Some Rock Worlds Are So Big Their Outer Layer is Soft

The outer layer of a rock world in space is usually strong. This is because rock is strong, especially if it is cold (and the upper part of a space body is often cold). A strong cold outer layer is important, especially if it is not too thick, because it allows hot rock to get from the inside to the outside along cracks, and to have some parts of the layer move around and under other parts, which can help to keep the rock body cool. Rock can also be soft, which is usually the case if it is warm—say, when it is deep inside a rock world. And a lot of force over area can also make rock soft, which is also what things are like deep inside a rock world. But very big rock worlds—a few times bigger than the rock world on which you are reading this right now—could have a lot of force over area very close to the outside, which might mean that the layer of strong rock would not be very thick on such bodies. To see if this idea makes sense, we put together a big set of studies of what rocks do when they are put under a lot of force over area. We found that rocks get soft at force over area numbers that are probably very close to the outside of some big rock worlds that go around other stars. In other words, it turns out that how big a rock world is plays a key part in how thick the strong outer layer is, and that, for very big rock worlds, the outer layer may not be very thick at all. In fact, for the very biggest rock worlds, there might not even be a strong rock layer! On such very big rock worlds, the outer layer is soft, and this is a very big deal. Why? Because if there is no strong outer layer, then there can not be any tall rocks that stick up, or big things made up of hot rock that has cooled down, or big round lows made by things that hit the ground very fast. So, such rock worlds with a soft outer layer will look very different to the worlds that go around the Sun. Actually, such a rock body with a soft outer layer might be like the low parts of the second world that goes around our Sun, where there are no tall rocks that stick up, and where the strong outer layer is only a few ten hundred steps thick. In the years to come, we should be able to see if this thinking is right by looking for tall rock things on rock worlds around other stars with new, very, very good things we use for looking into space.