Astro Bob: Perseverance rover finds evidence of raging river on Mars
How a rollicking river left its mark on the Red Planet. We also look ahead to the return of the young moon at dusk.
Mars is just a scintilla of light in the constellation Gemini seen from Earth. But thanks to the Martian rovers Curiosity and Perseverance we can imagine what it would like to be there in person, filling up our senses the way the home planet does each waking day.
NASA recently released a spectacular montage that shows curving bands of rock deposited as sediments by a deep, fast-flowing river. The river was part of a network of waterways that flowed into Jezero Crater, where the Perseverance rover has been exploring for more than two years. When Mars was a wetter, more life-friendly place 3.5 billion years ago, rain and rivers turned the 28-mile-wide (45 kilometers) crater into a lake you could have gone sailing on.
Sometime after, the combined effects of the planet's lower mass (weaker gravitational pull) and lack of a global-wide protective magnetic field caused some of the water in Jezero and elsewhere to evaporate into space. But to this day a significant amount remains locked up in minerals such as clays, carbonates and sulfates in the Martian crust.
So, yes. Mars still retains plenty of water albeit hidden from direct view. Discovery of water-rich minerals as well as abundant evidence of rivers, lakes and even oceans motivate the continuing search for signs of ancient microbial life , precisely the purpose of the Perseverance rover. Since early 2021 it's been drilling, sampling and caching promising samples for later return to Earth and keeping an eye out for any potential fossils.
Those strikingly curvy rows of rocks in the photos sit atop an 820-foot-tall (250 meters), fan-shaped pile of sedimentary rock in the middle of a massive river delta. The delta formed billions of years ago when a river and its tributaries deposited loads of silt as they drained through a gap in wall of the crater.
While NASA's Curiosity rover found evidence of shallow streams in Gale Crater several years back, Perseverance has photographed coarse sediment grains and cobbles — water-rounded rocks larger than pebbles but smaller than boulders — that indicate a high-energy river carrying a lot debris. Fast-flowing, deep water moves larger pieces of material more easily.
While mission scientists are convinced a powerful river once flowed here they're still debating whether it was like the Mississippi which winds snakelike across the landscape or a braided river dotted with sandbars like Nebraska's Platte. The curved layers ripple out across the Martian landscape — were they riverbanks that shifted over time or former sandbars? The layers were likely much taller in the past but have been sandblasted by wind over the eons:
“The wind has acted like a scalpel that has cut the tops off these deposits,” said Michael Lamb of Caltech, a river specialist and Perseverance science team collaborator. “We do see deposits like this on Earth, but they’re never as well exposed as they are here on Mars. Earth is covered in vegetation that hides these layers.”
Scientists continue to study the rover photos as well as the data from its ground-penetrating radar for a better understanding of the planet's aqueous past. All the evidence is there, but like those easy meals where you "just add water," we can employ our imaginations to to do the same when viewing these barren landscapes. To peruse the rover's daily image upload check out the Perseverance raw images site .
Moon returns at dusk
Just a heads up on the moon. The thin day-and-a-half-old crescent will grace the northwestern sky Saturday, May 20. You'll need an unobstructed view in that direction to see it best. Look for it about 30-35 minutes after sunset well below and to the lower right of Venus. The view through binoculars should be inspiring — as long as the wildfire smoke parts!