ESA Rosetta: First measurements of Comet 67/P’s water ratio

ESA Rosetta’s measurement of the deuterium-to-hydrogen ratio (D/H) measured in the water vapour around Comet 67P/Churyumov–Gerasimenko. 

The measurements were made using ROSINA’s DFMS double focusing mass spectrometer between 8 August and 5 September 2014.

Credit: ESA, DLR.

Deuterium is an isotope of hydrogen with an added neutron.

The ratio of deuterium to hydrogen in water is a key diagnostic to determining where in the Solar System an object originated and in what proportion asteroids and/or comets contributed to Earth’s oceans.

The graph displays the different values of D/H in water observed in various bodies in the Solar System.

The data points are grouped by colour as planets and moons (blue), chondritic meteorites from the Asteroid Belt (grey), comets originating from the Oort cloud (purple) and Jupiter family comets (pink). Rosetta’s Jupiter-family comet is highlighted in yellow.

Diamonds represent data obtained in situ; circles represent data obtained by astronomical methods.

The lower part of the graph shows the value of D/H measured in molecular hydrogen in the atmosphere of the giant planets of the Solar System (Jupiter, Saturn, Uranus, Neptune) and an estimate of the typical value in molecular hydrogen for the protosolar nebula, from which all objects in our Solar System formed.

The ratio for Earth’s oceans is 1.56 ×10–4 (shown as the blue horizontal line in the upper part of the graph).

The value for Comet 67P/Churyumov–Gerasimenko is found to be 5.3 x 10–4, more than three times greater than for Earth’s oceans.

The discovery fuels the debate on the origin of Earth’s oceans and whether asteroids or comets played the bigger role in delivering water.