Transient Lunar Phenomena Studies

Image of TLP taken by Leon Stuart, 1953 November 15.
The TLP is the small, bright spot in the center of the image.

Introduction - The Basics of TLPs

What is a TLP?

Transient lunar phenomena (TLPs) are described as short-lived changes in the brightness of patches on the face of the Moon. They last anywhere from a few seconds to a few hours and can grow from less than a few to a hundred kilometers in size. Most instances of TLPs are described as increases in the overall luminosity of a spot on the Moon; however, sometimes observers report a decrease in a region's brightness or even a change in its color to red or violet. Reports of TLPs have described them as "mists", "clouds", "volcanoes", among other provocative terms. Even today, they are poorly understood; however, our team is attempting to remedy this.

Who has observed a TLP?

Hundreds of amateur and professional astronomers have documented their observations of TLPs since the invention of the telescope. A number have been confirmed by multiple observers or even photographed. A few TLPs were even reported by Apollo astronauts as they orbited the Moon.

What causes a TLP?

The physical mechanism responsible for creating a TLP is not well understood. Of the many theories proposed, we believe that they are the outcome of lunar outgassing, that is, gas being released from the surface of the Moon. To learn more about the evidence supporting this hypothesis, please read our scientific papers below.

Are TLPs real?

Because the evidence for TLPs is almost entirely anecdotal and based on visual descriptions by observers, many scientists remain skeptical as to whether or not TLPs are real. We, too, were skeptical when we began our investigation of TLPs, but our statistical analyses suggest that most reported TLP events are real, even though there are some spurious reports. Our Lunar Monitor project, as described below, is an attempt at gaining an objective database of these events through the use of modern technology in a systematic search.

Published and Submitted Papers on TLP Basics

Transient Lunar Phenomena: Regularity and Reality (Astrophysical Journal, 697, 1-15 - 2009 May 20)
Author: Arlin Crotts
This is a statistical evaluation of the repeatability of TLP observations despite the changing collection of observers. We find that certain sites are robust to changes in the characteristics of the observers; these sites include craters Aristarchus, Plato and the major, young impacts (Copernicus, Kepler & Tycho), plus Grimaldi and perhaps the basalt plain Mare Crisium. Leading to this analysis, we characterized the extent to which TLP reports from some observers were known by subsequent observers, and find that until the year 1956, TLPs were poorly known, but after that time many reports were generated by observers who were aware of and even searching for TLPs. Since 1956, several common features appear that were nearly absent in TLP reports prior to this, thus failing our robustness tests. Examples of such features are TLP observations in craters Alphonsus, Gassendi and Ross D.

Lunar Outgassing, Transient Phenomena and The Return to The Moon, I: Existing Data (Astrophysical Journal, 687, 692-705 - 2008 July 1)
Author: Arlin Crotts
There are other classes of transient events on and in the Moon: moonquakes and outgassing events traced by radioactive 222Rn gas. These can be localized and we find an amazing correspondence: all of the 222Rn events land in the robust TLP sites that we found previously, which has a probability of only 10-4 of occurring at random. Furthermore, TLPs (both the robust sample and the original sample before the robustness tests) are strongly correlated with the boundary between the basalt maria and the heavily cratered highlands. Surprisingly, the alpha-particle radioactive signal from 210Po, a product of 222Rn decay, also correlates with the same boundary. This is an independent indication of a correlation of TLPs with outgassing, also at the 10-4 random probability level. We conclude that TLPs are associated with lunar outgassing.

Lunar Outgassing, Transient Phenomena & The Return to The Moon, II: Predictions and Tests of Regolith/Outgassing Interactions (to appear in The Astrophysical Journal, Dec. 10, 2009, also astro-ph/0909.3832, originally astro-ph/0706.3952 and astro-ph/0706.3954)
Authors: Arlin Crotts & Cameron Hummels
We follow Paper I with predictions of how gas leaking through the lunar surface could influence the regolith, as might be observed via optical Transient Lunar Phenomena (TLPs) and related effects. We touch on several processes, but concentrate on low and high flow rate extremes, perhaps the most likely. We model explosive outgassing for the smallest gas overpressure at the regolith base that releases the regolith plug above it. This disturbance's timescale and affected area are consistent with observed TLPs; we also discuss other effects. For slow flow, escape through the regolith is prolonged by low diffusivity. Water, found recently in deep magma samples, is unique among candidate volatiles, capable of freezing between the regolith base and surface, especially near the lunar poles. For major outgassing sites, we consider the possible accumulation of water ice. Over geological time ice accumulation can evolve downward through the regolith. Depending on gases additional to water, regolith diffusivity might be suppressed chemically, blocking seepage and forcing the ice zone to expand to larger areas, up to square km scales, again, particularly at high latitudes. We propose an empirical path forward, wherein current and forthcoming technologies provide controlled, sensitive probes of outgassing. The optical transient/outgassing connection, addressed via Earth-based remote sensing, suggests imaging and/or spectroscopy, but aspects of lunar outgassing might be more covert, as indicated above. TLPs betray some outgassing, but does outgassing necessarily produces TLPs? We also suggest more intrusive techniques from radar to in-situ probes. Many of these approaches should be practiced in a pristine lunar atmosphere, before significant confusing signals likely to be produced upon humans returning to the Moon.

Ongoing Projects

Robotic Lunar Imaging Monitor

Collaborators: Arlin Crotts, Paul Hickson, Cameron Hummels & Thomas Pfrommer
Description: We have constructed a lunar monitor at Cerro Tololo Interamerican Observatory in Chile for the purpose of detecting and investigating TLPs as well as any other short-lived lunar events. The monitor consists of two cameras, one low-resolution and high frame-rate (10km/pixel, 5Hz) and the other high-resolution and low frame-rate (1.2km/pixel, 0.1Hz), each sensitive to different phenomena. This monitor is continuously taking images of the Moon, and by using an algorithm to search for differences between consecutive images, it is capable of detecting TLPs more sensitively than the human eye. The monitor has acheived full operation.

Above, we see two images depicting a potential TLP event. First is a section of the Moon where we've added a synthetic TLP event to our dataset, which is circled here. Note, however, that without the circle, your eye would be unable to detect the low-level TLP that has been added to this image. Next are the results (the signal to noise image) of our image-differencing program, clearly indicating the detection of our synthetic TLP.

Below are two images of the lunar monitor with its two cameras. First is a picture of our monitor in its housing, located at Cerro Tololo, high in the Chilean Andes. Second is a closeup of the monitor with each camera wrapped in plastic to protect it from moisture. The larger, high-resolution camera on the left is attached to a 6" telescope, whereas the high-speed camera is mounted in the center of the bar. The vertical tubes that give our monitor its strange appearance are actually part of a separate, unrelated instrument designed for measuring atmospheric turbulence. Our two telescopes and their horizontal bar mount rotate slowly as the night progresses, tracking the Moon as it passes across the sky. In fact, because the Moon is so luminous, out instrument is able to take data during the daytime, increasing our ability to detect and observe a TLP. As of August 2008, a second monitor has begun operations located on the main campus of Columbia University. (Telescope on red mount.)

We plan for this monitor (and maybe others) to be operating in September 2007 when the Japanese spacecraft SELENE goes into orbit around the Moon with its 222Rn alpha-particle detector, which will be uniquely sensitive to lunar outgassing. SELENE's alpha particle detector and our automated lunar imaging monitor(s) will be a powerful combination.


AEOLUS (which stands for "Atmosphere seen from Earth, Orbit and the LUnar Surface") is a group of scientists* determined to understand the nature and effects of the lunar atmosphere. This could be a very important issue not only for lunar geology, but also as a potential resource for human exploitation. We are not only implementing ideas from Paper III above, but also developing ways to study gas directly at the lunar surface. A summary of some aspects of our efforts are summarized in the paper below.
*Collaborators: Daniel Austin, Andrew Barclay, Alex Bergier, Ara Chutjian, Arlin Crotts, Patrick Cseresnjes, Murray Darrach, Denton Ebel, Steve Gorevan, Paul Hickson, Cameron Hummels, Jan Kratochvil, John Longhi, Dragan Lukic, Szabi Marka, Zsuzsa Marka, Yosio Nakamura, Jani Radebaugh, Daniel Savin, Caleb Scharf, Edward Spiegel

Published Papers by AEOLUS

Probing Lunar Volatiles: Initial Ground-Based Results
Monitoring Lunar Surface Changes During and After The Kaguya Mission

Popular Media Coverage

Scientific American, June 26, 2007.
Columbia News, June 27, 2007
PhysOrg, June 27, 2007
India Daily, June 27, 2007
The Hindu, June 27, 2007
New Scientist, June 28, 2007
CNews R&D, June 28, 2007 (in russian)
La Stampa, July 25, 2007 (in italian), July 30, 2007
MSNBC, July 30, 2007
USA Today, July 30, 2007
Yahoo! News, July 30, 2007
Fox News, July 30, 2007
Columbia, August 20, 2007
Discover magazine (Oct. issue), September 10, 2007
Hirek, January 12, 2008 (in hungarian), January 14, 2008 (in hungarian)
New Scientist magazine, March 28, 2008
BBC Radio 4 - Science Frontiers, November 3, 2008
BBC World Service - Discovery, February 4, 2009
Ciel et Espace, March 2009 issue (in french)
National Geographic News, March 2, 2009
Nature News note - May 13, 2009

For more information contact Arlin Crotts:
Webpage design by Cameron Hummels and Arlin Crotts.