After almost a half-century--numerous space probes and six manned lunar landings later--what had become known in astronomy circles as "Stuart's Event" was still an unproven, controversial theory. Skeptics dismissed Stuart's data as inconclusive and claimed the flash was a result of a meteorite entering Earth's atmosphere. That is, until Dr. Bonnie J. Buratti, a scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and Lane Johnson of Pomona College, Claremont, Calif., took a fresh look at the 50-year-old lunar mystery.
"Stuart's remarkable photograph of the collision gave us an excellent starting point in our search," said Buratti. "We were able to estimate the energy produced by the collision. But we calculated that any crater resulting from the collision would have been too small to be seen by even the best Earth-based telescopes, so we looked elsewhere for proof."
"Using Stuart's photograph of the lunar flash, we estimated the object that hit the Moon was approximately 20 meters (65.6 feet) across, and the resulting crater would be in the range of one to two kilometers (.62 to 1.24 miles) across. We were looking for fresh craters with a non-eroded appearance," Buratti said.
Part of what makes a Moon crater look "fresh" is the appearance of a bluish tinge to the surface. This bluish tinge indicates lunar soil that is relatively untouched by a process called "space weathering," which reddens the soil. Another indicator of a fresh crater is that it reflects distinctly more light than the surrounding area.
Buratti and Johnson's search of images from the Clementine mission revealed a 1.5-kilometer (0.93 mile) wide crater. It had a bright blue, fresh-appearing layer of material surrounding the impact site, and it was located in the middle of Stuart's photograph of the 1953 flash. The crater's size is consistent with the energy produced by the observed flash; it has the right color and reflectance, and it is the right shape.
Having the vital statistics of Stuart's crater, Buratti and Johnson calculated the energy released at impact was about .5 megatons (35 times more powerful than the Hiroshima atomic bomb). They estimate such events occur on the lunar surface once every half-century.
"To me this is the celestial equivalent of observing a once-in-a-century hurricane," said Buratti. "We're taught the Moon is geologically dead, but this proves that it is not. Here we can actually see weather on the Moon," she said.
While Dr. Stuart passed on in 1969, his son Jerry Stuart offered some thoughts about Buratti and Lane's findings. "Astronomy is all about investigation and discovery. It was my father's passion, and I know he would be quite pleased," he said.
If the Moon is Struck, and No One is There to See it...
While the Moon as taught in astrophysics classes is largely inactive today, the details of its formation are of considerable ongoing interest. The Moon is believed to play an important role in Earth's habitability. Because the Moon helps stabilize the tilt of the Earth's rotation, it prevents the Earth from wobbling between climatic extremes. Without the Moon, seasonal shifts would likely outpace even the most adaptable forms of life.
"A Moon-less Earth with the same mass, rotation rate, and orbit as today would have the direction of its spin axis vary chaotically between 0 and 90 degrees on time scales as short as 10 million years," says Darren Williams, Assistant Professor of Physics and Astronomy at Penn State University and NAI member. "At high obliquity, temperatures over mid-to-high latitude continents would reach near boiling 80 to 100 Celsius around the summer solstice under a 1-bar nitrogen- dominated atmosphere. Such temperatures would be damaging to all forms of water-dependent life on Earth today."
The Earth-Moon system is unusual in several respects. The Moon has an abnormally low density compared to the terrestrial planets (Mercury, Venus, Earth, and Mars), indicating that it lacks high-density iron. While the Earth's iron core is about 30 percent of the planet's total mass, the Moon's core constitutes only a few percent of its total mass. In addition, the angular momentum of the Earth-Moon system is quite large. It implies that the terrestrial day was only about five hours long when the Moon first formed close to the Earth. These characteristics provide strong constraints for giant impact models that try to explain the Moon's formation.
The "giant impact" theory, first proposed in the mid-1970s to explain how the Moon formed, has recently received a major boost, based on computer simulations. Previous models had shown two classes of impacts capable of producing an iron-poor Moon, but both were problematic. One model involved an impact with twice the angular momentum of the Earth-Moon system. The second model proposed that the Moon-forming impact occurred when Earth was about half of its present mass.
To predict the effects of various impact scenarios, simulations performed by Southwest Research Institute (SwRI) and University of California at Santa Cruz (UCSC) researchers show that a single impact by a Mars-sized object in the late stages of Earth's formation could account for an iron-depleted Moon and the masses and angular momentum of the Earth-Moon system. This is the first model that can simultaneously explain these characteristics without requiring that the Earth-Moon system be substantially modified after the lunar forming impact.
One open question that will likely remain a mystery is whether the original November 15 observations was a case of an earth-crossing meteor shower--like the annual November Leonids. Meteor showers producing up to 100 per hour have seasons and are named after the background constellations: April and June = Lyrids, August = Perseids, October = Orionids, November = Taurids, Leonids.
The Leonids are grains of dust from comet Tempel-Tuttle colliding into the Earth's atmosphere. Most Leonid particles are tiny and will vaporize very high in the atmosphere due to their extreme speed (about 44 miles per second, or almost 71 km/sec), so they present no threat on a global (or lunar) scale. As it progresses in its 33-year orbit, the comet releases dust particles every time it comes near the Sun. Earth intersects the comet's debris trail every year in mid-November, but the intensity of each year's Leonid meteor shower depends on whether Earth ploughs through a particularly concentrated stream of dust within the broader debris trail.
While not of the necessary size to produce the kind of large impact of Stuart's event, there are anecdotal cases logged of visible Leonids perhaps striking the moon as seen from Earth. The 1999 Leonids shower may have started a new chapter in the "Stuart event" logs.
Buratti and Johnson's study appears in the latest issue of the space journal, Icarus. The NASA Planetary Geology and Planetary Astronomy Programs and the National Science Foundation funded Buratti's work. The California Institute of Technology manages JPL for NASA.