The Heterodyne Instrument for the Far Infrared (HIFI) is a high-resolution heterodyne spectrometer. It works by mixing the incoming signal with a stable monochromatic signal, generated by a local oscillator, and extracting the frequency difference for further processing in a spectrometer. HIFI will have seven separate local oscillators covering two bands from 480-1250 gigaHertz and 1410–1910 gigaHertz. HIFI was developed by a consortium led by SRON (Groningen, The Netherlands). Credits: ESA (image by C. Carreau). HI-RES JPEG (Size: 1020 kb)
However, until now, astronomers' observations have failed to back up the idea that comets provided Earth's water. The key measurement they make is the level of deuterium – a heavier form of hydrogen – found in water.
All comets previously studied have shown deuterium levels around twice that of Earth's oceans. If comets of this kind had collided with Earth, they could not have contributed more than a few percent of Earth's water. In fact, astronomers had begun to think that meteorites had to be responsible, even though their water content is much lower.
Now, however, Herschel has studied comet Hartley 2 using HIFI, the most sensitive instrument so far for detecting water in space, and has shown that at least this one comet does have ocean-like water.
"Comet Hartley's deuterium-to-hydrogen ratio is almost exactly the same as the water in Earth's oceans," says Paul Hartogh, Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany, who led the international team of astronomers in this work.
The other comets previously studied by astronomers are all thought to have formed near to Jupiter and Saturn before being thrown out by the gravity of those giant planets, only to return much later from great distances.
Herschel is now looking at other comets to see whether this picture can be backed up.
"Thanks to this detection made possible by Herschel, an old, very interesting discussion will be revived and invigorated," says Göran Pilbratt, ESA Herschel Project Scientist.
"It will be exciting to see where this discovery will take us."
Contact for further information:
Markus Bauer ESA Science and Robotic Exploration Communication Officer Email: email@example.com
Tel: +31 71 565 6799 Mob: +31 61 594 3 954
Max-Planck-Institut für Sonnensystemforschung
Tel: +49 5556 979 342
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Notes to editors
Herschel studied comet Hartley on 17 November 2010 using the Heterodyne Instrument for the Far Infrared (HIFI).