Europa & Enceladus Might Harbor Oceans With Tides, Life

The scoop: Our eyes might be on Saturn's icy moon Enceladus, but scientists haven't forgotten about Jupiter's moon Europa.
Richard Greenberg, Galileo spacecraft scientist and author of "Unmasking Europa," chats about the potential for oceanic life off Earth.

Dave on Earth (9:14 AM): Good morning Dr. Greenberg!

Europa Explorer (9:17 AM): Hi Dave.

Dave on Earth (9:18 AM): Hello there -- thanks for doing this.

Europa Explorer (9:20 AM): Ok, but I'm still trying to get used to the IM process!

Dave on Earth (9:21 AM): No worries! If Ed Stone can do it, I think anyone can.

Europa Explorer (9:21 AM): Ed Stone is a very competent guy!

Dave on Earth (9:22 AM): That he is. You said on the phone that you're typing from Jackson Heights in Queens, which means you're about 10 minutes by train away from me.

Europa Explorer (9:23 AM): Yes, I'm looking across at the skyline (including the Chrysler Building).

Dave on Earth (9:23 AM): Yep -- I can see that one, and the Empire State Building. Er, the top of it at least.
Now we could chat all day about New York City, but why would I when I have an expert on moons around Jupiter?

Europa Explorer (9:25 AM): Ok, ask away...

Dave on Earth (9:25 AM): Sure.
To get started: how did you get to be so familiar with Jupiter's famous icy moon Europa?

Europa Explorer (9:29 AM): Well, I was working on understanding the complex orbits of Jupiter's satellites for some time.
When it came time to pick members of the Galileo spacecraft imaging team, I was selected.
My idea was that tides (like those caused by Earth's moon) might affect Jupiter's satellites

Dave on Earth (9:30 AM): Ah, like moon causes tides in Earth's oceans?

Europa Explorer (9:30 AM): Correct. So it made sense to have someone with my specialty on the imaging team.
When the first images arrived at Earth in the 1990s, it turned out that much of what we saw could be explained by tides.

Dave on Earth (9:31 AM): Thanks for that.
Backtracking to the Galileo spacecraft itself for a moment -- what was it intended to do around Jupiter?

Europa Explorer (9:33 AM): The Galileo spacecraft orbited Jupiter from late 1995 until late 2003, with multiple flybys of the four largest Jovian moons originally discovered by Galileo.
The idea was to study Jupiter, its environment and find out as much as we could about the planet's moons.

Dave on Earth (9:33 AM): If I'm not mistaken, Galileo started in the 1970s.
Why did it take so long to get the robot to Jupiter?

Europa Explorer (9:35 AM): The project started in 1977, but was delayed for a number of reasons.
One was technical problems with the rocket designed to launch it on after the space shuttle put it into orbit.
Then the Challenger disaster shook up the space shuttle program. So there were many years of delays.

Dave on Earth (9:38 AM): I see. Going back to what you said about tides on Europa...
How can you have tides on an ice-covered moon?

Europa Explorer (9:43 AM): As I discuss in my book "Unmasking Europa," we tend to think of tides as something we see at the shore: the sea level rising and falling against the land.
From a planetary perspective, however, tides are really the elongation of a planet -- sort of a stretching into an oval from the gravitational pull of other bodies.
That is what really happens to the Earth as the moon orbits it. Likewise on Europa, as the moon orbits Jupiter, the planet's gravity changes Europa's shape over every orbit.
The result? Stress on the crust and heating inside from all of the friction.

Dave on Earth (9:44 AM): So Europa's ice melts from the inside-out...
How long has this been happening to Europa?
(My guess is that the longer, the better the chance would be for life to develop.)

Europa Explorer (9:49 AM): Correct, most of the heat comes from friction in the rock below the liquid ocean, but also in the ocean water itself.
Then some from friction in the ice.
The heating indirectly depends on the orbital resonance, and much of my work has shown that, in the past, this resonance was stronger than it is now.

Dave on Earth (9:50 AM): "Orbital resonance" = what in layman's terms? When I hear the term I think of vibrating guitar strings.
Is it similar -- do each of the moons have a "resonance" at which their insides are heated?

Europa Explorer (9:52 AM): A guitar string resonates when it is driven at a frequency matching its natural frequency.
Similarly, when the moons Io and Ganymede pull on Europa, they do it at frequencies that resonate with Europa's frequency (by the way, Europa's period is about 3.5 Earth days, meaning its frequency is 1 cycle per 3.5 days).
Amazingly (and I still find it amazing), the time it takes Io, Europa and Ganymede to orbit around Jupiter is in a ratio of 1:2:4.
In other words: Each time Ganymede goes around once, Europa goes around twice and Io goes around four times.

Dave on Earth (9:54 AM): And that leads to tides?