Title: Deposits from giant floods in Gale crater and their implications for the climate of early Mars (Nature, open access)
Authors: E. Heydari, J. F. Schroeder, F. J. Calef, J. Van Beek, S. K. Rowland, T. J. Parker & A. G. Fairén
First author affiliation: Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State University
Thanks to the 8-year trek of NASA’s intrepid Curiosity Rover (Fig. 1), Gale Crater is arguably the best-studied place on Mars. The crater has had a tumultuous history – it’s been filled to the brim with rock, then hollowed out again by wind to form a hill at its center, known as Mt. Sharp. It has housed small lakes and had parts of its rim destroyed by rivers. However, to fully understand Gale’s place in Mars’ potentially habitable past, these snapshots aren’t enough. Rover images show tantalizing hints of ancient water inside Gale crater perhaps a billion years before the most recent lakes, and where there was liquid water, there might have been promise for life. But life doesn’t just appear on a planet overnight! For an environment to go from habitable to inhabited takes time. So, how long did wet conditions last in Gale? So far there’s been an air of cautious optimism, but the re-examination of the rocks in Gale crater in today’s paper stands to turn everything we thought we knew about Gale’s history on its head.
In the conventional version of Gale’s sedimentological story, rivers washed sand and pebbles and from the crater rim down into a lake over hundreds or thousands of years. Only the fast-moving water in rivers can carry sand and pebbles downstream, so when a lake stops a river in its tracks, all the rocks, sand and mud fall to the bottom, forming deltas. The Earth is covered in deltas like the Bengal Fan off the coast of India, and the Mississippi delta in the Gulf of Mexico, so we have a good idea of what the rocks left behind by deltas look like. As lake levels change, repeating patterns of lake mud, sand, and pebbles build up. These are brought back to the surface (where rovers can see them) when the material above them is removed by wind (think slow-motion sandblasting!). If Gale’s rocks formed in a delta, it would suggest a long-lived warm, wet climate, which would be very promising for scientists searching for traces of life on Mars. Unfortunately, rocks in unexpected orders, mud and sand in the wrong places, and mysterious ridges (Fig. 1) fly in the face of this delta story, and there hasn’t yet been a satisfying explanation as to why.
Instead of comparing Gale’s rocks to calm lake and river environments, where sand and gravel accumulate slowly in rivers and lakes, the authors of today’s paper noticed similarities between the appearance of rock within Gale and rocks left behind by the most dramatic flooding events the Earth has ever seen – megafloods! These catastrophic events were generated by the sudden melting of enormous ice caps that used to cover the northern hemisphere (Fig. 3)!
Today’s authors propose a single, catastrophic flood with roiling waters 24 meters (72 feet) deep which left behind enormous ripples, hundreds of meters wide (Figs. 2 & 4), like those observed in Washington’s Channeled Scablands (Fig. 3). Gale’s perplexing pattern of pebbly ridges (Fig 2.) is one of the features the delta hypothesis struggles most to explain, and formation in deep, fast-flowing floodwaters (Fig. 4) is an elegant (if terrifying) alternative.
But where could all this water have come from, and so suddenly? To explain how a lake could exist for thousands of years on Mars, planetary scientists often suggest a thicker past atmosphere with a mixture of greenhouse gasses like water vapor and methane released by volcanoes. The authors of today’s paper propose a more dramatic explanation. While volcanic eruptions take a long time to change the atmosphere, giant asteroid impacts can radically change a planet’s climate by providing an instant injection of heat into the atmosphere. This heat could have been enough to melt – and even evaporate – glaciers all over Mars, forming rivers, kickstarting rainfall, and releasing methane trapped in Martian permafrost for an extra warming kick. However, climates caused by asteroid impacts can’t last. So, while they might be able to generate lots of liquid water through melting ice caps and rainfall, the water might only stick around for a few months – not nearly long enough for life to get established!
The jury is still out on whether deltas or megafloods fit Gale’s geology best, but how scientists choose to interpret these rocks could rewrite Mars’ history, and completely change our search for life on the red planet. The difference between the two theories could be the difference between a Mars that spent hundreds of millions of years warm, wet, and with promise for life, and a cold, dry Mars where brief snippets of habitable conditions occurred only at the whim of giant asteroid impacts.
Edited by: Laila Linke
Featured image credit: NASA/JPL-Caltech