Mars as we know it is a cold, red desert but in the past, it might have had long-lived water features, despite the frigid temperature. Rivers, lakes, even a large ocean may have once marked the surface of what we now think is a dry (or mostly dry) planet. And while there’s no certainty about an ancient ocean yet, there’s an increasing body of evidence.

Over the last decade, detailed observations from spacecraft orbiting the Red Planet have provided more evidence of water-made structures on ancient Mars. In particular, scientists have spotted terrain known as "thumbprint terrain" near where they think the ocean once existed that they believe was formed by the chaotic force of displaced water, most likely from not one but two mega-tsunamis. Mega-tsunamis differ from regular tsunamis in that they are created by water being displaced rather than underwater tectonics. Given the lack of tectonics on Mars, planetary scientists think that these mega-tsunamis were caused by meteor impacts. Now researchers believe they have found the crater of at least one of them.

According to research published in Journal of Geophysical Research: Planets, the Lomonosov Crater appears to have geological features consistent with being the source of this type of event. The rim of the crater looks like it was broken by water rushing back in. The presence of these fractures on all sides makes a strong case for water rushing in trying to fill the void left by the impact.

The crater itself is roughly 120 kilometers (75 miles) across and is surrounded by smooth young terrain, similar to how it would have been as a potential shallow ocean seabed. According to the researchers, some 3 billion years ago an object between 15-20 kilometers (9-12 miles) across impacted the northern hemisphere of Mars, hitting the coastline and generating a huge tidal wave.

There are other pieces of evidence regarding this and other ancient tsunamis left as scars on the surface of Mars. Coastal terrains have been observed with structures consistent with water rushing in and then flowing back down to lower altitudes. Researchers have attributed these structures to different tsunamis that may have happened during times the climate had changed significantly.

The climate is a crucial element in this discussion. Ancient Mars is expected to be very cold for many reasons, including the fact that the Sun emitted a lot less radiation back then than it does now. A long-lived ocean seems very unlikely in those freezing conditions, instead requiring warm and wet conditions to maintain. That said, in this study, the researchers make a compelling case for the ocean's existence. 

If the Lomonosov Crater’s features were indeed caused by rushing water, it would suggest that an ocean was indeed present and given how rare impacts of this size are, this ocean must have been there for a very long time.

[H/T: The New York Times]