Two Moons and a Ringed Planet

Marcus Chown’s book, Infinity in the Palm of Your Hand, asks and answers many, many fascinating questions. I’ll mention a few of them here.

 

Why is there no pic of Neil Armstrong taking his famous “one small step for man”? The iconic photo of the moon landing, instead, is a pic of the other astronaut, Buzz Aldrin! How on earth moon could that have happened? Well, the camera was mounted on the astronaut’s chest and:

“It had no viewfinder and they had to guess what was visible through the lens.”

Unbelievable, right? But more problematically:

“The surface of the moon poses unique challenges for photography.”

It’s got to do with the fact that there’s no air on the moon. Yes, but what’s the connection? On earth, sunlight is scattered by the air, “softening its harshness” and “spreading it even into the shadow so they are not completely black”. On the moon, with no air:

“A camera must contend with dazzling brightness alternating with utter blackness, the two zones separated by a knife-edge boundary. Not an easy thing for an exposure meter to contend with.”

Not an unsolvable problem, but back in 1969, NASA’s priority probably wasn’t to ensure that Armstrong and Aldrin got great selfies!

 

Which (known) object in the solar system generates the most heat, on a heat per unit of mass basis? The sun, obviously, right? Wrong! It is Jupiter’s tiny moon named Io. But what could possibly be the source of so much heat?!

 

Remember how gravity causes tides? Well, the difference in the gravitation force acting on the side facing the planet vs the opposite side also stretches the rocks on the moon. Yes, rocks get stretched, then contract back. This repeated stretching and contracting generates a lot of heat. In Io’s case, this is amplified further because of the add-on effect of two of Jupiter’s other moons:

“For every four circuits Io makes of Jupiter, Europa completes two and Ganymede one.”

Because their orbits are integer multiples of each other, the two much larger satellites (Ganymede is bigger than Mercury!) tend to line up perfectly frequently with Io very often:

“The effect is to yank Io, elongating its orbit; so it swings in close to Jupiter and then flies back out again, repeatedly. And it is this motion, it turns out, that is behind Io’s prodigious heating.”

Poor Io gets stretched even more as it flies closer to Jupiter thanks to its elliptical orbit. To compound the effect, unlike our moon, Io takes only 1.7 days to go around Jupiter once. So you see how frequently the combined effects of Jupiter and those two moons add to Io’s heating? So much so that Io’s “interior is heated to melting point”! And that is why Io generates more heat (on a heat per mass basis) than even the sun.

 

And here’s the last one for this blog. When Galileo pointed his telescopes at Saturn in 1610, based on what he saw, he declared it to be “a planet with ears”. Huh, why? In 1611, he changed his mind and thought it had 2 moons, one on either side. In 1612, the moons had vanished. In 1613, the moons reappeared. What was going on?

 

Poor Galileo never got to know. His telescope wasn’t powerful enough to reveal Saturn’s “big secret”: it had rings. Ok, but how does that explain Galileo’s weird observations? The answer lies in the fact that Saturn’s rings are tilted (the way the earth is tilted at 23 ½ degrees). This tilt of the rings in turn results in the view from earth showing different things as Saturn moves around the sun. At one angle of view, they vanish, and at another, yes, they appear like ears!

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