Earth’s sister planet is closer, brighter, bigger and more relevant than the overrated Mars

Science | by Gregory Beatty

With all the space probes and rovers studying Mars these days, and all the plans being made to visit it in the next decade or so, the Red Planet is getting a lot of attention.

Sure, it’s exciting to think about people one day landing on Mars and setting up camp to explore. But what about our other planetary neighbour Venus? It’s closer to us than Mars, and was the first planet we actually visited — via flybys by the Venera 1 (USSR) and Mariner 2 (U.S.) space probes in 1961–62.

When Venus is visible in the night sky you can’t miss it. Behind the Sun and Moon, it’s the third brightest celestial object. You’ll find it either shining jewel-like in the east before sunrise or in the west after sunset. Not only is Venus beautiful to look at [see sidebar], it’s got a critical lesson to teach us.

More on that later. For now we’ve established that Venus is closer, shinier and more relevant than the Red Planet. So suck it Mars, you overhyped also-ran. Today we’re talking about Venus, a planet that is just plain better than you.

Sister Planet

Venus is often described as Earth’s sister planet. To the extent that they’re both terrestrial (i.e. rocky) and have similar size and mass, that’s true. But from there, they part ways.

Venus doesn’t have a moon, so that’s one difference. It also rotates in the opposite direction as Earth, so the Sun rises in the west and sets in the east. Even weirder, its day is longer than its year by a 243 to 224.7 Earth-day margin.

That’s a slow spin.

“It’s not known why Venus rotates so slowly,” says University of Saskatchewan astronomer Simone Hagey. “But at the equator you could apparently jog faster than Venus rotates. So the day is longer than the year because it actually takes longer for Venus to rotate once on its axis than to make one trip around the Sun.”

Mercury is like that too. It doesn’t have an atmosphere, so with one side facing the Sun for long periods there’s a huge gap between daytime (427 degree C) and nighttime (minus 173 degree C) temperatures. Venus, though, is different.

“One side of Venus will face the Sun for a very long time, so you’d think the dayside would be hotter than the nightside,” says Hagey. “But Venus has such a thick carbon dioxide atmosphere that it traps the heat and distributes it evenly around the planet.”

Ah yes, the carbon dioxide atmosphere. That’s the biggest difference between Venus and Earth.

Tortured History

Because Venus is shrouded in clouds of sulphur dioxide (with drops of sulphuric acid mixed in), it isn’t an easy planet to study. For several years in the 1990s, the U.S. probe Magellan mapped its surface with radar before plunging into the atmosphere on one final experiment. Two decades earlier, in 1975, the USSR landed Venera 9 on Venus where it transmitted for 53 minutes before losing radio contact.

That was no mean feat. Venus’ atmosphere is so thick and heavy that the pressure at the surface equals 90 Earth atmospheres. But through data gathered by Venera 9, Magellan and other missions, scientists have pieced together an understanding of Venus’ tortured four billion year history.

“The prevailing theory is that going back to Venus’ formation, large areas of the surface were covered in oceans,” says Hagey. “There might even have been a landscape similar to what we’ve seen on Earth. It’s unknown whether there was any life. For that, we’d have to do testing on the surface.”

As the Sun gradually warmed over the next billion plus years, though, conditions began to change, says Hagey.

“Because Venus is a bit closer to the Sun than Earth, the oceans began evaporating,” she says. “That released a lot of water vapour, which is a greenhouse gas.”

That kickstarted a greenhouse effect which increased the rate of evaporation and released CO2 trapped in the oceans. “Then CO2 actually started to get baked out of the rock, and that made it even worse,” Hagey adds. “It turned into a runaway greenhouse effect, with all the oceans boiled away, and the planet turning into a 460-degree C wasteland.”

Wasteland isn’t an exaggeration. In addition to the hellish heat and pressure, Venus has the most volcanoes of any planet in the solar system (over 1,600). Infrared imaging has detected magma, but few volcanoes seem to be currently active.

There is evidence, though, of recent geological activity.

“It’s thought that maybe 500 million years ago there was some catastrophic event that resurfaced the entire planet,” says Hagey. “We know that, because Venus has a lot of craters from meteor impacts but they’re all about the same age and have the same weathering pattern. That’s really weird, as you’d expect to see [some] craters that were billions of years old and others that were younger.

“At some point Venus was resurfaced, and then new impact craters appeared.”

On Earth, crust is continuously being created and remelted via plate tectonics involving rifts in the Atlantic and Pacific Oceans. With no oceans to facilitate plate tectonics — which helps Earth moderate its internal temperature — what’s thought to have happened on Venus is that heat built up in the interior until there was a giant tectonic event that resurfaced the planet over millions of years.

Venus’ atmosphere is 96.5 per cent carbon dioxide, so it’s pretty well run the greenhouse gas gauntlet. Earth, as scientists have warned for the past three decades, is just starting its run, so Venus holds an important lesson for us, says Hagey.

“The term ‘greenhouse gas effect’ gets thrown around a lot, but I’m not sure how many people actually understand it,” Hagey says. “On Earth, the effect is absolutely necessary for our survival. Without it, Earth wouldn’t be habitable.”

What we’re doing now through our reckless use of fossil fuels is increasing the concentration of CO2, methane and other greenhouse gases in the atmosphere. That, in turn, is trapping more heat and beginning to trigger the type of feedback loops that proved so devastating on Venus.

“It’s called a greenhouse effect because it works the same as a greenhouse, where the light can get in, but the heat can’t get out,” Hagey says.

With the Sun continuing to warm over the next one to three billion years, Earth is destined, one day, to suffer the same fate as Venus, says Hagey.

“But what we’re doing with all our greenhouse gas emissions is accelerating that process. Instead of waiting for the Sun to heat us up, we’re heating ourselves up.”

Some quotes in this story have been lightly edited.

Sidebar

Deadly Beauty, Deadly Lesson

Because Venus shines so brilliantly in the night sky, it’s long been an object of fascination. In ancient times, some cultures even mistook the planet for two separate bodies: the proverbial Morning Star that rises in the east and Evening Star which sets in the west.

Venus takes its name from the Roman goddess of love and beauty. Associating Venus with those qualities goes back at least as far as the Sumerians in 4000 B.C.E., who named it after their goddess of love, beauty and fertility, Inanna (later known as Ishtar by the Babylonians).

Even scientists have been seduced by that tradition, says astronomer Simone Hagey.

“Based on international agreement, all craters and surface features on Venus are named after goddesses and women from different cultures,” she says. “So that’s pretty cool.”

Science-fiction treatments of Venus have been strongly influenced by that idea too. Because of its thick cloud cover, Venus is impenetrable to even the most powerful telescopes. That gave writers (and later, filmmakers) wide leeway in how they depicted the planet.

Often, Venus was shown as being relatively Earth-like. If it was inhabited, the “aliens” skewed toward the female/hot end of the spectrum in keeping with the planet’s Roman goddess namesake and general lasciviousness.

Probably the most notorious example is the Hollywood B movie Queen of Outer Space (1958), about four male astronauts who crashland on Venus and become involved in a palace revolt led by Zsa Zsa Gabor against a cruel queen who has killed or imprisoned all the men, leaving her subjects pining for love.

It has the tagline “voluptuous Venusians” and has to be seen to be believed.

Nowadays, most fictional treatments incorporate an element of terraforming to try to make the planet habitable. One proposal to deal with the extreme heat is to erect a giant shade or mirror in space (giant, as in four times Venus’ size) to block sunlight from reaching the planet. As Venus slowly cooled, carbon dioxide in the atmosphere would start to freeze and fall as dry ice snow.

Sounds preposterous, I know. Mind you, a similar proposal has been made here on Earth as part of a broader strategy to geo-engineer our planet to protect against rising greenhouse gas emissions and fight climate change.

If we fail to tackle that challenge, says Hagey, we just have to look to our closest planetary neighbor to see what lies ahead.

“Venus is a really good lesson about the runaway greenhouse effect and climate change,” she says.