As they headed north across Lunae Planum they marked their route with little green transponders,  dropping one every few kilometers.  They also cleared rocks from their path that might disable a robot-driven rover, using the snowplow attachment  or the little crane at the front end of the first rover.  So in effect they were building a road.  But they seldom had to use the rockmoving equipment on Lunae; they drove northeast at nearly their full speed of thirty kilometers an hour, for several days straight.  They were heading northeast, to avoid the canyon systems of Tempe and Mareotis, and this route took them down Lunae to the long slope of Chryse Planitia.  Both these regions looked much like the land around their base camp, bumpy and strewn with small rocks; but because they were heading downhill they often had much longer views than they were used to.  It was a new pleasure to Nadia, to drive on and on and see new countryside continually pop over the horizon:  hillocks, dips, enormous isolated boulders, the occasional low round mesa that was the outside of a crater.

When they had descended to the lowlands of the northern hemisphere, they turned and drove straight north across the immense Acidalia Planitia, and again ran straight for several days.  Their wheel tracks stretched behind them like the first cut of a lawnmover through grass, and the transponders gleamed bright and incongruous among the rocks.  Phyllis, Edvard and George talked about making a few side trips, to investigate some indications seen in satellite photos that there were unusual mineral outcroppings near Perepelkin Crater.  Ann reminded them irritably of their mission.  It made Nadia sad to see that Ann was nearly as distant and tense out here as she was at back at base;  whenever the rovers were stopped she was outside walking around alone, and she was withdrawn when they sat together in Rover One to eat dinner.  Occasionally Nadia tried to draw her out:  "Ann, how did all these rocks get scattered around like this?"

"Meteors."

"But where are the craters?"

"Most are in the south."

"But how did the rocks get here, then?"

"They flew.  That's why they're so small.  It's only smaller rocks could be tossed so far."

"But I thought you told me that these northern plains were relatively new, while the heavy cratering was relatively old."

"That's right.   The rocks you see here come from late meteor action.  The total accumulation of loose rock from meteor strikes is much greater than what we can see,   that's what gardened regolith is.  And the regolith is a kilometer deep."

"It's hard to believe," Nadia said.  "I mean, that's so many meteors."

Ann nodded.  "It's billions of years.  That's the difference between here and Earth, the age of the land goes from millions of years to billions.  It's such a big difference it's hard to imagine.  But seeing stuff like this can help."