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Space Topics: VenusFacts and Pictures
Venus is Earth’s nearest neighbor in the solar system, and it is also the planet nearest to Earth in size and composition. But telescopic observers of Venus attempting to see the planet’s surface are frustrated by a thick, cloud-filled atmosphere, which permanently hides Venus’ surface from view. The light-reflecting clouds in Venus’ atmosphere, combined with its proximity to both Earth and the Sun, are responsible for its brilliant appearance in the evening and morning sky. Venus is the second brightest object in the night sky, after the Moon. Venus’ atmosphereAmong the terrestrial bodies with atmospheres -- Venus, Earth, Mars, and Titan -- Venus’ is by far the densest. At the surface, the pressure of the atmosphere is 90 times the pressure at Earth’s surface -- or the same as the pressure at a depth of 1 kilometer (3,000 feet) in the ocean. The crushing atmosphere contains 96% carbon dioxide, and 4% nitrogen. Scientists believe that Earth and Venus started out with very similar primordial atmospheres, but the situation now is wildly different:
Scientists believe that a history of widespread, active volcanism on Venus and a runaway greenhouse effect have kept nearly all of its volatile elements up in the air -- and caused most of its hydrogen (and thus water vapor) to be lost to space. Because there is more gas on Venus than Earth, Venus’ atmosphere is taller than Earth’s. Earth’s highest visible clouds are about 12 kilometers (7.5 miles) above the ground, while Venus’ clouds -- composed of sulfuric acid -- accumulate in opaque layers between 50 and 80 kilometers (30 and 50 miles) above the ground. The high elevation of Venus’ atmosphere means that the atmosphere doesn’t “feel” the topography of the surface underneath it nearly to the extent that Earth’s atmosphere does. That’s one reason for the relatively simple global atmospheric circulation patterns that have been observed in Venus’ clouds. Because of Venus’ very slow rotation rate and thick atmosphere, its atmospheric circulation patterns should be dominated by a pattern called Hadley circulation. With the Sun parked over one hemisphere, it creates a hot area on the day side near the equator. That hot air rises, inducing a circulation pattern where high altitude air flows toward the poles, sinks at the pole, and flows along the surface toward the equator. Hadley circulation does happen at Venus, but observers have also discovered that Venus’ atmosphere has a latitude-parallel component as well. In fact, Venus atmosphere super-rotates, meaning that it moves in the same direction as the planet rotates (from right to left in global views). Super-rotating winds in Venus’ atmosphere can reach speeds of 100 meters per second (220 miles per hour) at the cloud tops. Why the atmosphere rotates so fast is another Venus mystery. The “greenhouse effect”Venus is hotter than it should be. At its distance from the Sun, it receives an amount of solar insolation enough to maintain its surface temperature at about the boiling point of water (373 Kelvin, 100 Celsius, 212 Fahrenheit). But radio measurements from the Earth proved that Venus has the hottest solid surface in the solar system, at a constant 750 Kelvin (480 Celsius, 900 Fahrenheit), day and night. That temperature is hot enough to melt lead, and to give Venus’ rocks a warm glow. Where did all the heat come from? The carbon dioxide in Venus’ atmosphere is partially transparent to relatively short-wavelength, visible and near-infrared radiation coming from the Sun. That radiation is absorbed by rocks, which then re-emit the radiation at a longer wavelength (called “thermal” or “mid-infrared”). Carbon dioxide is much less transparent to the thermal radiation, so much of the radiation is bounced back to the planet or absorbed and re-radiated, in part back to the surface, keeping its energy inside the blanket of Venus’ atmosphere. This process is called the “greenhouse effect” even though it’s not why greenhouses keep warm. If Venus ever had any oceans, the greenhouse effect heated them so much that the oceans would have boiled and evaporated long ago. Liquid water is a necessary ingredient in chemical reactions on Earth that trap volatile carbon and sulfur compounds and sequester them in rocks. Without water on Venus, these volatile gases hang out in Venus’ atmosphere, contributing to a runaway greenhouse, where more heat meant more atmosphere, which trapped more heat, and so on. Venus’ surfaceWith its opaque atmosphere, Venus’ surface could not be observed until radio astronomy came of age in the 1960s and 1970s. Now, though, Earth-based radio telescopes and orbiting spacecraft, particularly Magellan, have produced global maps of the surface, and there are a few precious photos from four Venera landers. Venus does not look like Earth. On Earth, the patterns of continents, ocean floors, and chains of mountains and volcanoes are dominated by a process known as “plate tectonics,” in which pieces of the Earth’s crust move independently, colliding and pulling apart, creating geologic activity mostly on plate edges. Venus shows no evidence of plate tectonics. It does not have Earth-like ocean basins and continents. It does have high areas and low areas, but most of its elevation is in the middle. There are plenty of tectonic features -- chains and belts of ridges and deformed plateaus called “tesserae” -- but they don’t seem to tell a systematic story like the Earth’s plate boundaries do. The nature and origin of the tesserae and other tectonic features on Venus are an area of hot debate.
Venus shows plenty of evidence for volcanism everywhere across its surface. Some volcanoes are very large, while some are very small. Small volcanoes can cluster in the hundreds into “shield fields,” which have no equivalent on Earth. (Volcanoes on Earth form roughly linear patterns.) All over Venus, locally low topography appears to be filled with layer upon layer of broad lava flows. While some are small, many lava flows appear to have spread for hundreds or even thousands of kilometers across the surface before solidifying. Even with Venus’ high surface temperature and pressure, it is a great mystery how lava could remain so fluid for enough time to flow such a long distance on the surface.
Images from the Venera landers show that the plains appear to be filled with platy, basaltic volcanic rock, like a recent flow on the flank of a Hawai’ian volcano.
Craters on Venus
Venus has approximately 900 impact craters. The thick atmosphere acts to protect Venus from smaller asteroids, but any sufficiently large asteroid can get through. Over its history, Venus should have been completely covered with large impact craters, like the Moon or Mercury. The relatively low number of craters means that the geologic record of older craters has been destroyed, as is true on Earth. So Venus, like Earth, has been geologically active relatively recently in the past. The number of craters yields an estimate of an average age of the surface of Venus of 500 million years or so. However, this could mean either that the whole surface of Venus convulsed and was replaced in a global volcanic cataclysm 500 million years ago, or that, like Earth, it experiences a more gradual and continuous style of resurfacing, with some parts being geologically active but most parts dormant at any one time, averaging out to a whole-surface replacement rate of 500 million years. No one has ever proved the existence of active volcanism in the present on Venus; whether it’s active or not is a mystery. Radar images of Venus’ craters are particularly beautiful. When an asteroid strikes Venus and tosses material into the air, the thick atmosphere traps most of the crater ejecta close to the crater. It makes a splashy looking bright-colored blanket (as seen in radar images). When the asteroid comes in at a shallow angle, the resulting ejecta blanket often has a symmetrical butterfly-shaped form. But some of the finer ejecta material seems to be tossed high into Venus’ atmosphere, where the prevailing winds can carry it long distances. As the fine ejecta settles out, it can form a dark parabola-shaped deposit, opening in the direction of the prevailing winds, with the crater at its focus. |
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