Photograph courtesy of NASA/JPL/MIT/U. S. Geological Survey
Radar affords a peek at Venus’s dynamic surface. Elevated regions (colorized pink) are dotted with volcanoes, faults, and fractures. Lowlands (blue) are mostly covered with smooth, hardened lava.
Mars is all over the media these days, grabbing attention because of its Earth-like qualities, but if you look just at vital statistics you might wonder if astronomers are focusing on the wrong planet. Venus is much closer to Earth’s size (95 percent as wide), bulk (0.82 times as massive), and distance from the sun (72 percent as far). Glance west after sunset this month and you’ll see that Venus also dwarfs the brightness of Mars, dangling nearby in the sky.
Right now Venus is almost impossible to miss for anyone living north of the equator. As the month opens the planet lies nearly 46 degrees from the sun, so it stands high above the fading twilight. Peaking at magnitude –4.5 toward the end of April, Venus blazes away 10 times as brightly as Jupiter, the next most prominent planet right now. No star even comes close.
Examine this brilliant world more closely and the sister-planet comparisons begin to break down, however. One reason Venus shines so intensely is that it is covered with unbroken sulfuric-acid clouds that reflect sunlight almost as effectively as freshly fallen snow. Beneath the clouds, the planet chokes under a carbon dioxide atmosphere whose surface pressure is 92 times that on Earth. An extreme greenhouse effect traps so much heat that the surface is hotter than an oven set to broil—and the thermometer does not budge. Venusian meteorologists could play the same forecast forever: Overcast tomorrow, with a high of 850 degrees Fahrenheit and a low of 850.
Venus also has the slowest spin of any body in the solar system, taking 243 of our days to rotate once. That sluggishness may explain why, unlike Earth, it has no magnetic field deflecting the wind of charged particles from the sun. The planet’s equator moves at just four miles per hour. A jogger with enough stamina, and heat protection, could avoid seeing nightfall on Venus. Yet its upper-atmosphere winds howl along at hurricane speeds, circling the planet in just four Earth days. Nobody understands the cause of these winds.
|Other than the sun and the moon, Venus is the only celestial body bright enough to cast shadows on Earth. A white background located away from all artificial lights will reveal shadows of Venus to be extremely sharp-edged because the planet is essentially a point source of illumination.|
Radar images created by the Magellan
spacecraft reveal that Venus’s surface is odd too. The oldest impact craters formed around 500 million years ago, which means lava must have flowed not long before then. The big surprise is that the lava poured out everywhere, all at once. On Earth, internal stresses are relieved continually by plate tectonics. On Venus, pressures evidently build up until eruptions engulf the whole world in a molten layer.
The European Space Agency’s Venus Express, set for launch next year, may answer some of the questions posed by Earth’s wayward sister. The spacecraft, which reuses many spare components built for the current Mars Express probe, will examine Venus’s complex weather and atmospheric chemistry. Further ahead, NASA may send a heat-resistant rover to explore the Venusian landscape. Even if there is no hope for life there, and little chance that humans will step on its sizzling surface, Venus is still a fascinating world whose geology may offer clues about why two similar planets developed in startlingly different ways. We can’t ignore it.
This spring, we surely won’t. Every eight years Venus grows unusually prominent, and this apparition will reprise the planet’s fabulous 1996 showing. Earth’s Northern Hemisphere is angled toward Venus just as the planet is farthest from the sun’s glare. As a result, Venus does not set until midnight for most residents of North America, Europe, and Asia. Close meetings with the Pleiades star cluster on
April 3 and with the crescent moon on April 22 and 23 will be dazzling enough to make you forget all about Mars—until the next press conference, at least.