The Sibling Science is a series exploring the ways our brothers and sisters can influence us, from our money and mental health to our very molecules. We will be sharing these stories in the coming weeks.
Ask which planet in the solar system is closest to Earth and many people may point to Mars. It orbits nearby, just a little farther from the Sun. It was born at the same time and with the same things as Earth. And it is believed that there were once rivers and lakes, even oceans. NASA has sent rovers to its surface to help us find out if life once existed on the red planet.
But there are planetary scientists who would tell you to look the other way, to a planet that’s much less explored, but actually closer to Earth in size, appearance, composition, and actual distance … that is, to Venus .
Scientists who study Venus affectionately call themselves Venusians. They like to call Venus Earth’s twin.
Martha Gilmore is a proud resident of Venus and Professor of Earth and Environmental Sciences at Wesleyan University. She says that in the past our planet would not have looked so different from its two neighbors.
“If you were an alien visiting our solar system 4 billion years ago, you would see three rocky planets, each with oceans,” says Gilmore.
These planets – Earth, Mars and Venus – look very different from each other today. Earth is a temperate, blue-green marble transformed by living things. Meanwhile, its siblings have migrated to two extremes: Mars is a dry, cold, dusty planet with a paper-thin atmosphere, and Venus is the hottest planet in the Solar System, covered in a thick atmosphere that quickly destroys even non-living visitors from Earth.
This is not an exaggeration: ten probes that have reached the surface of Venus; none of them lasted more than two hours. Venus experiences temperatures in excess of 800 degrees Fahrenheit and pressures that are more than 75 times what we experience on Earth.
So what happened to these ancient oceans on our two nearest planets—and why is the surface of Venus such a harsh environment today?
For planets, size and location matter
Temperature and pressure are what distinguish Venus from Earth. Gilmore says these differences stem from several factors: the distance from the sun and the internal heat of the planet itself.
All planets are born with a certain amount of heat from the time of their creation, says Gilmore, who explains this phenomenon to her students using a holiday dinner.
“It’s like Thanksgiving. You have a hot potato, you know, a baked potato, and you have peas, and you want to eat that potato, but it’s too hot. But the peas, they are ready because their heat is radiated, because they are small.”
Mars is a smaller potato, so it loses its heat faster. Venus and Earth were similar in size, so they should have cooled at the same rate.
But besides internal heat, there was something else that kept Venus warm: the sun. Because Venus sits much closer to the sun, it receives more of its energy. And this extra amount of energy delivered over billions of years is a big reason why Venus’s atmosphere has become much more intense than what we experience on Earth.
A delicate balance
Atmospheres act like “cozy blankets,” says Gilmore. On Earth, for example, the atmosphere helps make the planet habitable by shielding life from radiation and also keeping the surface at temperatures where we humans can live. And to have a stable atmosphere, a planet needs several things: volcanism, sufficient mass, and oceans.
“You have to be hot enough, big enough to have volcanism,” Gilmore says, because volcanoes are fueled by the planet’s internal heat. These volcanoes pump out the gases that make up the atmosphere. But once that atmosphere is in place, a planet must also be large enough for its gravity to actually “hold [that] a blanket.”
While Mars initially looked similar to Venus and Earth, its smaller size meant that its gravity was not strong enough to retain the small amount of atmosphere it had developed when it had oceans in its early life. And as Mars cooled, its volcanic activity slowed … and eventually stopped.
The problem is that planets actually need volcanoes to continually replenish their atmospheres because those atmospheres are constantly being lost to space, Gilmore explains.
“At the top of our atmosphere right now, there are all kinds of nasty rays that eat away at the atmosphere, like cosmic rays and solar rays,” she says.
Thus, the Martian atmosphere slowly erodes – becoming so thin that water can no longer remain liquid on the surface. Part of it escaped into space, and part of it ended up frozen in ice.
Meanwhile, Venus was so close to the sun that its oceans boiled. Volcanoes also continued to pump out things like carbon dioxide—which is a powerful greenhouse gas.
Earth’s atmosphere also has carbon dioxide, but our oceans help moderate its heat-trapping effects by sucking up excess carbon and eventually turning it into rock. That’s why it’s so important to take care of our oceans, says Gilmore.
“Once you get rid of the ocean, you turn off the primary mechanism for storing carbon dioxide in the rock,” says Gilmore. “And therefore it just stays in the atmosphere. And the greenhouse effect takes over and you get a super, super hot blanket.”
The greenhouse effect that makes Venus uninhabitable for life as we know it on Earth is something scientists worry about when studying the effects of climate change. Humans currently pump 100 times more greenhouse gases into the atmosphere than volcanoes annually.
Venus is the nearest exoplanet
Exploring Venus could give scientists insights into what the world looks like when there are no carbon sinks left. But it also happens to be the closest “Earth-like” planet that researchers know of.
Hundreds of planets discovered outside our solar system are Earth-sized and potentially habitable. But these planets are so far away that sending spacecraft to explore them won’t be feasible for many generations.
Venus, Gilmore says, is much closer. It’s somewhere we’ve visited before and can visit again. Earth is scientists’ first data point on what a habitable planet might look like — but Venus’ past may give us a glimpse of another planet that was once habitable before it was changed forever.
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