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For Lori Glaze, Venus sometimes seems as remote as Pluto. A planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, Glaze has spent years pushing for a return to Earth’s nearest neighbor, which NASA hasn’t visited since the early 1990s. In the agency’s latest competition for future Discovery missions—its line of low-cost planetary probes—two out of five final candidates targeted Venus, including a mission Glaze would have led. The odds were good. “I really thought it was time,” she says.

It wasn’t. On 4 January, NASA announced the selection of two asteroid missions, Lucy and Psyche, for launches in 2021 and 2023, respectively (see related article). A third project, a space-based telescope to detect near-Earth objects, will receive design financing for another year. “Basically everything that wasn’t Venus got funded,” says Patrick McGovern, a geophysicist at the Lunar and Planetary Institute in Houston, Texas. The message seemed to be: Venus can wait.

The decision was about scientific merit and risk, says Jim Green, NASA’s planetary division director in Washington, D.C. Psyche and Lucy will use tested instruments on asteroids with no atmospheres to contend with. In contrast, Venus’s thick clouds block most sensors on orbiting spacecraft, and those that venture too far below the cloud deck face sulfuric acid droplets, scalding temperatures, and crushing pressures. Venus may simply be too risky for a $450 million Discovery mission, says Bob Grimm, a geophysicist at the Southwest Research Institute in Boulder, Colorado. “Venus is more complicated than an asteroid.”

But Venus scientists say there is no shortage of tantalizing scientific questions. For example, they want to measure the planet’s inventory of stable noble gases, such as xenon and krypton, which change little in abundance over billions of years and hold clues to planetary origins. Glaze’s mission would have targeted these gases by parachuting an armored ball, studded with instruments, through the atmosphere. Uncertain krypton measures from earlier probes presented two possibilities: Venusian levels could match Earth’s, indicating the two planets probably formed from similar ingredients in the primordial nebula, or they could differ enough to force scientists to toss out their theories on Venus’s origins. As it floated through the last 12 kilometers to the surface, the probe would have sniffed for sulfur and carbon, possible evidence of recent volcanic venting. “A history of Venus is stored in the atmosphere,” says Larry Esposito, a planetary scientist at the University of Colorado in Boulder.

Glaze’s probe would also have imaged mysterious highlands, or tessera, mapped in detail by radar on NASA’s Magellan orbiter in the 1990s. Scientists believe the tessera may have been continents that, several billion years ago, stood out of shallow venusian oceans. Detailed images could reveal the marks of ancient venusian plate tectonics.

Worlds apart

Early in the Space Race, Venus had the most hardware thrown at it (often unsuccessfully), mostly because of Soviet efforts. But in recent years, Mars has gotten all the love.

Although NASA has neglected Venus in favor of regular Mars missions (see graphic, above), Venus Express, a European mission that ended in 2014, observed the motion and structure of Venus’s atmosphere, which whips around the planet 60 times faster than it rotates. Wind speeds increased over 6 years of monitoring, though the drivers remain mysterious. Peering through the clouds in infrared “windows”—wavelengths that penetrate the murk—Venus Express also saw surface hot spots that change in temperature from day to day, suggesting active volcanism. The second Discovery candidate, led by Suzanne Smrekar of NASA’s Jet Propulsion Laboratory in Pasadena, California, would have targeted these hot spots with better sensors and provided much higher resolution radar mapping, helping settle whether Venus is still venting heat from its interior or is geologically dormant.

Venus scientists have not lost all hope. NASA is taking bids until 28 April for its $850 million New Frontiers mission, and Venus is one of the six potential targets. To meet NASA’s requirements, such a mission would probably have to include a short-lived lander and perhaps an orbiter. Glaze and Esposito are both planning bids—and are close-mouthed about what they envision.

If they fail, it could be a decade or more before NASA returns to Venus, perhaps in a joint mission called Venera-D, now under discussion with Russia. A successor to the long line of Soviet Venera probes, which in the 1970s and 1980s returned the only images ever from the surface of Venus, Venera-D would include a lander designed to last a few hours, if not longer. Meanwhile, McGovern laments, “We’re losing a whole generation” of young scientists interested in Venus. 

Venus is unlikely to harbor life or allow human exploration, points of resonance for NASA. But in the past few decades, scientists have found another theme to emphasize: climate change. The venusian atmosphere went to hell thanks to a runaway greenhouse effect, and understanding how that happened could guide humanity’s response to global warming. But, Glaze says, “Under the incoming administration, I’m not sure how strong that argument will play.”

If climate is out, exoplanets are in. Researchers have found a host of Earth-like planets, and are trying to understand what conditions might be like at the surface of a planet with a rocky core and a thick atmosphere. Venus and Earth present a tantalizing study in contrasts. “If you want to understand these planets with atmospheres,” Glaze says, “you have to go into those atmospheres.”

Quelle: Science AAAS