The planet Venus moves to astronomy's center stage this week with the arrival of the European Space Agency's spacecraft.
The Venus Express orbiter is slated to reach Earth's neighbor early Tuesday morning. Roughly three weeks later, it will reach its operating orbit, where it will spend at least two Venusian days - 486 Earth days - probing the atmosphere in unprecedented detail and piercing the planet's cloudy veil to image the surface. Among the questions scientists would like to answer: Does the thick layer of acidic clouds actually represent a potential habitat for simple forms of life?
Unlike Mars, which has become a magnet for space missions over the past decade, "Venus has been somewhat of a forgotten planet," says Larry Esposito, a planetary scientist at the University of Colorado at Boulder and a member of the Venus Express science team. Last July, NASA established an advisory panel to explore options for its own return to Venus.
The "morning star" is actually closer to Earth than Mars is, and as Earth's twin, Venus raises tantalizing questions about the factors that led such geologically similar planets down such radically different paths.
Venus's layer of highly acidic clouds is some 50 miles thick. Its surface temperature would melt lead. Its atmosphere is 96 percent carbon dioxide. And the air pressure on the surface is 90 times greater than the pressure on Earth.
The factors that make Venus such a tempting scientific target also help explain the tiptoe approach to exploration over the past decade. For landers or rovers to last long enough to make such missions worthwhile, they would have to withstand deep-sea pressures and searing temperatures. They would also need to keep ticking for weeks.
For now, this leaves the field to orbiters, and the European Space Agency just happened to have one on the shelf. The Venus Express craft was built largely from spare parts for the agency's Mars Express orbiter, which is currently returning data from the red planet. One result: Scientists can expect to make similar measurements of two planets simultaneously with the same instruments, making comparisons a bit easier to draw, notes David Grinspoon, a researcher and curator of astrobiology at Denver's Museum of Nature and Science.
Past Russian and US missions have yielded odd differences in sulfur dioxide concentrations in the atmosphere, he continues. Some missions recorded higher levels than others, leading some to suspect they may have indirectly witnessed volcanic activity on the planet, since sulfur dioxide is a byproduct of vulcanism.
Other gases, such as carbon dioxide, break up readily in sunlight, yet CO2 is the atmosphere's main gas. Researchers expect Venus Express to shed more light on the processes that keep such gases abundant. Moreover, says Dr. Grinspoon, the clouds contain some mysterious "absorber" of ultraviolet light.
For planetary geologists like Martha Gilmore, an assistant professor at Wesleyan University in Middletown, Conn., Venus Express should open a window on the surface itself. In the early 1990s, the Magellan spacecraft orbited Venus and built a nearly global map with radar. Venus Express will be able to image it through the clouds by taking advantage of a particular wavelength of light that penetrates the layer.
The current suite of instruments also may be able to distinguish, if barely, different minerals at the surface, she adds. That information could help determine whether Venus has continental crust, similar to Earth's.
Astrobiologists are also casting fresh eyes on Venus. The least controversial questions involve those comparing Earth, Mars, and Venus to better understand the conditions that allow life to take hold, says Grinspoon. One might even find chemical signatures in the atmosphere that indicate the planet once harbored life.
More controversial, but most intriguing, would be the presence of life today, he adds. Several insights over the past few years suggest that life on Venus can't be ruled out yet, some researchers say. On Earth, microbes can thrive quite happily in extremely acidic conditions. Early in Venus's history, the planet appeared to have had an ocean.
The jury certainly is still out on the issue, notes Dirk Schulze-Makuch, a planetary scientist at Washington State University in Pullman. But when he looks at what he dubs "indicators," such as hydrogen sulfide and sulfur dioxide coexisting in the atmosphere when "they shouldn't ... the most straightforward explanation for them is microbial life."
That suggests to him, he says, that the next mission to Venus should be a sample-return mission aimed at scooping particles out of the cloud layer.