Re: Effect of planetary core temperatures on lifeforms

Your question is an excellent one. We know that the Earth has a molten interior based on the travel time of seismic waves through the Earth. Unfortunately, we have never put a series of seismographs on any other planet, except the Moon. Actually placing these instruments on other planets would probably give us pretty definitive answers to your question, but there are other, somewhat less definite, ways we can learn about the interiors of planets.

For example, we can see that there are volcanoes on Jupiter's moon Io, so there is some molten material within the planet (although it is worth pointing out that Io is heated by tidal stresses and not like other bodies are expected to be heated). Similarly, Mars has had volcanoes up until fairly recently (geologically speaking), so it at least had a molten interior recently. And based on the composition of Venus atmosphere, it is likely that Venus has a molten interior and some volcanic activity even now.

Another trick we might be able to use is to look at how the planet wobbles as it spins. The presence of a liquid, rather than a solid, interior would create tiny changes in the spin behavior. There are projects being considered now to look at Mercury (see below for more of why Mercury is weird) in this way.

The Moon, on the other hand, is almost certainly nearly without a liquid core. Yet we see places where lava once flowed on the surface (the dark mare). What's up with that?

Permit me to detour slightly and explain why most planets would have molten interiors (Io being a notable exception!). When planets form, a lot of mass comes together. In doing so, a lot of energy is converted from gravitational energy to heat. This causes planets to start their lives molten. But over time, the heat leaks out. In fact, if this were the end of the story, all of the terrestrial planets (Earth included) would be solidified by now. But there is also radioactive decay going on within the planets. Not much, but enough to keep the heat up so that the planet can stay molten longer. Unfortunately, radioactivity falls off with time, so eventually cold and frozen will win out. This has already happened to the Moon. To a planet the size of Earth (or Venus, which is roughly the same size), this wouldn't have happened yet. Mars is heading down that path, but probably is not likely to be totally solid yet.

What about Mercury? Mercury isn't that different in size from the Moon, so it seems that it should be solidified by now. But Mercury has a rather sizable magnetic field, which we believe probably tells us that the interior is molten. How can this be? There are a number of theories, ranging from explaining away the magnetic field using something other than a liquid interior to guessing that the core is chemically different from our Moon and the Earth, so that it can stay liquid a lower temperatures. But right now, no one knows the answer. We hope that the upcoming Mercury MESSENGER mission will help answer some of these question.

Sorry to be so long-winded, there. Now, for your other question: is a liquid interior needed for life? Maybe. No one knows what is required for life, but our best guess right now is that life on Earth started near volcanic vents on the ocean floors. This is by no means the only way life can start, as far as we know. But it's a good place to start looking.

The main "planet" we are looking at for possible life right now is Jupiter's moon Europa, which may have a water ocean under its icy crust. There may well be volcanic vents on Europa's ocean floors (although some planetary scientists are predicting that there are not). If so, with the right set of chemicals in the water, this could lead to an environment favorable to life! But, right now, no one knows for sure on any of this. Stay tuned as we explore more!

If you want to read some more on this subject, I recommend the textbook, The Cosmic Perspective, but Bennett, Donahue, Schneider and Voigt. The section on planets covers the topic of liquid interiors fairly well. There is some information available at The Nine Planets Website, but not a lot. Finally, for more on the search of life elsewhere in our solar system, you might try The Search of Life on Other Planets by Bruce Jakosky.