Venus Spinning Slower Than Thought

[Andre]

Apparantly this one was missed, at least nothing turned up on a search.

Venus Spinning Slower Than Thought—Scientists Stumped...

According to the new data, Venus is rotating 6.5 minutes slower than it was 16 years ago, a result that's been found to correlate with long-term radar observations taken from Earth.

Obviously you can speculate all you want about atmosphere and winds but you can't explain away the loss of angular momentum.

But then again, a long time ago, when thinking out the box was still allowed, there was a thread speculating about how Venus could have lost its spinning and what happened to the related energy. For those who are familiar with that, there is no reason at all to be 'stumped'.

 

[Constantin]

It's more likely the outer crust has been migrating over the molten interior.

I've read something in that article about the features on the surface being 20 km from where they should be. A 20 km migration of the crust doesn't seem that much to me, considering Venus' volcanic past, with a lot of its surface covered with lava.

 

[Andre]

It's more likely the outer crust has been migrating over the molten interior.

I've read something in that article about the features on the surface being 20 km from where they should be. A 20 km migration of the crust doesn't seem that much to me, considering Venus' volcanic past, with a lot of its surface covered with lava.

Oh but before thinking out of the box, one should be thorougly familiar with the box first, to paraphrase Lisab.

Here is some study material:

http://www.imcce.fr/Equipes/ASD/preprints/prep.2002/venus1.2002.pdf
http://www.gps.caltech.edu/classes/ge151/references/phillips_and_hansen_1998.pdf

 

[Constantin]

The best theory I can remember for the rotation of Venus is that the planet was hit by another proto-planet in the distant past, close to the time the solar system was created.
Such impacts are also attributed for the the creation of Earth's Moon, for an extremely large crater on Mars which covers most of Mars' north side, for the peculiar inclination of Uranus, and maybe other examples.

As for the current observed slowing down, I don't believe it is possible to slow down an entire planet. You can however slow down the movement of it's outer crust. Whether it's the atmosphere doing this, or some other phenomenon, that's a matter for research.

 

[DaveC426913]

As for the current observed slowing down, I don't believe it is possible to slow down an entire planet. You can however slow down the movement of it's outer crust. Whether it's the atmosphere doing this, or some other phenomenon, that's a matter for research.

It's all those gravity-assists by those Earthers and their probes!

First air pollution, light pollution and noise pollution - now they've invented celestial mechanical pollution!

 

[Hurkyl]

I don't believe it is possible to slow down an entire planet. You can however slow down the movement of it's outer crust.

If you believe Wikipedia, the moon lengthens an Earth day by 15 microseconds per year, and the 2004 Indian Ocean earthquake shortened the Earth day by over 2 microseconds.

 

[Hurkyl]

Apparantly this one was missed, at least nothing turned up on a search.

Do you have a reference that isn't science entertainment?

 

[Andre]

Do you have a reference that isn't science entertainment?

http://www.esa.int/SPECIALS/Venus_Express/SEM0TLSXXXG_0.html

 

[Constantin]

If Venus has a rather thin crust over a molten layer, that crust can shift under the influence of the atmosphere.
The thinner the crust, the bigger this effect can be.

Atmospheric weather and earthquakes only affect the movement of the crust, not of the entire planet.
Tidal forces slow down the entire planet, but this effect is very very slow.

 

[PhilDSP]

If Venus has a rather thin crust over a molten layer, that crust can shift under the influence of the atmosphere.
The thinner the crust, the bigger this effect can be.

Wouldn't the viscosity of the magma also be higher in the areas near the crust because more heat is trapped there than on Earth? Or least the magma might be more completely molten throughout the border areas. The very frequent intrusion of magma onto the planet's surface, not just from volcano tops, is very much evident from the Magellan imaging.

 

[D H]

Venus' atmosphere is very weird. Venus as a planet rotates very slowly, once per 243 days, and retrograde with respect to the solar system's angular momentum. Venus' upper atmosphere rotates very quickly, once every four days, and posigrade with respect to the solar system's angular momentum (so retrograde with respect to Venus).

One obvious (and probably the most obvious) explanation is that there must be some unexplained decades-long variations in Venus' atmosphere that result in transfer of angular momentum between Venus proper and it's atmosphere. The problem is the mechanism. Scientists do not know, and this site is not the place to be making speculations.

We do see this transfer of angular momentum between planet and atmosphere right here on the Earth. The Earth's rotational rate varies with the seasons (the Earth slows down a tiny bit in January and February, then speeds up again), and it turns out that the atmosphere is the key cause.

We also see decadal variations in the Earth's rotation rate. Those variations remain an unexplained mystery, at least to some extent. There are too many possible culprits, too little data to narrow down the cause. We have millions and millions of measurements for the Earth and yet the Earth's rotation is not a fully solved problem. We have orders of magnitude fewer measurements of Venus. Is it any surprise that Venus' rotation is not a fully solved problem?

 

[Hurkyl]

http://www.esa.int/SPECIALS/Venus_Express/SEM0TLSXXXG_0.html

While further study is needed

So grain of salt, and all of that. You indicate there has been no further news, so I'm forced to conclude that there really wasn't anything noteworthy.I must say, I'm rather befuddled that claims of the day having slowed down were made by observing a 0.00186% error in an extrapolation from 20 year old data rather than, y'know, re-measuring the rotation of Venus.

Amusingly, the error in the length of the Venusian day between the two articles is nearly as large as the reported error of 6.5 minutes.

IMO the fact we haven't heard anything again is far more likely to be publication bias than anything else: that the announcement was made prematurely, and nobody thought it was worth publishing a retraction.

Or, it is true but uninteresting, and the fact it was even mentioned in the first place is an anomaly.

 

[D H]

If Venus has a rather thin crust over a molten layer, that crust can shift under the influence of the atmosphere.

Venus has a thick crust over a solid mantle. Planetary mantles are solid, not liquid.

 

[Constantin]

Venus has a thick crust over a solid mantle. Planetary mantles are solid, not liquid.

Parts of the Earth's mantle are made of "plastic flowing rock".
I've seen no evidence for "Venus has a thick crust over a solid mantle". Do you have a reference for that ?

You're obviously not an expert on this subject, but as you're a moderator you give infractions and your words on a subject your knowledge seems limited on, are supposed to carry weight.

 

[Andre]

About the data, the article states:

Magellan was able to watch features rotate under the spacecraft , allowing scientists to determine the length of the day on Venus as being equal to 243.0185 Earth days.

I learned on school that if no further details are given, one has to assume that the error margin is equal to the last digit: 243.01865 +/- 0.00001, that's almost 9 seconds.

However

However, surface features seen by Venus Express some 16 years later could only be lined up with those observed by Magellan if the length of the Venus day is on average 6.5 minutes longer than Magellan measured.

That's well over an order of magnitude compared to the error margin

Furthermore it's interesting to understand how the accuracy in astronomy and space exploration can decay over time, when far before Magellan, success or failure depended on many more digits accuracy.

 

[Hurkyl]

I learned on school that if no further details are given, one has to assume that the error margin is equal to the last digit: 243.01865 +/- 0.00001, that's almost 9 seconds.

But how confident can one really be in that assumption?

But it's interesting to note that web searching for the figure already shows much greater error than that. The two articles you linked show 243.015 and 243.0185. Wikipedia reports the latter. NASA's page on Venus says 243.018, and also reports the figure in hours: 5832.4 -- a figure with listed precision three digits less than yours!

I could not find any references that have error bounds on this figure.

Incidentally, your post is the only Google hit for "243.01865 venus".

 

[D H]

I learned on school that if no further details are given, one has to assume that the error margin is equal to the last digit: 243.01865 +/- 0.00001, that's almost 9 seconds.

I'll give you the benefit of the doubt here and assume you meant 243.0185 ± 0.0001 days. The ESA article said 243.0185 days, not 243.01865, and your 9 seconds is just about 0.0001 days.

That said, that's a bad assumption. You need to look deeper if you aren't given further details and you need those details. You might have just gotten lucky with that assumption of a 9 second error. The scientific literature is always a good place to look deeper.

The two articles you linked show 243.015 and 243.0185. Wikipedia reports the latter.

The latter figure is correct. The NatGeo article linked in the OP made the opposite mistake that Andre did. They omitted a digit.

NASA's page on Venus says 243.018

That site is for kids!

I could not find any references that have error bounds on this figure.

From Davies et al., The Rotation Period, Direction of the North Pole, and Geodetic Control Network of Venus, Journal of Geophysical Research, 97:E8 (1992) DOI:10.1029/92JE01166 http://www.agu.org/pubs/crossref/1992.../92JE01166.shtml

"A rotation period estimate of 243.0185 ± 0.0001 days was determined via the ephemeris improvement technique applied simultaneously to two overlapping orbit blocks with many common points and separated by two full Venus rotations."

 

[Andre]

Sorry about the typo. Obviously I hit two keys with that figure and overlooked it.

And indeed that was a too hasty publication of ESA. They should have investigated first and then they would have quickly found http://www.ssec.wisc.edu/venus-workshop-submission/files/schubert_gerald-2.pdf.

Comparison of Magellan topography and VIRTIS thermal mapping data suggests a rotation period of about 243.023 days compared with the 243.0185 days recommended by the IAU (Mueller et al. , 2010).

That's consistent with the 6.5 minutes, but if that would have been the average decay in spinning since Magellan, then the last measurements would have to be much slower. Obviously Magellan was not accurate.

Sorry about the fuss.

 

[D H]

And indeed that was a too hasty publication of ESA. They should have investigated first and then they would have quickly found http://www.ssec.wisc.edu/venus-workshop-submission/files/schubert_gerald-2.pdf. That's consistent with the 6.5 minutes, but if that would have been the average decay in spinning since Magellan, then the last measurements would have to be much slower.

Andre, ESA didn't miss your "this". Your this is a conference paper on some preliminary results from the VIRTIS, one of the scientific instruments aboard the Venus Express. Your this is not just consistent with that reported 6.5 minute change in Venus' rotation period, it is the source of that reported change.

The problem is that your this was just a conference paper, the finding was but one part of that paper, and the finding was preliminary. I suspect that there's a journal paper somewhere in the pipeline, particularly since this change has now been confirmed independently. Problem: We don't know if there is such a paper. The best thing to do at this point is to wait. If a paper comes out, great. We can continue the discussion then.

 

[Andre]

I'm confused. Maybe I should have quoted this sentence:

Earth-based radar observations of Venus covering the period 2006-2009 are
consistent with the 243.023 day period (Margot, private communication)

.

I wonder if somebody at ESA had picked up the phone and did the same, wouldn't they have brought out another story?

Also (oh please don't ley me make such a horrible typo again...) the difference between 234.023 and 234.0185 days, 0.0045 days is 6 min and 29 sec. So if Margot sees that 'now' too (2007 - 2009) - and the difference in real and assumed position of the geography -as per Magellan- is consistent with an average slowdown of 6:30 min per evolution, then, expecting a gradual decay, the first revolution after Magellan would not have been so much less, hence the last revolutions would have been much slower ie some 13 minutes less - to meet that average. But obviously that's not what Margot appears to have observed. So things seems only consistent if Magellan was just off by that bit.

 

[D H]

I wonder if somebody at ESA had picked up the phone and did the same, wouldn't they have brought out another story?

It's right there in the ESA press release to which you linked in post #8: "This also agrees with the most recent long-duration radar measurements from Earth."

I suspect that there's a journal paper somewhere in the pipeline, particularly since this change has now been confirmed independently. Problem: We don't know if there is such a paper.

Problem: I typed before I searched. There is a paper, published in the February 2012 issue of Icarus. The ESA press release web page cited in post #8 is dated 10 February 2012. The press release was connected to a paper; it just didn't specify the paper. I've seen that behavior before with ESA pronouncements. Here's the paper:

N.T. Mueller et al., Rotation period of Venus estimated from Venus Express VIRTIS images and Magellan altimetry, Icarus 217:2 (2011) DOI: 10.1016/j.icarus.2011.09.026

 

[Andre]

That's here.

Reading the abstract, it's still not clear to me, if Magellan was off or not or if it's length of day variation.

Incidently, if you want to have the atmosphere exchange momentum with the planet, you need to apply a net torque between the two. I would expect that to be noticable at the surface of Venus in the form of wind. I seem to remember that the Venera's only reported light winds. Also with the high mass of the atmosphere, and some wind forming, you'd expect clear wind erosion features on the surface. But I don't recall that anybody suggested that after analysing the Magellan imagery

 

[Andre]

Meanwhile Mueller et al 2011 conclude:

Therefore it might be possible that the long time baseline estimates represent the average spin rate while the Magellan radar and gravity observations were made during a time when the spin rate deviated from its average. All discussed estimates that do not exclusively use Magellan data have a time baseline of at least 8 years and are formally consistent with a period of rotation of 243.023 ± 0.002 days. The Magellan radar (Davies et al., 1992) and gravity (Konopliv et al., 1999) estimates are not consistent with this value, but observe each a 2 year period between 1990 and 1994. Thus a short, singular or periodic length of day excursion could explain why the Magellan radar period of rotation estimate differs by 5 min from the estimates with longer time baselines. A possible explanation for such spin period variations is angular momentum exchange between the solid body of Venus and its thick, superrotating atmosphere (e.g. Schubert, 1983). Assuming relative atmospheric angular momentum exchanges similar to Earth (Hide et al., 1980), length of day variations about 1 h are possible (Golitsyn, 1982; Schubert, 1983).

... which returns to my question, if the exchange of momentum via surface winds can be supported by observed evidence of wind erosion or by measurements of the Venera landers.

 

[Hurkyl]

"A rotation period estimate of 243.0185 ± 0.0001 days was determined via the ephemeris improvement technique applied simultaneously to two overlapping orbit blocks with many common points and separated by two full Venus rotations."

Thanks! This, and the ensuing discussion, have answered (or, at least, responded to) all of the questions I've since thought to ask.

 

[Andre]

... which returns to my question, if the exchange of momentum via surface winds can be supported by observed evidence of wind erosion or by measurements of the Venera landers.

Anyway an essential read about the transfer of angular momentum between planet and atmosphere is Karatekin et al 2011

Reading suggests that they confirm that the momentum transfer medium is wind and they they state:

Spacecraft observations on Venus suggest the presence of Hadley cells, reaching to about 50–65 km altitude above the surface and an anti-solar mesospheric circulation at the top of the atmosphere (e.g.Sevdhem,2009)

Unfoirtunately Svedhem 2009 is not in the references. Anyway for Hadley cells to exist, one needs a vertical atmospheric lapse rate that is potentially unstable, ie temperature drop with altitude bigger than the dry adiabatic lapse rate., which can be calculated for Venus to 10.468 degrees Kelvin/km

Now, about the actual lapse rate of Venus, (nightside) we see at the right some 490k at 38 km altitude. At the surface we have temperatures of about something like 740 kelvin hence we have an average lapse rate of about roughly 6,6 Kelvin/km in the lower part of the atmosphere, which makes it very stable. So obviously it's hard for a vertical convection mechanism to exists -at the surface-, which drives the Hadley cell.

Incidentely, the Venera landers observed as surface wind:

The transport time of the dust cloud above the landers is shown to correspond to a wind speed of 0.5 to 1 m/s.

So it seems that's going to be hard to find factual evidence of surface winds that could cause transfer of momentum

 

[D H]

Thread locked pending moderation.