Water signatures in Herschel/HIFI spectrum of comet 103P/Hartley 2
This graph shows the relative signatures of two different 'flavours' - technically known as isotopologues - of water in the spectrum of comet 103P/Hartley 2. The spectrum has been acquired with the HIFI instrument on board Herschel during the comet's approach to Earth in late 2010.
The blue curve refers to the isotopologue H218O, composed of two hydrogen (H) and one oxygen (18O) atoms, and the red curve shows the much weaker signature of another isotopologue, semi-heavy water (HDO), where one of the hydrogen atoms appears in its isotopical form of deuterium (D, or 2H). H218O was used as a proxy for 'ordinary' water (H216O). The HDO feature is also shown, in greater detail, in the inset to the top right of the image.
From the relative abundance of these two molecules in various Solar System bodies, astronomers can infer the deuterium-to-hydrogen ratio (D/H) in water. By comparing the D/H ratio measured across the Solar System to that measured in Earth's oceans, it is possible to constrain the nature of the bodies that contributed most to delivering water to our planet.
The data acquired with Herschel on comet 103P/Hartley 2 represent the first evidence for Earth-like water in a comet, and suggest that comets may have played an important role in bringing water to our planet.