Venus may have Earth-like lithospheric thickness and heat flow.

Credit: Pixabay/CC0 Public Domain
Credit: Pixabay/CC0 Public Domain

On October 22, 1975, the Soviet Union's Venus lander, Venera 9, broke away from its orbiter and plunged violently through the planet's dense atmosphere, landing forcefully on a circular shield intended to crumple and deflect the impact. While transmitting information on clouds, light irradiance, temperature, and atmospheric chemistry, as well as the first image ever taken of the surface of another planet, it only managed to survive the harsh surface conditions for 53 minutes. Then it passed away. But given that Venus and Earth are comparable terrestrial planets that are thought to have formed via comparable processes, its findings are significant.

While Earth and Venus are related and have a similar size and composition, it is fair to say that their personalities are very different, making them the Dennis and Randy Quaid of the inner solar system. (In this analogy, Venus is not Dennis.) The conditions on Earth are favorable for life; in contrast, to say Venus is uninhabitable is a hilarious understatement.

The atmosphere of Venus, the densest and hottest of the four terrestrial planets, is primarily made of carbon dioxide, and the pressure there is roughly 92 times that of the atmosphere of Earth at sea level. The average temperature of the planet is around 464 degrees Celsius (867 degrees Fahrenheit). Very bad! It also doesn't have a moon, but even if it did, any beautiful nighttime views would be obscured by the thick sulfuric acid clouds that cover the entire planet.

But Venus also emits heat into space, a characteristic that it also has in common with Earth. On Earth, heat radiates from the locations where plates separate due to plate tectonics, but little is known about the internal dynamics of Venus.

Now, scientists at the Jet Propulsion Laboratory in Pasadena, California, have calculated the thickness of the crust on Venus using data collected by the Magellan spacecraft in the 1990s. Their findings show that, despite having very different personalities, Earth and Venus share a similar heat flow and lithospheric thickness, which places limitations on Venus' evolution and internal dynamics. The findings have been released in Nature Geoscience.

Mobile tectonic plates on Earth move around, collide with one another, and then separate, allowing for effective heat loss. Previous models predicted that the lithosphere of Venus either existed as a "stagnant lid"—basically, as a cold, immobile lithosphere covering the entire planet—or as a "episodic lid," where an unstable stagnant lid occasionally erupted into tectonic activity. Recenter models and data analysis, however, do not back up these claims. Instead, the "squishy-lid" model with active lithospheric flexure is what the JPL researchers advise.

By measuring the flex in surface formations called coronae, quasi-circular features created by geologic and volcanic activity, the researchers were able to calculate the thickness of the lithosphere. They calculated an average heat flow from Venus that is higher than the Earth's average but similar to the values observed at actively extending tectonic areas. They did this by using data from the Magellan altimeter to determine the average thickness of the lithosphere at 75 locations within 65 coronae: 117 kilometers.

The writers state, "Our analysis shows where active extension is most likely to occur while also pointing to Venus's similarity to Earth in terms of lithospheric thickness and global heat flow ranges. Our results support a squishy-lid convective regime that depends on plumes, intrusive magmatism, and delamination to increase heat flow, along with the geologic history of the planet."

This is significant because Venus may have resembled Earth during the Archaean Eon 4 to 2.5 billion years ago, and plume-induced subduction is thought by many scientists to be the origin of plate tectonics on Earth. Earth's heat flow was roughly three times as high during the Archaeon than it is today, and despite the planet being covered in water, it was much hotter.

Overall, the authors claim that the squishy-lid model is a good fit with other observations, including limited surface mobility, intrusive magmatism, lithospheric delamination (where a material fractures into layers), and coronae formation through upwelling and downwelling. Additionally, it has a different planetary geodynamic mode from Earth's that is interestingly different.

Another use of these findings: Information about heat flow on planets orbiting other stars would be necessary to determine whether exoplanetary systems are habitable. But closer to home, if any of this decade's upcoming Venus observation missions can support the group's "squishy lid" hypothesis, it will probably prompt a reevaluation of theories regarding Venus surface features as well as the evolution of the planet's mantle, and it could even have implications for the early formation of the solar system.

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