Now that Spirit has arrived at the foot of the Columbia Hills, we're getting some interesting perspectives into erosional processes on Mars.

What I see in the images are a range of fragments, from tiny pebbles to giant house-sized boulders to outcropping small cliffs. Most are fractured and fragmented, and tend to break-up at the margins and even internally into smaller nearly-connected pieces. One often sees "outcrops" of material that is a whole set of tiny fragments, all arranged in order with a common fabric, as if one had broken and reassembled a larger piece.

In some ways, we see similar textures from Oppy, but these are less-fractured and less fragmented.

What's going on here?

Well, I think we need to look at analogues and differences between Earth, Mars, and the Moon.

On the Moon, there's lots of cratering, and there are vacuum-welding processes that rebuild rocks from fragments - this is where the Lunar breccias come from. Many are multi-generation breccias. A singe Moon rock sample may contain fragments from many terrains and ages, recemented into hard rock.

On Mars, cementation of Breccias does not seem to occur. All the rocks are loose. Weathering is much slower than on Earth, but adds its overprint of fracturing and spallation of exposed rocks, and perhaps fluid percolation through the rocks at depth and maybe even near-surface sulphate activity, but the cementing processes are hugely overwhelmed by the fracturing processes on Mars.

What we see appears, to me, as if a team of mad giants had taken hammers to the surface of Mars and pounded it to scrap. Some larger fragments remain, but these are fractured and partly dislocated. These mad giants are the impacts that have formed craters across the surface of Mars, to saturation density at some diameters (most of Mars has had every square centimetre of its surface affected by ~10m scale craters). Some areas are pounded to small fragments, but not to dust, like we see on the Moon - because there are only giant and medium-sized hammers on Mars - the micrometeorites get screened out.

The surface of Mars is a thick layer of fragmented and unconsolidated rock. New impacts mostly just fling the existing fragments around, rather than break them up more and more - try pounding thick gravel with a sledgehammer, and see how hard it is to break down the shifting mass of grains into smaller fragments. Your sledge just gets buried in the gravel, absorbing the impact.

What about Meridiani, and Opportunity? Why is it less fragmented here? The answer lies in the nature of the sulphate-rich bedrock. This material is so soft that it erodes in the gentle wind of Mars - sand-blasted by basalt grains. It breaks down to wispy light grains that are carried away in the wind, occasionally collecting in selected spots - like the light tints in the interior dunes of Endurance crater.

The blueberries are rare in the rocks at Meridiani, yet the ground is covered by a solid carpet of them. Hundreds of metres of layered sulphates must have been eroded to produce a lag deposit this rich. So in this surface stripping, we have lost much of the record of small craters in Meridiani. We see occasional 10m-scale fresh craters such as Fram, where again we see blocks scattered across the surface and a fragmented appearance to the local outcrop, but these exposed materials soon weather away to dust, leaving rolling plains of blueberries and occasional exposures of deeper, less-fragmented bedrock in larger and older craters such as Eagle and Endurance.

Of course, when I say "soon", I don't mean anything like as soon as on Earth. If we look at canyons and craters on Mars, these have experienced at most a few hundred metres to a kilometre of wall erosion in ~3 billion years (i.e. 3,000 to 30,000 years for 1 mm or erosion). On Earth, the Grand Canyon formed in the last 17 Ma and is 1750m deep and its walls are set back 8km from the centre. This is an erosion rate of 1 mm in 2-10 years. At Pathfinder, the surface appears to have eroded less than 1m in 3 billion years!

It may take 100,000 years to erase the tracks of the rovers, and 1-10 million years to erase a 1m-deep crater like Fram. Endurance is probably 100 million years old (yes, I know it looks like Meteor Crater, Az, and that's only 40,000 yeas old, but there are dozens - hundreds of similar craters scattered across Meridiani, while Meteor Crater lies in solitary splendour in the entire south-western USA).

So, in summary, the two rovers have provided a useful compare-and-contrast between processes and rates at two different locations on Mars, and help set Mars in a context between Earth and Moon. Mars is not closely like either of these analogs, but it does have elemnts in common, and with care we can begin to understand how Mars works, and how it is different from the earth we are used to, and the Moon that we have learned much about through Apollo and other missions.

Mars is Mars-like, and we have to be careful how we interpret observations and extrapolate to new areas. Mars is going to take a lot of thinking, to properly understand...