1.   Finding an assortment of rocks carried from a range of geological formations upstream would enable them to sample a variety of bits of Martian history.

2.   The best explanation for these valleys is that they are outflow channels for major floods that may have occurred at times in Martian history.

3.   The spacecraft has given scientists their best look yet at Martian history.

4.   Tracking data showed the craft headed for its planned destination, a broad plain where ancient floods presumably deposited rocks bearing clues to early Martian history.