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The Planetary Society Weblog

By Emily Lakdawalla

Discovery of a whole new kind of comet

Apr. 28, 2006 | 12:44 PDT | 19:44 UTC

Every Friday I check out Science and Nature to see what they've tapped as the biggest science news of the week, and there were a couple of key stories this time. The first one was published in Science by Henry Hsieh and David Jewitt at the University of Hawaii, and it's kind of big: the discovery of a new reservoir for comets in the solar system, within the main asteroid belt. So far, it has generally been assumed that all comets originate from the other two known reservoir regions: the Oort cloud and Kuiper belt. The main asteroid belt is home to, well, asteroids, not comets. But Hsieh and Jewitt have proven that there are comets in the main asteroid belt, and that they almost certainly formed there. There was apparently a press release on this story in late March, but I missed it.

The main belt comets
The main belt comets
Three bodies with orbits that identify them as main belt asteroids are shown here to have cometary comae and tails: 133P/Elst-Pizarro, P/Read (2005 U1) and asteroid 118401 (1999 RE70). They are now considered the first objects in a new class of bodies: the main belt comets. Their discovery has important implications for the conditions that prevailed when the asteroid belt was forming, and they may represent the kinds of bodies that formed the source of Earth's water during our planet's formation. Credit: Hsieh and Jewitt, 2006

Plot of semimajor axis versus eccentricity for all numbered asteroids and comets
Plot of semimajor axis versus eccentricity for all numbered asteroids and comets
Asteroids are represented by small black dots and comets by large blue dots. The main belt comets are shown as red dots. The vertical dotted lines represent the semimajor axes of Mars, the 2:1 mean-motion resonance with Jupiter, and Jupiter. Curved dashed lines represent the loci of all orbits with perihelia equal to Mars' aphelion and orbits with aphelia equal to Jupiter's perihelion. Asteroids (black dots) are almost exclusively found below those two curves; comets are almost exclusively found to the right of the Jupiter perihelion curve. But the main belt comets sit squarely among the asteroids. Credit: Hsieh and Jewitt, 2006
Hsieh and Jewitt base their conclusions on observations of three bodies: 133P/(7968) Elst-Pizarro, P/2005 U1 (Read), and asteroid 118401 (1999 RE70). They show images of all three bodies having clearly cometary comae and tails. Elst-Pizarro has even been observed to have periodic cometary behavior according to its position in its orbit, with the most cometary activity occurring near its perihelion (closest approach to the Sun) in 1996 and 2002. The cometary activity was continuous enough, over several months, that it couldn't just be a dust cloud from an impact; these things definitely behave like comets.

But their orbits put them squarely in the realm of the asteroids. All three bodies occupy orbits in the outer part of the asteroid belt, at a little more than 3 astronomical units from the Sun, with reasonably low eccentricities and inclinations. In the paper Hsieh and Jewitt show a graph of orbital distance versus eccentricity, with all numbered comets and asteroids plotted; the asteroids almost all occupy a well-definied region that is bounded by the orbits of Mars and Jupiter, and the comets are outside this region. The neat division is at least in part because bodies found to lie in orbits bounded by those of Mars' and Jupiter's have pretty much been defined to be asteroids. They're only called comets when you can see cometary activity, and so far cometary activity has only been observed for objects that orbit outside that narrowly-defined region. But not anymore -- 133P/Elst-Pizarro, P/Read, and asteroid 118401 all sit in that asteroidal region. Hsieh and Jewitt's analysis demonstrates that it's really unlikely that they could have arrived at such orbits from Kuiper belt or Oort cloud origins.

This makes life more complicated but also much more interesting for people who study comets and asteroids to understand how our solar system formed. This isn't the first time that water has been implied to exist in asteroidal bodies. After all, Ceres must have lots of water; and lots of asteroids have minerals that contain water bound up in their chemical structure. But Ceres is unusually big, so it would have had an easier time holding on to its primordial water. Actual little tiny comets require actual water ice, not just bound water, and that water ice somehow managed to stick around since the formation of these comets in their current positions four and a half billion years ago.

This isn't completely insane. Mathematical modeling suggests that water ice can survive at some depth (1 to 100 meters) below the surface of a body in the main belt over the age of the solar system; in order to see cometary activity on such a body, there would have to be some recent triggering event, like an impact, to expose the ice to the surface. Once that happens, cometary activity can only take place for a thousand years or so before the exposed ice gets used up and covered up by a lag deposit that will protect it again. What that means is that however many main belt bodies are observed to have cometary activity, there must be many times that many that have ice inside them but are dormant -- in fact, they could be common! At least in the outer part of the belt. All three of the bodies under discussion here do sit near the outer edge of the belt; careful surveys are necessary to discover more main belt comets and see how close they are found to the Sun.

But the main reason that this discovery is important is that some formation models for Earth have difficulty explaining why it's so wet -- Earth formed much closer to the Sun than the main asteroid belt, so it should be drier than the asteroid belt. One good way to get lots of water to Earth is to deliver it after Earth's formation on comets and asteroids. During the early days of the solar system, it would have been fairly common for asteroids -- or comets -- from the outer part of the main belt to be jogged into Earth-crossing paths. Maybe that's where our water came from. Hsieh and Jewitt suggest that one of these new main belt comets would be a terrific target for a sample return mission; ice samples from such a body could be tested to see if their isotopic ratios and noble gas contents match those in our oceans and atmospheres.

Cool. To read more, you can check out Hsieh's website.