Herschel sheds light on role of water

Herschel image of comet HArtley 2
The location of Comet Hartley 2 and the Herschel image of it (inset). Click on the image for more information. Image credit: ESA/NASA/Herschel/HSSO

One of the most interesting molecules that astronomers like to study is water, which is so abundant on the Earth and though to be crucial for life. Water can form relatively easily, providing the temperature is not too high, and the outer regions of our own Solar system are full of icy bodies. Observations of water are very hard to observed from the Earth as the atmosphere interferes with the measurements, and so studying water requires spacecraft. Far from Earth, the HIFI instrument on board Herschel is studying the role of water in not just our own Solar System, but also in others.

In late 2010 the comet Hartley 2 passed relatively close to Earth, though it has not always ventured this close to the Sun. Hartley 2 originated in an outer region of the Solar System called the Kuiper Belt, but was put on an orbit that brings it in to the inner Solar System after encounters with planets and other large bodies. The HIFI instrument onboard Herschel measured the composition of the water in the comet, and found that it is very similar to that of the water in the Earth's oceans.

The idea that comets seeded the water on Earth has been a favourite among many astronomers for decades, as there are few other ways to explain where the water came from. However the composition of the water in most of the comets studied doesn't match that of the ocean water, and this has long been a thorn in the side of the proponents of the idea. Most of these comets formed relatively close to the Sun, around where Jupiter orbits, but Hartley 2 is the first comet to be investigated that formed much further out. The fact that the composition of the water in Hartley 2 is so similar lends further weight to the theory that comets seeded our oceans, though perhaps it was mainly this particular type of comet that did the seeding.

Artist's impression of TW Hydrae
Artist's impression of the disc of gas and dust around the star TW Hydrae. Click on the image for more information. Image credit: ESA/NASA/JPL-Caltech/WISH

HIFI is sensitive enough to look at the material orbiting other stars, such as TW Hydrae in the constellation of Hydra. This star is a little less massive than the Sun but which is much, much younger. At only 10 million years old, TW Hydrae is still in its adolescent years, and has a disc of gas and dust around it. It is thought that our own Sun would have started out like this, with the planets forming after tens or hundreds of millions of years.

TW Hydrae is 175 light years away from the Sun, far too distant for any of Herschel's instruments to see in detail. The sensitivity of the HIFI instrument allows it to detect the faint signature of water vapour, and a careful study of the signature allows astronomers to work out the temperature of the water vapour and where it is in the disk. The water vapour is stripped from grains of ice by the light from the star, but is found at temperatures below 100 K (-170 Celsius). This is much colder than has been found previously around other stars, and indicates that the water vapour is present throughout the disc.

Water plays a crucial role in planetary systems, as it allows grains of dust to clump together and form asteroids, which later collect together to form the planets. Studying the presence of water, and the conditions in which it is found, is vital for understanding how Solar Systems such as our own formed.