For centuries, astronomers have watched brief, unexplained flashes light up the lunar surface, a phenomenon so persistent and strange that it earned its own scientific label: Transient Lunar Phenomena, or TLP. While many of these flashes are now confidently linked to meteoroid impacts, a growing body of evidence points to a second, less intuitive cause: gas escaping from deep within the moon itself. The convergence of Apollo-era measurements, modern telescope monitoring programs, and statistical reanalysis of historical sighting records is bringing researchers closer than ever to sorting out which flashes come from space rocks and which may bubble up from below.
Centuries of Unexplained Lunar Flashes
Short flashes of light appear on the moon several times a week, ranging from pinpoint bursts lasting a fraction of a second to longer-lived glows that shimmer for minutes. Some of these events were recorded by telescope observers as far back as the 1500s, and the sheer volume of reports over the centuries made it difficult for scientists to dismiss them all as optical illusions or instrument errors. Yet for most of that history, no single mechanism could account for the variety of appearances, durations, and locations described in observing logs, from sudden star-like flashes to faint veils of light that seemed to cling to crater rims.
The scientific community long debated whether TLP sightings reflected real physical processes or simply observer mistakes. A quantitative reanalysis of the historical record by Arlin P. S. Crotts of Columbia University found that reported events cluster strongly at specific sites, most notably the Aristarchus crater region, rather than scattering randomly across the lunar disk. That clustering pattern is hard to explain as coincidence or bias, because the analysis filtered out obviously unreliable accounts and still revealed persistent “hot spots” for activity. If the sightings were driven purely by human error, they would be expected to spread more evenly in longitude and latitude. Instead, the geographic concentration suggests a real, localized physical process at work, one tied to something particular about certain lunar features and potentially linked to the moon’s interior.
Meteoroid Impacts Explain the Bright Bursts
The most straightforward explanation for many lunar flashes is simple kinetics: small space rocks slamming into the moon’s unprotected surface at tens of thousands of miles per hour. Without an atmosphere to slow incoming debris, even a pebble-sized meteoroid can release enough energy on impact to produce a visible flash from Earth. NASA’s dedicated impact monitoring effort, run by the Meteoroid Environment Office at Marshall Space Flight Center, has been systematically recording these bursts of light for years, measuring meteoroids in the size range of tens of grams to kilograms. By tracking the brightness and frequency of these flashes, researchers can estimate how often the moon is struck and how the impact rate changes during meteor showers or as Earth and the moon move through different streams of interplanetary debris.
One of the most dramatic confirmations came on March 17, 2013, when observers recorded an unusually bright flash that stood out even against the sunlit portion of the lunar disk. NASA’s Lunar Reconnaissance Orbiter later captured before-and-after images of the impact site, revealing a fresh crater approximately 18.8 meters wide with more than 200 related surface changes, or “splotches,” extending up to roughly 30 kilometers from the point of impact. That single event provided concrete, photographic proof that observed flashes correspond to real craters and real ejecta fields, validating the technique of using optical flashes as impact proxies. Over longer timescales, such observations feed into broader studies of the near-Earth environment, complementing work highlighted in recent coverage of impacts that underscores how the moon’s pockmarked surface serves as a running tally of the solar system’s bombardment history.
The Outgassing Hypothesis and Aristarchus
Impact flashes, however, do not account for every type of TLP. Some historical reports describe diffuse glows, color changes, or hazes that persist far longer than the split-second flash an impact would produce, and they sometimes appear to brighten and fade gradually rather than blinking on and off. This is where the outgassing hypothesis enters the picture. Crotts synthesized multiple independent datasets, including filtered TLP reports, radon-222 episodic outgassing detections, polonium-210 mapping, and moonquake correlations, and found a striking overlap between active regions and the most reliable TLP sites. The implication is that gas escaping from the lunar interior could disturb fine surface dust, creating transient reflective clouds, or interact with sunlight and charged particles in ways that produce visible effects from Earth-based telescopes.
The strongest piece of supporting evidence dates to the Apollo 15 mission, when an onboard alpha particle spectrometer detected enhanced radon-222 levels emanating from the Aristarchus crater region while in lunar orbit. Radon-222 is a radioactive gas produced by the decay of uranium in rock, and its presence near the surface signals that pathways exist for gas to migrate upward from the moon’s interior rather than being completely trapped in solid rock. Aristarchus is simultaneously the single most reported TLP site and the location with the clearest orbital evidence of active outgassing, an overlap that is difficult to attribute to chance alone. As reporting in Nature has noted, the scientific dispute around TLPs has long oscillated between dismissing them as observer error and treating them as genuine lunar phenomena, but the Aristarchus data tilt the balance toward at least some events being real and internally driven, hinting that the supposedly geologically “dead” moon may still be slowly venting its insides.
Two Mechanisms, One Moon
The critical insight emerging from this body of research is that lunar flashes are not a single phenomenon with a single cause. Impact flashes and outgassing events likely coexist, and failing to distinguish between them has muddied the scientific record for decades. The meteoroid impact explanation is now well supported by direct crater imaging, systematic monitoring programs, and energy calculations that match observed brightnesses. At the same time, the spatial correlation between TLP hot spots and outgassing tracers, together with Apollo-era radon detections, argues strongly that some of the longer-lasting, more diffuse events arise from gas release and dust lofting rather than from high-speed collisions. In this view, the moon’s surface behaves as both a passive target for external debris and an active interface where internal processes occasionally break through.
Untangling these overlapping signals matters for more than just solving a centuries-old astronomical puzzle. Understanding impact rates helps refine models of the meteoroid environment that spacecraft must navigate, while characterizing outgassing sheds light on the moon’s thermal evolution, interior composition, and the stability of volatiles buried in its crust. As NASA and other agencies plan sustained human and robotic activity near the lunar surface, including long-term habitats and resource extraction, knowing where and how often the ground is disturbed (by incoming rocks or by gas pushing upward) will shape site selection and engineering decisions. In that sense, the flashes that once seemed like mere curiosities are becoming practical data points for the next era of lunar exploration.
What Future Missions Could Reveal
Answering the remaining questions about TLPs will require combining historical archives, real-time monitoring, and on-site measurements. Modern high-speed cameras and sensitive detectors can watch the night side of the moon continuously, catching even faint impacts and subtle glows that would have escaped notice through an eyepiece. These efforts build on a growing ecosystem of lunar science outreach and mission updates, such as NASA’s public series on exploration, which regularly highlights how small robotic missions and new instruments are reshaping our view of Earth’s companion. Future orbiters equipped with improved particle and gas detectors could fly over regions like Aristarchus repeatedly, looking for episodic bursts of radon or other species and tying those directly to any optical changes seen from the ground.
On the surface, seismometers and dust sensors deployed by upcoming landers could capture the local signatures of both impacts and outgassing, revealing whether faint tremors or pressure spikes accompany the flashes seen from afar. Coordinated campaigns, where telescopes on Earth, orbiters overhead, and instruments in the regolith all watch the same region at once, offer the best chance of finally matching each type of TLP to its underlying mechanism. If those efforts succeed, the moon’s fleeting glimmers will no longer be mysterious anomalies but well-understood manifestations of two intertwined processes: the constant rain of cosmic debris and the slow, persistent exhalation of a still-evolving world.
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*This article was researched with the help of AI, with human editors creating the final content.
