Abstract
We present a new model of interstellar dust in which large grains are a single composite material, “astrodust,” and nanoparticle-sized grains come in distinct varieties including polycyclic ...aromatic hydrocarbons (PAHs). We argue that a single-composition model for grains larger than ∼0.02
μ
m most naturally explains the lack of frequency dependence in the far-infrared (FIR) polarization fraction and the characteristic ratio of optical to FIR polarization. We derive a size distribution and alignment function for 1.4:1 oblate astrodust grains that, with PAHs, reproduce the mean wavelength dependence and polarization of Galactic extinction and emission from the diffuse interstellar medium while respecting constraints on solid-phase abundances. All model data and Python-based interfaces are made publicly available.
The Taklimakan Desert（TD） and Gobi Desert（GD） are two of the most important dust sources in East Asia, and have important impact on energy budgets, ecosystems and water cycles at regional and even ...global scales. To investigate the contribution of the TD and the GD to dust concentrations in East Asia as a whole, dust emissions, transport, and deposition over the TD and the GD in different seasons from 2007 to 2011 were systematically compared, based on the Weather Research and Forecasting model coupled with Chemistry（WRF-Chem）. Dust emissions, uplift, and long-range transport related to these two dust source regions were markedly different due to differences in topography, elevation, thermal conditions, and atmospheric circulation. Specifically,the topography of the GD is relatively flat, and at a high elevation, and the area is under the influence of two jet streams at high altitudes, resulting in high wind speeds in the upper atmosphere. Deep convective mixing enables the descending branch of jet streams to continuously transport momentum downward to the mid-troposphere, leading to enhanced wind speeds in the lower troposphere over the GD which favors the vertical uplift of the GD dust particles. Therefore, the GD dust was very likely to be transported under the effect of strong westerly jets, and thus played the most important role in contributing to dust concentrations in East Asia. Approximately 35% and 31% of dust emitted from the GD transported to remote areas in East Asia in spring and summer, respectively. The TD has the highest dust emission capabilities in East Asia, with emissions of about 70.54 Tg yr.1 in spring, accounting for 42% of the total dust emissions in East Asia. However, the TD is located in the Tarim Basin and surrounded by mountains on three sides. Furthermore, the dominant surface wind direction is eastward and the average wind speed at high altitudes is relatively small over the TD. As a result, the TD dust particles are not easily transported outside the Tarim Basin, such that most of the dust particles are re-deposited after uplift, at a total deposition rate of about 40 g m.2. It is only when the TD dust particles are uplifted above 4 km, and entrained in westerlies that they begin to undergo a long-range transport. Therefore,the contribution of the TD dust to East Asian dust concentrations was relatively small. Only 25% and 23% of the TD dust was transported to remote areas over East Asia in spring and summer, respectively.
We quantify the effect of dust accumulation at Jezero crater by means of a Dust Correction Factor (DCF) for the solar radiation measured by the photodiodes of the Radiation and Dust Sensor of the ...Mars 2020 mission. After one Mars Year, dust on the photodiode surface attenuated 25%–30% of the incoming solar radiation. The DCF did not decrease monotonically; we use a model to reproduce its evolution and to derive dust deposition and lifting rates, showing that dust removal is 9 times larger at Jezero crater than at InSight's location in western Elysium Planitia. The model fit obtained using observed opacities is further improved when fed with dust sedimentation rates simulated by a GCM that considers a particle size distrtibution. Projections show seasonal net dust removal, being encouraging for the long‐term survival of solar‐powered missions to Jezero or similarly active dust lifting regions.
Plain Language Summary
Dust is ubiquitous in the Martian atmosphere, accumulating on both natural and artificial surfaces. Dust particularly affects the performance and lifetime of missions: the termination of InSight and MER‐B operations are recent examples. Dust accumulation shows a seasonal behavior, and attenuated 25%–30% of the incoming solar radiation on Perseverance after the first Mars Year of the mission. Dust removal is almost 10 times larger than at InSight's location: projections indicate that surfaces at Jezero will be periodically partially cleaned. The estimations of the effect of the accumulated dust as a function of time are encouraging for solar‐powered missions to regions with similar amounts of dust lifting, which might be determined from orbital data on where dust storms originate, dust devils or their tracks are found, or seasonal albedo changes are noted. In addition, the quantification of the effect of accumulated enables future studies requiring more accurate knowledge of incoming solar radiation at the surface.
Key Points
We present the evolution of dust accumulation at Jezero crater for more than one Mars Year
We derive dust deposition and removal rates: removal is 9 times more efficient than at the InSight location in western Elysium Planitia
Projections show that surfaces at Jezero will experience seasonal net dust removal, encouraging solar‐powered missions
We present a synthesis of the astronomical observations constraining the wavelength-dependent extinction, emission, and polarization from interstellar dust from UV to microwave wavelengths on diffuse ...Galactic sight lines. Representative solid-phase abundances for those sight lines are also derived. Given the sensitive new observations of polarized dust emission provided by the Planck satellite, we place particular emphasis on dust polarimetry, including continuum polarized extinction, polarization in the carbonaceous and silicate spectroscopic features, the wavelength-dependent polarization fraction of the dust emission, and the connection between optical polarized extinction and far-infrared polarized emission. Together, these constitute a set of constraints that should be reproduced by models of dust in the diffuse interstellar medium.
The diurnal cycle of dust aerosols on Mars is studied by analyzing lidar observations at the Phoenix landing site under cloud‐ and fog‐free conditions and in the absence of elevated, long‐range ...transported dust layers. There is a pronounced diurnal cycle in the dust‐layer height with minimum heights of 4–6 km occurring between 11:00 and 17:00 local time. The ratio of the aerosol optical depth (AOD) within the lowermost 2 km to the total AOD reaches peak values at the same time. This can be explained by local dust emissions driven by the diurnal cycle of heating and cooling in the boundary layer. Analysis of wind and pressure measurements show that the gustiness of surface winds and the frequency of convective vortices undergo diurnal variations resembling those of AOD, indicating that these processes are the main drivers for local dust emissions.
Plain Language Summary
Dust particles suspended in air are important for the radiative energy budget of Mars. By interacting with solar radiation, these aerosols impact the temperature, dynamics, and composition of the atmosphere, as well as the surface temperature of Mars. Dust aerosols are produced by wind‐lifting processes ranging from small‐scale eddies to planetary‐scale dust storms, resulting in spatially and temporally varying concentrations. Here the focus is on the diurnal cycle of dust emissions as observed by the Phoenix lander. By jointly analyzing light‐scattering observations by a lidar instrument and measurements of wind speed and air pressure, one can correlate the concentration of aerosols in air in proximity to the ground to meteorological processes. The results reveal distinct maxima in aerosol loads, the intensity of wind gusts, and the frequency of dust devils around local noon and early afternoon, and corresponding minima around midnight. This indicates that gustiness and dust devils, both fueled by solar heating during the day, are among the main causes for local dust emissions. During daytime the aerosols are concentrated at altitudes of 4–6 km. This can provide us with an indication for the degree of vertical mixing in the planetary boundary layer of Mars.
Key Points
Diurnal variation of local dust emissions, wind gustiness, and convective vortices are analyzed for the entire Phoenix mission
Peak values are observed in early afternoon, indicating that gustiness and convective vortices are main drivers for local dust emissions
Typical dust‐layer heights in early afternoon are 4–6 km, consistent with typical boundary‐layer heights
Mineral dust plays a pivotal role in the Earth’s system. Dust modulates the global energy budget directly via its interactions with radiation and indirectly via its influence on cloud and ...precipitation formation processes. Dust is a micro-nutrient and fertilizer for ecosystems due to its mineralogical composition and thus impacts on the global carbon cycle. Hence, dust aerosol is an essential part of weather and climate. Dust suspended in the air is determined by the atmospheric dust cycle: Dust sources and emission processes define the amount of dust entrained into the atmosphere. Atmospheric mixing and circulation carry plumes of dust to remote places. Ultimately, dust particles are removed from the atmosphere by deposition processes such as gravitational settling and rain wash out. During its residence time, dust interacts with and thus modulates the atmosphere resulting into changes such as in surface temperature, wind, clouds, and precipitation rates. There are still uncertainties regarding individual dust interactions and their relevance. Dust modulates key processes that are inevitably influencing the Earth energy budget. Dust transport allows for these interactions and at the same time, the intermittency of dust transport introduces additional fluctuations into a complex and challenging system.
The light scattered from dust grains in debris disks is typically modeled as compact spheres using the Lorenz-Mie theory or as porous spheres by incorporating an effective medium theory. In this work ...we examine the effect of incorporating a more realistic particle morphology on estimated radiation-pressure blowout sizes. To calculate the scattering and absorption cross-sections of irregularly shaped dust grains, we use the discrete dipole approximation. These cross-sections are necessary to calculate the β-ratio, which determines whether dust grains can remain gravitationally bound to their star. We calculate blowout sizes for a range of stellar spectral types corresponding with stars known to host debris disks. As with compact spheres, more luminous stars blow out larger irregularly shaped dust grains. We also find that dust grain composition influences blowout size such that absorptive grains are more readily removed from the disk. Moreover, the difference between blowout sizes calculated assuming spherical particles versus particle morphologies more representative of real dust particles is compositionally dependent as well, with blowout size estimates diverging further for transparent grains. We find that the blowout sizes calculated have a strong dependence on the particle model used, with differences in the blowout size calculated being as large as an order of magnitude for particles of similar porosities.
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•Investigated particle size, dust concentration and dust-dispersion-air pressure effects on MIT of coal dust cloud.•Used particle size ranges <38, 38 to <75, 75 to <212, 212 to <425 ...and 425 to <850 µm.•The limiting coal dust particle size was determined in between 38 and <75 µm.•MIT increased exponentially from 420 to 730 °C with increase in coal dust particle size.•MIT decreased with dust concentration till stoichiometric value and then increased.
The present study investigates the effects of three important parameters, namely, particle size, dust concentration and dust-dispersion-air pressure on Minimum Ignition Temperature (MIT) and explosion process of coal dust cloud using Godbert-Greenwald furnace. The coal sample collected from Jharia Coalfield, India was used for experimentation. The effects of particle size and concentration of coal dust on MIT of coal dust cloud were studied at five different particle size ranges, that is, <38 µm, 38 to <75 µm, 75 to <212 µm, 212 to <425 µm and 425 to <850 µm. However, the effect of dust-dispersion-air pressure was studied only with the initial three finer particle size ranges, because at coarser sizes it was not possible to maintain a dust cloud of sufficient concentration due to high particle settling velocity and the dust cloud became non-explosive beyond dust-dispersion-air pressure of 30–35 kPa, even at ignition temperature of 1000 °C, which is the limiting temperature of the apparatus. The results indicate that MIT of coal dust cloud increases exponentially from 420 to 730 °C with increase in coal dust particle size. MIT was also found to decrease with the coal dust concentration till a stoichiometric concentration was reached, beyond which MIT slightly increased for the three finer particle size ranges. Nevertheless, in the case of the coarser particle size ranges tested, initially a trivial decrease in MIT was observed only up to certain dust concentration, after which, the MIT increased faintly. The results and the underlying mechanism presented are extremely useful, not only in research and advancement of knowledge on the coal dust explosion process, but also in taking necessary measures for preventing coal dust explosions in underground coal mines.
The Emirates Mars Mission (EMM) science phase began in Martian Year 36, solar longitude 49, which is outside of the classical Mars dust storm season. EMM observed a distinct dust cloud at northern ...mid‐to‐high latitudes on 10 September 2021 (Martian Year 36, solar longitude 97). The dust cloud is an arc‐shaped dust storm, typically observed at the northern polar cap edge. This type of non‐season dust storm is a well‐known phenomenon, but this particular case is interesting because the dust cloud has frontal structure. A large atmospheric front is unusual in this location and season. Moreover, EMM's unique observational coverage adds value to this observation. EMM provided a sequence of four camera images, which are separated by just 2–3 hr. The dust cloud showed very little motion over 7–8 hr, that is, it is quasi‐stationary. We discuss relevant dynamical processes, supported by a consistency check with the Mars Climate Database.
Plain Language Summary
The Emirates Mars Mission (EMM) officially started its science observations on 23 May 2021. We saw some, but not many dust storms until end of 2021. That is because Mars has a dust storm season and a non‐dust‐storm‐season, and EMM arrived during the latter. On 10 September 2021, EMM observed an arc‐shaped dust storm close to the northern polar cap. This type of non‐season dust storm is well‐known. What makes this observation interesting is that the dust storm is also a weather front. A large atmospheric front is unusual for this location and time during the Martian year. Moreover, it is unique how EMM observed this dust storm. EMM took a camera image every 2–3 hr, giving four dust cloud images in total. The dust cloud showed very little motion over 7–8 hr. We discuss how the motion of air masses in the Mars atmosphere may have caused the observed dust storm. The discussion is supported by wind and temperature data from the Mars Climate Database.
Key Points
We study a sequence of four Emirates Mars Mission images, showing a frontal dust cloud on 10 September 2021 (solar longitude 97)
The dust cloud extends from the Chasma Boreale of the northern polar cap to the midlatitudes; it shows very little movement for 7–8 hr
Large atmospheric fronts are unusual in this location and season; we discuss dynamical processes, supported by the Mars Climate Database
This study explores the direct and semi-direct radiative effect of North African dust in the present and future climate using the regional climate model RegCM4. The simulations cover a historical ...decade extending from December 1999 to November 2009 and a future decade that spans from December 2089 to November 2099 under the Representative Concentration Pathway 4.5 (RCP4.5), without considering land-cover/land-use changes. For each time-slice a set of two experiments was conducted, namely the “Control”, in which dust is radiatively inactive and the “Feedback”, in which dust interacts with shortwave and longwave radiation. The impact of North African dust on the regional radiative balance is assessed by comparing the “Feedback” and the “Control” experiments during the historical period. The results indicate that the combined effect of dust Direct + Semi-direct Radiative Effect (DSRE) on the shortwave is − 13.8 W m
−2
and − 10.7 W m
−2
over the Sahel and the Sahara, respectively. The Direct Radiative Effect (DRE) dominates over the Semi-direct Radiative Effect (SRE) in both winter and summer, although during summer over some parts of the desert the SRE in the longwave spectrum accounts for almost 50% of the DSRE. Part of this is due to a noteworthy statistically significant increase of clouds that reaches values up to 3% and stretches across the eastern and western Sahara desert. Dust DSRE intensifies moderately in the future period (− 15.8 W m
−2
and − 11.0 W m
−2
), while its spatial distribution remains the same, suggesting that the effect of climate change in the atmosphere will not alter the radiative effect of dust over North Africa considerably. When taking into account the dust radiative feedback in regional climate simulations the maximum temperature is altered by − 0.2/− 0.2 °C and − 0.3/− 0.6 °C over the Sahel and Sahara regions, respectively, during the summer/winter period, mainly as a result of changes in the shortwave radiative balance. On the contrary, the minimum temperature increases, since it is mostly controlled by the longwave radiation emitted from the Earth’s surface. In the future period the near surface air temperature increases by 1.5–2.5 °C and the fine dust column burden increases by + 4% to + 8% in comparison to the historical period, mainly due to the RCP4.5 forcing. When the dust feedback on climate is active in future simulations it can decrease the summer daily maximum temperature by 0.3 °C over Sahel, and decrease or increase it locally in Sahara by up to 0.2 °C. Prior to the Feedback-Control analysis an extensive evaluation has been conducted for dust optical depth, dust extinction, near surface air temperature and cloud fraction cover using the LIVAS, CRU and CM SAF datasets.
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