Mars is pretty weird.
I guess I’m biased, since I live on Earth and all. But everything about it is just so dang odd.
For example, there is a great honking wart on the planet called the Tharsis bulge, where four enormous volcanoes sit. The bulge is probably due to a hot spot, an upwelling of magma inside Mars, creating a mound that’s, oh, say, 5,000 kilometers across. The size of the continental United States.
To the west of Tharsis a few thousand kilometers is a region called Elysium Planitia, a large plain. A subsection of Elysium is Cerberus Palus, a smaller plain area a few hundred kilometers across. It’s a lava flood plain, created by volcanic activity in the area. The age has been estimated using crater counts* to be between 2–10 million years old.
That’s pretty young! Most geologic features we see on Mars are more like a billion years old or more. So it’s fair to call this lava flood “recent”.
But it gets weirder. Meet Cerberus Fossae:
This image was taken by the European Space Agency’s Mars Express orbiter. “Fossae” means “long, narrow depressions”, and you can see why the name fits. Cerberus Fossae consists of quite a few of these depressions that are roughly parallel, running pretty close to the same direction (northwest to southeast). The entire set is well over 1,000 km long.
Here’s the thing: You can see them running right through features like the plains, hills, and even craters. That means the cracks are younger than those features. An asteroid impacting after the crack formed would erase the crack underneath, or at least hugely deform it.
What caused these cracks? They are most likely due to tectonic extension, when the surface is stretched for some reason. You’ve seen something like this before if you’ve ever made a cake or muffins. The top will brown and harden first before the middle. As the interior of the cake cooks, it expands, pushing up against the already-cooked top. That pressure can cause the top to split and pull apart, creating a crack or rift.
The fact that the Martian fossae are mostly parallel implies a common origin, and not a random one. One idea is that the cause (I almost wrote “fault”, har har) is Tharsis! Lava running under the surface could have created enough pressure above it to split the ground, creating the rift.
In higher-resolution images you can see the bottoms of these cracks are flat. In geology you can get such a big rift that the ground in the middle between them will drop downward, creating a floor. That sort of valley is called a graben, and they’re relatively common on the Moon. In fact, you can even see sand dunes in the Cerberus Fossae graben as wind blows the particles around.
It’s likely that when Cerberus Fossae formed, lava erupted out from them, and may even be what formed Cerberus Palus, those lava-flood plains. There’s been some speculation water came out too; there’s ice deep underground, so it’s possible, but to what extent (including none) that occurred isn’t clear. There are obvious collapse and flow features extending outward from the fossae in many images, too.
We have rift valleys on Earth, too, and even river cut canyons that look similar to these, so it’s not like these are unique to Mars. But still, to see something like this cutting right across features like craters willy nilly… that just seems so strange to me. Of course, I’m an astronomer, not a planetary scientist, and my experience with geology is rather limited to hikes I’ve made and books I’ve read, as opposed to spending a lifetime studying it.
And it would take a lifetime to study Mars; many, many lifetimes. There’s just so much to know, from the gross to the minute, and even how they interact with other. If there’s one thing that Mars provides for scientists — besides joy, curiosity, wonder, and mystery — it’s job security.
*Assuming that over long periods of time craters are formed at some average rate, you can count the number of craters in an area to estimate the age. This works best for older areas, but isn’t bad on the several-million-year timescale.