Did Earth's Water come from Asteroids? (Jan 2011)
The discovery of water ice on asteroids has opened up new possibilities for astronomers keen to explain the source of Earth's abundant water.
It is now clear that the water signature of cometary ice is different to that of the water on Earth. As such, the long-held belief among scientists that much of the Earth's water was deposited by impacting comets has been severely challenged. I wouldn't say that this theory is now entirely rejected among astronomers, because at the same time a large grey area has emerged between the nature and composition of comets versus asteroids. In astronomical parlance, the 'snow-line' has shifted. Nevertheless, it is clear that if water was deposited here early on, it did not come from the edges of the solar system. Rather, the water here is consistent with that found in the asteroid belt.
Because of this finding, scientists are now speculating that water was deposited upon a desiccated Earth by asteroids (1). While most asteroids sit quite happily in the main asteroid belt safely located between Mars and Jupiter, there are asteroids which have more eccentric orbits, and there are many 'near Earth objects'. The problem for scientists is that the speculation revolves around the water surprisingly featured on main belt asteroids such as 24 Themis and 65 Cybele. They don't come anywhere Earth, and never have. If one believes the currently accepted models of planetary formation in the solar system, then the Earth has always been where it currently is, and the main belt asteroids where they are. The two do not mix socially - so how did the water actually get to Earth if it was supposedly located on distant asteroids in the first place?
Zecharia Sitchin's vision of the early solar system offers a solution to this question - the asteroids and Earth come from the same source. According to his readings of ancient texts, he argued that Earth was once a much larger, watery world. Vitally important to the issue of the origin of Earth's water, the Earth was initially located where the asteroid belt is now. That argument holds great promise here, because if the early Earth was once located up to four times further away from the Sun, then its early water need not have been driven off by the Sun early on. This would also explain why there's water on the Moon.
There's a lot of evidence for massive collisions upon the Earth early in its history. The leading scientific theory about the formation of the Moon holds that the Earth was struck by a planet the size of Mars. Furthermore, the Earth (along with other worlds) was pummelled by massive asteroids/comets/planetoids 3.9 billion years ago, during the late heavy bombardment. Surprisingly, water on the Moon is deeply embedded in rocks, suggesting water was part of the Moon's composition from the very beginning. If the Moon formed as a result of a collision of a planet with the early Earth, then it stands to reason that the early Earth also had water in abundance (2). In that case, the 'late veneer' theory is redundant anyhow. But the collisions themselves offer a suggestion as to how the Earth migrated:
Perhaps it's as simple as saying that the Earth got knocked off its perch, and the asteroid belt marks the spot of this catastrophic event. More likely, the close flyby of a larger, rogue body played a part - simultaneously providing the gravitational push for a substantial migration by the Earth, and the accompanying system of moons and comets that could have resulted in sizeable impacts during the flyby.
I think these latest findings lead us towards the conclusion that the Earth initially formed, water and all, much further away from the Sun than its current location. Furthermore, the asteroid belt is a by-product of whatever event drove the Earth deeper into the inner solar system. Such a situation would explain the Earth's abundance of water, the fact that the water did not come from comets, and the abundance of water located on even minor main belt asteroids far from the Earth.
Comet from the Kuiper Belt Has Earth-like Water (Oct 2011)
Comets in the solar system have variable origins. Over recent years scientists have looked at the water composition of comets in order to answer the question of where the Earth's water originated from. This is a trickier question than you might think. If the Earth formed in its current location (and note I say 'if') then the intense bombardment of the solar wind from the early hot Sun should have driven off all the volatiles from the Earth's primordial surface, including water. The fact that the Earth has abundant water, rather than being arid like Venus and to a lesser extent Mars, is a mystery.
The generally accepted answer to this mystery is the 'later veneer theory', whereby water was deposited onto a dry Earth as a result of the impacts of many, many comets and asteroids over a vast period of time. But this theory has been questioned recently because the isotopic composition of water in the half dozen comets so far analysed has been different to that of Earth water. If they are a representative sample of smaller bodies in the solar system, then the water must have come from somewhere else: Firstly, there aren't enough comets in the solar system to have created the Earth's oceans, and secondly, the isotopic composition seems to be wrong anyway.
The
latest comet to be analysed in this way has opened a door of
hope on the Late Veneer Theory:
"New measurements from the Herschel Space Observatory show that comet Hartley 2, which comes from the distant Kuiper Belt, contains water with the same chemical signature as Earth's oceans. This remote region of the solar system, some 30 to 50 times as far away as the distance between Earth and the sun, is home to icy, rocky bodies including Pluto, other dwarf planets and innumerable comets." (3)
The problem is where comet Hartley 2 originally came from (4). The Kuiper Belt is effectively a second asteroid belt beyond the planet Neptune, much larger than its famous cousin which lies between the orbits of Mars and Jupiter. It is a part of the solar system astronomers are only just starting to understand, and regular readers of this site will recognise that it has thrown up a large number of anomalies and new questions.
Many of these new findings are suggestive, in my opinion, of a different scenario for the early solar system than is presently held by the mainstream scientific community. The new puzzle is why bodies originating close to the Earth have a different isotopic composition to the water found on our planet, whereas one originating in the distant Kuiper Belt has exactly the same. So where this new discovery throws the Late Veneer Theory a bit of a lifeline (5), it does not explain why the Earth has more in common with a Kuiper Belt Object that has strayed back into the inner solar system, than it does with the nearby asteroids and other neighbouring cosmic debris. Comet Hartley 2 seems misplaced, to say the least:
"However, the new results also raise new questions. Until now, scientists assumed that the distance of a body’s origin from the Sun correlated to the deuterium-to-hydrogen ratio in its water. The farther away this origin lies from the Sun, the larger this ratio should be. With a “birth place” within the Kuiper belt and thus well beyond the orbit of Neptune, Hartley 2, however, seems to violate this rule. “Either the comet originated in greater proximity to the Sun than we thought”, says Hartogh, “or the current assumptions on the distribution of deuterium have to be reconsidered.” And maybe Hartley 2 is a so-called Trojan that originated close to Jupiter and could never overcome its gravitational pull." (6)
Competing theories like mine, originating in Zecharia Sitchin's controversial explanation for the cosmic catastrophes of the early solar system, may offer answers to this mystery, which don't require rewriting the laws of physics. There's the potential here to prove a very important point about the origin of Earth. Let me explain: Earth may have formed in one of two locations, (1) where it is now (what everyone assumes), or (2) at the distance of the asteroid belt (Zecharia Sitchin's solution (7)). How can we tell? Scientists think that water in the solar system contains different ratios of hydrogen to deuterium isotopes, depending upon the formation distance from the Sun, and that gives us the potential ability to determine where things should sit in the solar system.
Now, if the
Earth formed at the current distance, then the early waters were
blasted off the surface of the Earth by the sun's solar wind,
which was stronger initially than it is now. Like volatiles
being blasted off comets as they approach the Sun, the Earth
should have lost its water. It should have been dry when it
solidified. As we know, it's actually wet.
So the water either came from impactors like comets and asteroids, or...the Earth formed further away from the Sun than it is now. At the distance of the asteroid belt, water would not have been blasted away.
When scientists look at the isotopic ratio of water contained in various bodies in the solar system, they are building up a picture of the early solar system. As it turns out, things have not gone as expected. Comets studied so far have not had the same ratio as Earth, so that brings into question whether the Earth gained water from them. This latest comet, which is really a Kuiper Belt Object, is the first to contain the same ratio as the Earth.
Some scientists suggest that the bombardment of the inner solar system that took place 3.9 billion years ago (the 'Late Heavy Bombardment') consisted of a swarm of giant comets from the Kuiper Belt displaced by the migration of Jupiter and Saturn. Such a concept is supported by the recent discovery of the debris of a giant comet in a neighbouring star system whose chemical signature matches meteorite fragments found here on Earth. A pattern may be occurring here:
"About
4 billion years ago, some 600 million years after our solar
system formed, scientists think the Kuiper Belt was disturbed by
a migration of the gas-giant planets Jupiter and Saturn. This
jarring shift in the solar system's gravitational balance
scattered the icy bodies in the Kuiper Belt, flinging the vast
majority into interstellar space and producing cold dust in the
belt. Some Kuiper Belt objects, however, were set on paths that
crossed the orbits of the inner planets.
"The resulting bombardment of comets lasted until 3.8 billion
years ago. After comets impacted the side of the moon that faces
Earth, magma seeped out of the lunar crust, eventually cooling
into dark "seas," or maria.
"Comets also struck Earth or incinerated in the atmosphere, and are thought to have deposited water and carbon on our planet. This period of impacts might have helped life form by delivering its crucial ingredients." (8)
Despite the ready supply of water in comet clouds seen forming around other stars (9), the problem remains that the population of the Kuiper Belt is insufficient to have supplied enough water to have filled the Earth's oceans. What could have swept through the Kuiper Belt to have displaced all those missing KBO asteroids? Surely not distant Jupiter and Saturn? This is where the Dark Star concept could really provide the answer (10). But caution is needed: the dataset here remains small, and we will only be able to piece the puzzle together properly when a larger sample of comets and asteroids has been properly studied.
