Certainly the trough predated the chaos and the outbreak channels, which were no doubt located there because of the trough. The Tharsis bulge had been a tremendous source of outgassing from the hot center of the planet, all the radial and concentric fractures around it leaking volatiles out of the hot center of the planet. Water in the regolith had run downhill, into the depressions on each side of the bulge. It could be that the depressions were the direct result of the bulge, simply a matter of the lithosphere bent down on the outskirts of where it had been pushed up. Or it could be that the mantle had sunk underneath the depressions, as it had plumed under the bulge. Standard convection models would support such an idea— the upwelling of the plume had to go back down somewhere, after all, rolling at its sides and pulling the lithosphere down after it.

And then, up in the regolith, water had run downhill in its usual way, pooling in the troughs, until the aquifers burst open, and the surface over them collapsed: thus the outbreak channels, and the chaos. It was a good working model, plausible and powerful, explaining a lot of features.

So every day Ann drove and then walked, seeking confirmation of the mantle convection explanation for the Chryse trough, wandering over the surface of the planet, checking old seismographs and picking away at rocks. It was hard now to make one’s way north in the trough; the aquifer outbreaks of 2061 nearly blocked the way, leaving only a narrow slot between the eastern end of the great Marineris glacier and the western side of a smaller glacier that filled the whole length of Ares Vallis. This slot was the first chance east of Noctis Labyrinthus to cross the equator without going over ice, and Noctis was six thousand kilometers away. So a piste and a road had been built in the slot, and a fairly large tent town established on the rim of Galilaei Crater. South of Galilaei the narrowest part of the slot was only forty kilometers wide, a zone of navigable plain located between the eastern arm of the Hydaspis Chaos and the western part of Aram Chaos. It was hard to drive through this zone and keep the piste and road under the horizons, and Ann drove right on the edge of Aram Chaos, looking down onto the shattered terrain.

North of Galilaei it was easier. And then she was out of the slot, and onto Chryse Planitia. This was the heart of the trough, with a gravitational potential of -0.65; the lightest place on the planet, lighter even than Hellas and Isidis.