- 4629
- HOW EARTH
GOT WATER? -
Our planet started off bone dry. Then space sent ice balls and 'water
balloons.' Each time you take a sip of
water, you’re imbibing liquid that, in all likelihood, is up to 4.5 billion years
old. Earth is awash in a life-sustaining substance about as ancient as the
planet itself.
-
----------------- ---------------------- 4629 - HOW EARTH GOT WATER?
- Circumstantial evidence suggests that
water-containing meteorites might have pummeled an infant Earth. Those rocky
showers would have helped transform a bone-dry place into a unique wet world.
-
- Although our planet is covered by an
estimated 326 quintillion gallons of H2O, it’s drier than you’d imagine. Earth, which could be as little as 0.023
percent water. Crackers are around 2
percent water. That’s still a lot more moisture than we had at the beginning.
-
- When the solar system first came together,
some of the young planets were too hot for water. Earth and Mars should have formed extremely
dry, due to their locations in the solar system’s frost line.
-
- When the sun was coalescing out of a
collapsing cloud of gas and dust 4.6 billion years ago, its tremendous heat
made a boundary. Outside of it, space was cool enough for ice grains to
solidify. This helps explain why far-out
Jupiter and Saturn have ocean moons.
-
- Inside of it, heat vaporized water. Earth
and the other inner planets clumped together from the dry rock and dense metal
that remained. Something must have happened, some millions of years later, to
nourish those planets with water.
-
- Craters on the surface of our moon indicate
that our side of the frost line was constantly hit with space rocks, including
a particularly violent shower known as the Late Heavy Bombardment. Some experts
think those projectiles, specifically meteorites, the bits of asteroids that
fall to Earth, might have been more like cosmic water balloons.
-
- The hypothesis is supported by the 2010
discovery of a thin crust of frost on asteroid “24 Themis”. NASA found water-bearing clay minerals in
the near-Earth asteroid Bennu during a ground-breaking sample-retrieval
mission.
-
- Still, it’s possible that other processes
were involved in delivering water to Earth, such as gas from the cloudy solar
nebula that dissolved hydrogen into the planet’s magma layer. It’s also
possible that there were multiple sources and steps.
-
- One major clue that gives us an idea of
where the water may be coming from is the type of hydrogen that flows through
our aquatic systems. The most common
form of hydrogen in the universe has a lone proton orbited by an electron.
-
- But there’s a slightly different version
called deuterium with a proton and a neutron squished into the center.
Astronomers have measured the proportion of deuterium to regular hydrogen in
Earth’s water and looked for that “D-H ratio” in other objects around the solar
system.
-
- Carbonaceous chondrites, a kind of
meteorite, are a pretty good match. If our solar system was once an ancient
construction site, think of the chondrites as the unmelted rubble. They hail
from the asteroid belt’s outer section, closer to Jupiter than Mars, which
means they probably formed on the wet side of the frost line.
-
- A single ton of carbonaceous space rocks,
rich in ice and watery minerals, could have delivered 110 to 220 pounds of
water to Earth. When Jupiter and Saturn’s masses “grew big really fast,” the
gas giant kicked those rocks toward the sun and the inner planets.
-
- The meteorites “contain a lot of organic
goop” like carbon and other molecules associated with life. They also hold
volatile materials—substances that evaporate easily when heated—like water,
zinc, and hydrogen from the early days of the solar system. While those can be
found on our planet today, a few volatile materials are still missing.
-
- If the carbonaceous chondrites contributed
Earth’s water, they would have also contributed Earth’s noble gasses. But they don’t support those elements, so
something else must have filled the gap. Comet 67P, closely studied in the
mid-2010s by the European Space Agency’s Rosetta probe and Philae lander, has
the right noble gas content.
-
- This lends to the idea that a bunch of
space bodies hit Earth with noble gasses, H2O.
If most of the water gets contributed by asteroid impacts and most of
the noble gasses are contributed by comets, the elemental math seems to add
up.
-
- Local rocks, “enstatite chondrites” are
meteorites with a similar composition to the original building blocks of Earth.
Because they formed within the inner solar system—on our side of the asteroid
belt—astronomers classify them as “non-carbonaceous.” While they don’t have as
much water as their distant counterparts, they could pack some punch.
-
- Past astrophysics models vastly
underestimated the amount of hydrogen in them, killing off the old idea that
the rocks in Earth’s vicinity were dry. Even cooler, they have a promising D-H ratio.
-
- More recent studies have linked nitrogen
and other volatile elements on Earth to enstatite chondrites. An analysis of
Martian zinc, indicates that debris from the inner solar system transported the
metal to our neighbor. If zinc existed within those meteorites, they probably
carried other volatile materials, specifically, water. Mars had liquid water at
one point and may have some still lurking under an ice cap.
-
- If space rocks brought water to the Red
Planet, could they have done so elsewhere?
The possibility of water and organic molecules being delivered to
planets around other stars, which would give you an environment that could be
conducive to the formation of life.
-
- Putting these lines of evidence together
gives us a recipe that would have involved lots of damp local rocks and a few
of the more distant ice balls. Hydrogen, nitrogen, and zinc isotopes “all tell
the same story” of a wet Earth.
-
- Previously overlooked non-carbonaceous
meteorites probably supplied about 70 percent of the planet’s water, and just a
dash of carbonaceous meteorites touched up its vast blue surface.
-
November 30, 2024 HOW EARTH
GOT WATER? 4629
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------ “Jim Detrick” -----------
--------------------- --- Sunday, December 1,
2024
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