At the altitude where it happens: science goals for exploring the martian boundary layer with rotorcraft

With the success of the Ingenuity helicopter onboard the Perseverance Rover, there is increased interest in exploring aerial vehicle concepts to assist with long-range reconnaissance and geological sampling on Mars [1]. Preliminary studies of more capable rotorcraft have suggested that such craft could carry science payloads of at least 2 to 5 kg at altitudes of at least 200 m along ranges of at least 1 to 5 km [2,3]. Clearly, such a vehicle could revolutionize our exploration of the martian surface.

However, a next generation rotorcraft could also revolutionize our understanding of the near-surface martian atmospheric planetary boundary layer (PBL). This layer is largely inaccessible to orbiters and landers, yet links the datasets obtained by both. Within this zone, dust is lifted and deposited, water ice sublimates and falls as snow, and gas-solid chemistry alters the mixture of trace gasses within the bulk atmosphere [4,5]. Furthermore, any gasses which vent from the subsurface will form a plume within this layer that is detectable at large distances, allowing such vents to be localized for follow-on in-situ exploration.

This presentation will therefore outline atmospheric science goals that could be accomplished from a rotorcraft [2,3]. These goals will be divided into four categories

Meteorology:  Meteorological packages can be compact providing high-frequency in-situ data on atmospheric parameters (e.g. [6]) while generating relatively little data volume. The most advantageous parameters to measure would include (1) single or multi-axis measurements of wind speed and direction, e.g. from sonic anemometers, (2) temperature, (3) pressure, (4) relative humidity and (4) upwelling or downwelling radiometry in a variety of wavebands, e.g. by photovoltaic sensors.

These measurements could be used to improve numerical models of the atmosphere. Vertical profiles of meteorological parameters would be particularly useful. For instance, by determining the wind profile, friction velocities can be obtained which will inform studies of aeolian transport of materials in the near surface. Relative humidity as a function of height can be used to examine surface-atmosphere exchange of water. Pressure and temperature can be used to profile the cores of dust devils. Changes in upwelling and downwelling radiation as altitude changes provides information on atmospheric aerosol radiative properties. 

Dust and Ice Cloud Aerosols: The rotorcraft will need a downward facing camera not just for reconnaissance but also to be able to position itself relative to the landscape. Previous spacecraft cameras have shown themselves to be adept at retrieving the optical depth of dust at different layers within the atmosphere [7] and by allowing the rotorcraft to vary its altitude, a full tomographic profile of atmospheric dust can be obtained. However, beyond imaging clouds and dust directly, more active sensing techniques could be considered. For instance, a lidar or small laser could induce backscattering from dust or ice aerosols, allowing low-altitude clouds, fog, or dust plumes/devils to be investigated in situ. A nephelometer [8] could be used to observe the scattering properties of the aerosols, obtaining particle size and shape as a function of altitude.

Trace Gasses and Fluxes:  While many atmospheric gasses have been quantified from orbit, the processes that take place within the PBL prevent these gas concentrations from being directly compared to landed measurements and to subsurface fluxes. For instance, TGO is incapable of observing the methane that is regularly seen by the Curiosity rover at Gale Crater [9,10]. Somewhere between the rover and the airmass aloft being sampled by the orbiter, the gas is chemically transformed [4]. Furthermore, there are changes observed in oxygen (the fourth most abundant atmospheric species), carbon monoxide [5] and peroxide that are poorly explored over altitude, to say nothing of isotopic ratios within each chemical constituent.

Measuring bulk composition and trace gas composition require different instrumentation. However, no matter the species, a great deal about surface chemistry on Mars could be learned by creating profiles at different times of day and season. Advanced spectroscopic systems using techniques such as OA-ICOS [11] allow measurements of even trace gasses to be performed on the scale of minutes using instruments capable of being carried aloft. The most tantalizing possibility here is the ability to use the capabilities of the rotorcraft to detect and localize subsurface vents by tracing gas plumes in the PBL.

Aeolian and Geophysical Platform:  Rotorcraft also permit an ideal vantage point from which to observe aeolian systems and to perform geophysical measurements. A rotorcraft can observe ripple forms and can conduct change detection of aeolian features through repeat observations at locations beyond a rover’s line-of-sight even over terrain that is inaccessible to rovers. Meanwhile, modern gravimeters may be able to help identify different types of subsurface geology, including deep deposits of aeolian sediments or dust/loess. Magnetic field sensors could add richness to these datasets and have been entirely unexplored at these geographic scales on Mars previously.

Conclusion: Just as the InSight Lander was targeted as a pathfinder to demystify the deep Martian interior, a PBL explorer could address significant scientific questions about the atmosphere of Mars today. Modern instrumentation can enrich the science return from such a rotorcraft at relatively little cost, given the compact nature of most atmospheric instrumentation.

References: [1] Wadhwa and Farley (2022) AGUFM abstract P56A-03 [2] Withrow-Maser et al (2020) ASCEND 2020 Conference https://doi.org/10.2514/6.2020-4029  [3] Johnson et al (2020) NASA/TM-2020-220485 [4] Korablev et al. (2021) Science Advances 7(7) doi: 10.1126/sciadv.abe4386 [5] Trainer, M.G. (2019) JGR: Planets v124 (11) pp. 3000-3024 doi: 10.1029/2016JE006175 [6] Gómez-Elvira et al (2012) SSR 170 pp 583-640 [7] Smith et al (2020) JGRE doi: 10.1029/2020JE006465 [8] Ragent et al. (1996) Science 272(5263) pp 854-856 [9] Webster et al. (2018) Science 360 pp. 1093-1096 doi: 10.1126/science.aaq0131 [10] Korablev et al. (2019) Nature 568 pp 517-520 [11] Walters et al (2024) Acta Astronautica doi: 10.1016/j.actaastro.2024.02.031