Thursday, July 30, 2009

Venusian Atmosphere

Chemically, the atmosphere of Venus consists mostly of carbon dioxide (96.5 percent). The remainder is mostly nitrogen. These are organic gases, which might lead one to jump to the conclusion that life—some form of life—may exist on Venus. Indeed, during the 1930s, spectroscopic studies of Venus revealed the temperature of the planet’s upper atmosphere to be about 240 K—close to the earth’s surface temperature of 290 K. Some speculated that the environment of Venus might be a dense jungle.
In the 1950s, radio astronomy was used for the first time to penetrate the dense cloud layer that envelops Venus. It turned out that surface temperatures were not 240 K, but were closer to 600 K. Those temperatures are incompatible with any form of life we know. But the outlook got only worse. Spacecraft probes soon revealed that the dense atmosphere of Venus creates high surface pressure—the crushing equivalent of 90 Earth atmospheres—and that surface temperatures actually top 730 K. And what about those clouds?
On Earth, clouds are composed of water vapor. But Venus shows little sign of water.
Its clouds consist of sulfuric acid droplets.

The Sun Sets on Venus (in the East)

As we’ve seen, Mercury’s peculiar rotational pattern can be explained by its proximity to the sun. But no such gravitational explanation is available for the peculiar behavior of Venus. If at 59 days, Mercury rotates on its axis slowly, Venus is even more sluggish, consuming 243 Earth days to accomplish a single spin. What’s more, it spins backwards! That is, viewed from a perspective above the earth’s North Pole, all of the planets (terrestrial and jovian) spin counterclockwise—except for Venus, which spins clockwise.
Nobody knows why for sure, but we can guess that the rotational peculiarities of Venus were caused by some random event that occurred during the formation of the solar system—a collision or close encounter with another planetesimal, perhaps. A violent collision, like the one that formed the earth’s moon, might have started Venus on its slow backward spin.

Forecast for Venus: “Hot, Overcast, and Dense”

Venus’s thick atmosphere and its proximity to the sun are a cruel combination. The planet absorbs more of the sun’s energy (being closer to the sun than the earth) and because of its heavy cloud cover, is unable to radiate away much of the heat. Even before astronomers saw pictures of the planet’s surface, they knew that it would not be a welcoming place.
Until the advent of radar imaging aboard space probes such as Pioneer Venus (in the late 1970s) and Magellan (in the mid-1990s), the surface of Venus was a shrouded mystery. Optical photons bounce off the upper clouds of the planet, and all we can see with even the best optical telescopes is the planet’s swirling upper atmosphere. Modern radio imaging techniques (which involve bouncing radio signals off the surface) have revealed a Venusian surface of rolling plains punctuated by a pair of raised land masses that resemble the earth’s continents. Venus has no coastlines, all of it’s surface water having long ago evaporated in the ghastly heat. These land masses, called Ishtar Terra and Aphrodite Terra, are plateaus in a harsh waterless world.
The Venusian landscape sports some low mountains and volcanoes. Volcanic activity on the surface has produced calderas (volcanic craters) andcoronae, which are vast, rough, circular areas created by titanic volcanic upwellings of the mantle.
Venus is surely lifeless biologically, but geologically it is very active. Volcanic activity is ongoing, and many astronomers believe that the significant, but fluctuating, level of sulfur dioxide above the Venusian cloud cover is the result of volcanic eruptions. Probes sent to Venus thus far have not detected a magnetosphere; however, astronomers still believe that the planet has an iron-rich core. Scientists reason that the core of Venus might simply rotate too slowly to generate a detectable magnetic field.