Planetary scientists at the University of Colorado Boulder have discovered how Venus, Earthโs scalding and uninhabitable neighbor, became so dry.
The new study fills in a big gap in what the researchers call โthe water story on Venus.โ Using computer simulations, the team found that hydrogen atoms in the planetโs atmosphere go whizzing into space through a process known as โdissociative recombinationโโcausing Venus to lose roughly twice as much water every day compared to previous estimates.
The team will publish their findings May 6 in the journal Nature.
The results could help to explain what happens to water in a host of planets across the galaxy.
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โWater is really important for life,โ said Eryn Cangi, a research scientist at the Laboratory for Atmospheric and Space Physics (LASP) and co-lead author of the new paper. โWe need to understand the conditions that support liquid water in the universe, and that may have produced the very dry state of Venus today.โ
Venus, she added, is positively parched. If you took all the water on Earth and spread it over the planet like jam on toast, youโd 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, youโd wind up with only 3 centimeters (1.2 inches), barely enough to get your toes wet.
โVenus has 100,000 times less water than the Earth, even though itโs basically the same size and mass,โ 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 planetโs 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 Venusโ atmosphere may be the culprit behind the planetโs escaping water.
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.
โWeโre trying to figure out what little changes occurred on each planet to drive them into these vastly different states,โ said Cangi, who earned her doctorate in astrophysical and planetary sciences at CU Boulder in 2023.
Spilling the water
Venus, she noted, wasnโt 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 Venusโ 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 Venusโ water evaporated into steam, and most drifted away into space.
But that ancient evaporation canโt explain why Venus is as dry as it is today, or how it continues to lose water to space.
โAs an analogy, say I dumped out the water in my water bottle. There would still be a few droplets left,โ 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.
Hereโs how it works on Venus: HCO+ is produced constantly in the atmosphere, but individual ions donโt 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 entirelyโrobbing Venus of one of the two components of water.
In the new study, the group calculated that the only way to explain Venusโ dry state was if the planet hosted larger than expected volumes of HCO+ in its atmosphere. There is one twist to the teamโs findings. Scientists have never observed HCO+ around Venus. Chaffin and Cangi suggest thatโs because theyโve 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 teamโs newly discovered escape route.
โOne of the surprising conclusions of this work is that HCO+ should actually be among the most abundant ions in the Venus atmosphere,โ Chaffin said.
In recent years, however, a growing number of scientists have set their sights on Venus. NASAโs planned Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission, for example, will drop a probe through the planetโs atmosphere all the way to the surface. Itโs scheduled to launch by the end of the decade.
DAVINCI wonโt be able to detect HCO+, either, but the researchers are hopeful that a future mission mightโrevealing another key piece of the story of water on Venus.
โThere havenโt been many missions to Venus,โ Cangi said. โBut 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.โ
IMAGE CREDIT: Aurore Simonnet / Laboratory for Atmospheric and Space Physics / University of Colorado Boulder
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