Rivers run, and an ocean nobody can prove
Thousands of branching valleys and a delta in Jezero crater show water flowed for a long time. Whether it ever pooled into a northern ocean is still argued.
The southern highlands of Mars are cut by valley networks: branching, dendritic drainage systems, some of them thousands of kilometres long, the kind of landform that needs sustained precipitation or groundwater sapping rather than a single flood, and that takes a long time to carve. Most of them date to the late Noachian and early Hesperian, around 3.8 to 3.6 billion years ago — several hundred million years after the dynamo had already shut down. Losing the magnetic field did not dry Mars out overnight, and anyone who tells that story as a clean sequence is smoothing over the hardest part.
Almost all of that is read from orbit. One basin the networks fed has been walked: Perseverance resolved the sediment fan in Jezero crater into a true delta, inclined strata laid down where a river lost speed running into standing water, capped by boulders no lazy river moves. That is a single confirmed lake among thousands of candidates, and it confirms the landform rather than the climate.
Nobody can make the climate work. The Sun then was 25 to 30 percent fainter than it is now, and models of early Mars struggle to lift the surface above freezing even with a thick carbon dioxide atmosphere, because CO₂ begins condensing out of the air before it can trap enough heat. The proposed rescues are all partial: hydrogen mixed with CO₂ producing collision-induced absorption, bursts of methane, impacts delivering transient warmth. A growing camp argues Mars was mostly frozen and only episodically wet — which fits the physics better than it fits the valleys.
Then there is the ocean. Since the 1980s people have traced two candidate shorelines around the northern lowlands, the Arabia and Deuteronilus contacts. The objection looked fatal: a real sea surface is an equipotential, and these contacts wander by kilometres in elevation. Robert Citron and colleagues argued in 2018 that the wandering is real and that Tharsis caused it: the shorelines formed before and during the rise's emplacement, and the growing load then flexed the crust under them. The older Arabia contact should be deformed more than the younger Deuteronilus, which is what is observed — and if that is right, any martian ocean is early, contemporaneous with the valley networks rather than a late leftover.
In April 2026 Abdallah Zaki and Michael Lamb changed the target. On Earth the clearest topographic fingerprint of an ocean is not the shoreline but the broad flat band of coastal plain and continental shelf behind it, which occupies a narrow elevation range of roughly −410 to −15 metres. Hunting for the martian equivalent, they found a circumglobal flat zone between about −1,800 and −3,800 metres: some 10.2 million km², around 7 percent of the surface. It is the strongest geomorphic case made so far. It is still not proof.