As the most dramatic activity in the Martian climate system composed of the surface and atmosphere, dust storms on Mars have significant seasonal variations. During each Mars year, as Mars approaches its southern summer solstice, local, regional, and global dust storms (
Figure 1) become very active on the surface of Mars. In this section, we review the basic properties of dust storms and their impacts on Mars environment.
2.1. Basic Properties of Dust Storms
The Martian dust storms can be categorized based on different standards. According to the scales of the dust storms, they can be categorized as local dust storms, regional dust storms, and global dust storms [
17,
18], as shown in
Figure 1. A regional dust storm is defined as dust events occupying a spatial area ≥1.6×10
6 km
2 and a duration longer than 2 days. The lofted dust can travel great distances, even around Mars in a latitude zone, and be deposited far away from the source region. Isolated dust events with smaller sizes or shorter durations are classified into local dust storms which generally last less than 3 days. Global dust storms are also referred as planet encircling dust storms which sweep the Martian globe. The rapid expansion and combination of multiple local and regional dust storms finally become a global dust storm which usually continues tens of days.
The local dust storms can occur in any season at any place on Mars. Regional dust storms are common during southern hemisphere spring and summer seasons, with a solar longitude (
LS) range between 180º and 360º. The dust storms occur in preferred locations, such as Acidalia, Utopia, Arcadia, and Hellas [
19]. The global dust storms are unpredictable nonperiodic events, both in time and location. Only few confirmed global dust storms were observed in years 1956, 1971, 1973, 1977, 2001, 2007, 2018 [
20].
In terms of season and time scale, Martian regional-scale dust storms can be divided into three types: A, B and C [
21], and an example in Martian year 36 is shown in
Figure 2 (please refer to
Section 3 for the details of column dust optical depth (CDOD) [
22,
23]). Their active time is generally in the southern hemisphere spring and summer seasons (
LS range 180º~360º), while the southern hemisphere autumn and winter seasons (
LS range 0º~180º) are generally a calm period of dust activity. Type A dust storms generally intensify in the middle and low latitudes after the southern hemisphere spring equinox in each Martian year (
LS=180º), covering more than two-thirds of the latitude of Mars and lasting for about four months. Type A dust storms generally have the longest duration, widest coverage, and highest intensity, and in some Martian years even evolved into global dust storms (such as 2001, 2007, 2018), and have the most significant impact on Mars exploration activities. Nearly at the end of Type A dust storm, near the summer solstice in the southern hemisphere, Type B dust storms will erupt in the south polar region of Mars, lasting about two months. Type B dust storms are mainly local and regional, and the scope of activity is mainly limited to the high latitude region near the South Pole (south of 60º S). After the end of type A and B dust storms, type C dust storms will occur in the middle and low latitudes (around
LS=330º), lasting about one to two months and may be developed into global dust storms, ending before the southern hemisphere autumn equinox, returning to a calm state, and entering the cycle of the next Martian year.
The occurrence of Martian dust storms follows an overall seasonal pattern as described above, for example, Fourier analysis on the long-term global mean CDOD during Martian years 24–36 (
Figure 2a) shows that Martian dust storms have a precise 668.615-day period (
Figure 2c). However, in different Martian years, the intensity, frequency, eruption and duration of dust storms, and their influence range all show significant irregular interannual variation characteristics, which brings great difficulty to the prediction of dust storms on Mars.
The effective radius of the lofted dust particles is near 1.5 µm and varies from ~1 µm during weak local dust storms near Martian northern hemisphere summer solstice to ~2 µm during regional and global dust storms in Martian southern hemisphere spring and summer [
24,
25]. The effective radius of dust particles generally increases with the dust content in the atmosphere, that is, the particle size in southern hemisphere spring and summer is generally larger than that in southern hemisphere autumn and winter [
26,
27,
28]. In the vertical direction (10-70 km height range), the distribution of dust particle size in the Martian atmosphere showed a bimodal distribution: the main peak of the particle size was 0.7~1.2 µm and the secondary peak of the particle size was 0.04~0.07 µm [
29]. The altitude-dependent particle size distribution of Martian dust storms significantly affects the absorption, scattering and re-radiation of the incoming solar radiations and plays a key role in the variability of radiative balance, thus affecting large-scale atmospheric circulation and climate on Mars [
30,
31].
2.2. Driving Mechanisms of Dust Storms
The interannual variability of dust storms occurring in the southern hemisphere summer season significantly dominated the seasonal climate on Mars. Therefore, uncover the driving mechanisms of the dust storms is essential to understand the climate on Mars. Many mechanisms have been proposed to interpret the generation of seasonal dust storms and they may be categorized into three classes.
The first class is related with large-scale atmospheric circulation. When Mariner 9 arrived Mars in 1971, it observed for the first time in Martian orbit the decaying phase of a global encircling dust storm [
32]. These observations have led to a number of efforts to understand the driving mechanisms of the global dust storm. Leovy et al. [
33] reviewed such works and proposed an energy balance model. During southern hemisphere spring, dust devils and the local cyclonic dust storms [
34], strong surface winds in the intense baroclinic regions [
36], or the strong mass outflow wind [
36] generates local dust storms, building up dust content in southern, middle and subtropical latitudes. The strong insolation, low static stability, and high atmospheric absorptivity and emissivity generate intense meridional circulations to promote the onset of a global storm [
33]. Later, observations at the Viking Lander #1 indicated that the local dust storm can be caused by baroclinic wave activity with a threshold wind speed of 25–30 m s
–1 [
37].
Now, it is generally accepted that the seasonal dust storms are driving by the seasonally varying solar radiation forcing on the physical system consisting of the Martian surface and atmosphere. The increasing solar radiations near the southern hemisphere spring equinox heat the Mars atmosphere and accelerate atmospheric flow. The strengthened near-surface wind stress lifts the dust particles into atmosphere [
38,
39]. The dust particles further heat the atmosphere to generate zonal temperature gradient, which intensify the equatorial easterly, forming positive feedback in the system to lift more dust particles into the atmosphere and finally forming large regional even global dust storms.
All the regional and global dust storms are originated from local dust storms, for which the second class of mechanisms attribute dust lifting to convective vortices known as dust devils [
34,
38,
40,
41]. Gierasch and Goody [
34] first proposed that an axisymmetric swirling flow field existed outside the central dusty core, and the solar radiation heating strengthened the horizontal temperature gradients to accelerate the cyclonic swirling flow inward into the core. Ryan and Lucich [
40] further found that the dust devils occurred more frequently during southern hemisphere spring and summer. The coexistence of multiple dust devils has been proposed as a mechanism to generate local dust storms.
The above-mentioned mechanisms are all related to the external forcing of solar radiations. The third class of mechanism link the dust storms to the changes in Martial orbital and rotational motions and their coupling to the atmospheric dynamics [
42,
43,
44,
45,
46]. Shirley [
42] found that all the regional dust storms developed into global dust storms during periods when the orbital angular momentum of Mars with respect to the solar system barycenter was increasing or near maximum. The latest simulations [
45,
46] further demonstrated that the global dust storms occur near times when orbit-spin coupling torques are peaking and near times when torques are changing most rapidly. The orbit-spin coupling generated circulatory intensification and relaxation superimposed upon the normal annual cycles of atmospheric circulation controlled by the external solar forcing. The non-synchronization of the two cycles introduces interannual variability of the dust storms [42-45].
To summary, until now, the exact mechanisms than control the interannual variations of the Martian dust storms, especially the global dust storms are still not clear. Modelling efforts achieved great success in the past decades using Mars global circulation models, such as the Mars Global Circulation Model (GCM) [
38,
39], the Ames Mars Global Climate Model (AMGCM) [
47], with dust lifting scheme embedded. A detailed review on status of Martian whole atmosphere model and dust activities can be found in Wu et al. [
48] and Zhou et al. [
49]. Shirley et al. [
46] recently pointed out that it is necessary to incorporate orbit-spin coupling accelerations with the above models to better reproduce the interannual variability of the Martian dust storms.
2.3. Impacts of Dust Storms
Dust activities on Mars will cause many physical and chemical effects and will bring a series of potential hazards to Mars landing exploration and manned missions. In this section, we will review the impacts of dust storms on atmospheric dynamics, atmospheric electric property, space environment, topography, and Mars explorations.
During the dust storm, the dust lifting into the atmosphere significantly absorbs solar radiation, causing the temperature of the Martian atmosphere to significantly rise at least 20 ℃ above the altitude of 25 km [
50,
51] and the drop of surface temperature. The nighttime surface temperature is raised during the main phase of dust storms, making the diurnal temperature difference significantly lower during dust storms [
52]. The radiation effect of dust changes the thermal structure and dynamic conditions of the atmosphere, which further affect the atmospheric tides and intensify the meridional wind and the Hadley circulation of Mars [
53,
54], strengthening the interhemispheric transport of dust. Dust storms will affect the sublimation and condensation of the CO
2 ice sheet in the polar region [
55], further changing the atmospheric pressure in the polar region. Apart from the strong orbital forcing on the water vapor saturation height, dust storms will significantly increase the vertical water vapor transportation, further deplete the ozone [
56].
Simulations, experiments and calculations have shown that during Martian dust storms, dust driven by strong winds is easy to generate friction discharge, which excites low-frequency electromagnetic disturbances [
57,
58,
59,
60]. During periods of high dust activities, the increased dust aerosols are favorable of attaching ions, resulting in the decrease of atmosphere conductivity [
61,
62].
The dust activities in lower atmosphere will significantly influence the upper atmosphere, ionosphere and magnetosphere through vertical transportation and coupling. During dust storms, rising of the lower atmospheric temperature induces the increase of the density of CO
2 in the upper atmosphere [
63,
64]. As a result, the ionospheric peak height will be raised by tens of kilometers, but without significant variation in peak electron density [
65,
66,
67]. The variations of atmospheric ion densities are species-dependent, with CO
2+ ions significantly increased, O
+ ions decreased, and no obvious change in O
2+ ions [
66]. More water vapor is transported to higher altitude during dust storms. These water molecules are dissociated and ionized, and the resultant hydrogen escape can be increased by 5–10 times [
3,
5,
68]. At the same time, the escape rate of CO
2+ ions can be increased by 3 times [
65].
On Mars surface, dust storms have direct impact on the topography. Dust storms can affect the deposition of dust [
69] and volatile (ice/snow) [
70] in the polar ice caps, which can seriously affect the analysis of Mars' climate evolution history. Dust storms can drive the redistribution of dust on Mars surface and the extremely strong winds (up to 100 km h
-1) play an important role in the evolution and reshaping of topography and landform []. In addition, dust storms also help to expose organic matter in the surface soil that has not been degraded by cosmic rays, which helps to assess the presence of life signals on Mars surface [
72].
Apart from the environmental effects, dust storms directly impact the operations of landers and rovers on Mars surface [
15]. The size of Martian dust particles ranges from 0.1 to 2000 µm, and the particles can be plate-like, angle-like, or round, with a particle density of 2.6 to 3.0 g cm
–3 [
73], and rich in Si, Al, Fe, Mg, Ca, Ti, S, Cl, Br and other elements [
74]. These tiny dust particles can cover the surface of the detector or penetrate the detector, causing mechanical failure and affecting the operation of related hardware and electronics. For example, the Spirit rover lost contact in 2010 when it got stuck in the sand, preventing its solar panels from turning toward the sun. Many studies have shown that dust activities will reduce the efficiency of solar panels and even cause damages [
75]. The Opportunity rover worked for fourteen years on Mars until lost contact during the 2018 global dust storm. In December 2022, the National Aeronautics and Space Administration (NASA) announced the ending of the Insight mission, which was launched in May 2018, due to the loss of power after a strong dust storm. Any landers or rovers using solar panels as power source will be significantly impacted by dust storms, even fatal damages. In addition, it was found that the Martian soil is rich in Cr (hexavalent chromium ions are highly toxic), S and Cl elements [
74,
76], therefore the Martian dust could be toxic and acidic, posing a significant threat to the health of astronauts. The great temperature, pressure, and wind variations during dust storms will significantly affect the entry, descending, and landing process, as well as the returning process.
In summary, dust storms have great impact on Martian environments and Mars exploration activities, comprehensive monitoring and forecasting are essential for the study of Mars multi-layer coupling system and for safe Mars exploration missions.