In-situ measurements of Ionizing Radiation and near-surface Water on Mars using Tumbleweed Rovers

The Tumbleweed Mission proposes a cost‑effective swarm of wind‑driven spheroidal rovers to acquire extensive in-situ measurements across the Martian surface [1]. Built from modular components and commercial off‑the‑shelf (COTS) parts, these passively propelled rovers can significantly reduce mission cost and complexity for surface mobility on Mars.

Figure 1 - Proposed Mission Architecture: mid-air deployment (1), landing (2), rolling towards the equator (3) and stationary phase (4).

 

Exploring the synergies amongst our pre-selected list of instruments, we arrived at the opportunity to use multispectral cameras, radiation and neutron spectrometers, and a triaxial fluxgate magnetometer to characterise Martian Habitability across the traversed paths of individual rovers [5]. We define areas of interest for the assessment of present-day Martian Habitability as the intersection of (a) low radiation exposure, (b) local availability of water/H‑bearing materials, and (c) magnetospheric shielding. These areas could constitute niches conducive for preservation of potential biosignatures [6]. These can be probed by a synergistic suite of instruments on-board a multi‑rover swarm of Tumbleweeds. For example, the neutron spectrometer shall continuously measure both fast and epithermal neutrons produced when Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) interact with the regolith, enabling high‐resolution WEH mapping along each rover’s path [7]. Neutron measurements will be complemented by continuous measurements of soil permittivity using patch electrode based sensors [8]. Meanwhile, the fluxgate magnetometer shall record vector magnetic fields at every stop and during locomotion, detecting localized magnetic anomalies that can act as mini‑magnetospheres [9]. Each Tumbleweed will continuously sample radiation flux, hydrogen abundance, magnetic anomalies, topography, and surface composition over hundreds of kilometers, collectively generating habitability maps that bridge the gap between orbital surveys and point measurements. 

Figure 2 – Tumbleweed rover prototype in the Negev Desert, Credit to OEWF/AMADEE20.

Several prototypes have been developed to evaluate feasibility of the platform’s mobility (figure 2) [9]. In 2025, Tumbleweed Mars achieved validation of the current box-kite spheroidal rover design through a wind tunnel campaign in Aarhus University Planetary Environment Facility supported by Europlanet’s Transnational Access program [10]. To further validate Martian surface operations, as well as high-quality scientific return during dynamic motions, an analog campaign in the Atacama desert demonstrated that the rover could carry out environmental measurements while in motion. During rolling operations, the prototype continuously recorded atmospheric and surface conditions, showing that the scientific payload remained functional under dynamic conditions (figure 3). 

 

Figure 3 - Tumbleweed rover prototype in the Atacama. 

Nevertheless, further testing and modelling is necessary to validate the capabilities of more specialized instruments aboard a Tumbleweed rover. 

Regarding necessary instrument requirements for radiation-specific passive instrumentation to successfully achieve measurement objectives, several layers of modelling are needed in order to complete a Science Traceability Matrix. An example is the passive neutron spectrometer, which is a statistical instrument, and thus requires modelling not only for the neutron counts it may receive onboard a tumbling rover but also for the broader geomorphological context in which the rover finds itself (modelled on a Unity-based Martian Digital Twin developed in-house (figure 4) [11].

Figure 4 - Unity-based Martian Digital Twin simulating a Tumbleweed rover traverse in the Jezero Crater.

Additionally, arriving at the minimum number of individual rovers required to successfully achieve mission requirements will require a larger-scale simulation of the swarm behavior and dispersion of Martian surface. This software has been already developed in-house (figure 5) [2]. 

Figure 5 - Global scale semi-stochastic spreading simulation, illustration of a sample run with 100 rovers, 80 sols fixed lifetime.

Thus, to validate the synergies of radiation-focused instrumentation, three layers of modelling are necessary. Radiation transport modelling (in GEANT4) is required to understand and derive measurement requirements during the rolling and the static phases of the Tumbleweed Mission. If we want to map water-equivalent hydrogen in a specific location, such as Deuteronilus Mensae, we might start by using the large-scale spreading simulation with a specific landing ellipse to infer how many rovers will reach the desired destination. Should x rovers reach Deuteronilus Mensae, the question becomes about their dynamic behavior on that location due to its geomorphological and meteorological context, which would be modelled by the Martian Digital Twin. Should y of these rovers find themselves within a lava tube, then it becomes important to model their radiation environment in GEANT4 where the number of rovers that traverses the lava tube, z, can be determined in order for achieving desired measurement coverage.This three-layer modelling can be executed in the reverse chronological order.The many relationships between the models/simulations and the science goals can be seen through the decoupling of the Science Traceability Matrix illustrated on figure 6.

Figure 6 - How the three modelling layers relate to the Science Traceability Matrix.

In conclusion, the Tumbleweed Mission represents a scalable and cost-effective approach for water prospecting and investigating Martian habitability through a distributed swarm of wind-driven rovers. Central to this concept is the neutron spectrometer, through which continuous measurements of fast and epithermal neutrons could provide high-resolution mapping of water-equivalent hydrogen along rover traverses, enabling the identification of subsurface volatile reservoirs and potentially protected habitable niches. When combined with magnetic field, radiation, and multispectral observations, these measurements can reveal regions where hydrogen abundance coincides with reduced radiation exposure and localized magnetic shielding. The Atacama campaign demonstrated the feasibility of conducting scientific measurements during rover motion, while ongoing multi-layer modelling efforts will constrain the operational requirements, statistical performance, and deployment architecture necessary for neutron spectroscopy to achieve meaningful scientific return on Mars.

References

[1] https://doi.org/10.5194/egusphere-egu24-20149 

[2] Renolder et al , IAC 2023

[3] https://doi.org/10.5194/epsc2024-1103

[4] https://doi.org/10.5194/epsc2024-790

[5] https://doi.org/10.5194/egusphere-egu25-19534

[6] Shanbhag et al, IAC 2023

[7] https://doi.org/10.1016/j.icarus.2021.114805.

[8] https://doi.org/10.3389/frspt.2023.1303180  

[9] Maxwell et al 2023

[10] https://doi.org/10.5194/epsc-dps2025-1775

[11] Holthuijsen et al, IAC 2024