Astronomy Cast on Mercury

I recently started listening to the Astronomy Cast podcast1. It does a marvellous job of explaining the crazy universal physics I enjoy letting blow my mind. Dark matter, dark energy, relativity, spaghettification, black holes, the (endlessly fascinating) cosmic microwave background…it’s all wonderful stuff.

But it’s not all far-out frontiers of science. Sometimes it’s a little more local. I recently learnt that geysers regularly explode out of Enceladus, one of Saturn’s moons. The water is flung out into the vacuum, where it instantly crystallises and over time has formed a ring. Isn’t that a beautiful image?

The title of the most recent episode was ‘Mercury‘. I was happy to listen while driving home from dancing, but didn’t anticipate anything special. I figured I knew the most interesting Mercury facts: it’s hard to see as it doesn’t get far from the sun, it was a good test of general relativity versus Newtonian gravity, something about a magnetic field…Otherwise, reasonably uninteresting, as planets go.

Despite years of ingesting popular science, it apparently hasn’t yet sunk in that a) I know nothing and b) everything is interesting. Mercury, unsurprisingly, is very cool:

  • Ice. Radio telescopes suggest there *might* be (water-)ice in the pole craters, which lie permanently in shadow. Ice! Probably from comets.
  • It’s made of iron, and extremely dense. The most dense object in the solar system, in fact. This presents a problem – how did this happen? Probably it’s the iron core of a once larger planet, in which case, what happened to the rest? Did something slam into it? Did a young sun blast its surface away?
  • Venus is hotter than Mercury, due to the former’s runaway greenhouse effect.
  • At times when Mercury is visible, we always see the same face. For decades it was assumed that the planet is tidally locked – it rotates once per orbit, like the moon to the Earth, only ever showing the same face to the sun. Radio telescopes showed this isn’t true: the back of the planet is incredibly hot, which shouldn’t be the case. It turns out that because Mercury’s orbit is particularly elliptical it’s become locked into a pattern of 3 rotations per 2 orbits.
  • It has a magnetic field. Weird. Magnetic fields are caused by molten cores. Mars used to have a molten core and magnetic field, but cooled. Theory suggests that, despite its proximity to the sun, Mercury should have cooled similarly. Why hasn’t it? Current theories suggest that the aforementioned weird orbit pulls and pushes on the planet in such a way that its core remains molten.
  • The far side of Mercury is mysterious. It’s incredibly difficult to observe, but tentative low-resolution imagery has hints of a massive crater and mountain. Did Mercury get hit by something enormous enough to seriously deform its iron structure? NASA’s Messenger spacecraft, en-route and scheduled for arrival in 2011, should provide answers.

There’s plenty more in the show.

It’s quickly become one of my must-listen podcasts, and is never less than fascinating. Large amounts of kudos and thanks to Fraser Cain and Dr. Pamela Gay for the hard work they put in – it must take some serious weekly research.

  1. sometimes only half of it, as my iPod is dying. Sob. []