Figure 1. (a) Portion of THEMIS VIX image V08198017 (18m/pixel) showing a typical example of a wrinkle ridge observed along the "shorelines" in Isidis Planitia. (b) Portion of THEMIS Vis image V02431006 (72 m/pixel) showing the only example of ridges observed at the THEMIS VIS scale along the "shorelines" within Isidis Planitia, nor immediately dismissed as wrinkle-ridges or Aeolian landforms.
Some planetary geologists infer that an ancient northern lowland-filled ocean once existed on Mars. As a result, this hypothesis has been repeatedly tested using numerous techniques and methods, culminating in very mixed results. Like many geologists before them, a team including Dr. Zimbelman finally took their turn at the wheel. Even though their new path yielded similarly inconclusive results, they provided ramifications for future studies in their wake.
This study looked for shorelines accompanying features such as barrier ridges/islands, beach ridges, spits, and looped barriers1 instead of searching for the shoreline itself. Out of the 447 THEMIS VIS and 735 MOC NA images examined, only four showed potential for these shoreline complimentary features1, one of which is featured above. Instead of ruling out the possibility of an ancient ocean once existing, five possible scenarios are identified explaining their results. These scenarios range from arguing an ocean never existed to justifying that it did exist and explaining why there is a lack of evidence1 today. Though no definite yes/no answer to the 'Did an ancient ocean exist?' question was determined from the study, it does provide a good stepping point for future investigation.
1. Ghatan and Zimbelman (2006), Paucity of candidate coastal constructional landforms along proposed shorelines on Mars: Implications for a northern lowlands-filling ocean.