A slushy cocktail of water-ice and organic materials has been directly detected on the surface of an asteroid for the first time. The finding strengthens the theory that asteroids delivered the ingredients for Earth’s oceans and life, and could make astronomers rethink conventional models for how the solar system evolved.
It has long been thought that asteroids, which lie in a belt between Mars and Jupiter, are rocky bodies that sit too close to the sun to retain ice. By contrast, comets, which form further out beyond Neptune, are ice-rich bodies that develop distinctive tails of vaporised gas and dust when they approach the sun.
However, this distinction was blurred in 2006 by the discovery of small objects with comet-like tails in the asteroid belt, says astronomer Andrew Rivkin of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
To investigate the composition of these “main-belt comets,” Rivkin and his colleague Joshua Emery, of the University of Tennessee in Knoxville, turned the infra-red telescope at Mauna Kea, Hawaii, onto the asteroid 24 Themis – the parent body from which two of the smaller comet-like asteroids observed in 2006 were chipped.
Emery and Rivkin took seven measurements of 24 Themis over a period of six years, each time looking at a different face of the asteroid as it travelled around its orbit. They consistently found a band in the absorption spectrum of light reflected from its surface that indicated the presence of grains coated in water ice, as well as the signature of carbon-to-hydrogen chemical bonds as found in organic materials. Rivkin and Emery’s work is published in Nature.
“Astronomers have looked at dozens of asteroids with this technique, but this is the first time we’ve seen ice on the surface and organics,” says Rivkin. The result was independently confirmed by a team led by Humberto Campins at the University of Central Florida in Orlando.
He and his colleagues observed 24 Themis for seven hours one night, as it almost fully rotated on its axis. “Between us, we have seen the asteroid from almost every angle and we see global coverage,” says Campins.Icy interloper
Because 24 Themis lies only about 479 million kilometers from the sun (roughly three times the mean distance from Earth to the Sun), it is surprising that the surface ice has not all been vaporised. Both teams speculate that more ice may be held in a reservoir beneath the asteroid’s surface, shielded from the sun, and that this ice is slowly churned up as the asteroid is struck by small bodies in the belt, thus replenishing the surface ice.
The findings lend weight to the idea that asteroids and comets are the source of Earth’s water and organic material. Geochemists think that the early Earth went through a molten phase when any organic molecules would have dissociated, so new organic material would have had to be delivered to the planet at a later time, says Campins. “I believe our findings are linked to the origin of life on Earth,” he says.
To assess the plausibility of this scenario, astronomers must determine whether the make-up of 24 Themis is typical of other asteroids and, if so, what exactly they hold, says Castillo-Rogez. A priority should be to search for water ice on near-Earth asteroids that could be targeted by NASA’s planned robotic and manned missions.
However, 24 Themis may not be a typical member of the belt – it could be an interloper that formed beyond Neptune, along with the comets, which was later knocked inwards, says Rivkin. If so, this would fit well with the controversial “Nice model” of the evolution of the solar system. Proposed in 2005, this model suggests that the giant planets Jupiter, Saturn, Uranus and Neptune and asteroids migrated to their present orbits after formation.
Either way, says Rivkin, “The old-fashioned picture of the solar system in which asteroids are asteroids and comets are comets is getting harder to sustain.”
NYT News Service
As old as the sun
The ice caps of Antarctica are largely untouched, clean, and – with the temperature around 60 below — very cold. This, it turns out, is the perfect combination for scientists who want to extract undamaged particles created billions of years ago in the early days of our solar system. “The deep question is, ‘How was the solar system formed?’ and to do that what you want to have in the lab is materials that didn’t change in 4.5 billion years,” said Jean Duprat, a scientist who researches nuclear and mass spectrometry at the University of Paris. Duprat and his colleagues drilled into the snow and in the cores they retrieved found two particles, each 100th of a micron in size, that were exactly what they were looking for.
Because of the depth at which the materials were found, about 12 feet, they were able to estimate that they were deposited into the Earth from 1955 to 1970. By studying the molecules and minerals in the particles, which, though tiny, each contain enough material to analyse, the researchers were able to determine that the sediments were formed in our solar system billions of years ago. Their results are published in a recent issue of the Journal of Science. To determine the particles’ origins, the scientists looked at their chemical makeup. They had larger amounts of carbon and deuterium, a form of hydrogen, than anything found on Earth, Duprat said.
He and his colleagues further analysed the chemical makeup, and found that the compositions of the particles were similar to remnants from comets, including Halley’s Comet. The grains may be as old as the sun, he said. Future research might reveal more about exactly how these particles formed, and how they arrived on Earth. But that will require more trips to the Concordia Research Station in Antarctica, and more samples. “We’ve got very few of these particles, and we need more of them,” Duprat said. “Each time you analyse such particles, you somehow use them. It’s destructive.”
Sindya N Bhanoo
NYT News Service