Scouting Ahead of Human Footsteps: The Role of the Tumbleweed Rover in Providing High-resolution Radiation and Water Mapping on Mars

Introduction

Mars’ feeble atmosphere and lack of a magnetosphere imply that the surface is exposed to ionizing radiation particles from solar winds as well as Galactic Cosmic Rays (GCRs). Thus, Martian radiation dose exposure is consequential for long-term human and robotic exploration. As such, it is critical to develop a rigorous and holistic understanding of the radiation surface environment using in-situ measurements acquired over large areas. Team Tumbleweed aims to revolutionise Mars exploration by offering a platform capable of navigating harsh terrain at lower costs and reduced mission risk. A swarm-based architecture, consisting of wind-driven Tumbleweed rovers, could open new doors for planetary science. Namely, a suite of miniaturized instruments hosted by this swarm would provide higher spatio-temporal resolutions for the characterisation of ionizing radiation and water equivalent hydrogen mapping at the near-surface level.

Problem Statement

Remote sensing measurements obtained from individual orbital instruments suffer from a lack of spatial and temporal resolution. For instance, the temporal resolution provided by orbiters is unable to satisfactorily capture time-dependent variations in quantities such as radiation exposure and particle spectra, resulting from episodic and cyclical events. 

Regarding spatial resolution, in the case of neutron spectrometers, this severely hampers the identification of mission-relevant water-ice deposits, as current spatial resolution provides data only at a resolution of 60 to 200 km (3600 km^2 per pixel, at best) from an altitude of 400 km (Malakhov et al., 2020).  As such, there are no high-resolution hydrogen or radiation exposure maps for Mars. Thus, ground truth measurements from the surface are indispensable. 

To date, investigations hosted on individual rovers have provided invaluable first measurements. However, forming a more complete understanding of Martian science necessitates investigations supported by infrastructures that can provide high spatio-temporal resolution and global scale coverage. In-situ measurements would not only provide higher spatial and temporal resolutions, but would also provide ground truth data for the corroboration of remote measurements made by orbiters. This calls for a diversification of exploration concepts and platforms to achieve maximum scientific return while reducing mission risk and cost.

 

Proposed Solution

Our proposed solution is a low-cost mission involving a swarm of wind-driven, box-kite-styled Tumbleweed rovers (Cohen et al., 2023).

To maximize the science return of each rover in the Tumbleweed swarm, we developed a methodology using scoring modifiers to assess instrument suitability based on mission goals. These were optimized against mass, volume and power constraints. This led to the following pre-selection of instruments:

• Hand-lens style imager
• Stereoscopic camera
• Radiation spectrometer/particle camera
• Neutron spectrometer
• Electric field sensor
• Wind sensor
• Dust sensor
• Pressure sensor
• Temperature sensor
• Soil pH sensor
• Relative humidity sensor
• Triaxial flux gate magnetometer

Exploring the synergies amongst our pre-selected list of instruments, we arrived at the opportunity to use radiation-focused instrumentation to simultaneously achieve high-resolution mapping of hydrogen in the near-surface environment.

Near-surface level water equivalent hydrogen (WEH) thermalizes neutrons when GCRs and Solar Particle Events finally interact with matter. Consequently, measuring the flux of epithermal neutrons is the best approach towards estimating hydrogen content in the Martian subsurface (Mitrofanov et al., 2022).

Hydrogen mapping at a high-spatial resolution is an essential factor in the definition of future human missions, and settlement, on Mars. As the atmosphere is composed of mostly carbon dioxide (95.32%), with WEH available a human mission would have access not only to water but to the products of the Sabatier reaction and subsequent water electrolysis (methane, water, hydrogen, oxygen), thus enabling ISRU and the production of Methalox, the rocket propellent that would be used for a safe return. 

Beyond hydrogen mapping, measuring cosmic rays may provide clues towards our comprehension of reality. Cosmic Ray Ensembles (CRE), for example, may exhibit large scale spatio-temporal correlations that would enable novel ways to study the properties of space-time, the nature of dark matter and the Lorentz invariance violation, empowering the scientific community in the research of these fundamental questions (Alvarez Castillo et al., 2023).

Additionally, our rovers would provide unprecedented data on Space Weather with the swarm architecture enabling multi-point measurements in the Martian planetary plasma system, illuminating the processes by which plasma interacts with the surface of celestial bodies lacking significant atmospheres. This would drive forward not only our understanding of atmospheric loss on Mars, but also deepen our understanding of its radiation environment and its dependence on the long-term solar cycle variation, altered during Coronal Mass Ejection events that shield the planet from GCRs (Holmstrom et al., 2024), for instance.

Combined hydrogen and radiation environment mapping are enabled by our current instrumentation pre-selection and magnified in scale by our mission architecture, potentially yielding a high-resolution spatio-temporal scouting of Mars. This, in turn, would allow a complete understanding of future possibilities for long-duration human missions to the Red Planet. Regions characterized by low radiation exposure and elevated concentration of sub-surface WEH would contribute towards identifying ideal candidate sites for future crewed missions.

Conclusion

The Tumbleweed Mission, featuring radiation and neutron spectrometers aboard a distributed network of spacecraft, holds the potential to revolutionize our understanding of Martian radiation environments and advance human exploration efforts by mapping hydrogen on the Red Planet. 

The Tumbleweed swarm would not only corroborate orbit-based measurements but also would provide invaluable information on the near-surface environment and their respective differences. Compared to orbital measurements, the Tumbleweed swarm can probe deeper into phenomena such as the effect of dust storms on ionizing radiation and the effect of diurnal and seasonal cycles. Furthermore, a Marswide network of radiation/cosmic-ray detectors would become a unique tool to study astrophysical phenomena, space weather and geophysics.