Alan Stahler: Searching for the source |

Alan Stahler: Searching for the source

Landscape on Comet Churry, made as Rosetta was falling fast.

Planet Earth was born 4 1/2 billion years ago, when gravity drew together humungous chunks of rock and metal. The chunks smashed together — and stuck.

The young Earth was more radioactive then than now. Radioactivity warmed the baby planet. More heat came from the colliding rock and metal. Earth grew hot, and melted. Rivers of molten metal flowed downward, down to the center, forming our planet’s iron core.

Platinum dissolves in molten iron; so does nickel; so does gold. Nearly all of the Earth’s stock of these metals was hauled down to the core with the iron.

Most of Earth’s gold resides in the core; so whence — from where — came the shiny stuff that drew the 49ers to California? Many believe asteroids later slammed into the solidified Earth, creating a veneer of ore-bearing rock.

Hot enough to melt iron, early Earth boiled off its water. Water vapor mixed into the air, then was blown away – with the rest of the atmosphere – by the sun.

You and I and everything alive are mostly water — whence came that water?

The word planet has a precise definition (why Pluto is no longer considered a true planet). But a world can be anything — anything we can stand on, walk on, swim on, explore.

Comet “Chury” (short for the unpronounceable names of the astronomers who discovered it) is a world, 2 1/2 miles across, 8 miles around. You could walk all the way around this world in a day. It’s a world of mountains and valleys, cliffs and ravines. It’s covered with rock, but just on the surface. Beneath the rock — protected from the sun — is ice.

Comets are rocky, dusty snowballs, made mostly of ice. It would seem logical that comets, smashing into a cooling Earth, might have delivered the water that fills our oceans, our lakes, our bodies.

Until last week, Chury has been under the gaze of the European Space Agency’s (ESA’s) Rosetta spacecraft.

Weather balloons are filled with helium because helium is lighter — less dense — than air. Helium floats in air.

Even lighter than helium is hydrogen — hydrogen is four times lighter than helium.

Well, most hydrogen atoms are four times lighter than helium. But there’s another form of hydrogen only twice as light as helium: heavy hydrogen.

Water contains two atoms of hydrogen, glued to an atom of oxygen: H2O. If one of the hydrogens is heavy, you’ve got heavy water.

Earth’s oceans and lakes and life-forms all contain the same percentage of heavy water, bequeathed by whatever brought that water to us, billions of years ago.

Rosetta measured how much of Chury’s ice is made of heavy water. It turned out to be a lot. There’s way more heavy water in Comet Chury than there is in Earthly water, meaning comets — at least, comets like Chury — did not bring water to Earth.

Rosetta was expected to confirm how Earth got its water. It has, instead, re-opened the debate.

Because a comet is small, its gravity is weak. An orbiting spacecraft would tend to drift away. With its mission over, rather than allow Rosetta to drift, the ESA put Rosetta down on the surface of the comet.

Coming in from space, a landing can be soft, or hard. Landings are softened by parachutes, or rockets, or air bags. But Rosetta last week made a hard landing. As planned, Rosetta crashed, gluing itself to the surface. It will now circle the sun with its comet forever.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages — kid to adult – and may be reached at

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