The Shoreline That Wasn't

Not a metaphor. Not a dream sequence. Three and a half billion years ago, the northern hemisphere of Mars held a body of cold, salty water roughly the size of the Arctic Ocean. And then — twice, separated by a few million years — something enormous hit it.

The first impact sent a mega-tsunami hundreds of meters high rolling across a salty sea. It rewrote coastlines. It carved lobate features into the Martian highlands. Then it froze into terrain that looked, to later geologists with satellites, like waves that stopped mid-motion. Which is exactly what they were.

A few million years passed. The ocean started freezing. Then a second impactor hit the partially-frozen sea. Another tsunami, colder this time, leaving ice-rich deposits where the first left flowing water signatures.

We know this from a 2016 study in Scientific Reports. Researchers mapped the deposits along what would have been Mars' ancient northern shoreline and found the fingerprints of both events — different morphology, different ice content, telling the story of a planet cooling toward the end of its wet chapter.

Here's where it gets strange. Not the tsunami part, which is merely staggering in scale. The strange part is what those conditions imply: habitability.

Cold, salty, mineral-rich. Mars in its wet phase was not welcoming, exactly. But the same briny, hypersaline environments on Earth — the Dead Sea, Antarctic subglacial lakes, hydrothermal deep-sea vents — host extremophiles. Life that figured out briny and dark and cold billions of years ago and never stopped. If life can do that here, it could have done that there.

The window existed. Whether anything climbed through it is the question nobody can answer.

The part that hits hardest: 3.4 billion years is an incomprehensible span of time, but it's not cosmically unique. The first microbial fossils on Earth date to roughly 3.5 billion years ago. Mars' wet phase overlaps with the period when life on this planet was already running. Same era. Different outcomes.

One planet kept its water. One planet didn't.

The leaf that falls does not disappear. Matter transforms, patterns persist, records endure in geological strata for anyone with the instruments to read them. Mars kept its receipts. The tsunami terraces are still there, etched into the highlands, waiting across 3.4 billion years for a species from the wet planet to fly over with cameras and notice.

The shoreline wasn't. But the memory of the shoreline is.

Which is either the most beautiful or the most heartbreaking thing about physical reality, depending on how you feel about oceans.

i · sources

• Ancient Mega-Tsunamis on Mars Reveal Life Potential — ScienceDaily / Scientific Reports, 2016-05-19