From Near Earth Asteroids to water on Mars - astronomers review latest ISO results
11 May 1999Near-Earth Asteroids - asteroids whose orbits bring them close to Earth - very likely originate from collisions between larger asteroids thatorbit the Sun between the planets Mars and Jupiter. This result, obtained byESA's infrared space telescope, ISO, was presented yesterday at the workshop onISO results on Solar System, held at ESA's Villafranca SatelliteTracking Station in Spain. Other findings related to the atmosphere of Mars and the giant planets - Saturn, Jupiter, Neptune and Uranus - were also presented during a press conference yesterday morning.
About 250 Near-Earth Asteroids (NEA) are known so far, with sizes ranging from one to 40 kilometres in diameter. But according to Alan Harris, of the German Aerospace Centre (Berlin), there are more than 2000 NEA not yet identified. Their origin is still unclear, but scientists are working with two main ideas: many probably originate from collisions among the larger bodies in the Main Belt of asteroids, between Mars and Jupiter, while others might be old, 'dead' comets that have undergone so many fly-bys of the Sun that they have lost all their coma - the beautiful comet tail of dust and gas.
The infrared observations made by Harris with ISO provide information about the surface and mineralogy of the NEAs, and confirm that many are fragments of larger asteroids. ISO sees a rocky surface without much residual dust; a dusty surface is what would be expected if the NEA were 'dead comets'. Harris, however, is still analyzing the data and has not yet ruled out the possibility of finding traces of dust in one of the asteroids observed, the orbit of which is very similar to that of a comet.
As Harris points out, knowledge on the composition of these objects is necessary to predict the consequences of a future impact on the Earth. "We still know just a few of them, and although the odds that any one will hit the Earth in the near future are very low, there's a possibility. Therefore, the more we know about these objects, the better", he said.
The new data about Mars refer to the water in the atmosphere: ISO sees that it condenses and freezes close to the planet's surface. This result will help to understand the behaviour of the Martian atmosphere.
"We got these observations while NASA's Mars Pathfinder mission was also studying the planet, and the results of both missions are in agreement. ISO confirms the Mars Pathfinder data", said Therese Encrenaz, of the Observatoire de Paris-Meudon.
Encrenaz also described the detection of new molecules in the atmospheres of the giant planets, mainly hydrocarbons like the radical CH3, never detected before, and benzene. Benzene is a common molecule on Earth (in petrol for instance), but is apparently less common on other planets.ISO's spectrometers see it now in Saturn.
The discovery of water in the atmosphere of Saturn's largest moon, Titan, was also reviewed by Athena Coustenis of the Observatoire de Paris-Meudon.