What if the key evidence of an ancient ocean on Mars has nothing to do with shoreline itself? For years, the issue of existence of a northern ocean on Mars has been mired in the question of how these alleged shorelines match up. While coastlines on Earth, aligned with the sea level, can be expected to be at some uniform elevation, many candidate shorelines on Mars are spread inconsistently on the map. This uncertainty left one of the largest questions in planetary science unresolved whether Mars in its early days consisted of merely separate lakes and rivers or possessed a fully fledged ocean which would impact its climate and even habitability.
Newer evidence has shifted the focus from controversial shoreline patterns to a planetary sized coastal feature continental shelf. Researchers compared the drainage patterns on Earth and created topographic simulations of such features on Mars. Comparing it to orbital images of Mars, they discovered a flat area consistent with what we expect to find at Earth’s ocean margins. Specifically, the feature sits somewhere 1,800 to 3,800 meters under a supposed ancient Mars sea level and surrounds areas in which the presence of water would be sufficient to occupy approximately one third of Mars surface.
Unlike shoreline patterns, continental shelf implies a geological formation developed for a long time span. It thus provides better evidence of existence of an ancient ocean than mere shoreline marks left on rocks. And unlike a fleeting body of water, a sustained presence of ocean means that liquid water was stable for millions of years.
This fact becomes particularly significant since long lived oceans change our picture of Mars drastically. For example, if an ocean indeed existed on the planet millions of years ago, then the river deltas observed near this continental shelf boundary should behave similarly to Earth’s deltas. Moreover, the finding was confirmed again in 2025 when orbital radar surveys done by the Zhurong rover revealed subsurface structures oriented towards the northern lowlands. These underground layers were very similar to coastal sediment deposits seen in Earth and were not compatible with a volcanic or a dune field deposit or a temporary river.
Overall, the ocean hypothesis on Mars dates back to decades ago and has gained many supporters. They refer to valley network patterns, delta elevations, sediments in Mars plains, and atmospheric conditions implying the presence of much more water on the planet in its early days. Some theories suggest that the deformities observed in these shorelines can be explained by later processes like true polar wander or evolution of the Tharsis region. At the same time, some researchers have criticized the ocean hypothesis due to inconsistent topology, absence of carbonates or insufficient amount of water on the planet at the time of the ocean’s existence.
Nevertheless, a continental shelf changes this picture completely. Not only does it provide stronger evidence but it opens up an array of new research opportunities. Geologically speaking, coastal sediments preserve the most valuable information from our planet’s history. The existence of a shelf on Mars increases the chances for researchers to find these deposits on Mars surface and learn more about environmental conditions on Mars millions of years ago, potentially discovering the earliest lifeforms that ever existed on this planet. Searching for such evidence has been limited to crater lakes before.