There's a mystery brewing a half-billion miles away as astronomers continue to study the collision of comet P/Shoemaker-Levy 9 with the planet Jupiter.
Call it the Case of the Missing Water.
If comets are made mostly of water ice, rock and dust, why are astronomers unable to detect any water in the fireballs and gas plumes produced by the broken comet's bombardment?
"It's kind of confusing," said Hubble Space Telescope astronomer Keith Noll of the Space Telescope Science Institute. While many predictions of the collisions' effects have been dead-on accurate, he said, "in many ways this has behaved contrary to what our initial assumptions were."
The missing water mystery goes to the heart of those assumptions, for if there really is no water there, it may mean that "comet" Shoemaker-Levy 9 is no comet after all, but a waterless rocky asteroid with an entirely different composition and natural history.
"That's a big question," said University of Maryland astronomer Lucy McFadden. And "we may not get the answer."
While they scratched their heads yesterday, astronomers around the world continued to watch the slow-motion train wreck taking place more than a half-billion miles away.
By today, 10 major fragments of the broken comet had plunged into Jupiter. Photos from Hubble and observatories around the world show a growing tattoo of dark spots around Jupiter's southern latitudes -- the slowly dissipating gas plumes thrown up by the explosive impacts.
Four more collisions were due between 6:21 a.m. and 8 p.m. EDT today. There will be five more collisions tomorrow, and a final smack early Friday morning.
A major part of scientists' worldwide vigil involves chemical analyses of the gases thrown up by the collisions. Because different elements absorb different wavelengths of sunlight, scientists using spectrographs can discover which chemicals are present by noting which wavelengths are missing from the reflected light.
So far, those studies have revealed increased amounts of ammonia at the impact sites. That's no surprise because heat from the fireballs was expected to vaporize ammonia crystals in the planet's high-altitude clouds.
Measuring how much ammonia vaporized may provide clues to the size of the fragment and the physics of the impact.
The missing water is a surprise. The search was a top priority for scientists for two reasons:
* Since Jupiter has no observable water, and comets do, any they detected would provide some measure of the size of the impacting fragment. By relating the size of the fragment to its observed effects, scientists could learn a great deal about how such collisions work.
* Theoreticians believe there are undetected layers of water clouds somewhere below Jupiter's visible ammonia clouds. Water in the plumes could confirm that theory, and show that the fragments shot deep enough to flush some of that water out with their fireballs.
So far, however, no one has detected any clear signs of water. Even if the fragments were atomized in the collisions, some trace of oxygen from the water should be detectable. But there's no sign of that either.
"It is disturbing," said Dr. McFadden. "This means either that our modeling is not correct, or the comet exploded before it reached [Jupiter's] water layers."
But that still wouldn't explain why nothing of the comet's own water has turned up.
Shoemaker-Levy 9 was originally classified as a comet because its fragments each appeared to have a "coma," or halo of water vapor, dust and gas.
Comets are believed to be rather loosely compacted, rocky iceballs, unchanged remnants of the solar system's primordial matter. They originate in a vast belt far beyond the orbit of Pluto, called the Oort Cloud.
Asteroids, on the other hand, are basically rocks floating in space, most of them in a broad belt between the orbits of Mars and Jupiter.
Asteroids have been changed by eons of sunlight and collisions with other asteroids. But "comets have been in cold storage," far from the sun, since the solar system formed, Dr. McFadden said. And that's what makes them important.
"What we learn of their material . . . can be applied to a time closer to the solar system's formation," she said, making them a window on the distant past.
In a sort of "Catch 22," however, astronomers may be unable to clearly classify Shoemaker-Levy 9 "because of our ignorance of the structure of asteroids and comets," she said.
For example, could an asteroid be pulled apart by a close encounter with Jupiter's gravitational field, as scientists believe Shoemaker-Levy 9 was in 1992? No one has ever seen it happen.
Perhaps a shattered asteroid could generate enough dust or gas to create what looks to Earth telescopes like a "coma" around Shoemaker-Levy's fragments. But no one knows that either.
Scientists might have solved the riddle had they found evidence of water in Shoemaker-Levy 9 before it reached Jupiter. Unfortunately, Dr. McFadden said, "the fragments were too darn faint, even with the largest telescopes, to collect enough photons [light] in a reasonable period of time."
"So we may be stuck," she said.
Nevertheless, Dr. Noll said, "I and others are going to keep looking."
A PLANET OF GASES
The fragments of comet Shoemaker-Levy 9 are not blasting craters on Jupiter because Jupiter is composed almost entirely of hydrogen and helium gas.
It has no hard surface like those of the four inner planets.
When the comet's fragments hit, they plunge an estimated 35 miles or more into Jupiter's atmosphere. In seconds, their enormous forward speed is converted into heat.
The heat creates a rapidly expanding gas "bubble," which rises back through the atmosphere, erupts from the cloud tops and rises a thousand miles or more in a 30,000-degree fireball not unlike a thermonuclear mushroom cloud.
In minutes, the cloud cools, and falls back on the cloud tops in a gas plume that will eventually dissipate in the Jovian winds.