I’m not a sedimentologist. In fact, in the course of my PhD I’ve developed the sort of attitude which goes: if it happened at less than a thousand degrees, who cares. My life at the moment is full of crazy experiments at ridiculous temperatures and rocks which were once so hot they were liquid (or IGNEOUS rocks to the sharp-witted).
However, when a robot on Mars stumbles across a particular kind of sedimentary rock; a conglomerate, even people like myself get interested.
Now here comes the basics… Sedimentary rocks are formed when a flow of some kind – water, ice or wind (generally) picks up and moves bits of rock/dust (i.e. sediments) and drops them somewhere else. We are all intrinsically familiar with these deposits of sediments – think about beaches, rivers, sand dunes, lakes. The sediments are moved by the water/wind/ice/etc, and as soon as that transport medium runs out of energy, it drops the sediments it can longer carry. A river hurtling down a mountain side will have lots of spare energy to carry sand, pebbles and boulders, but as soon as it hits a flood plain or estuary the energy of the flow drops, and the sediments are all removed from it.
Now, if we, the geologically savvy are walking along, looking at our feet (we do this because we prefer to look at rocks than at people, I think) and we find a rock made up of sediments, we are able to start to understand what sort of flow was responsible for that rock’s formation. The same is true for the Mars Rover ‘Curiosity’, even though it doesn’t so much walk as trundle. Curiosity went out for a walk and ended up spying a conglomerate, which is basically a sedimentary rock made of big, rounded pebbles. This means that there was not only enough energy in the ‘flow’ to move big pebbles, but also to round off the edges, which only happens by continued attrition.
So why do we care? Well, there is only one type of flow that can produce a rounded pebble conglomerate, and that is a stream. A flow of some liquid moving fast and picking things up, bashing them against each other, and dropping them again. It would be a lie if I said ‘where there is water, there is life’ so let’s not jump to Martian conclusions, but finding this rock type is a bloody good step in the right direction!
N.B. the interested should refer to New Scientist for a much better treatment of this problem…