APOD: Planets of the Solar System: Tilts... (2019 May 20)

[APOD Robot]

Planets of the Solar System: Tilts and Spins

Explanation: How does your favorite planet spin? Does it spin rapidly around a nearly vertical axis, or horizontally, or backwards? The featured video animates NASA images of all eight planets in our Solar System to show them spinning side-by-side for an easy comparison. In the time-lapse video, a day on Earth -- one Earth rotation -- takes just a few seconds. Jupiter rotates the fastest, while Venus spins not only the slowest (can you see it?), but backwards. The inner rocky planets, across the top, most certainly underwent dramatic spin-altering collisions during the early days of the Solar System. The reasons why planets spin and tilt as they do remains a topic of research with much insight gained from modern computer modeling and the recent discovery and analysis of hundreds of exoplanets: planets orbiting other stars.

<< Previous APOD

This Day in APOD

Next APOD >>

[Antony Rawlinson]

Interested to see this, as I hadn't seen this information in such a form. What I've seen in TV programmes on cosmology, is that the collision that led to the formation of our Moon, also set Earth's rotational axis at 23.4 degrees - thus giving us our seasons.

But this gives us the impression that rotation aligned with orbit is the norm, and in the video we see that only Mercury and Jupiter have less of a tilt than Earth. So how significant was the Moon in setting the Earth's rotation?

[Boomer12k]

Very interesting video...

:---[===] *

[De58te]

Interested to see this, as I hadn't seen this information in such a form. What I've seen in TV programmes on cosmology, is that the collision that led to the formation of our Moon, also set Earth's rotational axis at 23.4 degrees - thus giving us our seasons.

But this gives us the impression that rotation aligned with orbit is the norm, and in the video we see that only Mercury and Jupiter have less of a tilt than Earth. So how significant was the Moon in setting the Earth's rotation?

Actually Venus tilt of only 2.7 degrees to the ecliptic is less than Earth's tilt. Even less than Jupiter's. Just because Venus rotates retrograde doesn't mean its tilt is greater. Venus doesn't have any seasons like Earth.

[orin stepanek]

Like the workings of a clock!

[E Fish]

I knew Venus' rotation was retrograde, but I hadn't realized how slow it is. I may have to snag this video to show my science class when we get to the astronomy unit. It's a great visual.

[Chris Peterson]

Interested to see this, as I hadn't seen this information in such a form. What I've seen in TV programmes on cosmology, is that the collision that led to the formation of our Moon, also set Earth's rotational axis at 23.4 degrees - thus giving us our seasons.

But this gives us the impression that rotation aligned with orbit is the norm, and in the video we see that only Mercury and Jupiter have less of a tilt than Earth. So how significant was the Moon in setting the Earth's rotation?

Actually Venus tilt of only 2.7 degrees to the ecliptic is less than Earth's tilt. Even less than Jupiter's. Just because Venus rotates retrograde doesn't mean its tilt is greater. Venus doesn't have any seasons like Earth.

Yes, the fact that Venus rotates retrograde does mean its tilt is greater. It had to be externally tipped almost 180° to end up with its retrograde rotation. It could not have formed that way, could not have had retrograde rotation initially.

The tilt of Venus with respect to its orbital plane is small, but large enough that it would have detectable seasons if its atmosphere weren't so thick as to distribute heat uniformly around the planet.

[neufer]

retrograde Venus is keeping an eye on you!

---

I knew Venus' rotation was retrograde, but I hadn't realized how slow it is.

The synodic period between inferior conjunctions = 583.92 Earth days;
Almost exactly the same as 5 solar Venusian solar days = 583.75 Earth days.

Hence, every time Venus is at inferior conjunction,
Earth observes the same nighttime side of Venus.*

That nighttime side of Venus even slowly retrograde rotates
....as if it is constantly tracking Earth.

Venus seems to be very very leery of the Earth
-----------------------------------------------------------------------------------
* Over the long four transit Venus cycle [of 243 years]
there will be 152 inferior conjunctions during which
we will observe Venus to rotate only ~80º in latitude.

[BDanielMayfield]

Like the workings of a clock!

And a perpetual motion one at that.

[BDanielMayfield]

Interested to see this, as I hadn't seen this information in such a form. What I've seen in TV programmes on cosmology, is that the collision that led to the formation of our Moon, also set Earth's rotational axis at 23.4 degrees - thus giving us our seasons.

But this gives us the impression that rotation aligned with orbit is the norm, and in the video we see that only Mercury and Jupiter have less of a tilt than Earth. So how significant was the Moon in setting the Earth's rotation?

The present tilts and rotations of all the planets derive from the sum of all the impacts that built up the planets from the disk of gas and dust surrounding our Sun as a proto-star, less tidal effects which tend to slow rotations down over large time spans. The Earth's rotation continues to slow due mostly to lunar tides, so the Earth must have spun significantly faster than it does today [right] after the great impact that lead to the present Earth-Moon system.

Using numerical methods to simulate Solar System behavior, long-term changes in Earth's orbit, and hence its obliquity, have been investigated over a period of several million years. For the past 5 million years, Earth's obliquity has varied between 22° 2′ 33″ and 24° 30′ 16″, with a mean period of 41,040 years. This cycle is a combination of precession and the largest term in the motion of the ecliptic. For the next 1 million years, the cycle will carry the obliquity between 22° 13′ 44″ and 24° 20′ 50″.[26]

The Moon has a stabilizing effect on Earth's obliquity. Frequency map analysis conducted in 1993 suggested that, in the absence of the Moon, the obliquity can change rapidly due to orbital resonances and chaotic behavior of the Solar System, reaching as high as 90° in as little as a few million years (also see Orbit of the Moon).[27][28] However, more recent numerical simulations[29] made in 2011 indicated that even in the absence of the Moon, Earth's obliquity might not be quite so unstable; varying only by about 20–25°. To resolve this contradiction, diffusion rate of obliquity has been calculated, and it was found that it takes more than billions of years for Earth's obliquity to reach near 90°.[30] The Moon's stabilizing effect will continue for less than 2 billion years. As the Moon continues to recede from Earth due to tidal acceleration, resonances may occur which will cause large oscillations of the obliquity.[31]

[MarkBour]

I love this infographic, or, should I say info-video, and it's nice to see the variety in our system all collected in one image.

A different collection might list a lot more of the bodies that orbit the Sun mostly freely, like Pluto, the other dwarf planets, some large asteroids. There might be some interesting statistical patterns that hold. Looking at this set, it makes me think we did have good luck in our tilt and rotation, but it doesn't make me think we were wildly lucky in this regard. It looks like we're not all that special. Thanks @BDanielMayfield for the remarks on the Moon's stabilizing effect, though.

The rotation shown for Venus is that of its rocky surface, although it is not easy to view the surface from Earth. So they might instead have shown the rotation of its cloud cover. I'm not sure if its cloud cover rotation makes a consistent picture like one gets for the gas giants, but from what I've read I think that might be true. I.e. that Venus' atmosphere rotates somewhat consistently and coherently as a whole. I have read a rotation period of about 4 Earth days. (Wikipedia -- Venus Atmosphere.)

That makes me wonder about the gas giants. I'm trying to fill in some of the details of this APOD, but it's well beyond my knowledge. What this APOD makes me want to ask is clear enough at first: Do any of the gas giants have a solid core boundary (what we Earthlings think of as the ground, or the surface of our planet)? If any of them do, it may be very hard to ever get measurements of the rotation of the core, though I think some scientists would find the challenge a good one. For example, we may one day find that Neptune has a solid or liquid surface. If we did, what would be the likelihood that its surface rotates differently than the cloud tops? In the case of Venus, the only planet I know of where we do have the two measurements, the rotation is surprisingly different between the two. (Surprising to me, anyway.)

[neufer]

we may one day find that Neptune has a solid or liquid surface.
If we did, what would be the likelihood that its surface rotates differently than the cloud tops?

<<Neptune's sidereal rotation period is roughly 16.11 hours. Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet's magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours. This differential rotation is the most pronounced of any planet in the Solar System, and it results in strong latitudinal wind shear.>>

In the case of Venus, the only planet I know of where we do have the two measurements,
the rotation is surprisingly different between the two. (Surprising to me, anyway.)

Angular momentum keeps the planets from falling into the Sun.

Likewise, angular momentum keeps planetary jet streams from falling into the cold poles.

Most of the angular momentum of Earth's jet streams is due to Earth's rotation.

Most of the angular momentum of Venus's jet streams is due to their own jet stream rotation.

The retrograde angular momentum of Venus's equatorial rapid jet streams is not all that
different from the retrograde angular momentum of Venus's slow equatorial surface winds
thanks to the extreme density of the equatorial surface winds.
Momentum sharing between surface winds & jet streams may be due to atmospheric wave interaction.

[BDanielMayfield]

I love this infographic, or, should I say info-video, and it's nice to see the variety in our system all collected in one image.

A different collection might list a lot more of the bodies that orbit the Sun mostly freely, like Pluto, the other dwarf planets, some large asteroids. There might be some interesting statistical patterns that hold. Looking at this set, it makes me think we did have good luck in our tilt and rotation, but it doesn't make me think we were wildly lucky in this regard. It looks like we're not all that special. Thanks @BDanielMayfield for the remarks on the Moon's stabilizing effect, though.

The rotation shown for Venus is that of its rocky surface, although it is not easy to view the surface from Earth. So they might instead have shown the rotation of its cloud cover. I'm not sure if its cloud cover rotation makes a consistent picture like one gets for the gas giants, but from what I've read I think that might be true. I.e. that Venus' atmosphere rotates somewhat consistently and coherently as a whole. I have read a rotation period of about 4 Earth days. (Wikipedia -- Venus Atmosphere.)

That makes me wonder about the gas giants. I'm trying to fill in some of the details of this APOD, but it's well beyond my knowledge. What this APOD makes me want to ask is clear enough at first: Do any of the gas giants have a solid core boundary (what we Earthlings think of as the ground, or the surface of our planet)? If any of them do, it may be very hard to ever get measurements of the rotation of the core, though I think some scientists would find the challenge a good one. For example, we may one day find that Neptune has a solid or liquid surface. If we did, what would be the likelihood that its surface rotates differently than the cloud tops? In the case of Venus, the only planet I know of where we do have the two measurements, the rotation is surprisingly different between the two. (Surprising to me, anyway.)

Note this re Jupiter, from Wikipedia:

Internal structure

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen.[44] Beyond this basic outline, there is still considerable uncertainty. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). In 1997, the existence of the core was suggested by gravitational measurements,[44] indicating a mass of from 12 to 45 times that of Earth, or roughly 4%–14% of the total mass of Jupiter.[43][46] The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation that require the formation of a rocky or icy core massive enough to collect its bulk of hydrogen and helium from the protosolar nebula. Assuming it did exist, it may have shrunk as convection currents of hot liquid metallic hydrogen mixed with the molten core and carried its contents to higher levels in the planetary interior. A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely.[44][47]

The uncertainty of the models is tied to the error margin in hitherto measured parameters: one of the rotational coefficients (J6) used to describe the planet's gravitational moment, Jupiter's equatorial radius, and its temperature at 1 bar pressure. The Juno mission, which arrived in July 2016,[22] is expected to further constrain the values of these parameters for better models of the core.[48]

The core region may be surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet.[43] Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.[31][49] Rainfalls of diamonds have been suggested to occur on Jupiter, as well as on Saturn[50] and ice giants Uranus and Neptune.[51]

Above the layer of metallic hydrogen lies a transparent interior atmosphere of hydrogen. At this depth, the pressure and temperature are above hydrogen's critical pressure of 1.2858 MPa and critical temperature of only 32.938 K.[52] In this state, there are no distinct liquid and gas phases—hydrogen is said to be in a supercritical fluid state. It is convenient to treat hydrogen as gas in the upper layer extending downward from the cloud layer to a depth of about 1,000 km,[43] and as liquid in deeper layers. Physically, there is no clear boundary—the gas smoothly becomes hotter and denser as one descends.[53][54]

The temperature and pressure inside Jupiter increase steadily toward the core, due to the Kelvin–Helmholtz mechanism. At the pressure level of 10 bars (1 MPa), the temperature is around 340 K (67 °C; 152 °F). At the phase transition region where hydrogen—heated beyond its critical point—becomes metallic, it is calculated the temperature is 10,000 K (9,700 °C; 17,500 °F) and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K (35,700 °C; 64,300 °F) and the interior pressure is roughly 3,000–4,500 GPa.[43]

[neufer]

Tilts and Spins

---

Chap. 8 [planets]. Of the Good Success Don Quixote Had, in the Dreadful and Never-Imagined Adventure of the Windmills, with Other Accidents Worthy to Be Recorded

AS they discoursed, they discovered some thirty or forty windmills, that are in that field; and as soon as Don Quixote espied them, he said to his squire, ‘Fortune doth address our affairs better than we ourselves could desire; for behold there, friend Sancho Panza, how there appears thirty or forty monstrous giants, with whom I mean to fight, and deprive them all of their lives, with whose spoils we will begin to be rich; for this is a good war, and a great service unto God, to take away so bad a seed from the face of the earth.’ ‘What giants?’ quoth Sancho Panza. ‘Those that thou seest there,’ quoth his lord, ‘with the long arms; and some there are of that race whose arms are almost who leagues long.’ ‘I pray you understand,’ quoth Sancho Panza, ‘that those which appear there are no giants, but windmills; and that which seems in them to be arms, are their sails, that, swung about by the wind, do also make the mill go.’ ‘It seems well,’ quoth Don Quixote ‘that thou art not yet acquainted with matter of adventures. They are giants; and, if thou beest afraid, go aside and pray, whilst I enter into cruel and unequal battle with them.’ And, saying so, he spurred his horse Rozinante, without taking heed to his squire Sancho’s cries, advertising him how they were doubtless windmills that he did assault, and no giants; but he went so fully persuaded that they were giants as he neither heard his squire’s outcries, nor did discern what they were, although he drew very near to them, but rather said, so loud as he could, ‘Fly not, ye cowards and vile creatures! for it is only one knight that assaults you.’

With this the wind increased, and the mill sails began to turn about; which Don Quixote espying, said, ‘Although thou movest more arms than the giant Briareus thou shalt stoop to me.’ And, after saying this, and commending himself most devoutly to his Lady Dulcinea, desiring her to succor him in that trance, covering himself well with his buckler, and setting his lance on his rest, he spurred on Rozinante, and encountered with the first mill that was before him, and, striking his lance into the sail, the wind swung it about with such fury, that it broke his lance into shivers, carrying him and his horse after it, and finally tumbled him a good way off from it on the field in evil plight. Sancho Panza repaired presently to succor him as fast as his ass could drive; and when he arrived he found him not able to stir, he had gotten such a crush with Rozinante. ‘Good God!’ quoth Sancho, ‘did I not foretell unto you that you should look well what you did, for they were none other than windmills? nor could any think otherwise, unless he had also windmills in his brains.’ ‘Peace, Sancho,’ quoth Don Quixote; ‘for matters of war are more subject than any other thing to continual change; how much more, seeing I do verily persuade myself, that the wise Frestron, who robbed my study and books, hath transformed these giants into mills, to deprive me of the glory of the victory, such in the enmity he bears towards me.

[astroarchitect1]

Interesting visual. Is it correct to assume that Uranus behaves like a gyroscope and it's poles always point in the same direction no matter where it is located in it's orbit around the sun?

[neufer]

Interesting visual. Is it correct to assume that Uranus behaves like a gyroscope and it's poles always point in the same direction no matter where it is located in it's orbit around the sun?

The Earth behaves like a gyroscope and its polar axis "always" points in the same direction (i.e., Polaris) no matter where it is located in it's orbit around the sun...except for a tidal torque from a large Moon (and the Sun) on an oblate Earth causing a ~25,772 year precession of the equinoxes

Other planets act likewise except:

• 1) they have much smaller moons (relatively) and
  2) they are further from the Sun and/or they are less oblate.

Ergo the other planets have a much slower precession of the equinoxes.

(Furthermore: a planet with equatorial moons and an axial tilt near 90º is particularly immune to precession.)

[E Fish]

That nighttime side of Venus even slowly retrograde rotates
....as if it is constantly tracking Earth.

Venus seems to be very very leery of the Earth

Thanks for the extra info!

[bystander]

How Uranus ended up on its side
Popular Science | 2019 Jan 24

New research provides some clues about our solar system's most mysterious planet.

Click to play embedded YouTube video.

Uranus is arguably the most mysterious planet in the solar system – we know very little about it. So far, we have only visited the planet once, with the Voyager 2 spacecraft back in 1986. The most obvious odd thing about this ice giant is the fact that it is spinning on its side.

Unlike all the other planets, which spin roughly “upright” with their spin axes at close to right angles to their orbits around the sun, Uranus is tilted by almost a right angle. So in its summer, the north pole points almost directly towards the sun. And unlike Saturn, Jupiter and Neptune, which have horizontal sets of rings around them, Uranus has vertical rings and moons that orbit around its tilted equator.

The ice giant also has a surprisingly cold temperature and a messy and off-centre magnetic field, unlike the neat bar-magnet shape of most other planets like Earth or Jupiter. Scientists therefore suspect that Uranus was once similar to the other planets in the solar system but was suddenly flipped over. So what happened? Our new research ... offers a clue. ...

Consequences of Giant Impacts on Early Uranus for Rotation,
Internal Structure, Debris, and Atmospheric Erosion ~ J. A. Kegerreis et al

• Astrophysical Journal 861(1):52 (2018 Jul 01) DOI: 10.3847/1538-4357/aac725
• arXiv.org > astro-ph > arXiv:1803.07083 > 19 Mar 2018 (v1), 03 Jul 2018 (v2)