David Rothery, Open University
Was Mars once a surfer’s paradise? Possibly, if you like surfing on tsunamis! We know thanks to a succession of probes that there are strong signatures of water sitting beneath the surface of the red planet, and researchers have concluded that Mars was once dominated by a massive ocean. But if the ocean was there, where’s the shoreline that it once lapped up against? It’s only with an eye of faith that something fitting the bill can just about be seen on satellite images of the surface. Well now an international team of scientists think they know the answer. There was a shoreline, but some giant waves triggered by two massive meteor impacts washed it away… Planetary scientist David Rothery, who wasn’t involved with the study, has been taking a look at the data for Chris Smith…
David - So what’s been suggested is that the shoreline of the now vanished ocean has been washed over by tsunami waves. In one particular part of the shoreline, the team that’s produced this new paper, have shown two deposits that have washed up over where the shoreline would have been depositing boulders at a high level and then have washed back carving backwash channels. So we’ve got evidence of waves dumping stuff ashore and then the water draining back into the ocean from two tsunamis and these would have been generated by large meteorites, small asteroids, crashing into the ocean creating the tsunami.
Chris - And how big would those tsunamis have been?
David - Well, they’re taking about 30 kilometer size craters. It kind of depends whether the ocean was ice covered or free water at the surface how big the waves would be but once they reach shallow water at the edge of the ocean and rush on shore, they do ramp up. So we’re looking at waves in the order of 10 metres high running ashore and running uphill for several tens of metres under their own momentum carrying boulders with them and then draining back into the sea carving these channels.
Chris - What’s the backstory to all of this? We have evidence that Mars was once a very wet place so why is the whole idea of their being an ocean there contentious?
David - Well, you’re right Chris. There’s plenty of evidence that Mars has been wet in the distant past. There’s some very big channel systems draining through the high standing southern hemisphere of the planet into the low northern hemisphere. The northern hemisphere is low lying and covered in sediments. There may be some ice mixed in with those sediments still today and that is where the ocean would have been 3 or 3½ billion years ago. It’s now long since dried out and/or frozen into the sediments on the bottom but plenty of signs that there was an ocean there once upon a time.
Now the shoreline of this ocean has been hard to locate. You can trace it if you try and it’s not an obvious shoreline and perhaps now it’s been suggested that there have been tsunamis washing up and down across this shoreline, that’s why the usual shoreline markers aren’t so obvious to see because they’ve been obliterated by the occasional series of tsunami waves rushing ashore and then draining back down into the ocean basin.
Chris - How have the team who’ve come up with these predictions actually done this?
David - Well, they’ve been using a variety of images from spacecraft orbiting Mars, including some images with really fine spatial resolution. I’m looking here at a picture that’s showing angular boulders just a few metres in size in a big deposit above the shoreline that they say have been washed uphill from the sea by the waves crashing ashore. So, it’s a variety of high resolution and medium resolution images and they’ve traced the shorelines for over 1,000 kilometers.
Chris - Right, so they’ve starting with what we do know, which is we can see what we can see. We’ve got very good images of the surface of Mars and they’re asking what could have produced these images, and so they’ve backwards extrapolating what could have done that?
David - That’s right. I mean, I wish I’d thought of this because it’s pretty obvious that on a planet like Mars, if you’ve got a sea, doesn’t have much atmosphere to slow anything down and we do know in the distant past there were craters forming on Mars. There would have been a dozen or so 30 kilometer craters formed on the northern hemisphere of Mars during the time when this ocean was supposed to have been there. So it’s pretty obvious that impacts into that ocean ought to have caused tsunami waves. It happened in the Earth in the distant past as well. And these guys have found the deposits and the erosional features of the kind that would be produced by tsunami waves crashing ashore and that helps explain why the more conventional shoreline features are harder to see because in many places they’ve been obliterated by the effects of these very rare tsunamis, so it hangs together. It’s a seductive story. I’m not sure everybody’s going to believe it straight away but it’s one of these damned good ideas that you think with hindsight… yes, that works.