Hyperactive Comets

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Where did Earth’s water come from? The origin of water on Earth, or the reason why there is more liquid water on Earth than on the other rocky planets in our Solar System, is unknown. There are numerous more or less mutually compatible solutions to the mystery of how water may have pooled on Earth’s surface over the past 4.5 billion years, in sufficient amounts, to create our world’s deep, churning, frothing oceans. A new study, published in the May 20, 2019 issue of the journal Astronomy & Astrophysics proposes that water was delivered to Earth by one particular family of comets–the hyperactive comets–that contain water similar to that on our own planet.

In order to trace the origin of Earth’s water, the team of international astronomers studied the isotopic ratios, particularly the ratio in terrestrial water, of deuterium to hydrogen. This is known as the D/H ratio (deuterium is a heavier form of hydrogen). As a comet travels towards our Sun, and its ice sublimates to gas, this water vapor can then be analyzed by remote observations. However, the D/H ratio of comets measured so far have generally shown to be twice to three times that of ocean water. This suggests that comets only delivered approximately 10% of our planet’s water.

When comet 46P/Wirtanen wandered towards Earth in December 2018, it was analyzed using the SOFIA airborne observatory carried aboard a Boeing aircraft. This was the third comet found to display the same D/H ratio as Earth’s water. Like the two previously studied comets, it belongs to the family of hyperactive comets. When hyperactive comets travel towards our Sun, they release more water than the surface area of their nucleus should permit. This excess water is produced by ice-rich particles in their atmosphere.


Fascinated by the findings, the team of astronomers calculated the active fraction (the fraction of the nucleus surface area required to produce the amount of water present in their atmosphere) of all comets with a known D/H ratio. They found that there was an inverse correlation between the active fraction and the D/H ratio of water vapor: the more a comet tends towards hyperactivity–an active fraction exceeding 1–the more its D/H ratio decreases and becomes more like that of water on Earth.

The new research shows that hyperactive comets, whose water vapor is partly obtained from icy grains shot out into their atmosphere, possess a D/H ratio similar to that of Earth’s water. This is unlike other comet families whose gas halo is formed only by surface ice. The researchers propose that the D/H ratios measured in the atmosphere of the latter are not necessarily representative of the ice present in their nucleus. If this theory is correct, the water in all cometary nuclei may, indeed, be very similar to Earth’s water. This discovery could reopen the debate on the origin of our planet’s vast oceans.

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