CFD Analysis of Natural Convection Performance of a MMRTG Model Under Martian Atmospheric Conditions

Spacecraft-associated spores and four non-spore-forming bacterial isolates were prepared in Atacama Desert soil suspensions and tested both in solution and in a desiccated state to elucidate the shadowing effect of soil particulates on bacterial survival under simulated Martian atmospheric and UV irradiation conditions. All non-spore-forming cells that were prepared in nutrient-depleted, 0.2-microm-filtered desert soil (DSE) microcosms and desiccated for 75 days on aluminum died, whereas cells prepared similarly in 60-microm-filtered desert soil (DS) microcosms survived such conditions. Among the bacterial cells tested, Microbacterium schleiferi and Arthrobacter sp. exhibited elevated resistance to 254-nm UV irradiation (low-pressure Hg lamp), and their survival indices were comparable to those of DS- and DSE-associated Bacillus pumilus spores. Desiccated DSE-associated spores survived exposure to full Martian UV irradiation (200 to 400 nm) for 5 min and were only slightly affected by Martian atmospheric conditions in the absence of UV irradiation. Although prolonged UV irradiation (5 min to 12 h) killed substantial portions of the spores in DSE microcosms (approximately 5- to 6-log reduction with Martian UV irradiation), dramatic survival of spores was apparent in DS-spore microcosms. The survival of soil-associated wild-type spores under Martian conditions could have repercussions for forward contamination of extraterrestrial environments, especially Mars.

Relative humidity laboratory measurements in Martian atmospheric conditions

Clouds composed of CO2 ice form throughout the Martian atmosphere. In the mesosphere, CO2 ice clouds are thought to form via heterogeneous ice nucleation on nanoparticles of meteoric origin at temperatures often below 100K. Lower altitude CO2 ice clouds in the wintertime polar regions form up to around 145K and lead to the build-up of the polar ice caps. However, the crystal structure and related fundamental properties of CO2 ice under Martian conditions are poorly characterised. Here we present X-ray diffraction (XRD) measurements of CO2 ice, grown via deposition from the vapour phase under temperature and pressure conditions analogous to the Martian mesosphere. A crystalline cubic structure was determined, consistent with the low-pressure polymorph (CO2-I, space group Pa-3 (No. 205)). CO2 deposited at temperatures of 80-130K and pressures of 0.01–1 mbar was consistent with dry ice and previous literature measurements, thus removing the possibility of a more complicated phase diagram for CO2 in this region. At 80K, a lattice parameter of 5.578 ± 0.002Å, cell volume of 173.554 ± 0.19Å3 and density of 1.684 ± 0.002gcm−3 was determined. Using these measurements, we determined the thermal expansion of CO2 across 80–130K that allowed for a fit of CO2 ice density measurements across a larger temperature range (80–195K) when combined with literature data (CO2 density= 1.72391-2.53 × 10−4T-2.87 × 10−6T2). Temperature-dependent CO2 density values are used to estimate sedimentation velocities and heterogeneous ice nucleation rates, showing an increase in nucleation rate of up to a factor of 1000 when compared to commonly used literature values. This temperature-dependent equation of state is therefore suggested for use in future studies of Martian mesospheric CO2 clouds. Finally, we discuss the possible shapes of crystals of CO2 ice in the Martian atmosphere and show that a range of shapes including cubes and octahedra as well as a combination of the two in the form of cubo-octahedra are likely.

<p>Martian gullies are alcove-channel-fan systems that have been hypothesized to be formed by the action of liquid water and brines, the effects of sublimating CO<sub>2</sub> ice, or a combination of these processes. Recent activity and new flow deposits in these systems have shifted the leading hypothesis from water-based flows to CO<sub>2</sub>-driven flows, as it is hard to reconcile present activity with the low availability of atmospheric water under present Martian conditions. Direct observations of flows driven by metastable CO<sub>2</sub> on the surface of Mars are however nonexistent, and our knowledge of CO<sub>2</sub>-driven flows under Martian conditions remains limited. For the first time, we produced CO<sub>2</sub>-driven granular flows in a small-scale flume under Martian atmospheric conditions in the Mars Chamber at the Open University (UK). The experiments were used to quantify the slope threshold and CO<sub>2 </sub>fraction limits for fluidization. With these experiments, we show that the sublimation of CO<sub>2</sub> can fluidize sediment and sustain granular flows under Martian atmospheric conditions, and even transport sediment with grain sizes equal to half the flow depth. The morphology of the deposits is lobate and depends highly on the CO<sub>2</sub>-sediment ratio, sediment grain size, and flume angle. The gas-driven granular flows are sustained under low (<20º) flume angles and small volumes of CO<sub>2</sub> (around 5% of the entire flow). Pilot experiments with sediment flowing over a layer of CO2 suggest that even smaller percentages of CO<sub>2</sub> ice are needed for fluidization. The data further shows that the flow dynamics are complex with surging behavior and complex pressure distribution in the flow, through time and space.</p>

Generation and evolution of laser-induced shock waves under martian atmospheric conditions

The exploration of Mars has attracted increasing interest in these years. The experiments and simulations show that strong electric field triggered by the dust storms in the Martian atmosphere may cause CO<sub>2</sub> discharge. Studies on this phenomenon will not only help deepen our comprehension on the evolution of Martian surface, but also provide a possibility to realize the <i>in-situ</i> oxygen generation on Mars based on plasma chemistry. In this paper, a zero-dimensional global model is used to simplify the complicated description of CO<sub>2</sub> chemical kinetics, therefore a reduced chemistry can be obtained for detailed numerical simulation in the near future. At the beginning of simplification, the graph theoretical analysis is used to pre-treat the original model by exploring the relationship between reacting species. Through the study of connectivity and the topological network, species such as O<sub>2</sub>, e, and CO prove to be important in the information transmission of the network. While gaining a clearer understanding of the chemistry model, dependence analysis will be used to investigate numerical simulation results. In this way a directed relation diagram can be obtained, where the influence between different species is quantitively explained in terms of numerical solutions. Users could keep different types of species correspondingly according to their own needs, and in this paper, some species with low activeness such as C<sub>2</sub>O, <inline-formula><tex-math id="M5">\begin{document}$ {\mathrm{O}}_{5}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M6">\begin{document}$ {\mathrm{O}}_{4}^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.png"/></alternatives></inline-formula> and species with uncertainties such as <inline-formula><tex-math id="M7">\begin{document}$ {\mathrm{C}}_{2}{\mathrm{O}}_{2}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M8">\begin{document}$ {\mathrm{C}\mathrm{O}}_{4}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.png"/></alternatives></inline-formula> are removed from the original model. As for the reduction of specific reactions among species, the reaction proportion analysis based on the calculation of reaction rates is used to obtain the contribution of each reaction to the entire process of CO<sub>2</sub> discharge, through which the important reactions can be selected. Finally, a simplified chemistry model of CO<sub>2</sub> discharge based on Martian atmospheric conditions, including 16 species and 67 reactions, is established. The numerical simulations show that the trends of species densities based on the simplified chemistry model are highly consistent with those based on the original one, and considerations about the CO<sub>2</sub> conversion and the energy efficiency are also in line with expectations, which can help deepen the understanding of the essential process of CO<sub>2</sub> discharge under Martian atmospheric conditions. In addition, the quantitative results of the relationship between reactive species will lay a theoretical foundation for the accurate analysis of various products in Martian dust storm discharges and the realization of Mars <i>in-situ</i> oxygen generation technology based on plasma chemistry.

Plasma-induced luminescence spectroscopy in Martian atmospheric conditions

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) Lifecycle Testing Laboratory is operated by the University of Dayton Research Institute (UDRI), which is dedicated to conducting life-cycle testing of electrically heated versions of the MMRTG. Since there are only two MMRTG electrically-heated thermoelectric generators (ETG) available for testing, a Test Plan Development Working Group was established to determine and prioritize the performance testing that is being conducted with the Engineering Unit (EU) and Qualification Unit (QU) ETGs. This working group is comprised of subject matter experts from the U.S. Department of Energy (DOE), the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC), NASA Jet Propulsion Laboratory (JPL), Idaho National Laboratory (INL), Oak Ridge National Laboratory (ORNL), UDRI, Aerojet Rocketdyne, and Teledyne Energy Systems. The highest priority testing was concluded by the working group to be: 1) To determine the impact of thermal cycling on thermoelectric components by characterizing the evolution of the thermoelectric/electrical properties of the EU as a result of thermal cycling the ETG through a Martian Sol repeatedly; 2) to characterize the effect of a simulated cruise-phase environment on the QU by evaluating the change in performance of the ETG before and after a cruise-phase simulation; and 3) characterize and clear any potential MMRTG internal shorts to chassis by integrating the JPL derived active short technique between the internal electrical power circuit and chassis frame of the MMRTG. The data and risk mitigation techniques derived from this testing can potentially be incorporated into future missions that would employ the MMRTG or successor thermoelectric radioisotope power systems.

The Mars Sample Retrieval mission demands high-performance parachutes capable of surviving supersonic deceleration in Mars’ low-density atmosphere, specifically for the Lander. This study evaluates the suitability of a Ringsail parachute through computational fluid dynamics (CFD) simulations at supersonic (500 ms-1), low density (0.02 kgm-3) conditions. Velocity magnitude analysis revealed significant flow separation and turbulent wake formation. Stagnation zones posed localized stress risks, while asymmetric vorticity suggested susceptibility to collapse. This paper used computational f luid dynamics to evaluate the performance of a Ringsail parachute designed in Fusion360 in Martian atmospheric conditions through analysing lift and drag forces and coefficient of drag. The results are then validated against a previous study regarding the performance of Supersonic Ringsail Parachutes on Mars. Future work could explore transient inflation dynamics, material thermal limits, and varying porosity.

The MEDUSA and MicroMED Experiments for the ExoMars Space Programme to Perform In Situ Analysis of Martian Dust

Martian atmospheric conditions are very different to those on Earth, and present various challenges when designing rotorcraft. Specifically, Mars has a thin atmosphere with lower density compared to Earth. Additionally, the speed of sound on Mars is approximately 72% of that on Earth due to its low surface temperature and atmospheric composition, which puts a limit on rotor speeds in order to avoid high Mach numbers at the blade tip. Taken together, these conditions require Martian rotor blades to operate in a low-Reynolds-number (1,000 to 10,000) compressible flow regime; for which conventional aerofoils have not been designed. Recent studies by Munday et al. and Koning et al. have investigated the performance of aerofoils with flat surfaces and sharp leading edges under Martian atmospheric conditions. These aerofoils induce a higher adverse pressure gradient at their leading edge compared to conventional aerofoils, which can in turn lead to shedding of coherent vortices that do not trigger transition, but mitigate flow separation. In this talk I will present results from our latest GPU-accelerated high-order accurate Direct Numerical Simulations (DNS) of flow over a triangular aerofoil under Martian atmospheric conditions using PyFR, and compare them with results obtained for a triangular aerofoil in the Mars Wind Tunnel at Tohoku University. The importance of span and end-wall effects will be demonstrated. Finally, I will present our latest results from preliminary efforts to optimise a triangular aerofoil for Martian atmospheric conditions using Genetic Algorithms and GPU-accelerated high-order accurate DNS via PyFR.

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is a readily available, long-lived, small scale, versatile power supply that can generate uninterrupted power on the moon during the long lunar nights. Lunar surface conditions, however, can be very harsh. Lunar daytime highs could cause the critical hot junction temperature (T hj ) to exceed its operating limits, while the cold lunar nights can generate deep thermal cycles that could induce significant stress. An energy balance for objects on the lunar surface was performed, and an approximate lunar thermal sink for the MMRTG was obtained. This approximate sink does not incorporate potential self-shading from incident solar radation created by the MMRTG geometry. It is hypothesized, however, that the effect of this self-shading on the lunar surface will result in a decrease in operational temperatures. Recently developed thermal models have been improved so that they can calculate critical temperatures for an MMRTG on the lunar surface. Using the approximate lunar thermal sink, results from the model indicate that under the nominal operating conditions of 28 V and 2000 W th of thermal inventory, the MMRTG can operate on the lunar surface while remaining under the maximum T hj by a margin of greater than 25 °C. Operating under worst-case nominal conditions, 28 V and 2048 W th of thermal inventory, still allows operations on the lunar surface with over 15 °C of margin. Operating at higher voltages will increase temperatures within the MMRTG, while lower voltages are expected to cause T hj to decrease. An off-nominal 32 V system will still have at least 15 °C of margin if the system can be held at 2000 W th or less of thermal inventory, but that margin shrinks to almost zero if the maximum specification of 2048 W th happens to be implemented. Lowering the thermal inventory can lower T hj , but it will also decrease beginning-of-life power. The relationship between these three parameters appears to be linear, with a 48 W th decrease in thermal inventory causing T hj and beginning-of-life power to decrease by 11.2 °C and 4.8 We, respectively. Missions that want to use MMRTG on the lunar surface should carefully consider the voltage they want to pull from the generator. If the operating voltage chosen is high, then a decrease in thermal inventory is one potential option for keeping the T hj below the operational maximum. Results also indicate that the MMRTG will experience a thermal cycle of $\Delta\mathrm{T}=47 {}^{\circ}\mathrm{C}$ during the lunar diurnal cycle. Empirical testing of a prototypic system or subsystem is anticipated to be the best method for determining the effect of lunar induced thermal cycles on the MMRTG. Currently, the Engineering Unit is available and is not a critical testing resource for NASA. As such, potential damage to the unit from thermal cycle testing would not deprive NASA of valuable life test data on the MMRTG. Other thermal cycle testing options include an MMRTG thermoelectric module mounted in a suitable testing frame.

Survival of microorganisms in smectite clays: Implications for Martian exobiology

Investigation of the water sorption properties of Mars-relevant micro- and mesoporous minerals

The challenging range of landing sites for which the Mars Science Laboratory Rover was designed, requires a rover thermal management system that is capable of keeping temperatures controlled across a wide variety of environmental conditions. On the Martian surface where temperatures can be as cold as -123 C and as warm as 38 C, the rover relies upon a Mechanically Pumped Fluid Loop (MPFL) Rover Heat Rejection System (RHRS) and external radiators to maintain the temperature of sensitive electronics and science instruments within a -40 C to 50 C range. The RHRS harnesses some of the waste heat generated from the rover power source, known as the Multi Mission Radioisotope Thermoelectric Generator (MMRTG), for use as survival heat for the rover during cold conditions. The MMRTG produces 110 W of electrical power while generating waste heat equivalent to approximately 2000 W. Heat exchanger plates (hot plates) positioned close to the MMRTG pick up this survival heat from it by radiative heat transfer. Winds on Mars can be as fast as 15 m/s for extended periods. They can lead to significant heat loss from the MMRTG and the hot plates due to convective heat pick up from these surfaces. Estimation of this convective heat loss cannot be accurately and adequately achieved by simple textbook based calculations because of the very complicated flow fields around these surfaces, which are a function of wind direction and speed. Accurate calculations necessitated the employment of sophisticated Computational Fluid Dynamics (CFD) computer codes. This paper describes the methodology and results of these CFD calculations. Additionally, these results are compared to simple textbook based calculations that served as benchmarks and sanity checks for them. And finally, the overall RHRS system performance predictions will be shared to show how these results affected the overall rover thermal performance.