In a first for Martian water science, NASA’s Perseverance rover has discovered geological evidence of a large, fast-moving river in Mars’ ancient past. The high-energy river once emptied into Jezero crater, which the rover has been exploring since early 2021, and is a totally different water system than anything seen previously on the red planet.
“It’s the first time we’re seeing environments like this on Mars,” says Katie Stack Morgan, Perseverance’s deputy project scientist at the Jet Propulsion Laboratory. “We’re thinking about rivers on a different scale than we have before.”
These days, evidence for Mars’ wet history is plentiful. Back in 2004, the Opportunity rover landed near what was once the shore of a salty sea. The Curiosity rover is currently exploring an ancient lakebed in Gale crater, complete with wave-made ripples, and has seen evidence of small, shallow streams too. Meanwhile, China’s recently defunct (probably) Zhurong rover found evidence of liquid water in Mars’ more recent history.
What Perseverance is seeing now is new: a fast, high-energy river used to tear through the area, carrying large debris and building huge stacks of sedimentary rock that, though wind eroded, still stand out today.
One such outcrop, named Pinestand, is anomalously large, at 20 meters high, and has scientists wondering if there might be some other explanation. But on Earth, such a formation is most commonly caused by rivers.
NASA’s Perseverance Mars rover captured this mosaic of a hill nicknamed “Pinestand.” Credit: NASA/JPL-Caltech/ASU/MSSS.
Just under half a kilometer from Pinestand lies another telltale formation, which the team has named Skrinkle Haven. Skrinkle Haven consists of a series of curved bands of rock, and likely represents either sandbars that formed within the river, or the former banks of the river that shifted over time.
Wind erosion has cut the top off of the Skrinkle Haven formations, which may have once been much taller. But despite the passage of time, these exposed features remain helpful to scientists trying to understand the region’s geologic history.
In fact, Skrinkle Haven’s features are prominent enough that the geologic unit they are part of had actually been seen years ago from orbit. Perseverance is finally getting a close-up look.
The fan-shaped river delta that Perseverance is now exploring, even from orbit, looks like it must have been formed by water, but it hasn’t been clear until now whether it was a series of small streams or a large rushing river. Evidence now points to the latter.
Artist’s impression of Jezero crater as it may have looked in the ancient past, with ancient rivers flowing into it. Credit: NASA/JPL-CalTech.
The entire region features coarse sediment grains and cobbles which, according to JPL Postdoctoral researcher Libby Ives, “indicate a high-energy river that’s truckin’ and carrying a lot of debris. The more powerful the flow of water, the more easily it’s able to move larger pieces of material,” she said. “It’s been a delight to look at rocks on another planet and see processes that are so familiar.”
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Featured Image: NASA’s Perseverance Mars rover captured this scene at a location nicknamed “Skrinkle Haven” using its Mastcam-Z
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When Black Holes Merge, They’ll Ring Like a Bell
When two black holes collide, they don’t smash into each other the way two stars might. A black hole is an intensely curved region of space that can be described by only its mass, rotation, and electric charge, so two black holes release violent gravitational ripples as merge into a single black hole. The new black hole continues to emit gravitational waves until it settles down into a simple rotating black hole. That settling down period is known as the ring down, and its pattern holds clues to some of the deepest mysteries of gravitational physics.
Gravitational wave observatories such as the Laser Interferometry Gravitational-Wave Observatory (LIGO) have mostly focused on the inspiral period of black hole mergers. This is the period where the two black holes orbit ever closer to each other, creating a rhythmic stream of strong gravitational waves. From this astronomers can determine the mass and rotation of the original black holes, as well as the mass and rotation of the merged black hole. The pattern of gravitational waves we observe is governed by Einstein’s general relativity equations, and by matching observation to theory we learn about black holes.
General relativity describes gravity extremely well. Of all the gravitational tests we’ve done, they all agree with general relativity. But Einstein’s theory doesn’t play well with the other extremely accurate physical theory, quantum mechanics. Because of this, physicists have proposed modifications to general relativity that are more compatible with quantum theory. Under these modified theories, there are subtle differences in the way merged black holes ring down, but observing those differences hasn’t been possible. But a couple of new studies show how we might be able to observe them in the next LIGO run.
The modified Teukolsky equation. Credit: Li, Dongjun, et al
In the first work, the team focused on what is known as the Teukolsky Equation. First proposed by Saul Teukolsky, the equations are an efficient way of analyzing gravitational waves. The equations only apply to classical general relativity, so the team developed a way to modify the equations for modified general relativity models. Since the solutions to both the Teukolsky and modified Teukolsky equations don’t require a massive supercomputer to solve, the team can compare black hole ring downs in various gravitational models.
The second work looks at how this would be done with LIGO data. Rather than focusing on general differences, this work focuses on what is known as the no-hair theorem. General relativity predicts that no matter how two black holes merge, the final merged black hole must be described by only mass, rotation, and charge. It can’t have any “hair”, or remnant features of the collision. In some modified versions of general relativity, black holes can have certain features, which would violate the no-hair theorem. In this second work, the authors show how this could be used to test general relativity against certain modified theories.
LIGO has just begun its latest observation run, so it will be a while before there is enough data to test. But we may soon have a new observational test of Einstein’s old theory, and we might just prove it isn’t the final theory of gravity after all.
Reference: Li, Dongjun, et al. “Perturbations of spinning black holes beyond General Relativity: Modified Teukolsky equation.” Physical Review X 13.2 (2022): 021029.
Reference: Ma, Sizheng, Ling Sun, and Yanbei Chen. “Black hole spectroscopy by mode cleaning.” Physical Review Letters 130.2 (2023): 141401.
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There’s a Polar Cyclone on Uranus’ North Pole
Uranus takes 84 years to orbit the Sun, and so that last time that planet’s north polar region was pointed at Earth, radio telescope technology was in its infancy.
But now, scientists have been using radio telescopes like the Very Large Array (VLA) the past few years as Uranus has slowly revealing more and more of its north pole. VLA microwave observations from 2021 and 2022 show a giant cyclone swirling around this region, with a bright, compact spot centered at Uranus’ pole. Data also reveals patterns in temperature, zonal wind speed and trace gas variations consistent with a polar cyclone.
Uranus as seen by NASA’s Voyager 2. Credit: NASA/JPL
Scientists have long known that Uranus’ south pole has a swirling feature. When Voyager 2 flew past Uranus in 1986, it detected high wind speeds there. However, the way the planet was tilted did not allow Voyager to see the north pole.
But the VLA in New Mexico has now been studying Uranus the past several years, and observations collected in 2015, 2021, and 2022 were able to peer deep into Uranus’ atmosphere. The thermal emission data showed that circulating air at the north pole seems to be warmer and drier, which are the hallmarks of a strong cyclone.
“These observations tell us a lot more about the story of Uranus. It’s a much more dynamic world than you might think,” said Alex Akins of NASA’s Jet Propulsion Laboratory in Southern California, who is lead author of a new study published in Geophysical Letters. “It isn’t just a plain blue ball of gas. There’s a lot happening under the hood.”
The researchers said the cyclone on Uranus is similar to the polar cyclones observed by the Cassini mission at Saturn. With the new findings, cyclones (which rotate in the same direction their planet rotates) or anti-cyclones (which rotate in the opposite direction) have now been identified at the poles on every planet in our solar system that has an atmosphere. The researchers said this confirms a broad truth that planets with substantial atmospheres – whether the worlds are made of rock or gas – all show signs of swirling vortexes at the poles.
Uranus’ north pole is now in springtime. As it continues into summer, astronomers hope to see even more changes in its atmosphere.
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