I’ve been thinking a lot about where Earth’s oceans came from ever since we landed a probe on a comet, comet CG 67P.
It has been conjectured that comets colliding with early Earth provided the water for the oceans. The first person to come up with this idea as far as I know, was Dr. Michael Mumma of NASA. It sounds pretty good as far as a conjecture goes. Early Earth was hot – molten hot like a gigantic blob of molten rock hot so any water here from the beginning would be in the form of vapor anyhow.
But findings from examining probably the oldest rocks on Earth, rocks so old they were formed pretty close to when the crust of the earth first cooled, show that water in the form of large bodies of water, like ocean-sized bodied of water, were here like 200 million years after the earth first formed 4.5 billion years ago. So water was here, in large bodies comparatively early in Earth’s geological history. Where did all this water come from?
And just to back-track a bit, there had to be a magnetic field around earth to protect the atmosphere from having the lighter elements of its atmosphere being blown away by the solar wind; so it is conjectured that by this time the earth had already gone through a super-hot phase when everything was molten including the iron, and it being heavier sunk to the center of the earth and provided the physics for the magnetic field surrounding earth.
“Outer core convection rolls” by United States Geological Survey – http://geomag.usgs.gov/images/faq/Q6.jpg. Licensed under Public Domain via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Outer_core_convection_rolls.jpg#mediaviewer/File:Outer_core_convection_rolls.jpg
Venus for example, has a thick atmosphere, but little to no water or helium or hydrogen or oxygen because it has almost no magnetic field. Got to have that protective field; or you get a sterile atmosphere because the solar wind blasts light stuff all away into space and only the heavy species like CO2 stick to the planet because they are heavy enough to be held in place by planetary gravity. I guess that makes sense.
But there’s definitely a huge unanswered question of where all the water came from early in Earth’s history to make those enormous oceans.
First off, do comets really have enough water? Well apparently comet really do have a lot of water, even though from the outside they look rocky. Ends up the water is inside the comets and the outer part of comets is like a thin crust of collected space dust. http://www.nasa.gov/jpl/rosetta/why-comets-are-like-deep-fried-ice-cream/index.html#.VOKoMiwXc1L
For example, comet Hale-Bop is estimated to have enough water to fill up 10% of the great lakes. One comet. And you may have heard of the gigantic asteroid that probably hit the Yucatan Peninsua like 66 million years ago and wiped out practically all life on earth including dinosaurs.
The Chicxulub impactor had an estimated diameter of 10 km (6.2 mi) and delivered an estimated energy equivalent of 100 teratons of TNT (4.2×1023 J). By contrast, the most powerful man-made explosive device ever detonated, the Tsar Bomba, had a yield of only 50 megatons of TNT (2.1×1017 J), making the Chicxulub impact 2 million times more powerful. Even the most energetic known volcanic eruption, which released an estimated energy equivalent of approximately 240 gigatons of TNT (1.0×1021 J) and created the La Garita Caldera, delivered only 0.24% of the energy of the Chicxulub impact. http://en.wikipedia.org/wiki/Chicxulub_crater
An animation showing the impact, and subsequent crater formation (University of Arizona, Space Imagery Center)
That’s some impact! Apparently, during Earth’s early times when the solar system was young and wild, things like this happened monthly! Wild times. But the question remains: Did comets form the oceans?
Comets and Earth’s oceans both contain a small amount of so-called “heavy water” composed of equal parts hydrogen, oxygen, and deuterium. (Deuterium is an isotope of hydrogen with one extra neutron in its nucleus.) The chemical formula of heavy water is HDO; normal water is H2O.
There is growing evidence that comets born in the outer solar system (near Neptune’s orbit, for example) contain ices relatively rich in heavy water — too rich, in fact. They don’t match the isotopic composition of water in Earth’s oceans…
According to some laboratory experiments, a comet born billions of years ago ago in the neighborhood of Jupiter would contain about the same fraction of heavy water as Earth’s present-day oceans…
The “Jupiter zone” received more light and was warmer than the outer solar system. As a result, more reactions occurred in the gas near Jupiter and greater amounts of many complex organic molecules were available to wind up in comets. Also, Jupiter’s powerful gravity kept collision speeds between comets near it high, preventing them from growing very large. Both factors may have given a boost to life on Earth…
“It’s like being hit by a snowball instead of an iceberg,” said Mumma. “The smaller comets from Jupiter’s region impacted Earth relatively gently, shattering high in the atmosphere and delivering most of their organic molecules intact. Such comets would have had a greater portion of life’s building blocks — the complex organic molecules — to begin with. This means life on Earth did not have to start completely from scratch. Instead, it was delivered in kit form from space.”
Editor’s Note: In 1999 cosmochemist Geoff Blake and colleagues at Caltech showed that Comet Hale-Bopp contained too much heavy water to match Earth’s oceans.Those findings triggered widespread reports that comets couldn’t be a source of terrestrial water. Now Comet LINEAR has shown us that Earth’s oceans could hail from comets after all. But do they? Comets aren’t the only candidate water sources for Earth. Our oceans may have been delivered by asteroids (another controversial idea) or perhaps water was caught up in the body of Earth as it formed and later released by volcanic outgassing (as many of us learned in school). Or all of the above! The origin of Earth’s oceans remains a mystery, a topic for continued research. http://science1.nasa.gov/science-news/science-at-nasa/2001/ast18may_1/
Sounds weak to me. Why, in the early solar system would only comets from Jupiter hit earth? What happened to the Neptunian comets – of the comets we have actually checked out, they are overwhelmingly Neptune-style. But that’s now. Maybe way back then all the Jupiter comet got consumed. Why? How?
One awesome mission to find the answer was the mission Deep Impact, where we crazy Americans shot up a probe to a comet, Temple 1, and smashed a probe into the comet at high velocity. Yes, we bombed a comet:
https://www.youtube.com/watch?v=PklGEi9LHCg#t=11 Deep Impact
The impactor was maneuvered to plant itself in front of the comet, so that Tempel 1 would collide with it. Impact occurred at 05:45 UTC (05:52 Ground UTC, +/- up to three minutes, one-way light time = 7m 26s) on the morning of July 4, 2005, within one second of the expected time for impact.
The impactor returned images as late as three seconds before impact. Most of the data captured was stored on board the flyby spacecraft, which radioed approximately 4,500 images from the HRI, MRI, and ITS cameras to Earth over the next few days. The energy from the collision was similar in size to exploding five tons of dynamite and the comet shone six times brighter than normal.
“Deep Impact approach 2”. Licensed under Public Domain via Wikimedia Commons – http://commons.wikimedia.org/wiki/File:Deep_Impact_approach_2.jpg#mediaviewer/File:Deep_Impact_approach_2.jpg
Initial results were surprising as the material excavated by the impact contained more dust and less ice than had been expected. The only models of cometary structure astronomers could positively rule out were the very porous ones which had comets as loose aggregates of material. In addition, the material was finer than expected; scientists compared it to talcum powder rather than sand. Other materials found while studying the impact included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact. Clays and carbonates usually require liquid water to form and sodium is rare in space. Observations also revealed that the comet was about 75% empty space, and one astronomer compared the outer layers of the comet to the same makeup of a snow bank. Astronomers have expressed interest in more missions to different comets to determine if they share similar compositions or if there are different materials found deeper within comets that were produced at the time of the Solar System’s formation.
Astronomers hypothesized, based on its interior chemistry, that the comet formed in the Uranus and Neptune Oort cloud region of the Solar System. A comet which forms farther from the Sun is expected to have greater amounts of ices with low freezing temperatures, such as ethane, which was present in Tempel 1. Astronomers believe that other comets with compositions similar to Tempel 1 are likely to have formed in the same region.
And water from CG 67P?
On 10 December 2014, scientists reported that the composition of water vapor from Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water found on Earth came from comets such as Churyumov–Gerasimenko.
Measurements carried out before Philae ’s batteries failed indicate that the dust layer could be as much as 20 cm (7.9 in) thick. Beneath that is hard ice, or a mixture of ice and dust. Porosity appears to increase toward the center of the comet.
It’ also the “wrong” kind of water.
We just don’t know where earth’s water came from. Yet.
Here’s a cool video about early earth: