BepiColombo spacecraft records the sound of solar wind at Venus

The Mercury-bound BepiColombo spacecraft listened to the sound of the solar wind at Venus as it flew just 340 miles (550 kilometers) above the planet's surface during a maneuver designed to adjust its path. BepiColombo, a joint mission by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), recorded the audio with its magnetometer instrument, providing a rare glimpse into the interaction between the stream of charged particles flowing from the sun, known as solar wind, and the thick carbon dioxide-rich atmosphere of Earth's closest planetary neighbor. The audio is not the actual sound that could be heard in space but a so-called sonification, a translation of data into sounds, ESA said in a statement. BepiColombo passed by Venus on Aug. 10, just one day after another inner-solar-system explorer, Solar Orbiter, made its own close approach. This coincidence enabled scientists for the first time to make measurements of the environment around Venus from multiple points. Solar Orbiter, a joint mission by ESA and NASA, has a similar magnetometer in its instrument suite as BepiColombo. It has made its own measurements of the interactions between the solar wind and the planet as it zipped by at a distance of nearly 5,000 miles (8,000 km) on Aug. 9. Flybys are a common maneuver used by spacecraft operators to adjust the trajectory of a spacecraft. By flying close to a planet or another celestial body with a strong gravitational pull, the spacecraft loses or gains energy, which helps "slingshot" it toward its destination in the most fuel-efficient way.

The European/Japanese spacecraft BepiColombo has taken a selfie with Venus during its close flyby at the planet in August 2021. (Image credit: ESA)

Researchers are still analysing the data gathered by both spacecraft and hope that the Japanese mission Akatsuki, the only orbiter currently studying Venus, could contribute as well.

"This was the first time we could obtain such multi-dimensional measurements of the environment around Venus," Johannes Benkhoff, ESA BepiColombo project scientist, told Space.com. "That could enable us to see, for example, how the solar wind interacts with the planet and its atmosphere and how fast the processes are."

Mercury-bound BepiColombo has performed two flybys at Venus during its seven-year cruise to Mercury. (Image credit: ESA)

A detailed look at the composition of the atmosphere


BepiColombo could provide especially valuable data as the spacecraft swung closer to the surface of Venus during this flyby than Akatsuki gets at the closest point in its orbit around Venus.

According to Benkhoff, BepiColombo's Mercury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) instrument could therefore make unprecedented measurements of the middle layers of Venus's thick and cloudy atmosphere, known for its out-of-control greenhouse effect.

"We can look for carbon dioxide, sulphur dioxide and other aerosols, which has not been done with this type of instrument before," Benkhoff said. "There hasn't been a European mission to Venus since Venus Express [which lost contact with Earth in 2014]. We hope to make some measurements with BepiColombo that could be compared to the Venus Express measurements to see how things have changed."

For example, Benkhoff added, changes in concentrations of sulfur dioxide could indicate changes in the volcanic activity on the planet's surface.

ESA said in the statement that the MERTIS instrument captured high-resolution spectra of the atmosphere of Venus that are similar to those obtained by the early 1980s Soviet Venera 15 mission. No other spacecraft has made such detailed measurements since, ESA said.

An artist impression of BepiColombo flying by Venus on Aug. 10, 2021. (Image credit: ESA/ATG medialab)

Finding life on Venus?


There has been a revival in the interest in Venus following last year's surprising indications that the boiling planet might harbor life.

In September 2020 a team of scientists from the U.K. announced that they had detected phosphines, organic compounds that are usually produced by bacteria, in the planet's sulfur-rich clouds. The conclusions were based on measurements obtained by Earth-based telescopes. This year, however, a study co-authored by astrobiologist Chris McKay, of the NASA Ames Research Center in California, concluded that the amount of water in the atmosphere of Venus is so low that it is impossible for any life to exist there.

Benkhoff said that BepiColombo is unlikely to solve the ongoing dispute, even though it will look for phosphines in the atmosphere.

"Our MERTIS instrument is in principle able to detect phosphines," Benkhoff said. "But we don't think that it is sensitive enough to detect the low amounts that are expected at Venus."  

Getting ready for Mercury

The Aug. 9 flyby was the third of the overall nine required for BepiColombo to approach Mercury in the right way so that it can insert its two orbiters, the European Mercury Planetary Orbiter and the Japanese Mercury Magnetospheric Orbiter, into their correct orbits.

The innermost planet of the solar system is notoriously difficult to reach as the spacecraft has to continuously brake against the gravitational pull of the sun. For BepiColombo, this braking is achieved with the help of the gravity-assist flybys.

BepiColombo performed its first flyby at Earth in April 2020. Six months later, it made its first visit to Venus, passing at a much greater distance of 6,650 miles (10,700 km). On Oct. 1, the spacecraft will take its first look at Mercury from a distance of merely 125 miles (200 km). There will be five additional Mercury flybys to prepare BepiColombo for entering the planet's orbit in 2025.

The close Venus flyby, Benkhoff said, provided the first opportunity to test the spacecraft's instruments at a distance at which they will operate at Mercury.

"Our instruments were designed for orbiting Mercury at 400 to 1,500 kilometers," or 250 to 930 miles, Benkhoff said. "This Venus flyby provided us with the perfect opportunity to prepare not only for the mission but also for the upcoming first Mercury flyby."

BepiColombo has captured a sequence of photographs during the Venus flyby, which were released by ESA as a short video. The images were obtained by three low resolution 'selfie cameras' mounted on BepiColombo's propulsion module. The high albedo, or reflectiveness. of Venus, however, made it impossible for the cameras to capture any details of the planet's clouds.

The much darker Mercury, which lacks an atmosphere, will present a better photo opportunity, Benkhoff suggested.

"Venus was unfortunately quite overexposed in the images," Benkhoff said. "But we hope that at Mercury, even the selfie cameras might be able to identify some structures on the surface of the planet."

BepiColombo is fitted with a high-resolution stereoscopic camera, but that cannot be used during the cruise phase because of the spacecraft’s configuration in transit. The two orbiters and the propulsion module are stacked on top of each other, which blocks some of the instruments.

The October Mercury flyby will mark the first occasion any spacecraft will have visited the smallest and innermost planet of the solar system since the demise of the NASA mission Messenger in 2015.

NASA selects two Venus missions for Discovery program

For the first time in more than three decades, NASA has announced it will send a robotic mission to Venus, selecting two proposals in the latest round of its Discovery program. NASA Administrator Bill Nelson announced at the end of a “State of NASA” speech at NASA Headquarters June 2 that the DAVINCI+ and VERITAS missions will launch to Venus in the late 2020s, having beat out competing proposals for missions to Jupiter’s volcanic moon Io and Neptune’s large moon Triton that were also selected as finalists in early 2020. “These two sister missions both aim to understand how Venus became an inferno-like world capable of melting lead at the surface,” Nelson said. “They will offer the entire science community the chance to investigate a planet we haven’t been to in more than 30 years.” DAVINCI+, or Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging, will send a probe into the planet’s atmosphere, measuring noble gases and other elements that can provide information on how its runaway greenhouse effect developed. Cameras on the descent probe will provide high-resolution images of geological features known as “tesserae” that may be similar to Earth’s continents. VERITAS, or Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy, will map the planet from orbit using a synthetic aperture radar system. It will also search for infrared emissions that could help scientists determine if there is active volcanism.

“It is astounding how little we know about Venus, but the combined results of these missions will tell us about the planet from the clouds in its sky through the volcanoes on its surface all the way down to its very core,” said Tom Wagner, NASA’s Discovery program scientist, in a statement. “It will be as if we have rediscovered the planet.”

DAVINCI+ will be led by NASA’s Goddard Space Flight Center, while VERITAS will be run by the Jet Propulsion Laboratory. Lockheed Martin will build the spacecraft for both missions.

Each mission has an estimated cost of $500 million, with launches expected between 2028 and 2030. Launch contracts will be awarded later in each mission’s development.

In a briefing with reporters after the State of NASA speech, Thomas Zurbuchen, NASA associate administrator for science, said all four proposals, including the Io Volcano Observer and Trident mission to Triton, rated well. “This is not the case where one of these missions had a massively bigger risk. It’s not the case that one of them was ground-ruled out in some way,” he said.

While both DAVINCI+ and VERITAS are going to Venus, he noted that the two missions are “massively different” in their science, one focusing on the atmosphere and the other the surface. “In the end, those two swung, in terms of science return and programmatic match, to the top,” he said. “Those were the best missions, and that’s why we selected them.”

The two winning missions are updated versions of proposals that were finalists in the previous round of the Discovery program. NASA instead selected two asteroid missions, Lucy and Psyche, in early 2017.

Having been through the competition before helped DAVINCI+ and VERITAS. “I hardly recognized the proposal,” he said of VERITAS in particular. “The science was just so much better. The ratings were so much better than the previous round.”

Both missions will host technology demonstrations in addition to their primary science payloads. VERITAS will host an updated version of a deep space atomic clock first flown on an Earth-orbiting spacecraft in 2019 that will assist in radio science observations and autonomous spacecraft maneuvers. DAVINCI+ will fly a new ultraviolet imaging spectrometer.

The missions will be the first NASA missions dedicated to Venus since the Magellan radar mapper orbiter, launched in 1989. Magellan’s mission ended in 1994, and while scientists have been able to participate in European and Japanese Venus orbiter missions, or take advantage of limited observations by other spacecraft flying by Venus, many researchers had been advocating for years for a new Venus mission.

“In the science community, I can tell you, this is resonating,” Nelson said in the post-speech briefing. “They’re excited about this.”

Other countries and even private ventures are planning missions to Venus. Russia has been working for several years on a mission concept called Venera-D that includes an orbiter and lander, which could feature some NASA participation. India is developing a Venus orbiter mission called Shukrayaan scheduled for launch in 2024. The European Space Agency is considering a Venus orbiter called EnVision for its next medium-class mission.

Perhaps most intriguingly, Rocket Lab, the small launch vehicle and smallsat developer, has been leading efforts to send a small mission to Venus. That mission has been motivated in part by the potential discovery of phosphine in the planet’s atmosphere, a gas that could be evidence of life. Scientists announced in September 2020 the potential detection of phosphine based on analysis of ground-based infrared and microwave observations, but other scientists have questioned that detection.

Peter Beck, Rocket Lab’s chief executive, has said on numerous occasions that the company is working on a smallsat mission that would plunge into the atmosphere to search for phosphine or other biosignatures. That could launch as soon as 2023, although the company has provided few details about the mission’s development.

“Venus is the most underrated planet in our solar system,” he said during a May 27 webinar by Bessemer Venture Partners, citing the potential lessons from that planet’s runaway greenhouse that could apply to Earth. “I think we can learn a tremendous amount, scientifically, from Venus.”

Back to Venus armed with laboratory findings

Venus's impenetrable atmosphere has long made it difficult to conduct a thorough investigation of our neighbouring planet. In a step forward, by conducting laboratory experiments scientists from the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) have now developed a way of determining the nature of the planet's surface using new instruments from orbit. The entire surface of Venus can now be mapped mineralogically for the first time, addressing a large gap in planetary research. Venus is Earth's sister planet. It is almost exactly the same size and orbits on average only 40 million kilometres closer to the Sun. However, the two planets developed in very different ways. On Earth, continents formed, separated by oceans. Then some three and a half billion years ago, life emerged under its atmosphere and evolved into the vast variety of organisms that we know today. Things happened very differently on Venus, which is surrounded by an atmosphere of gas a hundred times thicker than that of Earth. Within it, the extreme greenhouse effect results in a constant surface temperature of 470 degrees Celsius - a temperature at which water would instantly evaporate and even lead would melt. The planet is permanently enveloped in thick clouds of sulphuric acid, making it impossible for telescopes on Earth or instruments on board spacecraft to acquire even a glimpse of the surface. Scientists have managed, however, to map its landscape using radar. And now, through a series of laboratory experiments, DLR researchers have developed a new method for determining the nature of the planet's surface from orbit.

"For a good ten years, we have been using a unique laboratory facility to measure the emission properties of various rocks of the kind we might expect to find on Venus under the same extreme conditions that prevail on the planet," says Jorn Helbert, Head of DLR's Planetary Spectroscopy Laboratory (PSL) and lead author of a research paper that has now been published in Science Advances magazine. "The reflectance and emissivity of rocks change when they are exposed to the high temperatures that you find on Venus. As a result, spectral profiles measured at terrestrial temperatures cannot simply be applied there. But now, we have a tool that we can use as the basis for new instruments on the next planned missions to Venus that will finally allow us to determine which types of rock exist there."

Concordance with existing surface measurements from Venus
The four-person research group made up of researchers from DLR, the Planetary Science Institute in Tucson, Arizona, and Mount Holyoke College in Massachusetts have used the results of their laboratory experiments to devise a new 'spectral library' for various types of rock. "Determining the emission spectra in this way has enabled us to reconstruct the iron oxide content at the landing site of the Soviet Union's Venera 9 and Venera 10 landers for the very first time," says DLR Planetary Scientist Alessandro Maturilli. "In 1975, the two landers transmitted images from Venus and provided important measurements. However, they were not equipped with an instrument that could directly measure iron oxide content."

The two landers provided the only direct spectral measurements of Venusian rocks. The emission profiles determined in the laboratory and the spectra determined by the Venera missions are in very good agreement. "As such, we have demonstrated the accuracy of our new method, which represents a big step forward," says Jorn Helbert. Although further Venus lander missions are currently under discussion, global mapping of the planet can only be conducted from orbit. However, the atmosphere of Venus is impenetrable to wavelengths of visible light - those that the human eye can see. To make mapping of the surface possible despite the planet's atmosphere, scientists are focusing on what are known as 'atmospheric windows'. These are narrow bands of wavelengths at which the atmosphere of Venus is transparent, and thus allow a view of the surface. Five such 'windows' exist at wavelengths close to 1000 nanometres (one micrometre). These wavelengths are in the near-infrared, which is adjacent to the visible portion of the electromagnetic spectrum (approximately 400-700 nanometres).

Venus returns to the focus of planetary research
Through their experiment, the researchers have shown that for rock types measured in the laboratory, the spectra and their characteristic profiles can be used to reliably identify them from orbit. "Our aim is now to create the first global map of rocks on Venus," says Helbert. "That would be a major achievement! After all, we know far too little about Venus. In its infancy, the planet may have had water, like Earth, and may also have been less hostile to life." The scientists are looking to use the Venus Emissivity Mapper (VEM) instrument to implement their plan, mapping emissions in the few atmospheric windows available at near-infrared wavelengths. VEM could be installed on the EnVision mission of the European Space Agency (ESA) and on NASA's VERITAS orbiter later this decade.

Venus, Earth's celestial neighbour and often described as its sister planet, was the first target for interplanetary spacecraft. The Soviet mission Venera 1 launched in 1961 and the American Mariner 2 in 1962. Despite a number of spectacular successes, particularly the Soviet space programme's initial orbiters and then eight landings between 1970 and 1983, the planet named after the Roman goddess of love later fell somewhat out of favour in planetary research. Between 1990 and 1994, NASA's Magellan orbiter mapped Venus using radar, revealing myriad details of its multifaceted surface in high-resolution. From 2006 to 2015, ESA's Venus Express mission investigated the planet across a series of seven experiments.

However, we still know little about the nature of the planet's surface. For Earth's other immediate neighbours, the Moon and Mars, determining rock types, mineralogical composition and the abundance of chemical elements was far easier. As a result, nowadays we have a fairly thorough understanding of them. The astronauts involved in the Apollo missions and the robotic missions of the Soviet Union returned 400 kilograms of sample material from the Moon to Earth. Half a dozen landers have been able to analyse the rocks on Mars. A landing on Venus would be a heroic feat, as the lander descends into an increasingly hot oven. The high temperatures and atmospheric pressure - which reaches 92 bar on the ground (equivalent to the pressure at 900 metres underwater on Earth) - would put the electronics under immense strain. The Venera 13 lander was able to withstand these conditions for the greatest amount of time so far and, on 30 October 1981, succeeded in transmitting data from the furnace-like surface of Venus for almost two hours. The new method developed in this laboratory study would allow the surface of Venus to be studied systematically from orbit over a period of many years, without having to run the risk of a landing.

Return to Venus on Indian Space Mission

For the second time the Swedish Institute of Space Physics (IRF) will explore Venus. On board the Indian Venus mission Shukrayaan-1, IRF's satellite instrument Venusian Neutrals Analyzer (VNA) will study how the charged particles from the Sun interact with the atmosphere and exosphere of the planet. Between 2006-2014 IRF's instrument ASPERA-4 (Analyzes of Space Plasma and EneRgetic Atoms) studied Venus on board the European spacecraft Venus Express. The satellite instrument measured the plasma properties around Venus in detail, as well as neutral atoms that escape into space from the atmosphere. ASPERA-4 consisted of four different sensors to analyze energetic neutral atoms (ENAs) and plasma particles. By analyzing ENAs, the instrument contributes to our knowledge of the interaction between the charged particles from the sun (solar wind) and the atmosphere of Venus. Associate Professor Yoshifumi Futaana, IRF, is the Principal Investigator responsible for both ASPERA-4 and now the VNA instrument that will explore the brightest planet in the sky. "Venus is the twin of our Earth, but these planets are very different. The atmosphere is dense and hot, but it is waterless. Water existed 4 billion years ago when Venus was formed, but it has been lost, probably to space. ASPERA-4 proved that water on Venus is escaping to space with energy obtained from the solar wind. Our results also provide essential information to help us understand exoplanets around other stars, planets which we cannot explore with instruments in place around them," says Yoshifumi Futaana.

IRF's satellite instrument on Shukrayaan-1 will be part of a larger Indian instrument package and will be the ninth generation of IRF's series of miniatured ion and ENA instruments. The first generation was named SARA (Sub-keV Atom Reflecting Analyzer) and was launched on board the successful Indian spacecraft Chandrayaan-1 that explored the Moon in 2008-2009.

SARA consisted of two sensors. One was a detector for energetic neutral atoms and the other was an instrument to measure the flow of ions in the solar wind. The instrument studied how the plasma around the Moon interacts with the moon where the surface is not protected by an atmosphere or a magnetic field.

For the first time ever, SARA could investigate energetic atoms that are knocked from the lunar surface when they are hit by the solar wind.

The SARA experiment was the first collaborative project between IRF and the Indian Space Research Organization (ISRO). The new Venus mission means that the collaboration between IRF and ISRO continues.

IRF's director Professor Stas Barabash was the Swedish Principal Investigator of the SARA experiment, and is the lead Co-Investigator of the VNA instrument.

"The SARA experiment was truly an exciting mission which revealed a number of unknown secrets about how the Moon interacts with space. Collaboration with the Indian team was the key of this successful experiment. The next target of our joint investigation is Venus in order to understand why the Earth Earth and Venus, which began as twins, evolved so differently," says Stas Barabash.

NASA wants your help designing a Venus rover concept

NASA's Jet Propulsion Laboratory in Pasadena, California, under a grant from the NASA Innovative Advanced Concepts program, is running a public challenge to develop an obstacle avoidance sensor for a possible future Venus rover. The "Exploring Hell: Avoiding Obstacles on a Clockwork Rover" challenge is seeking the public's designs for a sensor that could be incorporated into the design concept. Venus is an extreme world. With a surface temperature in excess of 840 degrees Fahrenheit and a surface pressure 90 times that of Earth, Venus can turn lead into a puddle and crush a nuclear-powered submarine with ease. While many missions have visited our sister planet, only about a dozen have made contact with the surface of Venus before quickly succumbing to the oppressive heat and pressure. The last spacecraft to touch the planet's surface, the Soviet Vega 2, landed in 1985. Now, engineers and scientists at JPL are studying mission designs that can survive the hellish landscape. "Earth and Venus are basically sibling planets, but Venus took a turn at one point and became inhospitable to life as we know it," said Jonathan Sauder, a senior mechatronics engineer at JPL and principal investigator for the Automaton Rover for Extreme Environments (AREE) concept. "By getting on the ground and exploring Venus, we can understand what caused Earth and Venus to diverge on wildly different paths and can explore a foreign world right in our own backyard."

Exploring and studying different geologic units across the surface of Venus could help us understand the planet's evolution, and could contribute to a better understanding of Earth's climate.

Powered by wind, AREE is intended to spend months, not minutes, exploring the Venus landscape. AREE could collect valuable, long-term longitudinal scientific data. As the rover explores the planet, it must also detect obstacles in its path, such as rocks, crevices and steep terrain. And NASA is crowdsourcing help for that sensor design. The challenge's winning sensor will be incorporated into the rover concept and could potentially one day be the mechanism by which a rover detects and navigates around obstructions.

The difficulty of this challenge is in designing a sensor that does not rely on electronic systems. Current state-of-the-art electronics fail at just over 250 degrees Fahrenheit and would easily succumb to the extreme Venus environment. That is why NASA is turning to the global community of innovators and inventors for a solution.

"This is an exciting opportunity for the public to design a component that could one day end up on another celestial body," said Ryon Stewart, challenge coordinator for the NASA Tournament Lab at the agency's Johnson Space Center in Houston. "NASA recognizes that good ideas can come from anywhere and that prize competitions are a great way to engage the public's interest and ingenuity and make space exploration possible for everyone."

Participants will have an opportunity to win a first-place prize of $15,000. Second place wins $10,000; and third place, $5,000. JPL is working with the NASA Tournament Lab to execute the challenge on the heroX crowdsourcing platform. Submissions will be accepted through May 29, 2020.

"When faced with navigating one of the most challenging terrestrial environments in the solar system, we need to think outside the box," Sauder said. "That is why we need the creativity of makers and garage inventors to help solve this challenge."

Is there life adrift in the clouds of Venus?

In the search for extraterrestrial life, scientists have turned over all sorts of rocks. Mars, for example, has geological features that suggest it once had - and still has - subsurface liquid water, an almost sure prerequisite for life. Scientists have also eyed Saturn's moons Titan and Enceladus as well as Jupiter's moons Europa, Ganymede and Callisto as possible havens for life in the oceans under their icy crusts. Now, however, scientists are dusting off an old idea that promises a new vista in the hunt for life beyond Earth: the clouds of Venus. In a paper published online in the journal Astrobiology, an international team of researchers led by planetary scientist Sanjay Limaye of the University of Wisconsin-Madison's Space Science and Engineering Center lays out a case for the atmosphere of Venus as a possible niche for extraterrestrial microbial life. "Venus has had plenty of time to evolve life on its own," explains Limaye, noting that some models suggest Venus once had a habitable climate with liquid water on its surface for as long as 2 billion years. "That's much longer than is believed to have occurred on Mars."

On Earth, terrestrial microorganisms - mostly bacteria - are capable of being swept into the atmosphere, where they have been found alive at altitudes as high as 41 kilometers (25 miles) by scientists using specially equipped balloons, according to study co-author David J. Smith of NASA's Ames Research Center.

There is also a growing catalog of microbes known to inhabit incredibly harsh environments on our planet, including the hot springs of Yellowstone, deep ocean hydrothermal vents, the toxic sludge of polluted areas, and in acidic lakes worldwide.

"On Earth, we know that life can thrive in very acidic conditions, can feed on carbon dioxide, and produce sulfuric acid," says Rakesh Mogul, a professor of biological chemistry at California State Polytechnic University, Pomona, and a co-author on the new paper. He notes that the cloudy, highly reflective and acidic atmosphere of Venus is composed mostly of carbon dioxide and water droplets containing sulfuric acid.

The habitability of Venus' clouds was first raised in 1967 by noted biophysicist Harold Morowitz and famed astronomer Carl Sagan. Decades later, the planetary scientists David Grinspoon, Mark Bullock and their colleagues expanded on the idea.

Supporting the notion that Venus' atmosphere could be a plausible niche for life, a series of space probes to the planet launched between 1962 and 1978 showed that the temperature and pressure conditions in the lower and middle portions of the Venusian atmosphere - altitudes between 40 and 60 kilometers (25-27 miles) - would not preclude microbial life. The surface conditions on the planet, however, are known to be inhospitable, with temperatures soaring above 450 degrees Celsius (860 degrees Fahrenheit).

Limaye, who conducts his research as a NASA participating scientist in the Japan Aerospace Exploration Agency's Akatsuki mission to Venus, was eager to revisit the idea of exploring the planet's atmosphere after a chance meeting at a teachers' workshop with paper co-author Grzegorz Slowik of Poland's University of Zielona Gora. Slowik made him aware of bacteria on Earth with light-absorbing properties similar to those of unidentified particles that make up unexplained dark patches observed in the clouds of Venus. Spectroscopic observations, particularly in the ultraviolet, show that the dark patches are composed of concentrated sulfuric acid and other unknown light-absorbing particles.

Those dark patches have been a mystery since they were first observed by ground-based telescopes nearly a century ago, says Limaye. They were studied in more detail by subsequent probes to the planet.

"Venus shows some episodic dark, sulfuric rich patches, with contrasts up to 30-40 percent in the ultraviolet, and muted in longer wavelengths. These patches persist for days, changing their shape and contrasts continuously and appear to be scale dependent," says Limaye.

"Venus has had plenty of time to evolve life on its own," explains Limaye, noting that some models suggest Venus once had a habitable climate with liquid water on its surface for as long as 2 billion years. "That's much longer than is believed to have occurred on Mars."

The particles that make up the dark patches have almost the same dimensions as some bacteria on Earth, although the instruments that have sampled Venus' atmosphere to date are incapable of distinguishing between materials of an organic or inorganic nature.

The patches could be something akin to the algae blooms that occur routinely in the lakes and oceans of Earth, according to Limaye and Mogul - only these would need to be sustained in the Venusian atmosphere.

Limaye, who has spent his career studying planetary atmospheres, was further inspired to revisit the idea of microbial life in the clouds of Venus by a visit to Tso Kar, a high-altitude salt lake in northern India where he observed the powdery residue of sulfur-fixing bacteria concentrated on decaying grass at the edge of the lake being wafted into the atmosphere.

Limaye notes, however, that a part of the equation that isn't known is when Venus' liquid water evaporated - extensive lava flows in the last billion years likely have either destroyed or covered up the planet's earlier terrestrial history.

In the hunt for extraterrestrial life, planetary atmospheres other than Earth's remain largely unexplored.

One possibility for sampling the clouds of Venus, says Limaye, is on the drawing board: VAMP, or Venus Atmospheric Maneuverable Platform, a craft that flies like a plane but floats like a blimp and could stay aloft in the planet's cloud layer for up to a year gathering data and samples.

Such a platform could include instruments like Raman Lidar, meteorological and chemical sensors, and spectrometers, says Limaye. It could also carry a type of microscope capable of identifying living microorganisms.

"To really know, we need to go there and sample the clouds," says Mogul. "Venus could be an exciting new chapter in astrobiology exploration."

The Wisconsin scientist and his colleagues remain hopeful that such a chapter can be opened as there are ongoing discussions about possible NASA participation in Russia's Roscosmos Venera-D mission, now slated for the late 2020s. Current plans for Venera-D might include an orbiter, a lander and a NASA-contributed surface station and maneuverable aerial platform.