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Rocks can reveal environment and history

Silvery Venus hangs close to the crescent moon Saturday night. Ruddy Mars lies to the left of the moon Monday night, to the right of the moon on Tuesday.

The first of the Mars Exploration Rovers is on the surface (the second makes planetfall tomorrow). Rover “Spirit” has found a surface covered with rocks – lots of rocks.

Not just any rocks – Mars rocks.



Rocks are made of atoms. Atoms stick to each other in specific ways – one sodium to one chlorine, for instance, to make sodium chloride (table salt); two calciums, a phosphorous and three oxygens make calcium phosphate (bone).

Early chemists envisioned atoms covered with hooks, the number of hooks determining which atoms and how many each could bond with. Today we envision atoms sharing one or more electrons.




A chemical reaction occurs when atoms change partners. A laboratory chemist mixes chemicals in flasks, a manufacturing chemist in huge reaction vessels – giant pots in which atoms are mixed and moistened, cooked and cooled.

A cook stirs the soup to ensure the different flavors can blend and to ensure no part gets too hot or cold, too moist or dry. Chemists must also ensure good mixing, or they will end up with a variable mix of products.

Imagine a reaction vessel 60 billion miles across – the size of the solar system. Such was the giant cloud of gas and dust – the solar nebula – that spun lazily through space some 41Ú2 billion years ago.

The solar nebula collapsed in on itself. Atoms collided and bonded according to their “hooks.” The center of the cloud grew dense enough to form a star – the sun. Around the new star, planets formed.

The solar nebula was not perfectly mixed. Different planets inherited different atoms. They cooked at different temperatures and bathed in different fluxes of radiation from the hot young sun.

The newborn planets were themselves reaction vessels. As in a bootlegger’s still, different atoms moved to different regions. Iron dropped down to form the Earth’s core, taking with it such “iron-loving” (siderophile) atoms as platinum and gold (the core begins 1,800 miles beneath our feet; the deepest hole yet drilled descends seven miles).

As the siderophiles dropped downward, “lithophile” (“rock-loving”) elements floated upward to form a rocky mantle over the core (and a thin rocky crust at the very top). Mantle and crust are enriched in lightweight atoms and those that easily combine with oxygen: silicon, aluminum, uranium.

Atoms continued to sort themselves out. Today we find iron ore here, copper ore there. The rocks are diagnostic of the environment in which they formed. Gypsum, for instance – the stuff of sheet rock and plaster – forms when large bodies of water dry out.

Geologists work out the history of the Earth by studying the rocks. To understand the history of our solar system, we need more rocks.

The Mars Exploration Rover “Opportunity” will make planetfall tomorrow night, close to where the prime meridian (zero degrees east or west) crosses the equator. Studying spectra – how rocks absorb or reflect different colors of sunlight – a team led by Phil Christensen (Arizona State University) has there discovered a large deposit of gray hematite – an iron mineral whose atoms have arranged themselves in a form that (on Earth, at least) usually occurs in lakes or oceans.

Opportunity will attempt to “ground truth” that discovery – to confirm it on the ground – while at the same time adding to our picture of how there came to be a solar system and a planet Earth.

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).


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