Background: Last fall, the world watched anxiously as we landed Philae (a robotic probe launched from the Rosetta spacecraft) on the surface of a comet; this being the first time any man-made object has intentionally landed intact on a comet’s surface. The comet itself is the Jupiter family comet 67P/Churyumov-Gerasimenko (I’ll call it 67P for short), and this mission was so exciting because it is much more difficult to land a machine on a relatively small, very rapidly moving comet (traveling at up to 84,000 mph and only 2.7 miles long) then, say, a planet like mars (traveling at about 53,688 mph and averaging 4,211 miles in diameter). The combined equipment present on Philae and Rosetta have enabled an array of analyses that we have never before been able to perform on a comet. This week in Science, a number of research articles have been published presenting data from these analyses collected over the first few months of study. The article I will discuss here by Altwegg et al. examines the characteristics of the organic compounds and water on 67P to try and determine where they came from.
Problem/Question: Is the composition of the water and organic compounds on 67P similar to those on other planets or moons that we have analyzed?
Results: There is a current debate about the extent of Earth’s water that was delivered via comets. Some scientists hypothesize that water from comets impacting with the Earth contributed negligibly to our water supply, while others hypothesize that comets provided most of Earth’s water. It has been very difficult to test this hypothesis for two main reasons, one is that there are many comets in our solar system and they could have greatly varying compositions. The second is that most missions examining comets have involved flying by the comet and not orbiting around it at close range, like Rosetta is doing. Measurements can be made from a distance, but there is more room for error.
To compare the water on different planets and comets, scientists measure the deuterium-to-hydrogen ratio (D/H ratio). The hydrogen atom has a single electron orbiting a single proton, while deuterium (also known as heavy hydrogen) has a single electron as well, but it orbits a proton and neutron. Oxygen atoms can also have variant forms (called isotopes) with different numbers of neutrons in them. Both of these can be measured by equipment on Rosetta and their ratios are known to vary in water throughout the solar system.
The D/H ratio of water on Earth is 1.5×10-4 (0.00015), and the control test performed by Rosetta gave the same value, ensuring us that the equipment is measuring properly. When Rosetta became close enough to 67P to reside within the coma, the gaseous layer surrounding the comet’s core and made mostly of water vapor, they were able to make measurements for the water coming from the comet itself. They found that the D/H ratio of water from 67P is about 5.3×10-4. The authors were also able to measure ratios of Oxygen isotopes and found that the values from the comet were similar to those found throughout the solar system, but the values obtained had relatively large fluctuations during the time that they were measured, making them less reliable than hoped. However, these values should stabilize when comet activity increases, and so future measurements will be of much better quality and use.
In interstellar space, the D/h ratio is usually around 2×10-5 (0.00002); much lower than objects in the solar system. The reasoning for this enrichment of deuterium on objects moving about our solar system is more complex than I feel I can properly explain without thorough study (as I’m not a physicist or astronomer), so if this interests you, I suggest reading the paper itself and the citations therein. The major trend appears to be that objects that originate or spend most of their time further from the sun have higher D/H ratios than those closer to the sun. This hypothesis, however, is under debate in part due to the data gathered by Rosetta. The D/H ratio for 67P is much higher than D/H values measured previously for other comets, even those supposedly originating further from the sun than 67P. This finding could mean that there is high variability in the positioning of these comets, thus creating a much more varied array of D/H ratios than originally expected, or that the breadth of objects covered by the distance hypothesis needs to be re-examined with regards to comets.
Big Picture: The Rosetta mission has created a plethora of new data on the comet 67P and will enable us to learn a great deal on comets in general. However, now that we have shown this can be done for one comet, additional targets will greatly improve our ability to generalize our findings, as well as better understand our solar system. Additionally, the findings presented herein provide support for the hypothesis that water from comets does not compose a substantial fraction of Earth’s water supply, and that other sources, like asteroids, may have had a more impactful (get it 🙂 ) role. In either case, this was a huge step forward in space exploration and hopefully additional missions will be undertaken.
The Ribosome 