With the recent news that Philae discovered that the water composition on 67P is much different than that of Earth (it has a much higher deuterium isotope content than Earth’s water), many scientists are reconsidering the theory that comets brought water to Earth. They are starting to generate new theories, such as thinking that asteroids brought water to Earth, or that Earth literally made its water by combining hydrogen and oxygen together. To me, To give up this theory so quickly based on very little data is ludicrous, especially given how numerously these balls consisting largely of frozen water pelted the Earth during the Late Heavy Bombardment. Somehow that water just disappeared, I assume?
I don’t think scientists should be ruling out the possibility that comets delivered water just based on one comet’s composition. I do think a mission should be launched that goes deep into the Kuiper Belt and Oort cloud to probe the ice and dust composition of several comets to see its composition. It may be that 67P is unique in its composition and is an outlier, or that all comets have entirely different compositions of water, or that there’s a difference between Kuiper Belt and Oort cloud comets chemically. Maybe one day I can lead such a mission?
Also, despite the fact the water on earth analyzed came from basalt formations, with plate tectonics and all, all of the truly ancient rocks that would have been exposed during the Late Heavy Bombardment would have long subducted. The water trapped in the basalt may be old, but it may not be old enough. By then, many other factors could have contributed to changing the deuterium ratio in water, which is what makes the composition of water found on 67P “different” from the composition of water found on earth today, including both cosmic and terrestrial factors.
I’m quite surprised that scientists are making any sort of verdict, no matter how weak the conjecture, based on only a couple of data points. Sure, at this moment, data points are scarce, but that is no reason to rule out this theory, even slightly, just yet. There may be no way to analyze the water from Earth’s past, since much like Earth’s impact crater history, much of the evidence has literally slipped under the Earth’s surface, wiped from existence; however, this has never stopped us from other endeavors.
There are other ways. It is possible to analyze the water composition of distant frozen bodies such as moons to see whether the composition of such matches that of various comets. Enceladus, for instance, may similarly have a “tainted” water composition due to the geochemical reactions and radiation it receives; however, especially under its surface, the water source would largely be uncontaminated by radiation, and comparatively little geochemical contamination with respect to Earth. There, the water source would be very pristine.
If in fact, the water is much like that detected in comets, then we can theorize that the water on Enceladus did come from comets. It would be relatively easy to collect samples of such–all we’d have to do is fly a probe through one of the geyser jets it spews out, collect the samples, analyze the deuterium isotope to the regular isotope of water, and compare it to that of comets. In fact, Cassini did something very similar in order to collect samples to analyze the organic chemistry of Enceladus. The difficult part would be sampling the comets in order to get a sufficient sample size of the isotopic chemistry of the water on the various comets.
To do such, I’d imagine a very lengthy (and sadly unimaginably costly) mission into various portions of space. We’d launch a mother-probe. This mother probe would be filled with hundreds of tiny “mini-probes.” The sole function of this miniprobe would be to analyze the isotopic composition of the comet’s water-ice; however, if such a mission were to actually happen, it would be highly cost effective if we essentially made each probe a mini-Philae of sorts. That way, we would not only make a comprehensive sample of the isotopic composition of water on the comets, but also could potentially make a comprehensive sample of the organic chemistry of each comet. I should say that each miniprobe would contain only a spectrograph of sorts; that way it could complete many chemistry experiments, observations, and analyses, making the most out of one instrument. It might also contain a small camera, just to compare landscapes on the different comets that will be sampled–particularly the difference between comets in the inner solar system, outer solar system, Kuiper Belt, and Oort Cloud.
While the mother-probe is waiting for the signals from the mini-probe, it would also analyze the composition of the comet’s tail, to see if it differs in any way from the ice on the comet. Why? If we find the composition of the tail is much higher in deuterium than on the surface of the comet, it shows that over time, the ratio of the deuterium isotope to the normal isotope slowly reduces, as deuterium gets blasted off into space to form the comet tail. It would also be good to analyze the organic chemistry of the tail while it’s waiting, to see how much organic material is being blasted out into space. If many comets are blasting out significant amounts of organic chemicals into space, this could show that there is a significant amount of organic dust in space, just waiting to be pulled in by the gravity of some astronomical body, quite literally scattering the seeds of life around the solar system.
We’ve analyzed such for a couple of comets–three if I remember correctly, but three comets can hardly be considered a sample size. As such, I think it’s crucial that we eventually plan some sort of a mission like this to collect a large sample of data. This mission may take a hundred years or so to complete, and that’s no hyperbole. It took the Voyagers 1 and 2 nearly half a century to get to the Kuiper Belt, and this mission would need to probe the Oort cloud to be truly comprehensive. It would span my entire lifetime–if I’m lucky enough to live that long. Even if it were launched today, by the time the century-long mission would be completed, I’d be 117 years old. It’s likely I will survive that long, but it’s still questionable.
It takes light about fourteen and a half hours to reach Voyager 2 from Earth, and it’s barely inside the Kuiper Belt. To put that in perspective, the entire Oort cloud reaches out to an entire light-year, which is a quarter of the way to our nearest star. Now of course such a reach would be unrealistic for many reasons, but one can imagine multiple “carrier” probes being released from the mother-probe.
These probes would carry a small handful of the miniprobes in various directions, dropping them off at comets surrounding it into the far reaches of the Kuiper belt and just inside the Oort cloud. The miniprobes would send signals back to the carrier probes, which would send signals back to the mother-probe. Once that region has been scanned, the mother-probe would move on to a different region and do the same task. This would also conserve fuel, which would otherwise generate unnecessary costs.
We would still need to work out what the most cost-effective way to collect samples from hundreds–at least a thousand–different comets would be. It would obviously be the most costly mission ever launched, but we could conclusively determine the composition of water in the comets, and how abundant organic compounds are in the solar system, among many other things. If we use this in conjunction with the compounds on Enceladus (and the data from the planned mission to there to analyze its organic chemistry in greater detail), especially if we find traces of microbial life there, then we can finally get a good picture of whether or not comets are bringers of water–and likely life. If we analyze other large untouched icy bodies and get a similar picture, then we can be almost certain that comets brought water to earth–and perhaps even life.
Perhaps the microbial life might be found on the comets, too. The greatest fear of looking for life extraterrestially is the fear of contamination from Earth biology. No matter how much sterilization we do, there always seems to be some trace of bacteria left on the probes. Surely if a large number of the hundreds of miniprobes we send out return positive, it can’t possibly be a false positive. That’s another plus to doing these mini-missions, among many. Some of the probes may fail, but if even only 20% of the thousand miniprobes we send out succeed in its mission, we still have a very large set of 200 data points to work from. Ah, I love Science! There’s never a shortage of things to learn and learn from!
I think I got a little carried away with postulating this hypothetical mission, but I do think a mission like this is necessary for truly discovering our origins. It may be expensive, but it’ll answer many of the biggest questions we have today either directly or as a result of its findings. It may not be finished within my lifetime, but I’d be happy to see it launched and start working at its mission. What do you think?