What Can Crater-Counting on Mars Tell Us About Its History?

Back in the 18th century, when geology was becoming a proper, systematic science, one of the early breakthroughs was the insight provided by the law of superposition. Simply put, this is the idea that younger rocks lie on top of older rocks. Following that observation, we discovered that we could order our own geology in terms of youngest, oldest and the layers in between, simply by comparing outcrops of rocks in various places—as well as natural cliffs and riverbanks, we could use information taken from railway cuttings, canal bottoms and mines.

After mapping all these out, we were able to tell with reasonable certainty which layers were the youngest, and then go all the way down to the oldest in an iterative process, until all the rocks were accounted for.

The Martian landscape might be different to ours, and our access to it very limited, but we can use the increasingly detailed photographs we’ve taken to map out the various landforms and rock types that are visible, and supplement those images with our knowledge of how these shapes were formed. We can do something similar for Mars as the first geologists did for Earth.

If lava flows into a valley, it has to be younger than that valley—we have no real way of telling how much younger, but if further on the valley appears to cut through a crater, we can tell that the crater is the oldest of these three features. More investigation might show that the crater wall has slumped down later, partially blocking the valley. This event, then, is younger than the valley, but it could be contemporaneous with the lava, or younger still than that.

We can do something similar for Mars as the first geologists did for Earth.

There are further tells the landscape gives us, and they are all interconnected and overlapping. Two separate lava flows from two separate volcanoes could reasonably appear to be the same age, but by noting the features each of them cross, or are crossed by, we can determine which is older, even though they themselves never meet.

We can tentatively identify other features as being formed by water, and by wind, and by ice, and know how these riverbeds, sand dunes and glaciers are connected. Add these to the structural elements—the areas where the land has been thrust up or brought down, the valleys and mountains, and underlying it all, the ancient Martian crust—and a careful, painstaking examination of the surface geology of Mars reveals a chronology of events.

What would be best of all would be to put exact dates on that relative history. But in the absence of radiometric dating of pristine rock samples, we are left with the less satisfactory but nevertheless powerful tool of crater counting.

Crater counting is a useless exercise on Earth because the planet renews its surface so often: craters here are transient features, eroded not just by the effects of our highly active weather but by the constant motion of the underlying crust. Earth has a system of plate tectonics: its mantle moves in a complex series of gyres, with hot rock rising from the depths and pushing against the crust as it cools, before sinking again, forming vast, ponderous convection cells. The crust in turn travels along the surface, driven by the mantle currents, splitting and moving and colliding, riding up and being dragged down. Mountains rise, basins sink, oceans form and close. Any record of impacts is reduced to the most recent or the very largest, and even then most visible signs become scoured from view.

A careful, painstaking examination of the surface geology of Mars reveals a chronology of events.

The situation is different on Mars. While, like Earth, Mars also had a mantle that moved, a full system of plate tectonics was only ever a brief episode—a couple of hundred million years, if that—after which any movement of different parts of Mars’s crust ground to a halt. From an active sheath of rock that split and moved and sank, it became a single, stagnant, solid shell.

As a consequence, the whole of Mars has a basement of crust that dates all the way back to when it froze out of the global melting event, 4.5 billion years ago. It undergirds every subsequent structure, and the record of every single impact made on it is potentially preserved from those earliest times.

So how does crater counting work? Imagine a big square of wet cement—this is our early Martian crust—and next to it, a bag of unsorted rocks ranging in size from big half-bricks through lumps of masonry all the way to bits of pea gravel: these are our meteorites. We have a shovel, which, at the risk of stretching the analogy too far, represents the number of meteorites that can impact Mars in a given time. We dig that shovel into our bag of rocks, and whatever is on it we fling at the cement.

What we’ve made is lots of craters. Small craters, medium craters, large craters, in a random pattern over the whole area of once-smooth cement. Let’s take another shovel-load of rock and throw that on too, just for good measure. Some of the craters will overlap; some of the new larger ones will have erased some of the older smaller ones. Large craters from our previous effort will now be pockmarked by gravel-sized ones.

We start to appreciate that the processes that sculpted Mars are explicable.

Now, we get some more cement—let’s call it a huge lava flow—and we cover half the original area. We can tell which is the original surface, because it’s covered in craters, and which is the new surface we’ve added on top, because it’s completely unmarked. Now we throw another shovelful of rock high into the air. Some of it comes down on the original surface, scarring it further. Some of it comes down on the new surface, marking it for the first time. But the important thing is that we can still tell the difference between the two—even though the newer surface now has some craters too—by comparing the size and number of craters over both areas.

Let’s do it again. Take some more cement—another, more recent lava flow—and cover half the original layer and half the second layer. We now have three ages of surface, but if we throw more rocks at them, we should still be able to work out which is youngest and which is oldest.

This is how we tell the relative ages of the different surfaces: the more heavily cratered a piece of ground is, the longer it’s been exposed to cratering events. There are caveats here, though. A surface may become so saturated with craters that any new crater might erase the memory of an older one. This means that two very old layers might record the same crater density, even though they’re not the same age.

At the other end of the scale, an area might be too young to have any craters at all, but there will be no way to tell how young it is—or if it’s a small area, whether its lack of craters is due to its size or its age. Then there’s the effect of an atmosphere: the thicker the atmosphere, the larger a meteorite has to be to keep enough of its interplanetary speed to make a crater when it strikes the ground. And as we’ll see, the thickness of the Martian atmosphere has changed a very great deal, more than enough to affect our calculations.

Reading the shape of Mars gives us the words to describe both its past and its present.

But the biggest problem with crater counting is that while we know that the rate of cratering is an uneven process, we don’t know how uneven it is. Look in the bucket of rocks we’ve been shoveling up and throwing down. We can see that in our first few throws we took almost all of the bigger rocks, in the next few we were left with several middling-sized pieces and some gravel, and now we’re scratting around at the bottom to get anything. Going backwards from our analogy, this is precisely what happened in the early solar system.

We just don’t know the make-up of the debris swarm that intersected Mars’s orbit early on. But we do know that, like the rocks in our bucket, meteorites can only ever strike an object once. They’re not a renewable resource. Shortly after formation, Mars was subjected to an intense bombardment of leftover space debris, some of which included kilometer-wide planetesimals. After these were cleared by collisions with planets, both the number of impacts per year and the size of the potential impactors decreased rapidly.

The biggest aid to dating the Martian surface comes not from Mars itself, but from the Moon. The two aren’t particularly alike, and they formed in different parts of the solar system by different mechanisms, but one vital similarity links them: their craters. Crucially, we have some samples of the Moon, which we can not only date using radiometric dating, but because we know exactly where they came from, we can compare landscapes.

With a few adjustments, we can devise a scheme to transfer the whole timescale of cratering rates on the Moon a hundred million kilometers away to Mars, and roughly—sometimes very roughly—date the various ages of its surface. Nothing trumps walking the landscape, examining the structures close up, taking samples and drawing on a paper map with colored pencils. But these methods are the best we have for now, and they work far better than they have any right to.

Mars’s landscape might appear alien at first, but by spending time looking at the folds of the land—the craters, ridges and valleys—we start to appreciate that the processes that sculpted Mars are explicable—each age dominated by impacts, water, fire and ice. Reading the shape of Mars gives us the words to describe both its past and its present.

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