The Invisible Rain from Space
Every second, approximately 10,000 high-energy particles called muons pass through your body. These subatomic particles, created when cosmic rays collide with Earth’s atmosphere, have been mainly ignored by climate scientists—until now. A breakthrough study published last month in the Environmental Monitoring and Assessment journal reveals how these quantum particles are becoming crucial tools for tracking soil moisture with unprecedented accuracy.
Researchers at the University of Bristol and the UK Centre for Ecology & Hydrology have developed a system called COSMOS-UK (COsmic-ray Soil Moisture Observing System) that uses the behavior of these cosmic particles to measure something seemingly unrelated: the water content in soil across vast landscapes. Unlike traditional methods that require physical sampling at specific points, this technique can continuously monitor moisture levels across several hectares simultaneously.
The cosmic rays that generate these particles originate primarily from supernova explosions throughout our galaxy. When these high-energy particles—mostly protons—reach Earth’s upper atmosphere, they trigger cascades of secondary particles, including neutrons and muons. For decades, these particles were primarily the domain of particle physicists rather than environmental scientists. The interdisciplinary leap to apply particle physics to climate science represents a fascinating convergence of fields that traditionally had little overlap.
How Muons Become Moisture Detectors
The science behind this approach is elegantly simple yet profoundly effective. When muons and neutrons from cosmic rays enter Earth’s atmosphere and eventually reach the ground, they interact differently with hydrogen atoms—abundant in water—than with other elements in soil.
Dr. Lucy Barker, who leads the COSMOS-UK network, explains: “Neutrons bounce and scatter when they hit hydrogen atoms in water molecules. By measuring the density of neutrons above the soil surface, we can accurately infer how much water is present below. Fewer neutrons detected means more water in the soil.”
The detection system consists of specialized tubes filled with helium-3 gas that capture these scattered neutrons. What makes this approach revolutionary is its scale—each sensor effectively samples an area approximately 300 meters in diameter, providing measurements representing a much larger area than traditional point-based soil sensors.
The physics underlying this process involves what scientists call “fast neutron thermalization.” Cosmic-ray neutrons arrive at Earth’s surface with high energy (fast neutrons). When these neutrons collide with hydrogen atoms in soil water, they lose energy much more efficiently than when colliding with heavier elements like silicon or oxygen. This energy loss process—thermalization—transforms fast neutrons into slower thermal neutrons. The proportion of fast to thermal neutrons detected above the soil surface creates a signature that correlates directly with soil moisture content.
The calibration process for these sensors involves comparing neutron counts with traditional soil sampling during installation. Once calibrated, the sensors provide continuous data streams that can detect changes in soil moisture within minutes following rainfall events. This temporal resolution represents a significant improvement over satellite-based methods that might only provide updates every few days.
Climate Crisis Applications
As of early 2023, the COSMOS-UK network has expanded to 51 stations across the United Kingdom, creating the world’s densest cosmic-ray soil moisture sensors network. The timing couldn’t be more critical as climate change intensifies drought and flood cycles.
The data collected has already proven valuable during recent extreme weather events. During the summer 2022 drought across Europe, COSMOS-UK provided real-time data showing soil moisture depletion rates were occurring 15-20% faster than historical averages. This information helped water resource managers implement conservation measures weeks earlier than expected.
Professor Sarah Bentley from Lancaster University’s Environment Centre notes: “What’s particularly valuable about cosmic-ray soil moisture sensing is that it bridges the gap between point measurements and satellite observations. It gives us the perfect middle scale that’s most relevant for agricultural management and flood prediction.”
Beyond drought monitoring, the technology has found applications in flood forecasting. Traditional flood models often struggle to account for antecedent soil moisture conditions—how wet the soil was before a significant rainfall event. The COSMOS network feeds data directly into the UK’s Flood Forecasting Centre, improving prediction accuracy by up to 28% in specific watersheds. During the 2020 winter storms, this improved forecasting allowed emergency services to evacuate vulnerable areas an average of 4.5 hours earlier than possible with previous systems.
Agricultural researchers have also begun incorporating cosmic-ray soil moisture data into crop yield models. A 2022 study in the Journal of Agricultural Science demonstrated that integrating COSMOS data improved wheat yield predictions by 17% compared to models using traditional soil moisture measurements. This improvement stems from the sensors’ ability to capture the spatial heterogeneity of soil moisture across entire fields rather than isolated points.
Beyond Earth: Interplanetary Applications
Perhaps most surprisingly, this technology has come full circle—from space to Earth and back to space again. NASA’s Perseverance rover on Mars carries a modified version of this technology called the Dynamic Albedo of Neutrons (DAN) instrument.
Scientists can map subsurface water ice and hydrated minerals on the Red Planet by measuring how cosmic radiation interacts with the Martian surface. The instrument can detect water or ice buried up to one meter below the surface, helping identify potential resources for future human missions.
Dr. Igor Mitrofanov, lead scientist for the DAN instrument, recently stated: “The principles we’re using to find water on Mars are essentially the same ones now being used to monitor Earth’s changing climate. It’s a remarkable example of how fundamental physics can address practical problems across planetary boundaries.”
The technology has expanded beyond Mars as well. The European Space Agency’s PROSPECT mission, scheduled to land near the lunar south pole in 2025, will utilize neutron spectroscopy to map hydrogen concentrations—likely in the form of water ice—in permanently shadowed regions. This information will be crucial for planning future lunar bases that might extract water for drinking, oxygen production, and rocket fuel.
The Future of Cosmic Climate Monitoring
The next generation of cosmic-ray soil moisture sensors is already under development. Researchers at Los Alamos National Laboratory are creating miniaturized versions that could potentially be deployed via drone or attached to agricultural equipment. These smaller sensors would sacrifice some spatial coverage. Still, they could be deployed in much greater numbers, potentially allowing farmers to create real-time high-resolution soil moisture maps of their fields.
Another promising direction involves combining cosmic-ray neutron sensing with machine learning algorithms. A team at the University of Bonn has developed neural networks that can distinguish between water held in soil and water contained in vegetation biomass—a longstanding challenge for this technology. This advancement could make the technology more valuable for monitoring forest health and carbon sequestration efforts.
As climate change accelerates and water resources become increasingly stressed, these cosmic particle detectors represent a rare bright spot—a new technology arriving precisely when needed most. They use the universe’s own particles to help us understand and respond to our changing planet. The convergence of particle physics, environmental science, and space exploration demonstrates how cross-disciplinary approaches often yield the most innovative solutions to our most pressing challenges.