Landscape formation at the Deuteronilus contact in southern Isidis Planitia, Mars: Implications for an Isidis Sea?
Two of the most widely studied landforms that are associated with a putative ocean that filled the northern hemisphere of Mars are (1) the Vastitas Borealis Formation (VBF), plain units that cover a larger portion of the northern lowlands of Mars, and (2) a candidate paleoshoreline, e.g., the Deuteronilus contact, which represents the outer margin of the VBF. The VBF and the Deuteronilus contact are interpreted to result from a short-lived Late Hesperian ocean that readily froze and sublimated. Similar landforms are also present in the impact basin of Isidis Planitia and suggest formation processes comparable to those that formed the VBF and the Deuteronilus contact in the northern lowlands. Our study of the Deuteronilus contact in Isidis revealed geologic evidence that possibly supports the existence of a Late Hesperian/Early Amazonian Isidis Sea. For example, numerous valleys that are incised into the plains of the southern Isidis basin rim between 82°/90°E and 3°/6°N and trend a few tens of kilometers to the north following the general topographic gradient toward the center of Isidis Planitia. A few of them reach the Deuteronilus contact and continue as sinuous ridges in the Isidis Interior Plains (IIP). Based on our findings we conclude that the geologic setting along the Deuteronilus contact, including the valleys and ridges is a result of (1) Late Hesperian short-term fluvial activity, (2) a Late Hesperian/Early Amazonian short-lived Isidis Sea that readily froze, (3) subglacial drainage and esker formation, and (4) subsequent sublimation of the proposed Isidis ice sheet. Although the fluvio-glacial model we introduce in our manuscript cannot fully explain the geologic setting, possible alternative formation models, including relief inversion and fluvio-volcanic scenarios are even less capable in explaining the observed geologic setting along the Deuteronilus contact.