Did Water on Earth Come From Comets?
Comets are both a delight and a nightmare for astronomers. They are beautiful to look at with their tails stretched across the night sky. However, it is hard to predict what they will do as they approach the sun. Will they be bright and shine with ease as they sublimate or will the sun devour it, breaking it apart? ISON and Kohotek are but two examples of comets that let astronomers down. But what are these mysterious objects of misfortune and occasionally of glory?
Before the understanding of comets that we currently have, people of antiquity felt that comets were harbingers of fate and destiny sent by deities from above. Their appearance would mean that a king would die or that a violent disaster was on its way. Of course any such incidents that seemed to coincide with the appearance of comets were purely coincidental, but that did not stop the legends and myths from spreading.
People also felt a comet came and was sent away, never to come back and visit Earth again. That changed in the early 1700’s when Edmund Halley showed that a particular comet would return but it would take years for the set cycle to appear. Not too long after, his prediction came true and now we have named that comet in honor of him. Not all comets visit us so often however, for some take 1000’s of years to complete an orbit. We are fortunate to have a few which visit us frequently.
Seeing comets has never been a difficulty, but knowing where they originate has been. Though we have never seen it, we can infer from gravity and the orbits of comets that they come from a structure in the outer solar system called the Oort Cloud. Trillions of comets reside in it, slowly orbiting the sun. They are the remnants of the formation of the solar system, seemingly frozen from that time frame. Occasionally, a gravitational disturbance will nudge them off their orbit and towards the sun at nearly 100,000 miles per hour, where solar particles begin to heavily bombard the surface of the comet. It is during this process that we learn much about what makes up a comet (Newcott 97).
Components of Life?
Comets are known as “dirty, lumpy snowballs” for a reason. They melt as they approach the sun, weakening their structure. As they break down, two tails emerge from the main body of the comet (called the nucleus): one made of dust and the other of gases that have been frozen inside the comet since its formation. These tails can stretch 100’s of millions of miles long and always point away from the sun, for it is the source of the solar particles hitting the comet (97, 102).
By looking at these tails with radio, infrared, and ultraviolet light, we know that hydrogen, oxygen, and several carbon compounds are present. Hale Bopp, one of the many comets to visit us, showed traces of nitrogen, sodium, and sulfur, all considered building blocks of life. This supports the theory that comets brought the ingredients needed for life to form on Earth, including precious water. However, Hale Bopp also provided evidence against this claim. Deuterium is a heavier variety of water, and Hale Bopp has nearly twice as much of it as does Earth’s water (97, 100, 106).
Instead of big comets, maybe smaller ones were responsible for the water brought to Earth. Simulations show that over a 20,000 year period the small comets in our early solar system could have deposited enough water to cover the entire Earth in an inch of water. In September of 1996, NASA’s Polar Satellite supposedly spotted a small comet entering atmosphere. It was mostly water with little dust according to the satellite, but not everyone is sure that it was not a glitch with the equipment (107, 109).
Why An Extraterrestrial Source for Water?
While we have gotten into depth about comets, we need to discuss why there is a need for them to even be a source of water on Earth. After all, don't we have all the material we started with? Definitely not, and the evidence is above all constantly: the moon. About 4.5 billion years ago, a Mars-sized planetesimal named Theia collided with us and thus knocking off a chunk of Earth while vaporizing the surface. Any water we had on top was lost as vapor or steam, and any that was present in the mantle is trapped in a non-liquid state because of the crust. So how did we get water back on top? (Jewitt 39)
Investigation and New Theories
Clearly, a probe needed to be sent to a comet to help resolve these confusing details about their chemistry and to see if they replenished us. On July 7th, 2005 the probe known as Deep Impact fired a mass of copper at comet Tempel 1 after years of travel. The 820 pound projectile collided with Tempel 1 and Deep Impact sat by to collect data. Based on how much debris was kicked off of Tempel 1, we know it does not have a hard surface but a nice soft one. Below that surface is a mix of water ice, dust, and frozen gases. Interestingly, the water levels were lower than expected but the carbon dioxide levels were higher than anticipated. Maybe a hidden layer of the gas exists as well as water (Kleeman 7).
After analyzing over 8 Oort Cloud comets, the deuterium levels did not correspond to those found here on Earth. In fact, they are twice as plentiful as those levels found on Earth and over fifteen times the amount that would have been present in the earlier solar system. But comets that were found to orbit closer to the sun do have deuterium levels that are closer to Earth’s water, such as those in the Kuiper Belt. And an article of the October 5 issue of Nature by Paul Hartogh (from the Max Planck Institute for Solar System Research) found that observations from ESA's Herschel IR camera show that comet 103P/Hartley has a deuterium level of 1 to 6200, a close match to Earth's 1 to 6400. All are encouraging finds (Eicher, Jewitt 39, Kruski).
However, as the 1990’s passed into the new millennium, scientists no longer felt comets were the answer. After the evidence that was already against comets, new simulations revealed that the comets which were closer to the sun could only have accounted for about 6% of the water on Earth. Noble gas studies also showed that if comets ever did deliver water to Earth, it was likely within the first 100 million years of its existence. It is important to note that this is all dependent on orbital positions, composition, and timing, all of which are estimates at best (Eicher).
Additionally, water elsewhere in the solar system matches comets better than Earth does. Titan's Nitrogen-14 and 15 levels do not match Earth's but they do correspond to comet values found earlier. The Titan readings were based off a NASA/ESA report along with work by Kathleen Mandt of the Southwest Research Institute. The findings indicate that comets may not have ventured deep enough into the solar system to deliver substantial water amounts (JPL "Titan").
Perhaps if we could understand the conditions comets formed under then maybe new insight could be gathered. In the early solar system, hydrogen and oxygen were the most prevalent elements around and a majority of it was claimed by the sun and the gas giants. The remaining oxygen bonded to various other elements such as leftover hydrogen. As one got closer to the swirling mass that would become the sun, things became warmer and more crowded but as you moved out it got cooler and more spacious. Therefore, icy particles would remain on the outskirts while the rockier components would remain inwards. On top of that, angular momentum caused different rates of rotation and so those rocky particles would accumulate through collisions and eventually could reach a size where water could find a refuge from the conditions around it. The comets would have migrated outward until they arrived in the Kuiper Belt and the Oort Cloud (Eicher, Jewitt 38).
In fact, there is a specific region known as the snow line, where the solar radiation and friction reached a low enough level for water to freeze. Located around this region was the asteroid belt. In fact, certain asteroids have been found to contain water and have deuterium levels which are near Earth-levels. They also have a tendency to strike objects courtesy of gravity nudges from Jupiter. The moon stands as a testament to this bombardment. In fact, models show that water may have been inside asteroids because of the snow line and where they formed. When aluminum-26 decays into magnesium-26, it releases heat which would have liquefied the water briefly and let it flow through porous rock before freezing again. Carbonaceous chondrites found on Earth seem to support this (Jewitt 42, Carnegie).
Perhaps even larger objects could have hung onto water as they cooled off. Whatever the source, the biggest problem is how water would be delivered over a long-term period. All simulations show that it happens over a short-period despite none of those time-frames matching when Earth would have received enough water, whether it was from asteroids or comets. Argon levels on Earth are low while in asteroids they are high, proofing to be a problem in the asteroid theory. And of course new findings from Rosetta put further doubt about comets being the originator of water on Earth, with the deuterium ratio being 3 times ours (Eicher, Jewitt 38, 41-2; Redd). The mystery endures.
Carnegie Institution for Science. "Solar system ice: source of Earth's water." Astronomy.com. Kalmbach Publishing Co., 13 Jul. 2012. Web. 03 Aug. 2016.
Eicher, David J. "Did Comets Deliver Earth's Oceans?" TheHuffingtonPost.com. The Huffington Post, 31 July 2013. Web. 26 Apr. 2014.
Jewitt, David and Edmund D. Young. "Oceans from the Skies." Scientific American Mar. 2015:38-9, 42-3. Print.
JPL. "Titan's Building Blocks Might Predate Saturn." Astronomy.com. Kalmbach Publishing Co., 25 Jun. 2014. Web. 29 Dec. 2014.
Kleeman, Elise. “Comets: Powdery Puffballs In Space?” Discover Oct. 2005: 7. Print
Kruski, Liz. "Comet Hints at Possible Source of Earth's Water." Astronomy Feb. 2012: 17. Print
Newcott, William. “The Age of Comets.” National Geographic Dec.1997: 97, 100, 102, 106-7. Print.
Redd, Taylor. "Where Did Earth's Water Come From?" Astronomy May 2019. Print. 26.