Banner image:听Illustration of what Venus may have looked like billions of years ago with water, left, and what Venus looks like today, right. (Credits: NASA;听NASA/JPL-Caltech)
Planetary scientists at 抖阴旅行射 Boulder have discovered how Venus, Earth鈥檚 scalding and uninhabitable neighbor, became so dry.
The new study fills in a big gap in what the researchers call 鈥渢he water story on Venus.鈥� Using computer simulations, the team found that hydrogen atoms in the planet鈥檚 atmosphere go whizzing into space through a process known as 鈥渄issociative recombination鈥濃�攃ausing Venus to lose roughly twice as much water every day compared to previous estimates.
In Venus' upper atmosphere, hydrogen atoms, orange, whiz into space, leaving听behind carbon monoxide molecules, blue and purple. (Credit:听Aurore Simonnet/LASP/抖阴旅行射 Boulder)
The team听 in the journal Nature. The results could help to explain what happens to water in a host of planets across the galaxy.
鈥淲ater is really important for life,鈥� said Eryn Cangi, a research scientist at the (LASP) and co-lead author of the new paper. 鈥淲e need to understand the conditions that support liquid water in the universe, and that may have produced the very dry state of Venus today.鈥�
Venus, she added, is positively parched. If you took all the water on Earth and spread it over the planet like jam on toast, you鈥檇 get a liquid layer roughly 3 kilometers (1.9 miles) deep. If you did the same thing on Venus, where all the water is trapped in the air, you鈥檇 wind up with only 3 centimeters (1.2 inches), barely enough to get your toes wet.
鈥淰enus has 100,000 times less water than the Earth, even though it鈥檚 basically the same size and mass,鈥� said Michael Chaffin, co-lead author of the study and a research scientist at LASP.
In the current study, the researchers used computer models to understand Venus as a gigantic chemistry laboratory, zooming in on the diverse reactions that occur in the planet鈥檚 swirling atmosphere. The group reports that a molecule called HCO+ (an ion made up of one atom each of hydrogen, carbon and oxygen) high in Venus鈥� atmosphere may be the culprit behind the planet鈥檚 escaping water.听
For Cangi, co-lead author of the research, the findings reveal new hints about why Venus, which probably once looked almost identical to Earth, is all but unrecognizable today.
鈥淲e鈥檙e trying to figure out what little changes occurred on each planet to drive them into these vastly different states,鈥� said Cangi, who earned her doctorate in astrophysical and planetary sciences at 抖阴旅行射 Boulder in 2023.
Spilling the water
Venus, she noted, wasn鈥檛 always such a desert.
Scientists suspect that billions of year ago during the formation of Venus, the planet received about as much water as Earth. At some point, catastrophe struck. Clouds of carbon dioxide in Venus鈥� atmosphere kicked off the most powerful greenhouse effect in the solar system, eventually raising temperatures at the surface to a roasting 900 degrees Fahrenheit. In the process, all of Venus鈥� water evaporated into steam, and most drifted away into space.
But that ancient evaporation can鈥檛 explain why Venus is as dry as it is today, or how it continues to lose water to space.
鈥淎s an analogy, say I dumped out the water in my water bottle. There would still be a few droplets left,鈥� Chaffin said.
On Venus, however, almost all of those remaining drops also disappeared. The culprit, according to the new work, is elusive HCO+.
Missions to Venus
Chaffin and Cangi explained that in planetary upper atmospheres, water mixes with carbon dioxide to form this molecule. In previous research, the researchers reported that HCO+ may be responsible for Mars losing a big chunk of its water.
Illustration of NASA's DAVINCI probe falling to the surface of Venus. (Credit:听NASA GSFC visualization by CI Labs Michael Lentz and others)
Here鈥檚 how it works on Venus: HCO+ is produced constantly in the atmosphere, but individual ions don鈥檛 survive for long. Electrons in the atmosphere find these ions, and recombine to split the ions in two. In the process, hydrogen atoms zip away and may even escape into space entirely鈥攔obbing Venus of one of the two components of water.
In the new study, the group calculated that the only way to explain Venus鈥� dry state was if the planet hosted larger than expected volumes of HCO+ in its atmosphere. There is one twist to the team鈥檚 findings. Scientists have never observed HCO+ around Venus. Chaffin and Cangi suggest that鈥檚 because they鈥檝e never had the instruments to properly look.
While dozens of missions have visited Mars in recent decades, far fewer spacecraft have traveled to the second planet from the sun. None have carried instruments capable of detecting the HCO+ that powers the team鈥檚 newly discovered escape route.
鈥淥ne of the surprising conclusions of this work is that HCO+ should actually be among the most abundant ions in the Venus atmosphere,鈥� Chaffin said.
In recent years, however, a growing number of scientists have set their sights on Venus. NASA鈥檚 planned Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission, for example, will drop a probe through the planet鈥檚 atmosphere all the way to the surface. It鈥檚 scheduled to launch by the end of the decade.
DAVINCI won鈥檛 be able to detect HCO+, either, but the researchers are hopeful that a future mission might鈥攔evealing another key piece of the story of water on Venus.
鈥淭here haven鈥檛 been many missions to Venus,鈥� Cangi said. 鈥淏ut newly planned missions will leverage decades of collective experience and a flourishing interest in Venus to explore the extremes of planetary atmospheres, evolution and habitability.鈥�
