Explosions on the Moon
During the 2001 Leonid meteor storm, astronomers
observed a curious flash on the Moon -- a telltale sign of meteoroids
hitting the lunar surface and exploding.
November 30, 2001: Vivid, colorful streaks of light. A ghostly flash. Strange crackling noises and twisting smoky trails. Add to those a cup of hot cocoa, and you have all the ingredients for a delightful meteor shower ... on Earth.
The recent Leonids were a good example. On Nov. 18th our planet plunged into a debris cloud shed by comet Tempel-Tuttle. Sky watchers saw thousands of meteors -- each streak of light a tiny bit of comet dust disintegrating in the atmosphere.
Right: A bright Leonid meteor recorded by Dennis Mammana on Nov. 18, 2001. [more]
A quarter of a million miles away, another Leonid shower was happening. But the recipe was different: Blinding flashes of light. Flying debris and molten rock. Sizzling craters. And certainly no hot cocoa! That's what the Leonids were like ... on the Moon.
David Palmer, an astrophysicist at the Los Alamos National Laboratory, saw just such an explosion from his backyard in White Rock, New Mexico. The 2001 Leonids were well underway when Palmer trained his 5-inch Celestron telescope and a low light video camera on the crescent Moon. "It was twilight," says Palmer. "Even so, the flash was bright enough to detect." He had spotted a Leonid crashing down near Sinus Media -- a lava plain on the lunar equator.
Far from New Mexico, observers on the east coast of the United States saw it, too. Using 8 inch telescopes equipped with video cameras, David Dunham in Maryland and Tony Cook in Virginia independently recorded the flash -- a double confirmation. "We estimate it was as least as bright as a 4th magnitude star," says Dunham, director of the International Occultation Timing Association.
This marks the second year Dunham and Palmer have seen lunar Leonids. They and others video-recorded six meteoroid impacts on the Moon during the 1999 Leonid meteor storm. The brightness of those flashes ranged from 7th to 3rd magnitude.
"Actually, we've known for many years that Leonids hit the Moon," notes Cooke. "Between 1970 and 1977, Apollo seismic stations detected impacts during the Leonids and several other annual meteor showers. What's new since 1999 is that we're actually seeing the explosions from Earth."
Left: At least 6 Leonids hitting the Moon in 1999 caused explosions bright enough to see from Earth. [more]
The first reports of bright lunar Leonids two years ago puzzled many scientists. Their calculations suggested that a Leonid hitting the Moon would need to mass hundreds of kilograms to produce an explosion visible through backyard telescopes. Yet there was little evidence for such massive fragments in the Leonid debris stream. Hundred-kilogram meteoroids hitting Earth's atmosphere would produce sensational fireballs, brighter than any sky watchers actually saw. Furthermore, lunar seismic stations operating for years had detected nothing larger than 50 kg.
To solve the mystery, Jay Melosh, a planetary scientist at the University of Arizona's Lunar and Planetary Lab and an expert on planetary impact cratering, teamed up with Ivan Nemtchinov, a Russian physicist skilled in computer simulations of nuclear explosions.
Below: A thermonuclear explosion at the Enewetak Atoll, Marshall Islands, 1952. Such explosions are more powerful than meteoroids hitting the Moon -- nevertheless, computer programs written to simulate nuclear blasts were needed to unravel the physics of lunar Leonids.
Experience with bombs came in handy solving this problem, says Melosh: "Leonid impacts aren't as potent as a nuclear warhead, but they are powerful. They hit the Moon traveling 72 km/s or 160,000 mph -- that's about 100 times faster than a rifle bullet. In fact, the energy per unit mass in a Leonid strike is 10,000 times greater than a blast of TNT."
Using computer programs designed to study bomb blasts, Melosh and Nemtchinov discovered that Leonids didn't have to be so massive to produce flashes as bright as those detected by Dunham and Palmer. Impactors massing only 1 to 10 kg could do the job.
"That's more like it," says Cooke. "We occasionally see kg-sized fragments burning up in Earth's atmosphere. They appear as very bright fireballs that disintegrate completely before hitting the ground." On the Moon, of course, there's nothing to stop them from reaching the surface.
According to Melosh, here's what happens when the Moon and a 10 kg Leonid collide:
Much of the ground within a few meters of the impact point would be vaporized, and a cloud of molten rock would billow out of a growing crater. "At first the cloud would be opaque and very hot, between 50,000 K and 100,000 K," explains Melosh. "But the temperature would drop rapidly. Milliseconds after the initial blast, the cloud would expand to a few meters in diameter and cool to 13,000 K. That's the critical moment," he says, "when the vapor becomes optically thin (transparent); then, all the photons rush out and we can see a flash of light from Earth."
Left: The Apollo 17 Lunar Seismic Profiling Experiment was one of several Moon-based seismic stations that detected meteoroid strikes between 1970 and 1977.
Fortunately for future Moon colonists, there's little chance of being hit. Cooke explains: "During an intense Leonid meteor storm like the one Earth experienced in 1966, the lunar flux of meteoroids more massive than 10-5 gm would be 1 per square-km per hour. If we assume really chubby or bulky astronauts with a cross-sectional area of 0.5 square-meters, then the probability of being hit by a 10-5 gm Leonid is only 0.00025." Such tiny particles carry enough energy to puncture a spacesuit, but the astronaut inside would remain mostly intact, says Cooke. "The probability of being hit by something that might totally vaporize you -- like a 10 kg fragment -- is a billion times less."
So ... lunar Leonid meteoroid showers might not be as scary as they sound. Future denizens of the Moon might even take up a new astronomical hobby: ground watching. "I saw a hundred puffs of moondust every hour," they might say after a good spate of Leonids. "And, ooh that fireball ... what a blast!"
Editor's Note: After this article was published, a second lunar flash was confirmed for the 2001 Leonid shower. Video tapes recorded by Roger Venable from eastern Georgia (USA) and Tony Cook of Virginia reveal a lunar Leonid on Nov. 18, 2001, at 23:19:16 UT near Tranquillitatis.More about lunar Leonids....
- How efficient are lunar Leonid explosions? "About 0.1% of the kinetic energy in a lunar Leonid impact is converted to visible light," says Melosh. That's a small fraction, but enough for a brilliant explosion.
- What's the best wavelength for detecting lunar Leonids? "So far we've seen lunar Leonids as flashes of visible light, but infrared wavelengths around 10 microns would be even better," says Melosh. The visible flash of a lunar Leonid comes and goes in milliseconds, but bright infrared radiation would persist for minutes, a result of the slow-to-cool crater formed by the explosion.
- How big is a lunar Leonid crater? According to computer simulations by Melosh and Nemtchinov, the explosion of a 10 kg Leonid meteoroid would leave behind a 4.5 meter-wide crater on the Moon.
- Could a 10 kg Leonid meteoroid make a crater on Earth? No, says Donald Yeomans, manager of NASA's Near-Earth Object program at JPL. "A 10 kg-sized particle entering the Earth's atmosphere would cause a fireball event that would be hard to miss. However, it would disintegrate entirely high in the Earth's atmosphere without getting close to the Earth's surface. Leonids are traveling at 71 km/s (the fastest meteoroids) and would completely disintegrate even in the very unlikely case they were solid iron."
- Why do lunar Leonids glow? When a Leonid hits the Moon it vaporizes the ground where it hits. The vaporous molten rock glows simply because it is hot.
- Why study lunar meteoroid impacts? Part I "Go out in your backyard and look up," says Cooke. "You can see about about 11,000 square kilometers [of Earth's atmosphere]. Now look at the Moon. Depending on its phase you could be looking at as much as 19 million square kilometers of dark terrain." The Moon is a huge meteoroid detector! Cooke believes that systematic observations of lunar meteoroid impacts might reveal new information about the largest fragments in comet debris streams.
- Why study lunar meteoroid impacts? Part II When meteoroids strike the Moon and explode, they vaporize a bit of the Moon itself. Studying those vapors might be one way of "lunar prospecting" from a distance. A team of scientists from the University of Texas and NASA tried something similar in 1999 when they crashed NASA's Lunar Prospector spacecraft into the Moon. They crashed the probe into a south polar crater, hoping that the impact would vaporize shadowed water-ice and eject a detectable cloud of water vapor over the lunar limb.
Confirmed and Canidate 2001 Lunar Leonid Impact Flashes -- from David Palmer, an astrophysicist at the Los Alamos National Lab.
Moon Phases -- The phase of the Moon was nearly New on Nov. 18, 2001, so there was plenty of dark terrain to scan for lunar Leonids -- but not much time to look. The crescent Moon sank below the western horizon less than two hours after sunset.
2001 Lunar Leonid Observations -- a summary of Leonid impact data so far received by the International Occultation Timing Association.
Association of Lunar and Planetary Observers Meteoritic Impacts Search -- Organized by Brian Cudnik, who discovered the first confirmed lunar Leonid in 1999.
IAU Circular 7320: Lunar Leonid Meteors -- (SpaceWeather.com) a summary of 5 lunar Leonid flashes from 1999
Meteoroid impact detection from an orbiter around the Moon: Kosarev I.B., Nemtchinov I.V. Institute for Dynamics of Geospheres, Lunar and Planetary Science XXXII (2001).
Lunar Leonids 2000 - (Science@NASA) Scientists look for vapors blasted off the Moon by meteoroid impacts.
Leonids on the Moon 1999 -- (Science@NASA) Leonid meteorite impacts on the Moon might be visible from Earth and provide a means for long-distance lunar prospecting.
A Leonid on the Moon? - (Science@NASA) The first recorded impact of a meteorite on the Moon was captured on video during the 1999 Leonids meteor storm.
Moon Struck - (APOD) Craters produced by ancient impacts on the airless Moon have long been a familiar sight. But now observers have seen elusive optical flashes on the lunar surface - likely the fleeting result of impacting meteoroids.
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