Hyperactive comets and an Earth sea mystery

A class of comets that behave in a way that contradicts the equations that best describe them could hold the key to the origin of Earth’s water, researchers suggest.

There is a general agreement among planetary scientists that when the Earth first formed – through the agglomeration of smaller bodies, known as planetesimals – it hosted very little if any water. Subsequent impacts by comets, which have icy nuclei, then delivered, at the very least, a significant portion of the water that the infant Earth came to enjoy.

The one problem with this otherwise neat hypothesis is the fact that observations reveal that the water carried by most comets is significantly different to that found on Earth.

The key is the ratio between “normal” water, which contains a stable isotope of hydrogen known as protium, and “heavy” water, in which the hydrogen isotope contains an extra neutron and is known as deuterium. In the jargon of the field, the difference is termed the D/H ratio.

The balance between the two isotopes can be measured when a comet’s path takes it close to the sun, which causes some of the core ice it carries to “sublimate”, or turn into vapour. Observations to date have recorded protium-deuterium ratios two to three times higher than that found on Earth, which implies that comets only delivered around 10% of the planet’s water. 

The conundrum, however, might now have been resolved following observations of a comet called 46P/Wirtanen. The object approached close enough to Earth in December 2018 to be closely measured by NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA), which is housed onboard a Boeing 747.

In results soon to be published in the journal Astronomy and Astrophyiscs – and available in preprint on the Cornell University server arXiv – a team of scientists led by Darek Lis from NASA’s Jet Propulsion Laboratory in California, US, reveal that Wiranten’s isotope ratio exactly matches that of the Earth’s sea water.

One comet doth not an ocean make, however – but three might constitute a clue.

Wirtanen is the third comet measured in recent years to reveal the required protium-deuterium balance.

Significantly, all three belong to a class known as “hyperactive” comets. They are described as such because of a definite equation-busting phenomenon: as they approach the sun, they produce more water vapour than the size of their nuclei should permit.

The anomaly, however, is more apparent than real. The excess is the result of sublimation by icy crystals present in their atmosphere.

When Lis and colleagues combed through the data, they found that the contribution of the ice crystals to the protium-deuterium ratio was significant, and remained so regardless of whether a comet originated from the Kuiper Belt or Oort Cloud.

The implication is that the contribution of comet ice crystals has been insufficiently factored in to calculations.

“This is the first time we could relate the heavy-to-regular water ratio of all comets to a single factor,” notes co-author Dominique Bockelée-Morvan, from the Paris Observatory in France.

“We may need to rethink how we study comets because water released from the ice grains appears to be a better indicator of the overall water ratio than the water released from surface ice.”

In their paper, the researchers conclude: “In this case, all comets may share the same Earth-like D/H ratio in water, with profound implications for the early solar system and the origin of Earth’s oceans.