Secrets of Martian riverbeds
Fred Watson
Fred Watson
Have you ever heard of the Martian dichotomy? No, it’s not a political confrontation among aliens but rather a distinct geographical contrast between Mars’s northern and southern hemispheres.
Discovered by orbiting spacecraft in the 1970s, it’s characterised principally by a difference in elevation – the southern hemisphere is 1–3km higher than the north, and has mountains instead of plains. There’s also a significant difference in crustal thickness – the 32km average thickness in the north is almost doubled in the south. That suggests a global origin for the dichotomy. Active tectonics and a giant impact in Mars’s early history have both been postulated.
Which of those scenarios was the actual cause remains to be established, but one thing most Mars specialists agree on is that 3–4 billion years ago, Mars had a climate similar to Earth’s, with bodies of water in its northern hemisphere. Ancient shorelines and river estuaries support this view.
What remains in doubt is the longevity and size of these bodies. Were they short-lived lakes and seas, or something more permanent such as an ocean?
New research carried out by scientists at the University of Arkansas adds weight to the suggestion of a long-lived ocean covering Mars’s northern hemisphere. The researchers looked at defining features in ancient riverbeds on Earth and compared them with what we see on Mars. In particular, they studied the fossilised signatures of channel belts – zones containing the meanders of a mature river.
Although the meanders themselves gradually evolve with time, they stay within the width defined by the channel belt. Crucially, that width decreases as a river approaches an ocean and its flow becomes more sluggish. This region of reduced channel-belt width is called the backwater zone.
The Arkansas researchers present results on nine Martian backwater zones, measured using data from NASA’s Mars Reconnaissance Orbiter. They find zone lengths ranging from 5–17km – modest by comparison with the Earth’s great rivers, but these are relatively minor watercourses. Nevertheless, the researchers argue that such zone lengths are consistent with the rivers draining into an ocean basin.
The finding is relevant to the idea of Martian life having emerged in an ocean environment, which is what we think happened on our own planet.
Although we’re still a long way from proving that microorganisms did once flourish on Mars, the new result can only lend support to that possibility.
