CALIFORNIA: It’s too early to tell for sure, but innovative ways to explore the planet and its atmosphere could provide a much more complete picture.
On September 17 a group of scientists using Earth-based telescopes announced that they had detected phosphine in the atmosphere of Venus. On Earth, phosphine is almost always produced by microbial activity. While geological processes are known that can, in principle, produce phosphine, the concentrations of the gas found on Venus, while small, were nevertheless much too high to be attributable to such sources. So how could the phosphine hove gotten there? The simplest explanation is life.
But how is that possible? Venus is nearly a twin for Earth in size and 30 percent closer to the Sun. Because it is closer it is warmer, but if solar distance were the sole consideration, the planet’s temperature would be expected to average about 60°C, with more clement climates near the poles. Thus, through the 1950s, many writers envisioned Venus as a world of steaming jungles, perhaps abounding with creatures comparable to the giant amphibians and reptiles that adorned Earth during its warmer prehistoric past. This glorious picture, however, was ruined forever when NASA’s Mariner 2 probe flew by Venus in 1962 and took measurements showing the planet’s surface temperature to be about 450°C — hot enough to melt lead. Apparently Venus’s thick carbon dioxide atmosphere was doing much more than hiding the planet’s surface from our view. It was creating a “greenhouse effect,” trapping heat beneath its blanket of clouds, with the result being a planet far too hot for life.
Some scientists, however, did not give up hope. While the Mariner findings spelled curtains for Venusian dinosaurs, perhaps other types of life might still have a chance. In 1967, Carl Sagan initiated such speculation by pointing out that while Venus’s surface temperatures would be instantly fatal to any known form of terrestrial life, the atmosphere becomes progressively cooler with altitude. Indeed, at 55 km, where the atmospheric pressure has fallen from the surface’s 90 bar (a bar is the average atmospheric pressure on Earth) to about half a bar, the same as Earth in the high Andes, the temperature is a pleasant 30°C. Anyone can live there; all they need to do is float. Earth’s atmosphere is thick with floating microbes: Couldn’t the same be true on Venus?
Not so fast, it was answered. While it is true that plenty of viable microbes can be found floating about in the Earth’s atmosphere, and even in the high stratosphere under nearly space-like conditions, they don’t actually live there. All microbial metabolism takes place on or below the Earth’s surface. Terrestrial microbes in flight are in suspended animation, just traveling around while taking a break from life.
This is a tough argument to refute, but nevertheless speculation continued, with possible answers being offered by a number of researchers, including notably American planetary scientist David Grinspoon in 1997, British astrobiologist Charles Cockell in 1999, and an international team in 2017. But until the phosphine discovery this week, there were no data supporting any such hypotheses. So what do we do now?
The first thing that needs to be done is to confirm the phosphine detection. The means for this are readily at hand. The European Space Agency’s BepiColombo probe, now on its way to Mercury, will swing by Venus on October 15, coming within 10,000 km of the planet, and pass by again next August 10 at a mere 550 km altitude. It should be able to take the needed measurements. Teams of astronomers with other Earth-based telescopes will no doubt be quickly on the case as well. So in all probability, we will have confirmation or refutation (I bet on confirmation — the phosphine-discovery team included some real heavy hitters) within a year. What next?
NASA has two proposals for Venus missions under current consideration, one of which, the DAVINCI+ mission led by planetary scientist James Garvin, would parachute a probe into Venus’s atmosphere in around 2026, taking measurements during a leisurely several-hour float downward before it expires from heat on the surface. The entrepreneurial Rocket Lab company also plans a parachute-probe mission to Venus, possibly as early as 2023. We could learn a lot from such missions, but I think we can do much better.
The right way to explore Venus is by balloon. Venus’s thick CO2 atmosphere makes ballooning easy; in fact, in the 1980s, the Soviets flew two Vega balloons, taking data over wide stretches of the planet, for about 50 hours. The Vega balloons were filled with helium and floated level at about 55 km altitude. My preferred approach, however, would be the solar Montgolfier balloon technology demonstrated by JPL’s Jack Jones in the 1990s.
A solar Montgolfier balloon is just a black balloon with an open bottom and a vent valve on top. It can be deployed from the air (or from an entry capsule) like a parachute, instantly inflating with air upon release. Absorbing sunlight, the balloon’s black skin then heats up the air inside, providing float gas. Then, if you want to go up, keep the vent valve closed; if you want to go down, open the valve a bit and vent some hot gas out the top, replacing it with cold air from the bottom. These systems are quite controllable. In fact, in 2004 my own company, Pioneer Astronautics, demonstrated the balloons’ ability to be used as soft landers, employing one to set down a payload on the Colorado prairie with an impact velocity of just 5 miles per hour — and then take off again. Similar maneuvers could be done in Venus’s atmosphere.
To explore Venus using such technology, we might send an orbiter carrying several balloon probes, each in its own entry capsule. The orbiter could drop these capsules off, one by one or all at once, and serve as their data-relay satellite. Entering the atmosphere, the capsule would use its heat shield to absorb the reentry heating and then employ atmospheric drag to slow down to subsonic speeds. The back shell of the capsule would then be blown off, and the solar balloon mortared out to be inflated like a parachute, pulling the gondola out of the capsule, which would then drop away. With its black skin heated by solar radiation, the balloon would rapidly heat up the air inside and begin to float, carrying a gondola equipped with scientific instruments, a set of solar panels, a container of water for thermal control, and a UHF radio system for communication with the orbiter. Then the fun begins.
As shown in the table below, Venus’s atmosphere offers temperatures ranging from 460°C to –70°C, depending on the altitude. It’s not clear where in this continuum might be the best place to look for signs of life, so we need to explore as much of it as possible. With a solar balloon, we can.