Fully understanding how a rocky planet becomes habitable and remains so is a fundamental challenge for planetary scientists and astrobiologists, given the diversity and complexity of intrinsic and extrinsic processes that contribute to sustain habitable conditions over geological and biological time scales. In the face of this challenge, it is imperative that the full range of rocky planet atmospheric evolution data within our Solar System be exploited. Although Venus represents a clear end-member of planetary habitability, its contributions to understanding the prevalence of long-term temperate surface conditions on large rocky worlds have yet to be fully realized. Upcoming missions to Venus, including NASA’s VERITAS and DAVINCI, and ESA’s EnVision mission, will begin to flesh out this understanding.
Kane & Byrne describe Venus as an anchor point from which planetary scientists can better understand the conditions that preclude life on exoplanets. Image credit: Kane & Byrne, doi: 10.1038/s41550-024-02228-5.
“We often assume that Earth is the model of habitability, but if you consider this planet in isolation, we don’t know where the boundaries and limitations are. Venus gives us that,” said Dr. Stephen Kane, an astrophysicist at the University of California, Riverside.
“Though it also features a pressure cooker-like atmosphere that would instantly flatten a human, Earth and Venus share some similarities.”
“They have roughly the same mass and radius. Given the proximity to that planet, it’s natural to wonder why Earth turned out so differently.”
Many scientists assume that insolation flux, the amount of energy Venus receives from the Sun, caused a runaway greenhouse situation that ruined the planet.
“If you consider the solar energy received by Earth as 100%, Venus collects 191%. A lot of people think that’s why Venus turned out differently,” Dr. Kane said.
“But hold on a second. Venus doesn’t have a moon, which is what gives Earth things like ocean tides and influenced the amount of water here.”
In addition to some of the known differences, more NASA missions to Venus would help clear up some of the unknowns.
Planetary scientists don’t know the size of its core, how it got to its present, relatively slow rotation rate, how its magnetic field changed over time, or anything about the chemistry of the lower atmosphere.
“Venus doesn’t have a detectable magnetic field. That could be related to the size of its core,” Dr. Kane said.
“Core size also gives us information about how a planet cools itself. Earth has a mantle circulating heat from its core. We don’t know what’s happening inside Venus.”
“A rocky planet’s interior also influences its atmosphere. That is the case on Earth, where our atmosphere is largely the result of volcanic outgassing.”
Schematic cross sections of Earth and Venus, showing the major internal components and atmospheric components, to scale. Image credit: Kane & Byrne, doi: 10.1038/s41550-024-02228-5.
NASA does have twin missions to Venus (DAVINCI and VERITAS) planned for the end of this decade, and Dr. Kane is assisting with both of them.
The DAVINCI mission will probe the acid-filled atmosphere to measure noble gases and other chemical elements.
“DAVINCI will measure the atmosphere all the way from the top to the bottom. That will really help us build new climate models and predict these kinds of atmospheres elsewhere, including on Earth, as we keep increasing the amount of carbon dioxide,” Dr. Kane said.
The VERITAS mission won’t land on the surface but it will allow scientists to create detailed 3D landscape reconstructions, revealing whether the planet has active plate tectonics or volcanoes.
“Currently, our maps of the planet are very incomplete. It’s very different to understand how active the surface is, versus how it may have changed through time. We need both kinds of information,” Dr. Kane said.
Ultimately, Dr. Kane and his co-author, Dr. Paul Byrne from the Washington University in St. Louis, advocate for missions like these to Venus for two main reasons.
One is the ability, with better data, to use Venus to ensure inferences about life on farther-flung planets are correct.
“The sobering part of the search for life elsewhere in the universe is that we’re never going to have in situ data for an exoplanet. We aren’t going there, landing, or taking direct measurements of them,” Dr. Kane said.
“If we think another planet has life on the surface, we might not ever know we’re wrong, and we’d be dreaming about a planet with life that doesn’t have it.”
“We are only going to get that right by properly understanding the Earth-size planets we can visit, and Venus gives us that chance.”
The other reason to research Venus is that it offers a preview of what Earth’s future could look like.
“One of the main reasons to study Venus is because of our sacred duties as caretakers of this planet, to preserve its future,” Dr. Kane said.
“My hope is that through studying the processes that produced present-day Venus, especially if Venus had a more temperate past that’s now devastated, there are lessons there for us. It can happen to us. It’s a question of how and when.”
The review paper was published in the journal Nature Astronomy.
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Stephen R. Kane & Paul K. Byrne. 2024. Venus as an Anchor Point for Planetary Habitability. Nat Astron 8, 417-424; doi: 10.1038/s41550-024-02228-5
