LATE DRAINAGE ALONG PORTIONS OF SAMARA VALLES, WEST OF JONES CRATER, MARGARITIFER TERRA, MARS

Relatively pristine valley segments along medial-to-distal Samara Valles in Margaritifer Terra originate within the continuous ejecta surrounding the Late-Hesperian Jones impact crater. Valley expression varies from well-incised to diffuse, some segments appear anastomosing or truncate one another, many are more than a kilometer across and 100s of meters deep, and some display interior terraces and depositional forms. More pristine segments continue north where they are typically less confined and are associated with finely layered, alternating relatively darker- and lighter-toned deposits partially filling a large crater well beyond the Jones ejecta deposit. The ultimate fate of the water that formed the relatively pristine valleys is unclear, perhaps due to mantling and partial burial by younger volcanic deposits near the southern margin of Margaritifer basin.

The relatively pristine valleys are continuous except where they enter two small basins west of and within the Jones ejecta (basin margins correspond to the -1275 m MOLA contour). The upper basin has an outlet through the northeast divide, encompasses ~46 km³, and displays benches and partially filled craters to near the –1275 m contour . Lower interior surfaces of the upper basin are not incised, expose relatively lighter-toned discontinuous deposits, include polygonal fractures, and are locally Mg/Fe phyllosilicate-bearing. The lower basin has a volume of ~7 km³, displays a fan across a large part of the interior floor, and there are multiple breaches across the downstream divide.

We interpret the formation of the relatively pristine segments to be the result of water from impact-melted ice in and under the Jones ejecta that drained westward and mostly into the upper basin. Water filled the upper basin over months to years before overtopping the northeast divide and drained at rates as high as 10⁴ m³s^-1 (see abstract 393213) before overwhelming the lower basin and breaching two additional downstream local divides. Drainage through the pristine valleys also persisted for a period of months or years at most and then ceased. If correct, our model indicates a geologically brief interval of transient potentially habitable environments relatively late in Mars history and likely unrelated to global climate change.