Sometimes geologists have it easy.
In astronomy, you can have two stars right next to each other, maybe even born in the same cluster, but you can’t be sure they’re the same age, or which one is older than the other. Differences can be really subtle, and have to be teased out of the data.
Geologists, on the other hand, get stuff like this:
WOW. That is a scene on Mars, taken by the wonderful HiRISE camera on the Mars Reconnaissance Orbiter. Its shows a small portion of Ganges Chasma, a huge, sprawling channel cut into the surface of Mars, probably due to catastrophic flooding a billion or more years ago. The violent rush of water cut through the landscape like God’s trowel, carving out the chasm.
Over time, erosion, weakness in the walls, or subsequent floods would have caused the walls to collapse inward further. Eventually, though, all that stopped, leaving behind the tableau we see now.
In the shot above, a crater about 3.5 kilometers (2.2 miles) across had the misfortune to be too near the flood. The surface just south of it collapsed, slicing the crater nearly in half.
But this is good fortune to Terran geologists! The crater was obviously there first, sitting in the surface from some ancient impact, minding its own business when the world collapsed next to it. The edge of the crater over the canyon is clean and sharp, and the crater itself quite nicely formed. If the crater had occurred after the flood, the shape would be distorted by the edge of the canyon.
The crater itself is interesting. It’s terraced, with circular ledges inside the main rim. Quite a few of these are seen on Mars. This can happen when there’s a layer of material under the surface with a different strength, like ice under rock. The shock wave from the impact carves out the main crater, then slams into the layer underneath and creates a smaller crater inside the first one. There are also smaller craters dotting the interior, and you can even see rippling sand dunes in the upper left of the floor.
Speaking of layers under the surface, using Google Mars to get some context, you can really see more of what happened after the channel formed:
The crater is arrowed. Note the weird raised features in the canyon floor; those are likely muddy or possibly dry material flows that happened when sections of the canyon wall collapsed. These landslides can create scalloplike indentations in the wall; many of them look like impact craters. But the featured crater here is clearly from an older impact.
Yegads. That really drives home the size and scale of this catastrophe.
Not that this in any way makes me less jealous of geologists, who sometimes have the timeline of events spelled out right in front of them. It’s a little ironic. Studying Mars used to be astronomy, and in some ways it still is. But now we go there, now we explore it, now we send robots to the surface to drill into it and zap it with lasers and sift through the dust to determine what it’s made of and what its history is.
Mars used to be astronomy, but now it’s a world, a place. Handing over the keys to the planet to geologists is hard, but oh, it’s so rewarding.