It's possible that Venus and Earth once simultaneously existed as pleasant worlds hosting mild temperatures and oceans.
Then, something went awry.
In new research, planetary scientists simulated how Venusian conditions today — with pizza oven-like temperatures, a crushing atmosphere, and past evidence of widespread volcanism — came to be. The results suggest that Venus, over a series of massive volcanic outgassing events and other geologic changes, transitioned from an Earth-like world to the hellish land we see today. Even the longest-lived robot sent to Venus survived for just two hours.
"It's hot enough to melt lead," Matthew Weller, a planetary geophysicist at Rensselaer Polytechnic Institute who coauthored the new research, told Mashable. "It's a very unpleasant place to be."
The study has been published in the peer-reviewed journal Science Advances.
Venus and Earth, although they've taken two dramatically different climatic roads, are considered sibling planets. They're about the same size. They're made out of the same rocky stuff. They both dwell in the inner part of the solar system. So what happened?
"You have these two planets sitting there, and then one spins off in another direction," Weller explained.
We can't, of course, go back in time to find out why. Nor can we send geologists there to sleuth the Venusian past. But the researchers used advanced computer simulations — which created 3D spherical models of geologic activity on Venus— of how the hot rock convecting beneath the surface, called the mantle, broke Venus' crust and ultimately drove the harsh outcomes present today. "Venus basically baked itself," Weller said.
"Venus basically baked itself."
Running the models showed that a once temperate, Earth-like Venus experienced a series of "stairstep" events, wherein deep interior motions rupturing the crust allowed for volcanism to reach Venus' surface. This led to molten rock resurfacing the planet and massive volcanic outgassing to fill the atmosphere, creating immense surface pressure. Over multiple periods of some 60 million years, each outgassing episode could have added three to 10 times more atmospheres (an atmosphere, or atm, is a unit of pressure representing one atmosphere on Earth) to Venus' atmosphere. Today, the pressure on the Venusian surface is 92 bars, or 1,350 psi. "To put this into context, imagine having 1,350 pounds (over 600 kilograms) resting on one square inch of your body; it would be like having a small car sitting on your thumbnail," the Planetary Society explains.
It's unknown when, exactly, these planet-altering events began. It could have been several billion years ago, or much more "recently" at hundreds of millions of years ago. But, for many eons, both Earth and Venus may have hosted expansive oceans and balmy temperatures, with water rippling onto shorelines. And they would have been just some 67 million miles apart from each other (a minute cosmic distance).
"It's possible both were habitable," Weller said.
Importantly, the Venus-Earth dichotomy reveals how much a planet can change. An exoplanet we see today, many light-years away in deep space, may look completely different in the future. Or Earth, without being pummeled by a colossal asteroid, could markedly transform, too. "Planets change dramatically over time," Weller emphasized. "It shows how easy it is for an Earth-like planet to become like Venus."
"It shows how easy it is for an Earth-like planet to become like Venus."
The "million-dollar question," however, is what initially pushed Venus onto this trajectory of repeated episodes of extreme volcanism, surface rupturing, and prodigious outgassing. It could have been an extremely large eruptive event at the wrong time that fed into a runaway cycle. Geologically, a world's atmosphere and inner-workings are inextricably connected, as conditions in the atmosphere influence what transpires below a planet's surface. For example, on Earth, rock weathering, over hundreds of millions of years, removes heat-trapping carbon dioxide from the air, acting to stabilize the climate. On Venus, dramatically cranking up surface temperature can kill plate tectonics, Weller explained, shutting off a planet's ability to stabilize itself.
This could have pushed Venus to veer sharply from its Earthly environs. And as the researchers simulated, it couldn't turn around.
In the coming years, Venus may grow much less mystifying. A NASA mission called DAVINCI — short for Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging — will drop a three-foot-wide titanium sphere through Venus' thick clouds. Over the course of just an hour, the probe will ingest gases, run experiments, and show us what Venusian mountains actually look like. It will dramatically improve our understanding of Venus, and why it's so unlike its rocky "twin," Earth.
