We often hear that studying asteroid material will help us better understand the origins of our Solar System. I don’t know about you, but to me that always sounds pretty vague (and probably complicated).
That’s why I like this new study led by the University of Glasgow (Nature Astronomy, November 29) that uses asteroid samples to help answer a straightforward question: Where did all the water on Earth come from?
Recently, two missions made headlines for fetching asteroid material for return to Earth: JAXA’s Hayabusa2 mission, which visited asteroid Ryugu and dropped samples back on Earth about a year ago, and NASA’s OSIRIS-REx mission, which is on its way back to Earth now from asteroid Bennu, with sample drop expected in 2023.
But the first of these missions (Hayabusa) returned in 2010 with granules from the near-Earth asteroid Itokawa. There were about 1500 of these, and most were smaller than 10 micrometers in diameter. The novelty of that was cool and everything, but the followup’s been pretty awesome as well. We’ve been learning a lot from them.
Itokawa’s orbit comes within about 5 lunar distances of Earth, and of course future interactions could possibly bring it closer than that. It’s actually gotten physically that close to Earth in 1905 and 2004, but luckily doesn’t figure to do so again anytime soon.
This proximity made Itokawa a good target for a sample-return mission, but it also gave us material from something that formed in the rough vicinity of Earth and was preserved in a more-pristine state, so maybe it could tell us about the formation of our own planet.
One puzzle about Earth’s formation is that our planet shouldn’t have nearly as much water as it does. Asteroids that formed closer to the Sun, such as those in the inner asteroid belt between Mars and Jupiter, have very little water, while those that formed farther out have much more. So that implies that Earth, which formed even closer to the Sun than those asteroids, started out pretty dry and must have gotten its water from some far-out source. But what could that source be?
Much of Earth’s water could very well have come from carbonaceous chondrite meteorites, flung to Earth from asteroids that formed far from the Sun, out around Jupiter/Saturn and beyond. Those weren’t exposed to much heat when they formed, and so their volatile components like water could stay put. Carbonaceous chondrite meteorites can contain up to 20% water.
It would take a whole lot of hits by these kinds of meteorites to produce our oceans, but even if we grant that possibility, when you take them as a whole, their water doesn’t quite match Earth’s water in one important way: it’s too heavy.
“Heavy” water is not H2O but rather D2O. Its hydrogen atoms are replaced by deuterium atoms. A hydrogen atom is simply a proton and an electron, but a deuterium atom is that plus a neutron, so it’s heavier.
On Earth we’ve got water with about 150 parts per million deuterium, but the average for those asteroids is more like 190. So we seem to be missing a significant source of lighter water to make all of this add up.
Enter the solar wind!
While the Earth was forming, it was quite hot in our neck of the woods, close to the young Sun, and so most of the volatile stuff like water evaporated and was lost. But after things started cooling down, and the Sun’s light started getting through the protoplanetary disk surrounding it, there was still a lot of fine dust around, and that dust continued to get sucked, or accreted, into Earth. But it didn’t have any water, right? How could it? Wellll, that’s where this new study comes in.
Fine surface particles (regolith) from the near-Earth asteroid Itokawa shouldn’t have a lot of water, either. It all evaporated away, right? But if you look carefully at the outer layer of these particles, they do indeed contain water, in fact so much that if you collected a cubic meter of regolith …
…you could recover 5 GALLONS of water from it! But how the fork did all that water get there?? The Itokawa particles show us that the answer seems to be the solar wind.
If we look carefully at Itokawa particles, we find that their exposed surfaces are weirdly enriched in hydroxyl (-OH) groups. Under slightly more-acidic conditons, that’s water. The authors explain how they measured this:
Atom probe tomography (APT) is a quantitative analytical technique capable of measuring the abundance of water and OH molecules within minerals in three dimensions at subnanometer resolution. We utilized APT to measure water abundances of space-weathered surfaces on two sides of a particle from the
asteroid Itokawa [...]
Some results of that are shown here. Close to the surface of an Itokawa particle (the outermost 50 nanometers, or the “space-weathered rim”) we see an enrichment in -OH. That’s shown in the graph but also displayed pictorially by blue dots in the APT image:
And the reason that -OH got there is because of the solar wind. The same solar wind that would have been nailing all those tiny dust particles that eventually accreted into Earth in the early Solar System. (And still are today, to the tune of 220,000 pounds per day).
This seems like a good time to stop and appreciate the solar wind.
The Sun’s surface is ”only” about 10,000°F, but the corona is more like 2,000,000°F, so hot that it expands quickly enough for some of it to escape the Sun’s gravity. There is your solar wind, streaming out from the Sun at over a million miles per hour. The corona is so hot that within it, electrons (negatively charged) get ripped away from their atomic nuclei (positively charged), so the solar wind is made up of these charged particles.
Almost all the atomic nuclei within the solar wind are hydrogen (95%, just one proton) or helium (5%, two protons and two neutrons), although there are traces of heavier atoms, too. Earth’s magnetic field deflects these ultrafast charged particles away from us, but dust and asteroids floating out in space aren’t afforded such protections. They get the solar wind head-on.
When you irradiate silicate minerals such as those found in asteroids with protons at about the same intensity as the solar wind, it turns out you generate water, to a depth up to 200 nanometers. This was confirmed by the authors using pristine Earthbound olivine (magnesium/iron silicate), which also makes up 2/3 of the Itokawa particles. They showed that simple laboratory handling didn’t cause any such -OH enrichment, but bombardment with charged particles did.
Itokawa is essentially a rubble pile, most likely formed from bits of another much larger asteroid that got blown apart by an impact with another body. Here’s a nice pictorial summary of Itokawa’s history as we understand it:
Particles on the surface of Itokawa tend to turn over pretty quickly (average surface exposure maybe 1000 years because of low gravity and impacts), so the water introduction we see in the Itokawa particles is a relatively short-term effect. The entire Itokawa regolith turns over no less frequently than once every 8 million years due to large-scale grain motion and escape by impacts. Here’s a cute picture of a teeny crater on an Itokawa particle probably formed by the ricochet of a speck from a larger impact:
For comparison, regolith on the calmer Moon has an average residence time of about 400 million years. And it can contain several percent water, perhaps also because of the solar wind.
Dust particles floating around in the Solar System are generally very small, less than a micrometer in diameter. It doesn’t take long for many of them to get saturated with water, to the point where water implantation by the solar wind and water loss (by solar heating and exposure to a vacuum) are happening at the same rate. Judging by the age of micrometeorites found on Earth, generally dust particles tend to hang around for a million years or so in the Solar System, plenty of time to get saturated with water:
Small particles with long exposure times such as this will be loaded with water to an even greater degree than the regolith particles on Itokawa, up to 100 times as much. And they’d be loaded with light water, as the solar wind has only a couple parts per million deuterium. Just what we need to correct the Earth’s water “heaviness” with respect to the water content of the carbonaceous chondrites.
So back to Earth’s formation, then: Once Earth had taken shape, it was still surrounded by a whole lot of interplanetary dust, but the Sun’s light was starting to get through, and hence also these charged particles, irradiating silicates in the dust and creating water. It’s thought that the last 0.5% to 1.5% or so of Earth’s mass (the “late veneer”) accreted from this kind of dust. That would have added up to a fair amount of light water, enough to complete the balance and explain what we have today.
The authors leave us with an intriguing statement:
As such, solar-wind-irradiated silicate minerals may represent a substantial renewable source of water on airless worlds throughout the Galaxy.
Hmmm, you don’t say….
Thanks to Dr. Luke Daly of the University of Glasgow, who graciously sent me this paywalled paper literally 13 minutes after I asked him for it.