An April 10 Science article incorrectly said that Venus's "day" is the equivalent of about 224 Earth days. Venus's "year" (the time the planet takes to orbit the sun) lasts 224.7 Earth days; its day (the time it takes to make one rotation around its axis) equals 243 Earth days. In addition, an accompanying chart expressed the density and surface gravity of Venus and Earth in the wrong units. Density should have been expressed as 5.52 g/cm3 (grams per cubic centimeter) for Earth and 5.24 g/cm3 for Venus. Surface gravity should have been 9.766 m/sec2 (meters per second squared) for Earth and 8.87 m/sec2 for Venus.
Parting the Shroud of Earth's Mysterious Twin
Monday, April 10, 2006
Eons ago, Venus may have been the gentle, tropical paradise that Earthlings once imagined. It was closer to the sun -- but not too close. It was almost Earth-size -- but not quite. And it had plenty of water, even oceans.
But that was then. Sometime in the distant past, the oceans started to heat up and then boiled away. The water vapor hung over the planet like a glove, trapping the heat below and creating a berserk greenhouse effect.
Today, Venus's atmosphere is 97 percent carbon dioxide, and the planet is wreathed in clouds of sulfuric acid. The planet is apparently condemned to an eternal cycle of global warming, with surface temperatures that hover around 900 degrees Fahrenheit.
There are, perhaps, lessons to be learned here. "Venus is very unpleasant," said Hakan Svedhem of the European Space Agency. "We know the greenhouse effect on Earth is a very interesting topic. Maybe with Venus, we can better understand how our own atmosphere works."
Early tomorrow Eastern time, ESA's Venus Express, a honeycombed aluminum spacecraft carrying seven instruments and cloaked in a metallic gold polymer to fend off the heat, is scheduled to begin a 51-minute rocket burn that will settle it into an elliptical polar orbit.
For the next 500 days, with the possibility of extending for another 500, the spacecraft will probe mysteries that have confounded and fascinated scientists since exploration of the planet first began with NASA's Mariner 2 in 1962.
Chief among them is what happened to turn Venus into a child's vision of hell, with a superheated toxic soup of an atmosphere that is 90 times denser at the surface than Earth's -- about the same pressure as the ocean at a half-mile depth.
NASA's Magellan mission, which ended in 1994, used radar to penetrate the cloud cover and map Venus's tortured surface, paved with lava flows and pocked with craters and volcanic mountain escarpments. Venus Express, by contrast, is "geared toward a very detailed study of the atmosphere," said ESA's Don McCoy, the project's manager.
There is a lot to understand. Measurements taken by early probes of Venus have made scientists all but certain that the planet once had extensive oceans that heated up and finally boiled off.
Quite probably the resulting cloud of water vapor provided the initial atmospheric blanket that turned the planet into a hothouse. "But where did [the water] go?" asked University of Michigan planetary scientist Stephen Bougher. "Nobody knows."
Heat could break the water into its constituent atoms, and the hydrogen could easily evaporate from the upper atmosphere and escape into space, but "something different" had to have happened to the heavier oxygen, Bougher said in a telephone interview. One possibility is that a magnetic field induced by the solar wind may have swept charged oxygen particles away from the planet, he said.
Venus Express has an instrument that can measure atmospheric erosion and perhaps provide data that will help scientists reconstruct how Venus lost its oceans.
Over time, carbon dioxide replaced the water vapor, probably as a result of the erupting volcanoes that resurfaced Venus about 700 million years ago and spewed clouds of sulfurous gas into the atmosphere.
Venus Express will search for traces of sulfur dioxide, an indication that volcanoes have been recently active. Scientists may also be able to deduce whether volcanoes are still erupting by using the spacecraft's infrared camera to penetrate the atmosphere and take pictures of Venus's surface.
"By correlating these images with the Magellan data, we can tell whether vulcanism today is altering the landscape," said NASA planetary scientist Adriana Ocampo, the agency's liaison with Venus Express. "This is a key question for Venus, and could be important in understanding climate change on Earth."
Another puzzle that has mystified scientists for decades is Venus's winds, which are negligible on the surface but reach speeds of 220 mph in the upper atmosphere, much faster than the planet rotates. Venus, the slowest-spinning planet in the solar system, has a "day" that is the equivalent of about 224 Earth days.
"There is no reason for the wind," Svedhem, Venus Express's lead scientist, said. The spacecraft will measure wind speeds at various altitudes and correlate them with temperatures. The spacecraft will also gather data on the whirlpool-like atmospheric vortices at Venus's poles, another phenomenon that has no explanation.
"It's really embarrassing how little we know," Bougher said. "The cloud top winds are so strong on a planet that rotates so slowly. Why?"
The $266 million Venus Express launched from Baikonur Cosmodrome, in Kazakhstan, on Nov. 9 last year. It was modeled on ESA's Mars Express, currently in orbit around Mars, and has some instruments that are identical to those on both the earlier spacecraft and on Rosetta, an ongoing ESA mission to a comet.
But unlike Mars Express, built to absorb warmth from a distant sun, Venus Express must shed heat. Besides its reflective coating, it has solar panels that are tiny, compared with those of Mars Express, and half the panels are mirrors. Getting electricity to operate the spacecraft, "is one of the easier aspects of the mission," McCoy said in a telephone interview.
Venus Express weighs 2,734 pounds fully fueled and will have traveled about 250 million miles to chase down Venus when it turns its thrusters in the direction of travel shortly after midnight tonight.
It will be moving at a speed of 18,000 mph relative to the planet and needs to lose 15 percent of its velocity to be captured into orbit. The initial ellipse will extend from 155 miles above Venus's surface to 205,000 miles away, and the spacecraft will need nine days to complete an orbit. McCoy said engineers will tighten the ellipse to a maximum distance of 41,000 miles with two more rocket burns and settle into a 24-hour orbit in about three weeks.
But the tension peaks during the capture burn that begins about an hour after engineers "slue" the stern of the spacecraft to point the thrusters properly. This delicately timed process, if it goes awry, could end up with the spacecraft missing its orbital "window" and careering off into space.
The spacecraft's fate should be decided 38 minutes after the burn begins, when it travels behind the planet, leaving ground controllers without a signal for 10 minutes. "It gives you a good indication of how the burn is going, so you want it to begin at just the right time," McCoy said. "It will be very interesting."