Venus Spinning Slower Than Thought

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 this.

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

Quote by Andre

And indeed that was a too hasty publication of ESA. They should have investigated first and then they would have quickly found this. 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

Quote by Andre

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."

Quote by D H

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 lenght 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

Quote by D H

"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

Quote by 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

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