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Register a new userDust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission
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Dust aerosol plays a fundamental role in the behavior and evolution of the Martian atmosphere. The first five Mars years of Mars Exploration Rover data provide an unprecedented record of the dust load at two sites. This record is useful for characterization of the atmosphere at the sites and as ground truth for orbital observations. Atmospheric extinction optical depths have been derived from solar images after calibration and correction for time-varying dust that has accumulated on the camera windows. The record includes local, regional, and globally extensive dust storms. Comparison with contemporaneous thermal infrared data suggests significant variation in the size of the dust aerosols, with a 1 {mu}m effective radius during northern summer and a 2 {mu}m effective radius at the onset of a dust lifting event. The solar longitude (LS) 20-136{deg} period is also characterized by the presence of cirriform clouds at the Opportunity site, especially near LS=50 and 115{deg}. In addition to water ice clouds, a water ice haze may also be present, and carbon dioxide clouds may be present early in the season. Variations in dust opacity are important to the energy balance of each site, and work with seasonal variations in insolation to control dust devil frequency at the Spirit site.
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We have produced a multiannual climatology of airborne dust from Martian year 24 to 31 using multiple datasets of retrieved or estimated column optical depths. The datasets are based on observations of the Martian atmosphere from April 1999 to July 2013 made by different orbiting instruments: the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey, and the Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO). The procedure we have adopted consists of gridding the available retrievals of column dust optical depth (CDOD) from TES and THEMIS nadir observations, as well as the estimates of this quantity from MCS limb observations. Our gridding method calculates averages and uncertainties on a regularly spaced, but possibly incomplete, spatio-temporal grid, using an iterative procedure weighted in space, time, and retrieval uncertainty. In order to evaluate strengths and weaknesses of the resulting gridded maps, we validate them with independent observations of CDOD. We have statistically analyzed the irregularly gridded maps to provide an overview of the dust climatology on Mars over eight years, specifically in relation to its interseasonal and interannual variability. Finally, we have produced multiannual, regular daily maps of CDOD by spatially interpolating the irregularly gridded maps using a kriging method. These synoptic maps are used as dust scenarios in the Mars Climate Database version 5, and are useful in many modelling applications in addition to forming a basis for instrument intercomparisons. The derived dust maps for the eight available Martian years are publicly available and distributed with open access.
In this paper we show that Sun-viewing images obtained by the Mars Science Laboratory (MSL) Navigation Cameras (Navcam) can be used for retrieving the dust optical depth and constrain the aerosol physical properties at Gale Crater by evaluating the sky brightness as a function of the scattering angle. We have used 65 Sun-pointing images covering a period of almost three Martian years, from MSL mission sol 21 to sol 1646 (MY 31 to 33). Radiometric calibration and geometric reduction were performed on MSL Navcam raw image data records to provide the observed sky radiance as a function of the scattering angle for the near-Sun region (scattering angle from 4{deg} to 30{deg}). These curves were fitted with a multiple scattering radiative transfer model for a plane-parallel Martian atmosphere model using the discrete ordinates method. Modelled sky brightness curves were generated as a function of two parameters: the aerosol particle size distribution effective radius and the dust column optical depth at the surface. A retrieval scheme was implemented for deriving the parameters that generated the best fitting curve under a least-square error criterion. The obtained results present a good agreement with previous work, showing the seasonal dependence of both dust column optical depth and the effectiveparticle radius.
It is not yet entirely clear whether Mars began as a warm and wet planet that evolved towards the present-day cold and dry body or if it always was cold and dry with just some sporadic episodes of liquid water on its surface. An important clue into this question can be gained by studying the earliest evolution of the Martian atmosphere and whether it was dense and stable to maintain a warm and wet climate or tenuous and susceptible to strong atmospheric escape. We discuss relevant aspects for the evolution and stability of a potential early Martian atmosphere. This contains the solar EUV flux evolution, the formation timescale and volatile inventory of the planet including volcanic degassing, impact delivery and removal, the loss of a catastrophically outgassed steam atmosphere, atmosphere-surface interactions, and thermal and non-thermal escape processes affecting any secondary atmosphere. While early non-thermal escape at Mars before 4 billion years ago (Ga) is poorly understood, particularly in view of its ancient intrinsic magnetic field, research on thermal escape processes indicate that volatile delivery and volcanic degassing cannot counterbalance the strong thermal escape. Therefore, a catastrophically outgassed steam atmosphere of several bars of CO2 and H2O, or CO and H2 for reduced conditions, could have been lost within just a few million years (Myr). Thereafter, Mars likely could not build up a dense secondary atmosphere during its first ~400 Myr but might only have possessed an atmosphere sporadically during events of strong volcanic degassing, potentially also including SO2. This indicates that before ~4.1 Ga Mars indeed might have been cold and dry. A denser CO2- or CO-dominated atmosphere, however, might have built up afterwards but must have been lost later-on due to non-thermal escape processes and sequestration into the ground.
An important and perhaps dominant source of dust in the martian atmosphere, dust devils play a key role in Mars climate. Datasets from previous landed missions have revealed dust devil activity, constrained their structures, and elucidated their dust-lifting capacities. However, each landing site and observational season exhibits unique meteorological properties that shape dust devil activity and help illuminate their dependence on ambient conditions. The recent release of data from the Mars Environmental Dynamics Analyzer (MEDA) instrument suite onboard the Mars 2020 Perseverance rover promises a new treasure-trove for dust devil studies. In this study, we sift the time-series from MEDAs Pressure Sensor (PS) and Radiative and Dust Sensors (RDS) to look for the signals of passing vortices and dust devils. We detected 309 vortex encounters over the missions first 89 sols. Consistent with predictions, these encounter rates exceed InSight and Curiositys encounter rates by factors of several. The RDS time-series also allows us to assess whether a passing vortex is likely to be dusty (and therefore is a true dust devil) or dustless. We find that about one-third of vortices show signs of dust-lofting, although unfavorable encounter geometries may have prevented us from detecting dust for other vortices. In addition to these results, we discuss prospects for vortex studies as additional data from Mars 2020 are processed and made available.
The Mars Express (MEX) mission has been successfully operated around Mars since 2004. Among many results, MEX has provided some of the most accurate astrometric data of the two Mars moons, Phobos and Deimos. In this work we present new ephemerides of Mars moons benefitting from all previously published astrometric data to the most recent MEX SRC data. All in all, observations from 1877 until 2018 and including spacecraft measurements from Mariner 9 to MEX were included. Assuming a homogeneous interior, we fitted Phobos forced libration amplitude simultaneously with the Martian tidal k2/Q ratio and the initial state of the moons. Our solution of the physical libration 1.09 +/- 0.01 degrees deviates notably from the homogeneous solution. But considering the very low error bar, this may essentially suggest the necessity to consider higher order harmonics, with an improved rotation model, in the future. While most data could be successfully fitted, we found a disagreement between the Mars Reconnaissance Orbiter and the Mars Express astrometric data at the kilometer level probably associated with a biased phase correction. The present solution precision is expected at the level of a few hundreds of meters for Phobos and several hundreds of meters for Deimos for the coming years. The real accuracy of our new ephemerides will have to be confirmed by confrontation with independent observational means.
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