As Comet ISON (C/2012 S1) slowly approaches, promising to give us a spectacular show this fall, it's tempting to think about the role comets may have played in our very existence.
For some years beginning in the 1990s it became trendy for scientists and lay people to jump on the bandwagon of the idea that comets supplied huge amounts of water to early Earth. Conferences included considerable talk about the notion. Writers pounded out books suggesting the idea was ironclad. Several studies cropped up adding apparent weight to the evidence that much or most of Earth's water -- and the conditions friendly to abundant life -- rained down from outside the atmosphere when Earth was in its formative years.
Because comets contain large amounts of water and other ices, the notion that bombardments by comets deposited much of Earth's water has seemed to be almost a measure of faith. But a variety of recent studies place this idea into a somewhat harsh context. And understanding how Earth got its water is certainly one of the most important unresolved questions about the early solar system.
In tracing where the water came from, scientists attempt to recreate the conditions of the protosolar nebula, the cloud of gas and dust that formed the Sun and planets. They agree that the nebular disk was hotter and denser toward the center and cooler and less dense away from the center. The varying degrees of temperature throughout the protosolar disk clearly affected where water and icy particles existed. The central region would have contained high concentrations of metals and silicates, whereas icy particles could have existed in far greater quantities away from the center. They also believe the earliest solid particles were tiny; these objects accreted into larger ones by sticking together through countless collisions. Where plentiful oxygen existed, carbonaceous chondrite meteorites formed, which can contain up to 10 percent water. But comets, on the icy perimeter, contain as much as 80 percent water by mass.
As hard as it is to believe when one stands on the shore of a great ocean, Earth has a small amount of water by mass -- only 0.02 percent in its oceans and a little more than that below ground on continents. Despite the small fraction of water on Earth compared to its total mass, our planet has plenty of water. For a planet at our distance from the Sun, it is exceedingly rich in water, containing far more than might exist here.
So how did the water get here? The possibilities include bombardment by comets, asteroids, and planetesimals; water absorbed by silicate grains in the protosolar nebula and transported into early Earth; and the production of liquid water through oxidation of a hydrogen-rich atmosphere. A significant, recent clue toward the likelihood of each of these scenarios comes from studying the ratios of deuterium to hydrogen from each of them as well as predictions made by computer modeling of how much water each of these methods would produce.
A years-long affair with the idea of comets delivering a huge amount of water to Earth seemed built of pure, simple logic. Made largely of water ice, and existing perhaps in the trillions, they were the leading suspects. They also presumably retained their isotopic properties -- the varieties of atoms they're made of --from the earliest days of the solar system. But recent measurements of the deuterium to hydrogen ratio (D/H ratio) of water in eight Oort Cloud comets delivers a heavy blow to this idea. Water can carry different isotopic signatures, depending on the hydrogen isotopes it contains, and the ratios of deuterium ("heavy hydrogen") to hydrogen in the comets are on average twice those of the Vienna Standard Mean Ocean Water (VSMOW), which defines the average isotopic water concentration on Earth. Moreover, they are 15 times greater than the D/H ratio of the early solar nebula. Although it's quite possible the D/H ratio of ocean water could have changed over time, most scientists now believe this incompatibility rules out comets as major sources of Earth's water.
But the arguments over comets providing water to early Earth didn't stop with the Oort Cloud. In the 1990s, several planetary scientists proposed that perhaps Jupiter-family comets, those in orbits much closer to us, were the source of Earth's water. These comets have lower D/H ratios due to the fact that they formed in a warmer region of the solar system. For example, the Jupiter-family Comet 103P/Hartley 2 has nearly the same D/H ratio as Earth's ocean water. But dynamical arguments stepped in to make the Jupiter-family comets unlikely as sources of water. With Jupiter and Saturn in their current positions, comets that bombarded Earth may have come from the region beyond Uranus. But computer modeling of the early solar system shows a "bombardment" of comets from this region likely would have produced only about 6 percent of the water needed to make up Earth's oceans.
A spate of recent studies supports this idea that comets played a relatively minor role in delivering water to early Earth. Several analyses show that comets probably contributed about 10 percent of Earth's water. But if the dynamics of orbits had changed drastically over time, then this answer may be misleading. Support for lots of water coming by comet from studies of isotopic ratios is also weak. Studies of ratios of noble gases suggest comets could not have delivered the majority of water to early Earth unless it happened very quickly, during something like the first 100 million years of the planet's history. Or perhaps if comets came from a region different than the Oort Cloud. These studies, coupled with dynamical simulations, suggest that comets contributed 10 to 15 percent of the water on Earth.
Could primitive asteroids have delivered a significant additional amount of water? Carbonaceous chondrites do have D/H ratios quite similar to Earth's ocean water. That match might suggest that these primitive asteroids, which could have been more hydrated in the past than now, might be a mechanism for delivering water. And relatively water-rich asteroids from far out could have been perturbed inward toward Earth by the giant planets. But the dynamics of orbits, once again, gets in the way. The efficiency of scattering asteroids toward Earth is very low and that the contribution in water to our planet must have been very small. If primitive asteroids had 10 percent water by mass, the likely rate of accretion would have required the mass of asteroids to be 4 times that of Earth in a region some 2.5 astronomical units away -- a figure that seems unrealistically high.
The timing of the delivery of water is also a problem. The scenario of primitive asteroids delivering lots of water suggests that it would have arrived to Earth very early on -- when the planet was young, still accreting, and only 60 percent of its current mass. This would have been before numerous major impacts would take place, with planetesimals and perhaps several small planet-sized bodies. These traumatic impacts would have made it very difficult for the planet to retain this water if it was deposited by carbonaceous chondrites.
But there is a leading idea among mechanisms to deliver significant water to early Earth. It seems clear that water must have been delivered to Earth during a long and sustained period during the plant's history, not just once, early on. Planetary scientists believe that the widespread impacting of water-bearing planetesimals and proto-planets into Earth probably delivered substantial amounts of water to our planet. These objects probably originated from the outer asteroid belt, which then becomes the leading suspected source of water in the solar system.
But the delivery by water-rich asteroids is not the only method that perhaps played a significant role. Large amounts of water could also have come directly from the solar nebula. Water molecules can adhere to dust grains so efficiently, even at relatively high temperatures, that significant amounts of water -- perhaps even the entirety of the oceans and more -- could have existed in and on dust grains that accreted in massive numbers to form Earth. And the cooler the temperatures, the greater the efficiency of the water molecule adhesion to dust grains.
But there are questions and problems here too. The D/H ratio of the solar nebula does not match the current ocean water well. And planetary scientists can't currently explain easily how the water would be retained as dust grains accreted into larger and larger particle sizes.
So the issue of where Earth's water came from is unresolved, but it seems that a large percentage of it may well have arrived from water-bearing asteroids from the outer asteroid belt, supplemented by comets bombarding us and by water molecules sticking to dust grains from the solar nebula. And as with most scientific questions, there are no easy answers as this world of one-off press releases would demand. The question is big and the research goes on. But clearly comets played a role in making our oceans -- it just wasn't the leading role on what was a very big, and very messy stage.
David J. Eicher is Editor-in-Chief of Astronomy magazine, author of 16 books on science and history, and president of the Astronomy Foundation. His book COMETS! Visitors from Deep Space will be out in October from Cambridge University Press.