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Sunday 14 August 2016

Ask An Astronomer Anything At All About Astronomy (XXVIII)

Let there be questions ! And let there also be answers both sarcastic and true !

1) Are black holes doorways to another dimension ?

2) How can we see the inner and outer planets at the same time ?
Because celestial mechanics, that's how.

3) How small a fraction of the deuterium was created during Big Bang Nucelosynthesis ?
All of it.

4) How about that spiral arms are density waves theory, eh ?
Yeah, it's great.

5) Which planets are the most and least spherical ?
Your mum.

6) Do solar winds keep star systems clear of dark matter ?
Dark matter isn't scared of no stupid stars.


  1. Hi, Rhys. Here's my question ...

    Q: When the Venus Pioneer mission was sent to investigate the source of Venus' heat, it was initially determined that Venus is not in thermal equilibrium -- that in fact the planet is releasing 15% more heat than it is taking in, an apparent violation of Carl Sagan's Super Greenhouse Theory (arguably the primary focus of this mission was to generate evidence for the greenhouse effect). A glance at the original papers reveals that the data was corrected to reflect the greenhouse theory as an assumption. It seems that when the data did not cooperate, theorists simply assumed their hypothesis.

    My question is: How do theorists rule out an alternative cause for this heat, such as a recent planetary catastrophe? Is it not possible that the planet is actually cooling down from some recent event?

    See snapshots of the original papers here:

    1. There's no way I'm going to read 23 papers (and books !) just to answer a question. It's not even clear which quote comes from which source.

      What I will say is that atmospheric physics is _hard_. For instance there's a strangely colder layer in the upper regions of Venus' atmosphere :
      Like asteroid impacts, planetary atmospheres are a grey area between astronomy and geophysics. So I don't feel at all comfortable in trying to understand the details of whether Venus is in thermal equilibrium or not, or whether the instrumental measurements had significant errors and were adjusted for good reason or because of malpractice. My experience in observational astronomy suggests that errors are usually larger than people like to claim, but I know nothing of this specific instrument or research area.

      Were Venus to have a negligible atmosphere like Mercury, I might try a simple calculation to work out how far in the past it would have been molten in order to get an estimate of when the catastrophe occurred. But the presence of that incredibly thick and complex atmosphere - which at the very least _is_ going to provide a lot of insulation, runaway greenhouse or no, strongly suggests that this wouldn't give a meaningful result. Someone with better knowledge of radiation transport could probably model it.

      So could we rule out cooling in some other way ? Hard to say, you should talk to a geologist or a meteorologist or a planetary scientists, or preferably all three. Depends what sort of catastrophe, I guess. The surface of Venus doesn't look much like Earth, or Mars, or Mercury - but then it's more massive than Mercury, much hotter than Earth or Mars, and has by far the thickest atmosphere. So probably it's not too surprising that it also has very different topography. The problem would be to differentiate between this naturally different morphology due to the different properties of the planet and the difference caused by whatever catastrophe is proposed. As far as I know, planetary models aren't sophisticated enough for us to predict things with this level of detail yet.


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