That system is three to four km deep at the ends, but seven in the centre. Most observers now seem agreed that these are fundamentally tension features resulting from cracking of the crust.

However, when first observed, these set the ball rolling for the notion that there were extensive rivers, lakes and oceans in the past. The sediments deposited inside these canyons are still cited as some of the best evidence of past water flows.

The edges of canyon after canyon show ridge and gully patterns that look as though they formed yesterday, and even fresher collapse basins eroding those. There is no way on any world with an atmosphere, gravity and spin, that the steep edges to be seen where these features meet the old undisturbed surface surrounding the canyons, could possibly have stayed up for millions let alone billions of years.

Valles Marineris and several other canyon systems seem to be growing now. The appearance of isolated lines of subsidence in the otherwise undisturbed original surface, some way back from and parallel to the developed canyons, shows clearly that the growth is not merely erosional extension of a tectonically dormant system, but is the result of continued and ongoing cracking of the crust.

The South Candor Chasm shows an interesting phenomenon. Much of the eroding material drains towards a narrow bottleneck, formed by a bed of resistive rock running at right angles to the flow (bottom right, below).

The material appears to move uphill over this barrier, without any evidence of ponding behind it, but an even more illuminating feature is apparent. Just below the barrier is preserved a slope of the same ridged and gullied soft material as forms the eroding canyon walls nearby.

If water had flowed there, it would have removed that soft material immediately it crossed the barrier, forming a steep nick or step and a waterfall.

For good measure an identical though smaller feature is to be seen in the very next channel across to the left, crossing the same bed of resistive rock. A wind-driven flow would conversely move up and over that soft material, and leave it as seen.

That geological catastrophism operates on Mars is quite clear. There are huge impact craters, huge volcanoes, massive tension gashes, and a vast unstable shield area. All are singular phenomena, not necessarily unique, but triggered by events quite outside the humdrum day- to-day geological business of Mars.

The Tharsis Dome with its attendant volcanoes is believed by some to be too high and large to be ancient and gravitationally stable. On that view, it must be subsiding now; Mars is a round planet with a temporary bulge, tending back to roundness under the force of its own gravity.