Comets played a ‘major’ role in making life on Earth possible, new study hints
Over the past decade the idea that comets delivered water to Earth fell out of favor, but a new look at data from the European Space Agency’s (ESA) Rosetta mission, an iconic “rubber ducky” comet, has reopened the possibility.
Water’s chemical structure is very simple: each molecule contains just three atoms (two hydrogen and one oxygen). It’s one of Earth’s most abundant molecules, with about a million trillion tons of the liquid filling our planet’s oceans.
How all this water got to Earth, however, remains a mystery. Some scientists believe that although Earth’s geological processes may have generated a small portion of it, most of the water was probably deposited by comets or asteroids through repeated, cataclysmic collisions.
Figuring out which of these two groups was responsible requires a special chemical signature that arises because the hydrogen in water occurs in two different isotopes, or forms. While most hydrogen atoms have just one proton in their nucleus, a small fraction have an extra neutron. The chemical signature involves measuring the amount of this heavier hydrogen isotope called deuterium relative to its lighter, regular form — a quantity called the deuterium-to-hydrogen ratio, or D/H.
“The D/H in water tells us at what temperature the ice formed and at what distance from the sun the comet formed,” Kathleen Mandt, a planetary scientist at NASA and corresponding author of a new study describing the reanalysis, told Live Science in an email. The lower the D/H value, the farther from the sun the asteroid or comet was born.
But the D/H value of “rubber ducky” comet 67P/Churyumov-Gerasimenko, determined in a 2015 study, essentially ended the case for comets. The D/H value, an average of more than 150 measurements collected by ESA’s Rosetta mission during the spacecraft’s 2014 rendezvous with comet 67P, was about three times that of Earth. The researchers took this as evidence that comets are very unlikely to have delivered water to Earth.
The results were puzzling, Mandt said, because the D/H value was much higher than that of other Jupiter-family comets. Also, he added, “the comet must have much more CO [carbon monoxide] and N2 [nitrogen] than the amounts measured by Rosetta because these ices also form at very cold temperatures.”
To understand comet 67P’s apparently high D/H ratio, Mandt and other astronomers from research institutions in the U.S., France and Switzerland decided to mine the entire Rosetta dataset. Using an innovative statistical technique developed by Jacob Lustig-Yager of the Johns Hopkins Applied Physics Laboratory, the team identified signals coming only from deuterium-rich water molecules, allowing them to collect nearly 4,000 D/H measurements.
The researchers found that D/H values varied greatly along the comet’s long axis, with the highest being near the “nucleus” — the rocky part that resembles a rubber ducky — and decreasing along the tail.
Such variation is likely caused by processes within the comet, the researchers wrote in their study, published Nov. 13 in the journal Science Advances.
As the comet approaches the sun, the comet’s surface heats up, releasing gas along with ice-covered dust particles in the coma (the halo that develops around the nucleus). Previous, unrelated laboratory studies had shown that deuterium-rich ice sticks to dust grains more than ordinary ice. Scientists realized that such dust particles, when entering the coma, could be responsible for the high D/H values recorded there.
However, the researchers found that dust grains about 75 miles (120 kilometers) from the nucleus have essentially dried up, meaning they contain no deuterium-rich ice that could produce extremely high D/H values.
Using data collected only at this distance, the authors calculated that comet 67P’s true D/H value was only 1.5 times Earth’s D/H value. The revised D/H value means that “the D/H of all the Jupiter family comets we measured is close to Earth’s water D/H,” Mandt said.
This means that comets played a large, not a small, role in irrigating Earth. Also, they said the low D/H value suggests that Comet 67P was born closer to the Sun than scientists previously thought.