High altitude clouds have been observed in the Martian atmosphere. However, their properties still remain to be characterized. Mars Climate Sounder (MCS) aboard Mars Reconnaissance Orbiter (MRO) is an instrument that measures radiances in the thermal infrared, both in limb and nadir views. It allows us to retrieve vertical profiles of radiance, temperature and aerosols. Using the MCS data and radiative transfer model coupled with an automated inversion routine, we can investigate the chemical composition of the high altitude clouds. We will present the first results on the properties of the clouds. CO2 ice is the best candidate to be the main component of some high altitude clouds due to the most similar spectral variation compared to water ice or dust, in agreement with previous studies. Using cloud composition of contaminated CO2 ice (dust core surrounded by CO2 ice) might improve the fitting result, but further study is needed.
Lower atmosphere variations in the martian water vapour and hydrogen abundance during the late southern summer regional-scale dust storm from LS = 326.1°–333.5° Mars Year 34 (18th-31st January 2019) and their associated effect on hydrogen escape are investigated using a multi-spacecraft assimilation of atmospheric retrievals into a Martian global circulation model. The dusty conditions led to an elevation of the hygropause and associated increase in middle atmosphere hydrogen at the peak of the dust storm, which was particularly intense in Mars Year 34. This response has an important effect on water loss during this time period, quantification of which can provide a more robust calculation of the integrated loss of water through time on Mars and better insight to planetary evolution. The influx of water vapour to higher altitudes led to an associated increase in hydrogen through photolysis of water vapour and a hydrogen escape rate of around 1.0×109 cm−2 s−1, meaning the late southern summer regional-scale dust storm in Mars Year 34 enhanced water loss rates on Mars to levels observed during global-scale dust storms. The water loss rate during the late southern summer regional-scale dust storm, also known as a C storm, led to around 15% of the total annual water loss during only 5% of the year and was at least three times stronger than the much less dusty C storm event in Mars Year 30. These results demonstrate that interannual variations in the strength of the late southern summer regional-scale dust storm must be considered when calculating the integrated loss of water on Mars, an important quantity to constrain in relation to the potential habitability of Mars.
Ever-increasing numbers of atmospheric observations from orbiting spacecraft, and increasingly sophisticated numerical atmospheric models, have recently permitted data assimilation techniques to be applied to planets beyond the Earth. Mars is the first extra-terrestrial planet for which reanalyses of the atmospheric state are now available.
The Thermal Emission Spectrometer (TES) on board NASA’s Mars Global Surveyor (MGS) has produced an extensive atmospheric data set during its scientific mapping phase between 1999 and 2004. Nadir thermal profiles for the atmosphere below about 40 km altitude, and total dust and water ice opacities, have been retrieved from TES spectra, covering almost three complete Martian seasonal cycles (each seasonal cycle on Mars corresponds to 668.6 mean solar days, and the Martian mean solar day is about 24 hours and 40 minutes). Note that dust on Mars plays a key role in the weather and climate, mainly through its strong absorption of short wave radiation with a short radiative relaxation timescale of 1-2 days. Assimilating dust opacities correctly is, therefore, particularly important for atmospheric data assimilation on the Red Planet.
TES retrieved observations have been analysed by assimilation into a Mars general circulation model (MGCM), making use of a sequential procedure known as the Analysis Correction scheme, a form of successive corrections method which has proved simple and robust under Martian conditions, even during the less-than-ideal MGS aerobraking period. The MGCM used at the University of Oxford and at The Open University consists of a spectral dynamical solver and a tracer transport scheme developed in the UK. Its package of state-of-the-art physical parameterization routines is shared with the LMD-MarsGCM, developed by the Laboratoire de Meteorologie Dynamique in Paris (France).
One limitation of TES is that relatively few limb profiles are available, compared to nadir soundings. Our MGS/TES reanalysis, therefore, does not include observations of temperature above about 40 km altitude, nor 3D information on dust opacity (the vertical distribution of dust opacity is prescribed assuming a well mixed dust layer with a rapid transition to a clear upper atmosphere at a height which depends on latitude and season.
In September 2006 NASA’s Mars Reconnaissance Orbiter (MRO) started its mapping phase. The Mars Climate Sounder (MCS) on board MRO is a radiometer with eight mid- and far-infrared
channels and one visible channel, which takes measurements in limb and off-nadir geometries. Retrieved vertical profiles of temperature, dust and water ice opacities from MCS observations can now be assimilated using the same scheme we used for TES, with the advantage of the extension in altitude (thermal profiles can extend to above 80 km altitude, although errors become larger at greater altitudes), the increased vertical resolution (~ 5km compared to > 10km for TES nadir retrievals), and the direct information on the vertical distribution of dust and water ice.
Overall, the application of our data assimilation scheme to retrieved observations from TES and MCS spans almost six complete Martian seasonal cycles. This represents a multi-annual climatology for Mars, which has the advantage of being a complete, dynamically-balanced, four-dimensional best-fit to observations for all the atmospheric variables, including those for which no direct measurements are available (e.g. wind and surface pressure) and with regions of no observations filled-in in a physically-consistent way.
The reanalysis represents, therefore, a unique opportunity to study the inter-annual variability of the Martian weather and climate with respect to all its components, such as the dust cycle, the water cycle, the CO2 cycle, the atmospheric tides and other prominent waves, such as high latitude baroclinic waves.
In this contribution we present the first results of a complete assimilation of both datasets, using a consistent model and data assimilation scheme, and highlight the challenges of combining TES and MCS data assimilation to produce a multi-annual climatology. Particular attention will be devoted to the inter-annual variability of the atmospheric thermal field in response to dust storm activity. We will also provide an insight into the dynamics, looking in particular at the high latitude winds, waves and polar vortices.
Our data assimilation products are freely available to the community for both science- and engineering-oriented purposes. The British Atmospheric Data Centre (BADC, http://badc.nerc.ac.uk) hosts our datasets, which, for the time being, are limited to the MGS/TES reanalysis. People may contact the corresponding author in order to register their interest and be updated about the status of the project. New versions of the MGS/TES reanalysis as well as the MRO/MCS reanalysis will be made available through the BADC in future.
Interested people can download the current TES reanalysis dataset by registering at the BADC and searching for the MACDA (“Mars Analysis Correction Data Assimilation”) project. The direct link to the project is provided by the following URL: http://badc.nerc.ac.uk/view/badc.nerc.ac.uk__ATOM__DE_095e8da2-cf02-11e0-8b7a-00e081470265
