More so than any other feature, the abundant evidence for past water activity on Mars has captured the attention and imagination of members of the public and scientists. From the giant outflow channels to the smaller valley networks (Fig. 1) to the relatively tiny fresh young gullies (Fig. 2), Mars displays evidence of fluvial erosion at all ages and scales. The interpretation of this evidence, three decades after the first signs of Martian water were found, remains elusive. Several origins have been proposed for these features, including glaciers, water runoff during a warmer, wetter period in Mars' history, and hydrothermally driven ground-water outflow. The keys to distinguishing which mechanisms were most active are likely found at the smallest scales within these fluvial features. By clearly resolving meter-scale features, HiRISE should be able to help differentiate between various formation mechanisms. The improvement in image resolution between HiRISE and the typical MOC image (Fig. 2) will be comparable to the improvement between the best Viking images (such as Fig. 1) and MOC.

Figure 1. Young fluvial valleys found on the northern slope of the volcano Alba Patera. The prominent crater is about 1 km in diameter. Viking high resolution image ~8m/px.

Figure 2. Very young gullies found on the slopes of a southern hemisphere impact crater on Mars. The image width is about the same as the crater diameter in Figure 1. MOC narrow angle image (MOC2-388).

Figure 3. This MOC image (MGS MOC Release No. MOC2-250) shows Dao, Niger, and Harmakhis Valles emerging from the flank of the Hecates volcano. The association between the volcano and the outflow channel may imply a hydrothermal origin for the channel. (Click for larger version.)

Ginny Gulick and daughter Alicia near a glacial melt water channel in Iceland.