About 3.4 billion years ago, a big meteorite impact triggered the first tsunami wave.
"This wave was composed of liquid water. It formed widespread backwash channels to carry the water back to the ocean," said Alberto Fairen, visiting scientist in astronomy at Cornell University.
The scientists found evidence of another big meteorite impact which triggered a second tsunami wave.
In the millions of years between the two meteorite impacts and their associated mega-tsunamis, Mars went through frigid climate change, where water turned to ice.
"The ocean level receded from its original shoreline to form a secondary shoreline, because the climate had become significantly colder," Fairen added.
The second tsunami formed rounded lobes of ice.
These lobes froze on the land as they reached their maximum extent and the ice never went back to the ocean - which implies the ocean was at least partially frozen at that time.
"Our paper provides very solid evidence for the existence of very cold oceans on early Mars," the authors noted.
These icy lobes retained their well-defined boundaries and their flow-related shapes, meaning the frozen ancient ocean was briny.
"Cold, salty waters may offer a refuge for life in extreme environments, as the salts could help keep the water liquid... If life existed on Mars, these icy tsunami lobes are very good candidates to search for biosignatures," Fairen said.
"We have already identified some areas inundated by the tsunamis where the ponded water appears to have emplaced lacustrine sediments, including evaporites," added lead author Alexis Rodriguez of the Planetary Science Institute in Arizona.
"As a follow-up investigation, we plan to characterise these terrains and assess their potential for future robotic or human in-situ exploration," he noted in Scientific Reports, a publication of the journal Nature.
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