Placeholder Image

Subtitles section Play video

  • SciShow Space is supported by Brilliant.

  • [♪ INTRO]

  • After studying the solar system for hundreds of years,

  • you'd think we'd at least have the basics figured out,

  • stuff like why planets spin and orbit the way they do.

  • Except, we totally don't.

  • And you only have to look at the planet next door to see it.

  • When scientists began observing Venus in detail in the 1950s and '60s,

  • they expected it to be pretty unremarkable.

  • Instead, it turned out to mostly be an inferno of acid rain,

  • one that, of all things, spins backwards!

  • It's been more than 50 years since then, and while we know a lot more about Venus's

  • climate, we still aren't totally sure why it's rotating the wrong way.

  • But we at least have some ideas.

  • In astronomy, a backwards spin is called retrograde rotation, andbackwardsis defined relatively.

  • Because the solar system formed from one cloud of spinning gas, the planets all orbit in

  • the same direction: counterclockwise, if you're looking down on the Earth's north pole.

  • They also rotate on their axes the same way they orbit, so also counterclockwise.

  • Except for Venus and Uranus.

  • Uranus sits sideways on its axis, probably thanks to a couple of collisions.

  • Venus, meanwhile, rotates clockwise, and it's much less clear why!

  • One of the earliest hypotheses was that Venus may have been hit

  • so hard by an asteroid that it reversed direction.

  • The hypothesis seems to have been thrown out there in 1965

  • by two scientists who worked with the original radar data.

  • And when I say, “thrown out there,” I mean it.

  • They followed up the suggestion by saying, and I quote,

  • The possibilities are limited mainly by one's imagination;

  • supporting evidence is rather harder to come by.”

  • Which is super convincing.

  • Either way, when you math it out, the idea kinda falls apart.

  • It turns out that anything big enough to reverse Venus's rotation

  • would also destroy the planet.

  • The impactor's kinetic energy would be some 10,000 times too high.

  • So, the impact idea was pretty much shelved.

  • Thankfully, it was replaced by a few actual evidence-based hypotheses.

  • One of the leading ones, proposed around 1970,

  • is that Venus spins the same way it always has.

  • Just at some point, much like Will Smith, its life got flipped, turned upside down!

  • Not by mom who got scared, though. Just by physics.

  • This could've happened because of processes within Venus's interior and atmosphere.

  • Venus is differentiated, meaning that it has layers like the Earth does:

  • a core, a mantle, and a crust.

  • As the planet rotates, the core and mantle can experience friction where they meet.

  • Venus also has a really thick atmosphere, which, thanks to the Sun's gravity and heat,

  • experiences tides along with the rest of the planet.

  • This hypothesis says that the core-mantle friction and those atmospheric tides could

  • both put some torque on the planet, and that instability could've flipped Venus over.

  • Some models suggest this might work only if Venus formed with an initial tilt of about

  • 90 degrees, but others show that it might work with less initial tilt.

  • Either way, the idea is pretty weird, and thinking about an entire

  • planet flipping is kind of mind-boggling.

  • But we have other ideas, too.

  • Another, first suggested in 1964,

  • is that Venus may have gradually slowed down and then reversed direction.

  • This could've been triggered by a few things, including interactions with the Sun's magnetic

  • field, or those atmospheric tides, or a combination of both.

  • Venus's atmosphere would have been the first part of the planet to

  • start rotating retrograde after that spin-down.

  • Then, that may have provided the rest of the force necessary to get the whole planet going backward.

  • As a bonus, this idea would also explain why Venus's days are so long.

  • But there's no clear winner between these two hypotheses yet.

  • To figure out which idea is most likely, we have to know more about Venus's early dynamics,

  • specifically its rotation rate and axial tilt.

  • According to a 2001 paper published in Nature, the axis-flip mechanism is most likely if

  • Venus had a rapid initial rotation rate.

  • But if it rotated slower than once every four Earth days and had a relatively small tilt,

  • like less than 70 degrees, then slowing down and reversing is the most probable mechanism.

  • Unfortunately, it's kind of hard to get evidence about Venus from four billion years ago.

  • So until we build a time machine, or at least some really good models, the jury is out.

  • Of course, that's not the whole discussion, either.

  • Because just to throw a wrench in things, that 1960s impact hypothesis

  • is actually making a comeback.

  • Or at least, a version of it.

  • In 2008, one researcher suggested that Venus may have gotten its weird spin

  • back when it was a wee li'l planetesimal.

  • They argue that, billions of years ago, another object about the same size slammed into it

  • and sent it spinning like a backward top.

  • But instead of destroying baby Venus,

  • those two pieces came together to form a full-sized planet.

  • Unlike in the '60s, there's actually some potential evidence for this now.

  • Based on Venus's topography, we don't think there's a lot of water in the planet's

  • interior compared with Earth's.

  • And a huge impact could have provided the energy to get rid of it.

  • This hypothesis would explain why Venus is so dry,

  • but there are other competing models, too.

  • Like, it's possible Venus lost its water through evaporation instead.

  • To learn more, it would help to do a mineralogical survey of the planet to determine if there

  • are any water-containing compounds there.

  • If there are, it means Venus couldn't have lost all its water so early in that big impact.

  • But we haven't done one of those yet, because Venus is a place where robots go to die.

  • The good news is, it's arguably easier to build a Venus-resistant rover than a time machine.

  • So hopefully we can get some evidence for at least one of those hypotheses soon.

  • And speaking of making hypotheses and gathering evidence, thanks to Brilliant.org for sponsoring

  • this episode and introducing me to this Science Essentials quiz

  • on Observations, Questions, and Hypotheses.

  • In this quiz, you're a scientist studying the fictional exoplanetXlyem,”

  • and making observations and hypotheses

  • based on what you experience on the planet, as well as what you already know from earth.

  • The questions and explanations walk you through the scientific process, but what's so special

  • about it is that in the quiz format, it reminds (and even requires) you to check your earthling

  • biases before moving on to the next question.

  • When I'm not making SciShow Space videos, I'm teaching kids how to learn and operate

  • in the world by using their imaginations, so the creativity and complex thinking required

  • in this lesson are right up my alley.

  • If you want to check it out, head to brilliant.org/scishowspace to sign up for free.

  • Brilliant is also offering 20% off the annual premium subscription

  • to the first 200 SciShow Space viewers to sign up.

  • So click on the link in the description to check it out and

  • let me know in the comments how you do on this quiz!

  • Thanks for watching and supporting SciShow Space.

  • [♪ OUTRO]

SciShow Space is supported by Brilliant.

Subtitles and vocabulary

Click the word to look it up Click the word to find further inforamtion about it