 Podcaster: Avivah Yamani
Organization: 365 Days Of Astronomy ; langitselatan Link : http://cosmoquest.org/x/365daysofastronomy; http://langitselatan.com Source for this podcast: Description: The question is why do Venus and Uranus rotate differently from all the others? Bio: Avivah is a project director of 365 Days Of Astronomy and astronomy communicator from Indonesia. Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at . Transcript: Venus, the second planet in Solar System and our neighbour, is also a twin of our blue planet. It has a similar size, mass, density and chemical composition. But of course just like any twin, there is always a difference between them. In this case, Venus’ high temperature and extreme pressure make it look very different from our world. How could you live in a such hot planet like that? And not just that. The day on Venus is much longer than one on the Earth. It takes 243 Earth days to complete its rotation but it only takes 224.7 days to complete each orbit. It means you already are in another year once the planet finishes the day. On top of it, Venus rotation is opposite to the Earth. It rotates backwards compared to the Earth and the other planets. Well not all, since Uranus also rotates in a different way. If you look at the solar system from its north pole, then you will see all the planets orbiting the Sun counter clockwise and rotating on their axis counterclockwise, except for Venus and Uranus. Venus rotates clockwise while Uranus rolls on its side as it orbits the Sun. The question is why do they rotate differently from all the others? Let’s take ourselves traveling into the past. Well not literally going to the past, but let’s explore the time when all the planets formed. At that time, all the planets rotated in the same direction. But something came along that changed the rotational direction of Venus and Uranus. Lets start with Venus. There are a few possibilities that we can take into account to explain why Venus rotates backwards. Venus is initially rotating counterclockwise like the other planets and it still does. In other words, it spins in the same direction it always has, just upside down, so that looking at it from the other planets makes the spin look backward. As I said before, there are a few explanations for this. Scientists have argued that the sun’s gravitational pull on the planet’s very dense atmosphere could have caused strong atmospheric tides. Such strong tides could have caused the flip to happen. Another explanation comes from the cratering evidence on each planet. Soon after the planets were formed, there still were many large and small objects or maybe we can classify them as mini planets that orbited the Sun. So.. this interaction with all the planets eventually cleared each planet’s orbit as we know now. This is what happened to the Earth/Moon system when a Mars-sized body collided with the Earth and the leftover material merged and formed the Moon. And Venus may have experienced an encounter with one of these larger bodies in a gigantic impact in which, unlike the Earth, the extra material didn’t form a separate moon, but stopped and even reversed its rotation altogether. Another simulation was made by Alex Alemi and David Stevenson from California Institute of Technology, to trace the oddness of Venus’s orbit. They proposed 2 large impacts in Venus’s history. The first impact’s result was a satellite for Venus but after 10 million years Venus encountered another gigantic impact on the opposite side of the planet from the first impact which reversed its rotation and the satellite it once had spiralled in and collided with the planet itself. That’s Venus. As for Uranus, it’s quite unique since it’s not rotating in the regular clockwise or counterclockwise direction. Well if Venus was rotating upside down then.. someone must have kicked this planet and made it roll on its side. Most planetary axes are perpendicular to the orbital plane. But Uranus’s axis is extremely tilted to 97.7º and its pole is pointed toward the equatorial plane of the other planets. This extreme tilt leads to the radical seasons that the planet experiences and makes the planet have unusual days at the poles. At the equator, Uranus experiences normal days and nights. But because it rotates on its side, at any given time one pole or the other is pointed more or less towards the Sun. This results in one pole experiencing 42 Earth years of day followed by 42 years of night. When the North Pole is toward the Sun, the South Pole is in darkness and vice-versa. So how could this have happened? Same as with Venus, Uranus also had counterclockwise rotation until a gigantic impact changed everything. The explanation for this is that in its formation history, Uranus collided with an Earth-sized object which lead to the change of its rotation. Another theory and simulation without collision by Gwenael Boue and Jacques Laskar from Paris Observatory shows that Uranus had very large moon with 0,1 % of Uranus’s mass. The gravitational interaction between the two changed Uranus’s axis to tilt the way it is now. And for the moon itself, it was ejected from the system when they encountered other massive planets. Thank you for listening. This is 365 Days Of Astronomy End of podcast: 365 Days of Astronomy The 365 Days of Astronomy Podcast is produced by NUCLIO. Audio post-production by Richard Drumm. Bandwidth donated by libsyn.com and wizzard media. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at . This year we celebrate cosmic light as light is our info messenger in the universe. Join us and share your story to celebrate the International Year of Light. Until tomorrow! Goodbye!
The first theory is that Venus began its life spinning in the same direction as the other planets. But, like Uranus, it was hit by a large object, like a huge asteroid, perhaps even something the size of a planet, or perhaps it was hit many times over time. This impact knocked the planet all the way over and flipped its axis, causing it to spin in the other direction. Another theory relates to the planet's interior. Within rocky planets there can be boundaries of different parts of the interior which have different viscosities, such as the mantle (the part near the surface) and the core (the part right in the middle). As either of these two parts moves around, it rubs at the boundary between them, creating friction. This effect, called core-mantle friction, can gradually change the spin of a planet over time. Combine this with the fact that Venus has an extremely thick atmosphere — in fact the atmosphere shows an effect called superrotation where it rotates 60 times faster than the planet and is so thick that it might affect the length of a day there — and you have a recipe for how a planet could change the direction of its rotation. Finally, there's a more recent theory that says Venus also started out rotating the same as the other planets, but over time its rotation slowed right down until it almost stopped rotating all together. Then, it started rotating in the opposite direction. This is because its initial rotation was unstable, but rotation in the other direction was a more stable state, and it could explain why Venus rotates so slowly compared to other planets. We still have a lot to learn about Venus, but with three missions heading there in the next few years, we should learn a lot more about this strange planet soon. Q. I read that Venus is the only planet to rotate clockwise. What dictates the direction of rotation? A. In fact, there are two planets that spin on their axes from east to west. The other is Uranus. The rotation of Venus is very slow, taking a little more than 243 Earth days to make a complete turn, while the planet’s orbit around the sun takes just over 224 days. An explanation for the backward, or retrograde, rotation is not certain. A long-held theory is that Venus once rotated as the other planets do, but was struck billions of years ago by a planet-size object. The impact and its aftermath caused the rotation to change directions or flipped the planetary axis. Another theory is that tidal effects from core-mantle friction and a thick atmosphere might have accounted for the change in direction. A more recent idea, put forth in the journal Nature in 2001, suggests that an initial counterclockwise rotation was an unstable state for various complex reasons and that the planet slowed down and slipped into a more stable state of clockwise rotation. As for the initial direction of rotation for all of the sun’s planets, it is assumed to be a legacy of their formation through a gradual accretion of the whirling original material of the solar system. |
Rotates in the opposite direction of most other planets
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