By Patrick Clinton
Why are planets basically round? You'd think they'd be more irregularly shaped, much like meteors.
--Hopie Brooks, Gastonia, South Carolina
Imagine you're building a stone tower, miles into the sky. At some point, you'll come up against the fundamental limitations of the materials you're working with: Your Tower of Babel will crumble from its own weight, the ground will sag beneath it--it simply won't get any taller. Something similar happens with celestial bodies. In effect, once a clod of
space rock gets up to a certain size, its gravity causes it to cave in on itself, smoothing out the biggest bumps, pocks, and other surface irregularities. Smaller rocks, like asteroids and meteors, don't have the same gravitational forces to contend with. "A celestial rock that's only 200 miles across can be quite irregular," notes David Morrison of NASA's
Ames Research Center. "But you'll find that once asteroids get up to, say, 700 miles across, they're usually pretty spherical." If Earth were made entirely of gases and liquids, its gravity would pull everything down to a smooth, even surface. But solid rock, to varying degrees, can withstand gravity's pull--which explains why, despite its overall orblike
shape, Earth tolerates mountains, valleys, hills, and other idiosyncrasies. Morrison says that the maximum difference that can be sustained between the highest and lowest points on Earth is about 15 miles. That maximum differential has been violated over the ages--but never for long. For example, scientists believe that billions of years ago Earth was
struck by a giant celestial body approximately the size of Mars. This cosmic impact, which is thought to have created our Moon, would have left one hell of a dent in our planet--yet there's no sign of it now.
What's alpenglow? Is it a scientific phenomenon, or a poetic term?
--Jack Meyers, Houston, Texas
It's scientific, but not very. The word has a vaguely medieval ring to it, but "alpenglow" was actually coined in 1871. It comes from the German alpen, "mountain," and glute, "fire," and describes the brilliant rosy light that you find when a sunrise or sunset reflects on a mountain peak. It can
make for a stunning après-ski effect to be sitting down in a dark valley, where the sunset has long since disappeared from view, while the west-facing crags above are ablaze with reds and pinks.
Alpenglow is most striking on snow-covered peaks, where all that white only intensifies the fiery hues. We have a similar phenomenon in Chicago, where I live-- Searstowerglow, it's called. Plate glass, we've found, has a poetry all its own.
In March the snow fleas appear in New England. What are they? What do they eat?
--J. Lumbard, Hollis, New Hampshire
Snow fleas are insects, but they aren't really fleas. They belong to the order Collembola, the springtails, whose name derives from an odd-looking forked appendage tucked under the body that allows them to hop around. Springtails are amazingly common, with something like 3,500 species found everywhere from pole to pole, living in soil, in leaf litter,
and on the surface of water, as well as on snow. And they're around pretty much all the time; it's just that their dark bodies are more conspicuous against a white background.
Unlike real fleas, snow fleas don't feed on blood and thus do not bite people (or dogs). They subsist on wind-carried pollen, fungi, spider dung, dust particles, and certain forms of algae that grow on the surface of snow. Uncanny survivors, they produce an antifreeze-like compound that prevents their body cells from rupturing in extreme cold.
By the way, snow fleas aren't the only creepy-crawlies lurking in your snow: There are also winter stone flies, spiders, ice worms--on and on it goes. So next time you're tempted to scoop up a thirst-quenching mouthful of "virgin" powder, think twice.
March will ride in on a comet: Hale-Bopp, one of the most spectacular of the late twentieth century and certainly one of the most exciting astronomical events of the year. The comet will come into view at twilight on the second and will continue to be visible through the middle of April. The new Moon, on the eighth,
will coincide with a partial eclipse of the Sun, visible in northwestern North America. The vernal equinox occurs on the 20th at 9:55 a.m. eastern time, marking the official start of spring in the Northern Hemisphere. This month's full Moon, on the 23d, will be accompanied by a partial lunar eclipse, climaxing with about 92 percent of the
satellite obscured at 11:39 p.m. eastern time.
--David N. Schramm
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