Probably the most striking thing about the Earth is the amount of water that it contains. The famous science fiction novelist, Arthur C. Clarke once remarked (as I'm sure many other people have): "How inappropriate to call this planet Earth, when clearly it is Ocean." Surely any visitor from beyond our planet would notice that it is covered with water (if they recognized the blue appearance as water). Yet, Earth is the name we go by and so we are stuck with it for now.
There are many ways to express the amount of water on the Earth (none of which give us a real sense of the enormity of it all). The short of it is that approximately 71% if the Earth's surface is covered by the world's oceans. The total volume of water on the Earth, including glaciers, rivers, lakes, groundwater and atmospheric water, is 1,398,898,300 cubic kilometers. For those of us who require a more practical unit of measurement, this amount is approximately equivalent to 40 thousand billion billion cans of your favorite beverage (~40 x 10^21 12 oz. cans). That's a lot of water!
Where did all this water come from? The formation of Earth involved the aggregation of planetesimals to form larger bodies which we now call planets. As the Earth formed, approximately 4.6 billion years ago (BYA), nuclear heating and the energy of thousands of collisions with smaller planetesimals and meteorites caused it to become molten, at which time the core of the Earth differentiated into layers based on the density of materials within them. As the Earth's core was differentiating, gases were released. These gases bubbled to the surface where they escaped to outer space (especially the lighter gases like helium and hydrogen) or were held by the forces of gravity to form our atmosphere. This process, called outgassing, was responsible for the release of gases from the interior of the earth that formed our early atmosphere.
In its initial stages, the Earth was too hot for an atmosphere to form. Any gases were superheated and released into space. However, once the Earth cooled sufficiently, sometime in the first 700 million years of its existence, clouds began to form in the atmosphere, and the Earth entered a new phase of development. It began to rain. And it rained and it rained and it rained.
The formation of an atmosphere also protected the Earth from smaller meteorites, which burned up before they impacted the surface. The cloudy atmosphere also reflected some of the radiation from the sun, allowing the Earth to cool further. Eventually, the surface of the Earth solidified and the crustal plates were formed. After several hundred million years, the Earth finally had oceans, and atmosphere, and continents.
In addition to outgassing, other processes released gases and were responsible for bringing water to the Earth. These include:
However, there is some evidence that these mechanisms alone could not account for all the water that we see on our planet. One popular explanation is that after the earth cooled sufficiently, it was bombarded by meteorites that brought water to the planet. Meteorites continue to bombard our planet every day, although we mostly see them as "shooting stars" that burn up before they hit the Earth's surface.
Recently, one scientist at at the University of Iowa, Louis Frank, went out on a limb and suggested that "mini-comets" that bombard our atmosphere every day are an important source of water on Earth. He was immediately ridiculed for this idea, yet satellite data released by NASA in 1997 appear to confirm the existence of the mini-comets at least.
Here's the report from Reuters News Service:
Satellite finds more proof comets are showering Earth
By Steven Young / Reuter 1997
CAPE CANAVERAL, Fla. -- Early results from the ozone-watching satellite released by the space shuttle Discovery appear to support a theory that Earth is being bombarded daily by thousands of house-size comets, a scientist said Sunday.
The satellite released Thursday has detected much more water vapor in the upper atmosphere at northern latitudes than current theories predicted, Robert Conway of the Naval Research Laboratory said.
One possible explanation is the Earth is being pelted by snowball-like mini-comets that release large clouds of water vapor into the atmosphere.
"It doesn't mean that's what's happening, but it certainly opens the question and it puts the burden on us to try to explain where the water is coming from," Conway said.
Louis Frank, a University of Iowa physicist, was ridiculed by the scientific community when he proposed the comet-pelting theory in 1986.
Frank's theory was backed up this May by NASA data that shows the Earth is being
bombarded by small comets every minute.
However, an article released this summer by scientists at Cal Tech casts doubt on Frank's hypothesis. Their article points out that the "composition" of water on Earth doesn't match the composition of water found in comets. (Note: water has many forms besides its simple H2O, one of which is called heavy water or deuterium. The deuterium data obtained by Cal Tech is what appears to refute the water-from-comets idea.)
Earth's water probably didn't come from comets, Caltech researchers say
PASADENA -- A new Caltech study of comet Hale-Bopp suggests that comets did not give Earth its water, buttressing other recent studies but contrary to the longstanding belief of many planetary scientists.
In the March 18  issue of Nature, cosmochemist Geoff Blake and his team show that Hale-Bopp contains sizable amounts of "heavy water," which contains a heavier isotope of hydrogen called deuterium.
Thus, if Hale-Bopp is a typical comet, and if comets indeed gave Earth its water supply billions of years ago, then the oceans should have roughly the same amount of deuterium as comets. In fact, the oceans have significantly less.
"An important question has been whether comets provided most of the water in Earth's oceans," says Blake, professor of cosmochemistry and planetary science at Caltech. "From the lunar cratering record, we know that, shortly after they were made, both the moon and Earth were bombarded by large numbers of asteroids or comets.
I want to add my two cents worth of scientific opinion here. I am not in favor of the mini-comet origin of the oceans. My biggest objection to the idea is this: where does the water go? We know that the amount of water on our planet has remained the same for at least a billion years, perhaps longer. How do we remove water from our planet in a way that balances water coming in to our planet?
How is it that scientists can have such widely different opinions? That, my friends, is what science is all about. Active and vigorous discussion of the fragments of data provided by nature makes science work. It is a process for discovering knowledge and debate sharpens our thinking, improves our approaches and eventually reveals something close to the truth.
Let me give an example: Imagine if you had a jigsaw puzzle without the box; in other words, you had no idea what picture was represented by the puzzle. Let's say you found a bunch of blue pieces and got those put together first. One person might think the puzzle represents "the sky", another might think "water" and yet another might think "a blue dress." Next you put together a bunch of white pieces; now someone might think "clouds and sky" or "water and ice" or a "blue-and-white dress." This process would go on until the puzzle was completed and the entire picture revealed (at which time it might be revealed that the puzzle was a famous painting!). The point is that scientists try to put "the pieces of the puzzle" together one-by-one. Until enough data is collected, any conclusions are open to interpretation.
Additional clues as to the origins of our oceans might exist in space. The Clementine mission to the moon revealed images of what looks like ice in the dark craters of the moon's south pole. The subsequent Lunar Prospector Mission provided further tantalizing evidence for water on the moon. So intrigued were scientists by the possibility that they crashed the Lunar Prospector on the lunar South Pole at the end of its mission in July 1999 in hopes of tossing up some water. For the latest findings from this mission [9.3.99], check this link.
NASA scientists also now believe that two of Jupiter's moons contain water. What looks like an icy ocean has been revealed in images of Ganymede taken by Galileo, an unmanned spacecraft studying the outer planets. There is also speculation that Europa houses an ocean of considerable dimensions. These new developments raise all kinds of possibilities, including life on other planets.
In 1996, astronomers discovered water in deep space. Astronomers have long speculated that water plays an important role in the formation of stars and planets but they have been unable to detect water in space because water in our atmosphere interferes with the measurement. The launch of the European Space Agency's Infrared Astronomy Satellite ISO enabled astronomers to detect water in space for the first time a carbon-rich planetary nebula (a fuzzy mass of interstellar gas and dust thought to be a place where planets form) in the constellation of Cygnus (about 2600 light years away). This instrument also detected water vapor -- appropriately -- in the giant star Hydra (which means water monster). Since then, astronomers have been able to observe water in a variety of other sources.
A new radio telescope has found water in the vast clouds of dust and gas
between stars. The Submillimiter-Wave Astronomy Satellite (SWAS), which orbits
400 miles above Earth's obscuring atmosphere, has given astronomers the first
clear view of water in space.
Interstellar dust clouds contain the ashes of previous generations of stars that have lived out their lives and exploded. They are also the nurseries for new generations of stars and planets. Astronomers believe that some of the water on Earth may ultimately have come from the water in interstellar dust clouds. For more information, visit the SWAS mission.
Astronomers consider these observations as a breakthrough in their understanding of the formation of stars and planets. Data from this satellite and others promises to provide keen insights into the complex chemistry that takes place in interstellar space. According to the Swedish Professor Rene Liseau of Stockholm Observatory, "Without exaggeration, these discoveries will be of utmost importance for our understanding of the highly complex chemistry and physics which govern the formation of stars and planets."
And here's another benefit of discovering water in space: human space exploration. Check out this article from Astronomy and Space News:
Human Trip To Water-Rich Asteroid Possible By 2015
By Deborah Zabarenko
ITHACA, N.Y. (Reuters) - Human beings could go prospecting on a tiny, water-rich asteroid and return samples to Earth by 2015, much sooner than any possible human mission to Mars, astronomers said Monday.
The water-rich asteroid -- a lumpy ball about 100 feet in diameter called 1998 KY26 -- could serve as a sort of cosmic filling station for space travelers, ``a space station waiting for occupants,'' according to Steven Ostro of NASA's Jet Propulsion Laboratory.
``We could have a demonstration mission to explore extraction of water ice from this object ... finished within six years if we wanted to, by the year 2006,'' Ostro said. ``We could have a human round-trip mission to KY26 completed by about the year 2015.''
Ostro, speaking at a conference on asteroids, comets and meteors at Cornell University, said such a mission would be in keeping with long term goals of the National Aeronautics and Space Administration.
``This is in the context of NASA's primary goal, which is trying to find missions that merge development in science technology and resource potential,'' he said.
Even though KY26 was first observed as it passed within 500,000 miles of earth in 1998, it posed no threat to humans: highly fragile, it would explode as it entered the upper atmosphere, creating a fabulous light show and sending small pieces drifting harmlessly to the ground.
Ostro and others at a news conference noted recent Hollywood attention paid to the prospect of large asteroids and comets striking earth with catastrophic results, but said quite the opposite would be true in the case of KY26.
``With the bigger objects, we fear them ... we see them as killer asteroids,'' Ostro said. ``From this object's point of view, it should fear us, because from this object's point of view, we will assimilate it.''
This asteroid is thought to be loaded with water, but the water is bound to organic chemicals, possibly including such basic building blocks of life as amino acids and nucleic acids. There is no life on KY26, but the basics for sustaining life could be there.
That means KY26 could be mined for samples, hollowed out and turned into a natural vessel for humans, who would be able to distill life-giving water and oxygen from the asteroid.
Ostro estimated the asteroid could be ``mined'' of enough water and oxygen to keep 1,000 people alive -- or enable many return visits to KY26 and make it a stepping stone to other space missions, possibly including Mars.
He estimated that any spacecraft sent to an asteroid would cost at least $50 million and possibly as much as $100 million.
Facing some skepticism from reporters, Ostro said, ``If you say this is unrealistic, then just forget Mars altogether.''
``Scientifically, that's to me a more interesting mission than the Mars mission, because the primitive materials that are in that asteroid come from the beginning of things,'' David Meisel, a radar astronomer at the State University of New York at Geneseo.
``Mars is a geologically interesting place, but like the earth, it's been turned up and down and sideways ... I personally am very much convinced that a mission to this asteroid would be very very beneficial,'' Meisel said.
NASA's Near Earth Asteroid Rendezvous is set to meet up with asteroid Eros on February 14, 2000, and has already taken pictures of its fast flyby of asteroid Mathilde.
These discoveries and their importance to human exploration may be debated from some time. However, one thing stands clear: our exploration of space helps us to understand our own planet. Water is arguably the most important substance on our planet. Knowing where it came from and where it could go are important questions for our own survival on this planet.
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