere to the other? Is the dust cycle ‘closed’ on seasonal, annual, multi-annual, or orbital change timescales?

To answer these fundamental questions, we need new fundamental and applied resarch to improve several key elements :

(1) There are uncertainties about the physics of dust lifting, including variations in grain type and/or cohesiveness that could affect the surface wind stress threshold (Kahre et al., 2017);

(2) Continued monitoring and new types of observations of the Martian atmosphere are needed to fully characterize the current climate (Smith et al., 2017);

(3) Characterisation of minor gases […] and of the vertical distribution of gases would provide important constraints for modellers (Smith et al., 2017);

(4) Remarkably, dedicated water vapor profiling measurements do not exist […] which considerably restricts diagnostic assessments of cloud microphysics, atmospheric volatile transport and photochemistry (Clancy et al., 2017);

(5) … a more complete description of [how] clouds influence the current Mars atmosphere […] would also significantly improve our understanding of the complex radiative, dynamical and photochemical environment constituted by today’s Mars atmosphere (Clancy et al., 2017);

(6) Fully interactive dust cycle models must be relied upon to provide the answers […], which highlights the need for continued dust cycle modelling studies and model development (Kahre et al., 2017);

(7) The radiatively active atmospheric aerosols (dust and clouds) constitute the greatest challenge going forward (Barnes et al., 2017);

(8) Several areas require further study […], such as the coupling of dust and water in the atmosphere and on the surface (Montmessin et al., 2017).