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The muon Moonshot: Moon subsurface tomography with upward-going muons

This paper proposes and simulates a novel method for lunar subsurface tomography using upward-going muons generated by hadron decays in the Moon's regolith, demonstrating their potential to non-invasively detect shallow voids and water resources with high temporal resolution.

Original authors: Zimo Hu, Leyun Gao, Zhengyun You, Qite Li, Qiang Li, Yuhong Yu, Liangwen Chen, Xueheng Zhang, Zhiyu Sun

Published 2026-07-14

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CC BY 4.0

Original authors: Zimo Hu, Leyun Gao, Zhengyun You, Qite Li, Qiang Li, Yuhong Yu, Liangwen Chen, Xueheng Zhang, Zhiyu Sun

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Moon as a giant, dusty cookie that has been sitting in a cosmic-ray oven for billions of years. When high-speed particles from deep space (cosmic rays) crash into this cookie, they smash the dust apart, creating a shower of new, tiny particles. On Earth, our thick atmosphere acts like a heavy blanket; it stops most of these new particles before they can turn into something else. But the Moon? It has no atmosphere. It's a vacuum.

This lack of a "blanket" changes everything. When the Moon gets hit, some of the new particles (called mesons) get kicked upward, escaping the dusty surface into the empty space above. Because there's no air to stop them, they get a chance to "grow up" and turn into muons (a type of heavy electron) while they are still flying through the air. These are upward-going muons, and they are the stars of this new idea.

The Big Idea: X-Raying the Moon from the Inside Out
The authors of this paper suggest a "Moonshot" concept: instead of looking at the Moon from the outside, we can use these upward-flying muons to take an X-ray picture of what's hiding just beneath the surface.

Think of it like this: If you shine a flashlight through a foggy window, you see the fog. But if you have a special camera that only sees light bouncing up from the glass, you can tell if there's a crack or a bubble inside the glass. In this case, the "flashlight" is cosmic rays, and the "camera" is a detector on the Moon or in orbit.

What the Simulations Show
The team didn't go to the Moon to test this yet; they built a super-accurate computer simulation (a digital twin of the Moon) to see what would happen. Here is what their digital experiment found:

The Sensitivity to Depth: Muons are unstable and decay as they travel through space. This means the number of muons detected depends heavily on how far they have to travel from the surface to the detector. Because of this decay, the detected muon flux is highly sensitive to the distance between the source and the detector. This sensitivity allows scientists to detect variations in the surface topography and shallow subsurface structures. Specifically, the method is effective for probing depths on the order of 100 meters (a few hundred meters), where changes in density—such as surface bumps or hidden cavities—alter the path and survival rate of the muons.
The Numbers: At sea level on Earth, we get about 1.7×10⁻² cm⁻²s⁻¹ of muons. On the Moon's surface, the simulation suggests we get about 4.57×10⁻³ cm⁻²s⁻¹ (when the Sun is quiet) or 3.15×10⁻³ cm⁻²s⁻¹ (when the Sun is active). At an altitude of 1 km, the flux is approximately 8.04×10⁻² cm⁻²s⁻¹ or 4.98×10⁻² cm⁻¹s⁻¹. While these numbers are significant, the key insight is not simply that "higher is better," but that the specific flux at a given altitude provides a precise baseline against which anomalies caused by subsurface features can be measured.
Finding Holes: The simulation suggests this method is great at finding empty spaces (cavities) under the surface. If there's a hole in the dust, the muon count changes because the particles encounter less material to scatter or absorb them. The computer says a detector with an area of 1 m² could spot these holes in less than a minute.
Finding Water: It's harder to find water or ice because it's denser than empty space but less dense than rock. The simulation suggests you'd need to collect data for about ten minutes (specifically, 12.5 minutes for a strong signal) with that same 1 m² detector to be sure you've found water.

What This Is NOT
It's important to know what this paper doesn't say.

They are not saying we have already found these muons or that we have mapped the Moon's underground. This is all based on computer models.
They are not suggesting we can see deep underground. The "microscopic" version of this idea only works for things very close to the surface, up to about 100 m deep, and mostly for things very shallow (like the top 0.5 m of dust).
They are not claiming this replaces other methods. Instead, they suggest it's a "complementary" tool, like having a second pair of eyes that looks at the Moon differently.

Why It Matters
The authors think this could be a game-changer for future missions, like the upcoming Chang'e 7 (planned for around 2026) and Chang'e 8 (planned for around 2029). If we put a detector on the Moon or on a satellite orbiting at a specific altitude, we could non-invasively scan for caves (which might be safe shelters for astronauts) or water ice (which is gold for making fuel and drinking water) by analyzing how the muon flux varies due to decay and subsurface interactions.

In short, the paper suggests that by listening to the "upward chatter" of particles escaping the Moon's dusty skin, we might finally get a clear look at the secrets hiding just beneath its feet.

Technical Summary: The Muon Moonshot – Lunar Subsurface Tomography with Upward-Going Muons

Problem Statement
While cosmic-ray muon tomography is a well-established non-invasive imaging technique on Earth (used for volcanoes, pyramids, and reactors), its application to the Moon has been limited by the lunar environment. Unlike Earth, the Moon lacks an atmosphere. On Earth, downward-going hadrons decay in the atmosphere to produce muons. On the Moon, the dense regolith absorbs most downward-going hadrons before they decay, while the near-vacuum above the surface allows upward-going hadrons escaping the regolith to decay in flight. Although this upward-going muon flux has been largely overlooked compared to backscattered muons or secondary X-rays and gamma rays, it represents a potential source of information regarding the shallow subsurface structure of the lunar regolith. The central problem addressed is whether this upward-going muon flux is detectable, quantifiable, and sensitive enough to variations in subsurface density (such as voids or water deposits) to serve as a tomographic tool.

Methodology
The authors propose a novel concept for lunar subsurface tomography based on the detection of upward-going muons. To investigate this, they developed a Monte Carlo (MC) simulation framework using FLUKA.

Geometry: The simulation models the Moon as a perfect sphere ( $R_m = 1737.1$ km) surrounded by a vacuum region and a "blackhole" absorbing layer. The lunar regolith is modeled using the Moskalenko–Porter 2007 (MP2007) material composition and density profiles.
Source: An isotropic galactic cosmic-ray source (protons, alpha particles, and heavier nuclei) is initialized 0.05 m above the surface, restricted to a $2\pi$ solid angle directed toward the Moon.
Physics Models: Hadronic interactions are modeled using DPMJET and RQMD event generators. Solar modulation is accounted for using potentials of 465 MV (solar minimum) and 1440 MV (solar maximum), with cosmic-ray spectra based on the Badhwar–O'Neill model constrained by AMS and BESS measurements.
Simulation Scope: The study simulates $10^6$ primary cosmic-ray events to generate differential yields for muons, charged pions ( $\pi^\pm$ ), and charged kaons ( $K^\pm$ ). These are analyzed at the lunar surface and at an altitude of 1 km.
Detector Assumptions: The study assumes ideal detection conditions with high efficiency and particle identification (PID), neglecting specific detector effects to focus on the physical flux.

Key Contributions and Results

Characterization of Upward-Going Muon Flux:
- The simulation confirms a non-negligible upward-going muon flux emerging from the lunar surface.
- Surface Flux: At the lunar surface, the integrated flux for kinetic energies $>100$ MeV is approximately $4.57 \times 10^{-3}$ cm $^{-2}$ s $^{-1}$ (solar minimum). The spectrum peaks around $10^2$ MeV.
- Altitude and Decay Sensitivity: The study highlights that muons decay in flight, meaning the detected flux is highly sensitive to the distance from the source. While the flux at 1 km altitude is significant ( $\approx 8.04 \times 10^{-2}$ cm $^{-2}$ s $^{-1}$ ), this is not simply due to an increase in muon count with height. Instead, the flux profile is shaped by the decay of charged pions (mean decay length $\approx 7.8$ m) in the vacuum above the surface. Crucially, because muons decay in flight, the detected signal is sensitive to surface topography and shallow subsurface structures. Variations in surface elevation or subsurface density at characteristic depths on the order of $10^2$ meters (a few hundred meters) significantly alter the path length and survival probability of these muons, making the flux a sensitive probe for such features.
Particle Identification and Background:
- The study identifies charged pions as the dominant source of secondary muons (flux $\sim$ 100 times that of kaons).
- It suggests that with fast timing (e.g., Time-of-Flight systems with $\sim$ 100–250 ps resolution) or dual-readout calorimetry, muons can be effectively distinguished from primary cosmic rays and other secondaries (photons, pions, kaons), creating a nearly background-free environment for tomography.
Feasibility of Subsurface Tomography:
The paper proposes two distinct tomography modes:
- Microscopic Tomography: Surface-based detectors probing shallow structures ( $\sim$ 100 m depth) via displaced meson decays.
- Macroscopic Tomography: Orbital detectors probing large-scale voids or heterogeneities near the surface via displaced muon decays.
- Case Studies:
  - Cavities: Simulations of shallow subsurface voids (extending from 0.1–0.5 m depth to 10 m) show that cavity-induced flux variations are observable with high statistical significance in less than one minute using a 1 m $^2$ detector.
  - Water Resources: Detection of water-rich regions is more challenging due to weaker signal modulation. However, the study indicates that a statistical significance of $\sim$ 1 $\sigma$ is achievable in 30 seconds, rising to $\sim$ 5 $\sigma$ with an exposure time of approximately 12.5 minutes.

Significance and Claims
The paper claims that lunar secondary muons offer a complementary, non-invasive tool for future lunar exploration missions. The primary significance lies in demonstrating that the lack of a lunar atmosphere, often viewed as a limitation, actually creates a unique environment where upward-going muons are generated and can be detected.

The authors assert that the flux is sensitive to detector altitude, surface topography, and subsurface density variations, validating the concept of using these muons for shallow subsurface tomography. The sensitivity arises because the decay-in-flight of muons makes the detected signal dependent on the precise geometry of the surface and subsurface structures at depths of hundreds of meters.
They highlight the potential to detect shallow voids and water resources with realistic detector areas and exposure times compatible with upcoming missions.
The work positions this technique as timely for missions such as Chang'e 7 (planned for ~2026) and Chang'e 8 (planned for ~2029), which aim to explore the lunar south pole and support the International Lunar Research Station.

The paper concludes by noting that while the current study uses idealized flat-terrain assumptions, the results encourage further research incorporating realistic topography, specific detector designs, and reconstruction methods to fully realize the potential of "Muon Moonshot" tomography.

Technical Summary: The Muon Moonshot – Lunar Subsurface Tomography with Upward-Going Muons

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