With surface temperatures hot enough to melt lead, Venus today is a veritable hellhole, despite being similar in size to Earth and orbiting in the habitable zone of the Sun. Yet studies suggest the planet may have once hosted oceans and even conditions suitable for life. Explaining how all that water disappeared has been a problem.
A study published today in Nature offers a solution, identifying a new water-loss mechanism operating high in Venus’s atmosphere that could have doubled the rate of water loss. Speedier drying could have allowed oceans to exist until later in Venus’s history—implying the planet might have been habitable for longer. “It fits in with Venus being a more active and maybe water-rich world,” says Sue Smrekar, a geophysicist at NASA’s Jet Propulsion Laboratory who was not involved in the study.
Today, Venus puts Earth’s greenhouse effect to shame. At 96% carbon dioxide, its atmosphere traps so much heat that surface temperatures reach more than 450°C. Yet spacecraft and telescopes have seen faint hints of water vapor in the atmosphere, and in the late 1970s, NASA’s Pioneer Venus orbiter detected a sign of long-vanished oceans: an enrichment of heavy hydrogen, deuterium.
Subsequent modeling studies have suggested the planet had enough moisture billions of years ago to cover the surface in 3 kilometers of water. But as volcanoes spewed carbon dioxide, a runaway greenhouse effect would have raised temperatures and boiled off most of the water. High in the atmosphere, the Sun’s ultraviolet light would have split the water vapor molecules into hydrogen and oxygen. In a process called hydrodynamic loss, the featherweight hydrogen got hot and energetic enough to escape the planet’s gravity, leaving the telltale deuterium enrichment.
But crucially, this process cannot account for the last 100 meters of water loss because hydrogen is also a greenhouse gas. Once enough hydrogen escaped, temperatures could no longer rise, and the water loss would have slowed. “You can’t lose all the water to match the present-day observations,” says Michael Way, who has modeled Venus’s climate at NASA’s Goddard Institute for Space Studies. “You’re left in this conundrum.”
The authors of the new study say they have identified new water-loss chemistry that can resolve the problem. For the highest gas molecules, some 150 kilometers above the surface, sunlight would not only split water vapor but also carbon dioxide, creating hydrogen and carbon monoxide that would combine into an unstable ion called HCO+. Almost immediately, the HCO+ ions would break apart to shed excess energy. “Because hydrogen is so much lighter than carbon monoxide, it gets most of the energy from this process and zips away superfast,” says Michael Chaffin, a planetary scientist at the University of Colorado (CU) Boulder and a co-lead author on the new study. “The hydrogen uses the carbon monoxide molecule as a launchpad to escape to space,” explaining how the last “dregs” of venusian water could have been lost even after hydrodynamic loss ceased.
Together the new HCO+ mechanism and the previously modeled water-loss processes could have enabled Venus to lose its water in half the time, a relatively brisk 600 million years, the researchers say. If so, Venus may have held onto its oceans until much more recently, perhaps 2 billion to 3 billion years ago. “If Venus did have this early habitable phase, understanding how it went from habitable to completely uninhabitable is extremely important,” Way says.
Co-author Bethan Gregory, a planetary scientist at CU Boulder, led a previous study last year that showed how the same HCO+ process could also explain how Mars lost its water, after NASA’s MAVEN orbiter detected energetic hydrogen escaping Mars in a way that corresponds to the breakup of HCO+. “The Mars and Venus atmospheres are both [carbon dioxide] dominated, so there are a lot of similarities there,” she says. Relatively low carbon dioxide levels on Earth mean there’s no threat that the same process could desiccate our atmosphere.
No upcoming mission will be able to definitively test whether the HCO+ mechanism was at work on Venus. Neither NASA’s VERITAS mission, which Smrekar leads, nor Europe’s EnVision mission, both to launch in 2031, have instruments to measure hydrogen loss from the upper atmosphere. NASA’s DAVINCI probe, also set to launch in 2031, will sample the atmosphere, but only below 70 kilometers. “There’s just nothing on the books to study the upper atmosphere,” Way says, but “people will propose [an instrument] I’m sure.”
Chaffin says finding out is critical, not just to better understand how Earth’s near-twin went down its darkling path, but also to apply the knowledge to rocky planets being discovered beyond the Solar System. “Venus and Earth may have formed from identical material, and yet they ended up very different,” he says. “Understanding how these processes work, and how they might limit the existence of habitable environments everywhere in the universe, is really important.”
More: https://www.science.org/content/article/hellish-venus-may-have-lost-its-water-quickly
