Meteorites whipped up mega-tsunamis on Mars

The Solar System is no stranger to mega-tsunamis. Just last year, on Earth, scientists analysed unusual boulders sitting 200 metres above sea level on the West African island of Santiago.

The only way those rocks could have ended up on the highlands, they reported in Science Advances, was if a 170-metre wave – pushed along when the oceanic volcano Fogo collapsed 73,000 years ago – dumped them there.

In a 2010 article in Planetary and Space Science, a North American team headed by Canadian Bill Mahaney proposed something similar might have happened on Mars during the Hesperian Period, around 3.4 billion years ago, when it was thought the Red Planet was flush with vast oceans.

But rather than landslides, meteorites could well have forced Martian mega-tsunamis – the Solar System was still being bombarded with asteroids and comets (although not quite on the scale of the Heavy Bombardment which finished around 400 million years earlier).

The problem, though, was tracing ancient Martian coastlines. Mars today lacks liquid water on its surface, let alone oceans. Finding the coasts of these ancient seas, and then locating evidence of huge waves, is tricky.

Mahaney and colleagues suggested the best places to start might be the Chryse Planitia and Arabia Terra regions of the northern plains – some of the oldest, most heavily cratered terrains on the planet. 

Rodriguez and colleagues examined infrared (or thermal) images of those areas and found what appeared to be coastal boundaries – thermally dark sediments abutting bright, bouldered, rocky segments – on different elevations.

These are evidence of two tsunamis: one which covered around 800,000 square kilometres and extended around 530 kilometres inland, and another that covered a million square kilometres with a reach of around 650 kilometres. 

The second flowed further because, over the intervening few million years since the first, the plain eroded and smoothed, easing the wave’s passage. It also dumped huge chunks of ice inland.

To reach such distances, both tsunamis were around 50 metres tall when they hit shore, and as tall as 120 metres in parts.

The team also saw what seemed to be backwash channels. Like those found on Earth when a tsunami’s water is dragged back to sea by gravity, the Martian channels were perpendicular to the ancient shoreline.

Simulations then showed the meteorites responsible for such massive waves would have left impact craters around 30 kilometres wide. 

The team saw seven impact craters in the region fit the bill, which turned out to be two such meteorites every 30 million years for the area at the time, or one every three million years striking anywhere on Mars.

Tracing mega-tsunami flows can help scientists nail down targets to search for life, says study co-author Alberto Fairén: “In spite of the extremely cold and dry global climatic conditions, the early Martian ocean likely had a briny composition that allowed it to remain in liquid form for as long as several tens of millions of years.

“Subfreezing briny aqueous environments are known to be habitable environments on Earth, and consequently, some of the tsunami deposits might be prime astrobiological targets.”