APOD: Venus at Night in Infrared from Akatsuki (2018 Jan 30)

[APOD Robot]

Venus at Night in Infrared from Akatsuki

Explanation: Why is Venus so different from Earth? To help find out, Japan launched the robotic Akatsuki spacecraft which entered orbit around Venus late in 2015 after an unplanned five-year adventure around the inner Solar System. Even though Akatsuki was past its original planned lifetime, the spacecraft and instruments were operating so well that much of its original mission was reinstated. Also known as the Venus Climate Orbiter, Akatsuki's instruments investigated unknowns about Earth's sister planet, including whether volcanoes are still active, whether lightning occurs in the dense atmosphere, and why wind speeds greatly exceed the planet's rotation speed. In the featured image taken by Akatsuki's IR2 camera, Venus's night side shows a jagged-edged equatorial band of high dark clouds absorbing infrared light from hotter layers deeper in Venus' atmosphere. The bright orange and black stripe on the upper right is a false digital artifact that covers part of the much brighter day side of Venus. Analyses of Akatsuki images and data has shown that Venus has equatorial jet similar to Earth's jet stream.

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[Boomer12k]

Interesting...kinda spooky...

:---[===] *

[johnemac]

I read an article about the slowing of the earth's rotation around 40 to 60 years ago, probably in the Scientific American
magazine, to the effect that one third of the slowing was caused by frictional losses due to surface winds. I remember
thinking at that time, "I wonder if Venus' loss of angular momentum was caused by that huge atmosphere and the
friction of its extreme winds" (they mentioned 400 mph winds in that article). I'm kinda thinkin that that had to be a major
factor, if not the primary cause of that spindown. Does this sound like a reasonable hypothesis?

[neufer]

I read an article about the slowing of the earth's rotation around 40 to 60 years ago, probably in the Scientific American
magazine, to the effect that one third of the slowing was caused by frictional losses due to surface winds. I remember
thinking at that time, "I wonder if Venus' loss of angular momentum was caused by that huge atmosphere and the
friction of its extreme winds" (they mentioned 400 mph winds in that article). I'm kinda thinkin that that had to be a major
factor, if not the primary cause of that spindown. Does this sound like a reasonable hypothesis?

The total rotational angular momentum of a planet (solid + liquid + gas) must transfer to another solar system body in order to change. In the case of Earth the gravitational ocean tides peak after they have passed under the Moon & Sun allowing for the Moon & Sun to then pull back on them to slow Earth's rotation. In the case of Venus the thermal & gravitational atmospheric tides peak after they have passed under the noontime Sun (thanks to rapid high altitude Venus winds) allowing for the Sun to then pull back on it to slow Venus's rotation in a long process of tidal locking.

<<All the planets in the Solar System orbit the Sun in a counterclockwise direction as viewed from above Earth's north pole. Most planets also rotate on their axes in an anti-clockwise direction, but Venus rotates clockwise in retrograde rotation once every 243 Earth days—the slowest rotation of any planet. Venus's equator rotates at 4.05 mph, whereas Earth's rotates at 1,037.6 mph. Venus's rotation has slowed down in the 16 years between the Magellan spacecraft and Venus Express visits; each Venusian sidereal day has increased by 6.5 minutes in that time span.>>

[sillyworm2]

One of the more fascinating & intriguing pictures I have seen here.

[Woobie]

I read an article about the slowing of the earth's rotation around 40 to 60 years ago, probably in the Scientific American
magazine, to the effect that one third of the slowing was caused by frictional losses due to surface winds. I remember
thinking at that time, "I wonder if Venus' loss of angular momentum was caused by that huge atmosphere and the
friction of its extreme winds" (they mentioned 400 mph winds in that article). I'm kinda thinkin that that had to be a major
factor, if not the primary cause of that spindown. Does this sound like a reasonable hypothesis?

The total rotational angular momentum of a planet (solid + liquid + gas) must transfer to another solar system body in order to change. In the case of Earth the gravitational ocean tides peak after they have passed under the Moon & Sun allowing for the Moon & Sun to then pull back on them to slow Earth's rotation. In the case of Venus the thermal & gravitational atmospheric tides peak after they have passed under the noontime Sun (thanks to rapid high altitude Venus winds) allowing for the Sun to then pull back on it to slow Venus's rotation in a long process of tidal locking.

<<All the planets in the Solar System orbit the Sun in a counterclockwise direction as viewed from above Earth's north pole. Most planets also rotate on their axes in an anti-clockwise direction, but Venus rotates clockwise in retrograde rotation once every 243 Earth days—the slowest rotation of any planet. Venus's equator rotates at 4.05 mph, whereas Earth's rotates at 1,037.6 mph. Venus's rotation has slowed down in the 16 years between the Magellan spacecraft and Venus Express visits; each Venusian sidereal day has increased by 6.5 minutes in that time span.>>

O.K., so then do you suppose the winds are in the process of slowing down due to fiction with the planet? When the planet is finally totally tidal locked will the winds also eventaully slow to a stop?

[MarkBour]

I'm just wanting to check my understanding of the basic statements in Wikipedia (paraphrased, rather than directly quoted).

• Venus has an orbit, proceeding in the usual direction for planets in our Solar system (a counter-clockwise direction, if viewed from a point above the ecliptic plane, that would look down on Earth's North pole), with an orbital period of 224 Earth-days.
• Venus has a north pole that is nearly straight up from the plane of its orbit (inclined only 2.6&deg; from it) and it rotates about this pole in the opposite direction of other planets, rotating clockwise, with a sidereal day of 243 Earth-days.
• Venus is undergoing a process of tidal locking with the Sun, so its rotation is being slowed, until eventually it will be tidally locked.

If we don't assume any change in Venus' orbit, then, when it becomes tidally locked, it will be rotating counter-clockwise in the above perspective with a sidereal day of 224 Earth-days. So, part-way through this process, Venus will experience a total stop of its rotation from the perspective of the fixed stars, with a sidereal day that is infinite (until it starts slowly rotating clockwise from this viewpoint).

If this is correct, then it would at that point be a great time to be an astronomer on Venus (in some ways), except for the unfortunate irony that the cloud-cover spoils any view of anything. Those of us who complain about cloud-cover spoiling everything neat that comes up in Earth's skies (I am guilty) can then, by comparison, count our lucky stars (literally).

Incidentally, if a planet's orbit is eccentric, then by Kepler's laws, I doubt it can ever become truly tidally locked (I mean I think the Sun would not be totally fixed in the planet's sky). It might be fun to work out how such a body would actually behave in its view of the Sun, and what tidal dynamics might remain, with what effect over long periods of time. Venus happens to have very little eccentricity in its orbit, so I assume this will not be an interesting case. It is interesting in the case of Mercury, where the orbit is eccentric and the effect of orbital speed-up near perihelion messes with the solar position quite visibly. But I think Mercury is far more complicated than my simple mental model, so there's probably more going on than I realize.

I also note that Mercury has almost zero axial tilt. Is this guaranteed for a tidally-locked body? I mean it seems as though the tidal locking process is going to result in that. Is that correct?

[Chris Peterson]

If this is correct, then it would at that point be a great time to be an astronomer on Venus (in some ways), except for the unfortunate irony that the cloud-cover spoils any view of anything.

The clouds only block certain parts of the electromagnetic spectrum. That need not include what would be accessible to the imaging organs of Venusian astronomers, and certainly not what would be accessible to their instruments.

[neufer]

O.K., so then do you suppose the winds are in the process of slowing down due to fiction with the planet? When the planet is finally totally tidal locked will the winds also eventaully slow to a stop?

Venus is slowly rotating clockwise now once every 243 days.

When (and if) Venus becomes tidally locked
... it will be slowly rotating counter clockwise once every 224 days.

There is no reason to believe that the winds will be significantly different then
than they are now (other than spinning in the opposite [i.e., "normal"] direction).

However, the thermal & gravitational tides will peak at noon rather than
in the afternoon so there will no longer be any torque on the planet.

[pferkul]

Here is a slightly enhanced version, without the very odd artifact colors.

[MarkBour]

I found this article interesting. Apparently those very fast winds are getting faster ...
http://www.esa.int/Our_Activities/Space ... ing_faster

[MarkBour]

If this is correct, then it would at that point be a great time to be an astronomer on Venus (in some ways), except for the unfortunate irony that the cloud-cover spoils any view of anything.

The clouds only block certain parts of the electromagnetic spectrum. That need not include what would be accessible to the imaging organs of Venusian astronomers, and certainly not what would be accessible to their instruments.

Good point. I found the following article, so folks have figured out a lot about the transmissivity of the Venusian atmosphere (clouds included).
https://www.sciencedirect.com/science/a ... 3516305851
So, if Venus had the conditions for complex life to evolve, I wonder what their eyes would be like (or if we'd call them eyes). It looks like a nice band of infrared is available, so they might not be very different from ours, just sensitive to a different band of wavelengths. Would they ever see stars at night? I guess it is reasonable that they could.

[Ann]

My computer crashed last week, so I haven't been able to really write anything here, and I missed this APOD. But I have to admit that this portrait of Venus is stunning.

Also I'm impressed that the Japanese scientist managed to save their probe despite its potentially catastrophic engine breakdown.

Ann

[neufer]

Click to play embedded YouTube video.

My computer crashed last week, so I haven't been able to really write anything here, and I missed this APOD. But I have to admit that this portrait of Venus is stunning.

Also I'm impressed that the Japanese scientist managed to save their probe despite its potentially catastrophic engine breakdown.