WATER, WATER, EVERYWHERE . . .
Just one month after announcing that there is a "high probability" that water ice exists at both poles of the Moon, NASA scientists are saying the amount of water present on the Moon may be much more than previously thought.
Initial estimates, announced in March 1998 and based on data returned by the Lunar Prospector, placed the amount of lunar water ice in the range of 11 million to 330 million tons. Scientists involved in analyzing the Prospector data are now cautiously asserting that the total may be closer to a billion metric tons.
"There's quite a bit more ice than we were thinking," said Lunar Prospector Principal Investigator Alan Binder, of the Lunar Research Institute in Gilroy, California. Early in the investigation, Binder said, "we knew we were seeing water, but we thought the data indicated a low mixing ratio."
Binder cautioned, however, that the new estimates are still based on "theoretical models" about the Moon's atmosphere, the rate of cometary impact, and how these factors affected the regolith mixing ratio over time.
In addition, Binder has been careful to reiterate that the Prospector's neutron spectrometer detected only the presence of hydrogen, and did not directly measure the presence of water ice.
"We're inferring that it's water. But I would be very surprised if it's just large hydrogen deposits," Binder said.
The discovery of water on the Moon could be a major catalyst to reviving interest in building a lunar base, Binder said, since the water could be used for fuel and life support. Binder said he believes a lunar base is the first logical step in the colonization of other planets and would also provide invaluable opportunities for lunar science and astronomy.
"Clearly, the post-Apollo plan that we had in the 1970s was to have a base by the end of the decade," he said. "If Lunar Prospector were to kindle this spirit again and we would have to rebuild some of the technology we might be able to build a base in 10 years. It's really a political question."
Binder said a sampler lander mission could definitively answer the questions surrounding the presence of ice on the Moon.
DISK DISCOVERY STIRS DEBATE
NASA astronomers using the new Keck II telescope have discovered what some are terming the "clearest evidence yet" of a budding solar system around a nearby star.
Scientists released an image of the possible site of planet formation around a star known as HR 4796, about 220 light-years from Earth in the constellation Centaurus. The image, taken with a sensitive infrared camera developed at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, shows what appears to be a swirling disk of dust around the star. Within the disk is an empty region that may have been swept clean when material was pulled into newly formed planetary bodies, the scientists said.
"This may be what our solar system looked like at the end of its main planetary formation phase," said Michael Werner of JPL, who discovered the region along with David Koerner and Michael Ressler, also of JPL, and Dana Backman of Franklin and Marshall College, Lancaster, Pennsylvania. "Comets may be forming right now in the disk's outer portion from remaining debris."
Koerner of JPL said the finding represents a "missing link" in the study of how planetary systems are born and evolve.
"In a sense, we've already peeked into the stellar family album and seen baby pictures and middle-aged photos," Koerner said. "With HR 4796, we're seeing a picture of a young adult star starting its own family of planets. This is the link between disks around very young stars and disks around mature stars, many with planets already orbiting them."
Other scientists, however, remain skeptical of the construction-zone interpretation.
"While the enthusiasm is understandable, and the scientific value of the observations is only just beginning to be appreciated, care should be taken to not overinterpret them," said David C. Black, director of the Lunar and Planetary Institute in Houston. "Observations have shown that most, if not all, stars younger than HR 4796 have disks associated with them. What is not known is the evolutionary fate of those disks, in particular which of them are likely to evolve to form planetary systems."
Black noted that there are three possible evolutionary tracks for circumstellar disks. Such a disk may form a binary system, evolve into a planetary system, or disperse into nothing of consequence. Black said that, at the present time, no clear evidence rules out any of these possibilities for HR 4796, which he terms "a modest disk at best."
"As more systems are discovered and analyzed, we may gain sufficient insight to know whether the disk associated with HR 4796 is a missing link, and if so, to what endpoint, or whether it is simply a snapshot of an infertile system, to continue the human analogy that seems to be so freely used in discussing these objects."
The discovery of the disk was made on March 16 from the 33-foot (10-meter) Keck II telescope atop Mauna Kea, Hawai'i. Keck II and its twin, Keck I, are the world's largest optical and infrared telescopes. Attached to the Keck II for this observation was the mid-infrared camera, developed by Ressler at JPL and designed to measure heat radiation.
The four scientists reported their discovery in a submission to The Astrophysical Journal Letters. The disk was discovered independently at the Cerro Tololo Observatory in Chile by another team of scientists, led by Ray Jayawardhana of the Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, Massachusetts, and Charles Telesco of the University of Florida, Gainesville.
The apparent diameter of the dust disk around HR 4796 is about 200 astronomical units (one astronomical unit is the distance from Earth to the Sun). The diameter of the cleared inner region is about 100 astronomical units, slightly larger than our own solar system.
HR 4796 is about 10 million years old and is difficult to see in the continental U.S., but is visible to telescopes in Hawai'i and the southern hemisphere.
The discovery of the HR 4796 disk was made in one hour of observing time at Keck, but the JPL team plans to return to Hawai'i in June for further studies. They hope to learn more about the structure, composition, and size of this disk, and to determine how disks around stars in our galaxy produce planets. They plan to study several other stars as well, including Vega, which was featured prominently in the movie Contact.
The Keck II image of HR 4796 is available at http://origins.jpl.nasa.gov/science/recentscience.html.
ASTEROID 1997 XF11 A NEAR MISS?
In March, asteroid 1997 XF11 came and went. A week after astronomers announced that the asteroid, discovered in December, would pass within 30,000 miles of the Earth in 2028, new calculations showed the passing distance to be a more comfortable 600,000 miles.
"We are saying now that the possibility of an impact is zero," Donald K. Yeomans, senior research scientist at NASA's Jet Propulsion Laboratory, told the Associated Press in March.
The flurry of media attention and doomsday speculations brought on by the original announcement prompted a panel of scientists, who were meeting during the LPSC conference to discuss near-Earth object discovery programs, to establish interim guidelines for releasing information to the public and press.
"The goal was to come up with guidelines for what will happen what should happen should another event like this happen in the future," said Yeomans. "The problem was that much of the analysis took place after the press releases and we would like to reverse that order."
During the meeting, scientists agreed to set up a committee, consisting of a half-dozen astronomers, that would work to verify and analyze information on potentially dangerous asteroids as quickly as possible.
"There would be a short period for more analysis, and a time to discuss the possibilities of what could be done should a real danger exist," Yeomans said.
Yeomans said such guidelines will help prevent gaps in credibility and will allow scientists to reach a concensus before releasing information to the public.
"This is the way science works you make mistakes and you put a process in place to make sure it doesn't happen again," he said. "If we lose our credibility we've lost everything."
Yeomans also noted that the distraction of the media attention surrounding 1997 XF11 hindered the scientific process.
"I was astonished by the media attention. It's very difficult to do analysis in that kind of climate," he said.
In addition, Yeomans and other scientists would like to see more funding and effort dedicated to searching for asteroids and comets, including those that might be on a collision course with Earth.
"There are some modest efforts being funded by NASA at JPL, in Tucson, and at Lowell to discover these objects," Yeomans said. "At the moment we're not searching the entire sky on a monthly basis and we should be. Eventually, we'll find them all, but it may take a while."
Images of 1997 XF11 are available at http://www.astro.washington.edu/deutsch/misc/asteroid/.