Q & A

Muse, Sep 2005 by Coontz, Robert, Spector, Rosanne

Q Where did the atmosphere come from?

-Justine P., age 13, California

A Rocks and ice, mostly. About 4.5 billion years ago, when the planets condensed out of gas and dust swirling around the sun, the earth's gravity was too weak to capture much gas. Instead, David Catling and Jim Kasting* told me, the gases now in the atmosphere must have arrived in solid form. Some were combined with other atoms in rocks and either bubbled out when the rocks melted or seeped out later through volcanoes. Others, such as water, probably smacked into the earth as big icy chunks that formed farther from the sun, where it was cold enough for chemicals like that to freeze.

Nitrogen gas, which makes up 78 percent of the atmosphere, probably has been here almost since the beginning, Catling and Kasting said. Oxygen, though, appears to have come along about two and a half billion years later, when some bacteria learned to pry it out of water using energy from sunlight (just as plants do today).

-Robert

Q What-scientifically-makes a person's face look older after puberty?

-also Justine P., age 13, California

A It comes down to hormones, as you probably already guessed. Craig Vander Kolk, a surgeon at Johns Hopkins University who repairs kids' facial defects, gave me this explanation. During puberty, hormone levels rise in boys and girls. The hormones trigger your facial bones to grow more than usual. They also cause the spaces inside your face to expand. Boys' faces change more than girls' do. Boys' changes are usually complete by the time they're 19. Girls' faces usually finish changing by 17.

The changes aren't all in the bones, though. The raging hormones increase facial hair growth. (This is mainly a guy thing.) They also set off acne outbreaks. Unlike the other changes, though, this one is just temporary. Phew!

-Rosanne

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* David Catling is an atmospheric scientist at the University of Washington. Jim Kasting is a planetary scientist at Pennsylvania State University.