The origin of Earth’s water is still something of an open debate. The popular theory at the moment is that the primordial Earth was far too hot to contain any form of liquid water, its molten surface still reeling from the cataclysmic events that led to its creation. However others postulate that the water was trapped deep below the surface, only to arise later on as the Earth cooled and an atmosphere developed. It’s an interesting question not only because of how fundamental water is to life but also because we seem to have a lot more of it than any other planet in the solar system. Thus the question of where it came from, and why it’s managed to stick around for so long, is one of continuous scientific enquiry, including such missions as the recently celebrated Rosetta probe.
If we run with the theory that Earth’s water came from some extraplanetary source then the question turns to what the original source might be. Comets seem like a good candidate as they’re primarily water ice by composition and were far more common during the early stages of Earth’s life than they are now. However measurements of isotopes within water of several comets, including Halley, Hyakutake and Hale-Bopp has shown that they are not likely the primary source of water that’s currently on Earth’s surface. The composition of water found on asteroids and other water formed minerals on the Moon seem to indicate that a source closer to home is far more likely which Rosetta’s latest data appears to confirm.
The comet that Rosetta was investigating, the romantically named 67P/Churyumov–Gerasimenko, has a ratio of isotopes that is completely different to anything that’s seen on Earth. The reason that this is important is due to it’s orbit as 67P is what we call a Jupiter class comet, a collection of various comets that have orbits that don’t extend far past Jupiter. It was thought that these kinds of comets would have been more likely to have been involved in the creation of Earth’s oceans than comets from further out, due to their proximity. However 67P, with its wildly different composition to Earth (and even other bodies in the same vicinity), lends credence to the idea that comets aren’t the likely source of Earth’s oceans. Indeed it’s far more likely that water and minerals trapped in asteroids are the likely source, based on how similar their composition is.
Now this doesn’t rule out comets completely as there’s potential for further out Kuiper belt class comets to have the composition we’re looking for but it’s looking far more likely that objects from within the asteroid belt are responsible for the oceans we have today. What the mechanism was for them making their way to Earth, whether it was early on in the cataclysmic forming of our solar system or later on when things calmed down, is something that’s still an open question. It’s one we might also have answers to very soon as Dawn is scheduled to arrive at Ceres early next year, the biggest object in the asteroid belt. What Dawn finds there might be the key to unlocking the secrets of our Earth’s oceans and, potentially, the asteroid belt itself.
I can remember for the longest time being completely unaware of the asteroid belt between Mars and Jupiter. After learning about it however I never paid much more thought to it, although I was curious about how there seemed to be a line that separated the smaller, rocky planets from the large gas giants of our solar system (negating Pluto, of course). As my interest in space grew I began to wonder how any spacecraft that had ventured past Mars (there have been 9 of them) hadn’t managed to have a run in with a stray asteroid. As it turns out there’s a few reasons for that, and I find them quite fascinating.
The first is that the average density of the asteroid belt is extremely low with the total mass contained within the entire system being less than 4% of that of our Moon. Our best calculations then put the odds of a satellite coming into (unintended) contact with an asteroid in this region at about a billion to one, or so unlikely that you wouldn’t even consider it a risk. The images of the asteroid belt that many are familiar with make it look far more densely packed than it really is, much like this series of pictures that shows all the artificial satellites of Earth. That’s not to say the amount of junk we’ve sent up around ourselves isn’t an issue, but an accurate scale representation of each satellite wouldn’t look anywhere near as packed as they do.
What fascinates me the most about the asteroid belt however is how the majority of the mass is concentrated within 4 objects, with the two largest of these being 4 Vesta and an object big enough to be classed as a dwarf planet called Ceres. In astronomical terms they’re right in our backyard but even with our most powerful space based telescope we’re still only able to capture relatively blurry representations of them, shrouding these little heavenly bodies in mystery. Ceres especially so, with a series of images showing a massive bright spot moving across its surface of which its nature is still unknown.
So fascinating are these objects that NASA launched a mission to both of them, named Dawn, back in 2007. This particular spacecraft is something of a novelty in of itself as well as it is the first purely exploratory mission to use only ion thrusters for propulsion. It needs these highly efficient engines as it will be the first spacecraft to launch to its target, orbit it for a set amount of time and then set off again to approach yet another target. To do this it is carrying with it over 400kg of propellant enough for it to change its velocity by over 10km/s, a figure well above that of any other spacecraft that has come before it. It may take its time in doing so, but it’s still an incredible achievement none the less.
Dawn is scheduled to arrive at its first target, 4 Vesta, in just over a month and it has already begun sending back pictures and video of this strange mega-asteroid. They’re not much to look at right now but once its closer the imagery will become much clearer, revealing the nature of all the blurry spots we’ve as of yet only been able to speculate about. Dawn will spend a year surveying 4 Vesta before it sets off on its long journey for Ceres, for which it is not expected to reach until February 2015. Its a long wait to get a better look at something that’s so small compared to nearly everything in our solar system, but the prospect still excites me immensely.
Perhaps its the combination of their close proximity yet relative lack of information about these two little bodies that makes them so interesting, they’re just sitting there begging to be investigated. The next year will reveal all sorts of insights into the asteroid belt and its second largest contributer which will in turn tell us something about Ceres itself. We’re still a long, long way away from seeing Ceres in the flesh (or rock, as it were) but any information that Dawn sends back is valuable and I can’t wait to see what it brings us.