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| Evidence of ancient tsunamis on Mars | Evidence of ancient tsunamis on Mars |
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| Scientists think they see evidence of two huge tsunamis having once swept across the surface of Mars. | Scientists think they see evidence of two huge tsunamis having once swept across the surface of Mars. |
| They point to satellite data suggesting a major redistribution of sediments over a large region at the edge of the Red Planet's northern lowlands. | They point to satellite data suggesting a major redistribution of sediments over a large region at the edge of the Red Planet's northern lowlands. |
| The US-led team argues that asteroid or comet strikes into an ocean of water could have triggered the giant waves. | The US-led team argues that asteroid or comet strikes into an ocean of water could have triggered the giant waves. |
| Such events could only have occurred more than three billion years ago when the planet was wetter and warmer. | Such events could only have occurred more than three billion years ago when the planet was wetter and warmer. |
| Today, Mars is dry and cold, and any impact would merely dig out a dusty hole. | Today, Mars is dry and cold, and any impact would merely dig out a dusty hole. |
| But researchers have long speculated that the low, flat terrain in Mars' northern hemisphere could have hosted an ocean if the climate conditions were just right. | |
| The nagging doubt with this theory has been the absence of an identifiable shoreline - something the new study could now help explain. | The nagging doubt with this theory has been the absence of an identifiable shoreline - something the new study could now help explain. |
| If tsunamis regularly inundated the "land", dumping sediments and scouring new flow channels, they could over time have disguised what otherwise would have been an obvious "coast". | |
| "For more than a quarter century, failure to identify shoreline features, consistently distributed along a constant elevation, has been regarded as inconsistent with the hypothesis that a vast ocean existed on Mars approximately 3.4 billion years ago," said Alexis Palmero Rodriguez from the Planetary Science Institute in Tuscon, Arizona. | "For more than a quarter century, failure to identify shoreline features, consistently distributed along a constant elevation, has been regarded as inconsistent with the hypothesis that a vast ocean existed on Mars approximately 3.4 billion years ago," said Alexis Palmero Rodriguez from the Planetary Science Institute in Tuscon, Arizona. |
| "Our discovery offers a simple solution to this problem: widespread tsunami deposits distributed within a wide range of elevations likely characterise the shorelines of early Martian oceans." | "Our discovery offers a simple solution to this problem: widespread tsunami deposits distributed within a wide range of elevations likely characterise the shorelines of early Martian oceans." |
| Dr Rodriguez and colleagues' tsunami findings are due to appear on Thursday in the Nature journal Scientific Reports. Their work centres on two connected regions of Mars, known as Chryse Planitia and Arabia Terra. | |
| The team interprets the sediments observed by satellite to betray the action of two ancient mega-tsunamis. | |
| The older event is perhaps easier to understand in an Earth context, where energetic waves can pick up and carry large boulders and other material and dump them at a higher elevation. The water, as it turns back, to run downhill, then cuts new channels. Dr Rodriguez's group points to such evidence. | |
| But the scientists go on to describe the traces of a second, younger event. This is calculated to have occurred a few million years later, when the climate had cooled significantly. In this case, the tsunami wave froze as it propagated across the land surface, depositing "lobes" of sediment but producing no backwash channels. | |
| On Earth, the frozen floes capping a sea or a lake can sometimes be pushed ashore by a storm surge. It is an unusual phenomenon but would be analogous to what is being suggested - albeit on a much larger scale - for Mars . | |
| Having lost some currency, the idea of an ocean on Mars is gaining popularity again. | |
| Investigations by Nasa's Curiosity rover at Gale Crater have revealed that the deep bowl likely contained persistent lakes in the past. | |
| Such water, it is argued, could only have been maintained if there was a robust hydrological system on Mars, cycling moisture between a large sea somewhere on the planet, its atmosphere and its land surface. | |
| Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos |