Comet Lovejoy Sheds Light On Source Of Earth’s Water

Comet C/2014 Q2 (Lovejoy) as seen from nearby Paris, in February 2015. Credit: Fabrice Noel/NASA

New information on Comet C/2014 Q2 (Lovejoy) could shed fresh information on the importance of cometary water in the formation of the Solar System.

In February 2015, comet Lovejoy was analyzed using infrared spectroscopy at the Keck Observatory in Hawaii. The observations were taken shortly after Lovejoy passed its closest approach to the Sun, or perihelion. The investigators were particularly interested in measuring deuterium (D), an isotope of hydrogen (H) found in water. The measurements of “D-to-H” in cometary water yield the amount of water these icy bodies delivered to Earth’s oceans.

Previous observations of water in most comets — including those of Comet 67P/Churyumov–Gerasimenko from 2014 to 2016 — found that they differ from water on Earth in the ratio of hydrogen to deuterium, except for Jupiter family comet 103P/Hartely 2. This difference in the “D-to-H ratio” implies that comets may have contributed only a small fraction of Earth’s water, making it more likely asteroids that did the job.

Now, the Lovejoy investigation reveals a new wrinkle in our understanding of cometary water. According to a recently published paper, “Ground-based Detection of Deuterated Water in Comet C/2014 Q2 (Lovejoy) at IR Wavelengths” in The Astrophysical Journal Letters, a comet’s D-to-H ratio may vary in its distance from the Sun.

“We did ground-based observations to measure water and heavy water (HDO) and compared to measurements before the comet reached perihelion using space and ground-based telescopes,” said lead author Lucas Paganini, a planetary scientist at Catholic University of America and the NASA Goddard Space Flight Center. He also is a research assistant at the Goddard Center for Astrobiology.

“We also studied the overall chemical composition of comets. Not only water, but we also studied volatiles of importance, like methane and ethane and so forth.”

The main astrobiological implication is this: the differences in the D-to-H ratios of comets helps explain where the comet was formed, and how water evolves in a comet’s lifetime. Knowing where comets formed helps investigators better understand how water collected and traveled in the early Solar System. As water is a key molecule of life, this is one of the larger questions that cometary scientists are seeking to understand.

Observations of Comet C/2014 Q2 (Lovejoy) were performed at the Keck Observatory in Hawaii. Credit: Ethan Tweedie Photography/W. M. Keck Observatory

Seeking deuterium

The team measured the abundance of HDO relative to water, and compared the February 2015 measurements to others performed before the comet’s closest approach to the Sun. Both Earth and comets have a fraction of deuterium in their water, but in some cases the deuterium is in different ratios. HDO on Earth occurs naturally, at a proportion of 1 in 3,200 molecules.

“As the comet approaches the Sun, we typically expect the output of both water and HDO to increase or decrease together, so the relative abundance should be constant regardless of distance to the Sun,” said Paganini.

But Lovejoy’s observations revealed that water remained constant, while the HDO output increased two to three times after the closest approach to the Sun. This contradicted work from other authors.

Paganini suggested that the “proper” D-to-H ratio of a typical comet may vary due to unaccounted effects. In other words, the amount of deuterium in the comet’s “exosphere,” so-called because it’s a tenuous atmosphere, would not be the same as the amount of deuterium locked in the comet’s icy nucleus. This is because the water trapped in the icy nucleus could sublimate first, before the deuterium. Alternatively, radiation near the Sun could modify water’s deuteration. If astronomers are measuring the wrong D-to-H ratio, perhaps cometary water played a more crucial role in Earth’s history after all.

“I think the key is to measure the exosphere’s D-to-H at different heliocentric distances for several comets,” Paganini said, “and to measure the D-to-H in ice form by sending a lander spacecraft and sample analyzer to the nucleus.”

Paganini added that the measurements are tricky. Earth has water in its atmosphere that could interfere with cometary observations, if astronomers are not careful. If the comet is moving quickly compared to the Earth’s motion, however, it’s possible to separate the terrestrial contamination from the cometary signatures, the so-called Doppler effect.

Paganini wants to continue the survey to better understand the D-to-H spread in comets originating from the Kuiper Belt beyond Neptune, and the outer edge of the Solar System in the Oort cloud. While some of these family of comets have D-to-H ratios that appear similar to Earth’s, a close-up look at Kuiper Belt Comet 67P revealed its D-to-H ratio is different than Earth’s. So, he says, more study is needed.

“As more comet samples are observed and instrument performance improves, we might get closer to a full understanding of D-to-H ratios,” he said.

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