Comet season for keeps!
With the astonishing success of the European mission to comet 67P, comets are back in the news. Rosetta the spaceship sent its slave spaceship, Philae, to land on 67P on Nov. 12, 2014. After Philae quit bouncing up and down in the comet’s microgravity, it landed in a place with limited sunlight. With insufficient electric power from its solar panels, the lander got down to business for about a day, and worked until it exhausted its batteries.
What have we learned from the European Space Agency’s Rosetta mission, which began traveling toward Comet 67P/Churyumov-Gerasimenko 10 years ago?
Not too dense! Density measurements released today by Rosetta scientists match well with those taken from Deep Impact in 2005, says comet expert Michael A’Hearn, of the University of Maryland. The Deep Impact mission fired a missile at Comet Tempel 1 and studied the results. To figure the density of the comet’s nucleus, they measured how fast junk lofted by the impact fell back under the comet’s gravity. Since gravity is a function of the object’s mass, the technique gives “a direct measure of gravity,” A’Hearn says. Using this and another method, Deep Impact scientists calculated a density of less than 0.5 grams per cubic centimeter.
That’s odd, since comets are believed to be a mix of ice (density 1) and dirt or rock (density 2 to 3). “This says the nucleus has to be very porous,” says A’Hearn, Deep Impact’s principal investigator. Gratifyingly, Rosetta direct measure of density, “got a number that’s in remarkably good agreement with Tempel 1.”
The unavoidable conclusion is that comets “are mostly hollow,” A’Hearn says, and likely were formed by individual “comitesimals” – small chunks that joined together (accreted) through a “very gentle” process. The low density also casts doubt on the theory that comets are fragments of large objects. (We’ll dig into more results from Deep Impact shortly.)
My beautiful regions! The highly variable surface of 67P is shown in a just-released study, showing at least 19 different regions, named for Egyptian gods. Based on appearance, the scientists found, “dust-covered terrains, brittle materials with pits and circular structures, large-scale depressions, smooth terrains, and exposed consolidated surfaces.” The study may lay to rest a common assumption about comets, the authors wrote. “The concept of cometary nuclei as rather uniform, pristine, proto-planetesimals that may have been subjected to collisional processing is persistent, despite evidence of regional differences” seen on other comets. The current observations, they added, “have revealed an irregular-shaped, processed nucleus surface with morphologically diverse units.” In other words, just like galaxies, stars and planets, objects that were once assumed to be similar and ultimately boring seem to be anything but. Viva la cometology!
Weird water: Even before Philae’s epic comet-touchdown, mother-ship Rosetta snared some provocative data2 about frozen water on the comet.
What is the origin of Earth’s water, the key to life? Earth was hotter than blazes for millions of years as the sun and its planets condensed from a fiery glob of gas and dust, so any water would have evaporated. Planetary scientists have thought that our water might have been delivered by comets, which are, indeed, dirty snowballs.
Hydrogen, an element with one proton and one electron, has a rarer, heavier isotope called deuterium, which also contains one neutron. But Rosetta, like some (but not all) other comet craft, found a deuterium-to-hydrogen (D-H) ratio totally out of whack with the oceanic ratio. The D-H ratio is thought to be a relic of the comet’s heritage, which depends on its birthplace:
“Oort Cloud” comets formed around Uranus and Neptune, and then moved far beyond Pluto.
“Jupiter family comets,” including 67P, formed in the Kuiper Belt further from the sun, then moved inward toward Jupiter’s orbit.
The D-H ratio, A’Hearn says, should reflect temperatures when the water formed, so each of the major comet families should have a characteristic D-H ratio that reflects its origin. The frigid conditions found farther from the sun produce a higher deuterium ratio. But if the Jupiter family comets all formed in the Oort cloud, as believed, why do comet studies show them having such a range of D-H ratios?
A second problem begs for solution: the new D-H ratio calculated for the Jupiter family comet 67P, is three times that found in Earth’s oceans, meaning that those comets may not have delivered water to Earth. Perhaps some or all of the water came from asteroids, even though they have a much lower percentage of water than comets.
The isotopic evidence of cometary heritage exemplifies the role of comets as messengers from the aborning solar system. The ensuing 4-plus billion years have completely and repeatedly transformed our planet, yet barely changed the comets.
Comet stages Mars flyby
Making a deep impact
– David J. Tenenbaum