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Water on Mars Probably Doesn't Explain These Weird Streaks

Water on Mars Probably Doesn't Explain These Weird Streaks

Yahoo11-07-2025
For years, scientists have been looking for signs of liquid water just beneath the surface of Mars. The problem, though, is that the observations from various orbiting probes have been maddeningly ambiguous, sparking a lot of debate.
New research recently published in Nature Communications may have dried up one of the most intriguing lines of evidence for subsurface water: it found that long streaks of material on the sides of slopes and crater walls is likely not from seeping liquid but from disturbed dry dust.
Does it even make sense to look for liquid water anywhere on Mars? When we look to Mars today, we see a desiccated, frozen world. No liquid water exists aboveground, and what water we do find is frozen solid, mostly at the poles.
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There's tantalizing evidence of liquid water in the Red Planet's distant past, however. Scientists have spied the sinuous courses of long-lost rivers, as well as the ancient shorelines of vanished lakes and seas carved in the world's rocks, and minerals formed in aqueous environments are relatively commonplace on the surface.
That wet phase of Martian history was billions of years ago, however, and all that water has since evaporated away into space or seeped deep underground. But here and there on modern-day Mars, we still see what might be evidence for liquid water lurking just beneath the Martian surface.
One of the most perplexing hints of hydration are a handful of slope streaks: narrow, long and sometimes bright but usually dark features that are commonly located near the tops of crater walls and scarps. Many are straight, and some wind a bit, but they do very much look like what you'd expect if water leaked out from behind the slope and caused a small flow downhill.
These streaks were first discovered in Viking data from the 1970s. The images were low-resolution and fuzzy by today's standards, but the advent of more advanced orbiters provided sharper views of these features. The streaks tend to be only a few dozen to a couple of hundred meters wide, but they can be a kilometer long. They're seen in dusty equatorial regions and relatively persistent: once formed, they fade over years and decades.
In the late 2000s similar markings were discovered. Called recurring slope lineae (or RSLs), they look much like slope streaks but are usually found in rocky southern areas. They tend to fade over the course of a Martian year (which is about twice as long as our Earthly year) and recur annually in the same spots during summer in Mars's southern hemisphere. RSLs are narrower than slope streaks, only a few meters wide, but also look very much like flow features.
Is this evidence of liquid water on Mars today? I remember, when these were first found, watching an associated NASA press conference and speculating with some colleagues that these could be from water frozen all winter but thawed by the spring and summer sun sending cascades of material downslope. It's cold on Mars, well below water's standard freezing point, but if the water were briny, it might stay liquid even in those frigid temperatures. (Salts are nature's antifreeze.) And we do have plenty of evidence for water ice frozen just beneath the surface in many locations on Mars, even down to midlatitudes.
If RSLs really are triggered by water, they could be the best places to search for extant Martian life (and potential oases for any future human explorers as well).
That's exciting! But is it true?
A problem with previous studies was the lack of a consistent, global database of streaks to investigate. To alleviate that issue, the authors of the new study examined more than 86,000 images from the Context Camera aboard NASA's Mars Reconnaissance Orbiter. This instrument takes images of the Red Planet's surface in long swaths about 30 kilometers wide. In the new study, the researchers used a machine-learning algorithm to find streaks in the images.
The algorithm identified about half a million streaks: roughly 13,000 bright and 484,000 dark. After accounting for streaks missed by the algorithm and other factors such as branching or overlapping streaks, the scientists estimate there could be as many as 140,000 bright and nearly two million dark streaks in the dataset. This is the first global, consistent database of Martian streaks, inviting deeper—and easier—analysis.
Next, the authors cross-referenced their streak database to others that instead track things such as temperature, wind and hydration across the surface of Mars. What they found supports a dry formation for RSLs and slope streaks alike.
For example, if the slope streaks are caused by sunlight-warmed water ice, you'd expect to find them forming overwhelmingly in slopes that face the sun. The researchers found only a weak tie to sunward-facing slopes, however. Another water-based expectation would be to find streaks where the temperature fluctuations are high, but instead they're typically found in locations where temperatures are relatively stable. And although Mars isn't exactly humid, there is some water vapor in the air, so streaks formed by wet cascades should occur mostly in slopes with higher humidity. But the study found them mostly in drier areas instead.
Interestingly, the more ephemeral RSLs do tend to favor sunward-facing slopes but, like the more longer-lived streaks, aren't found in areas with high temperature fluctuations or levels of humidity, again implying they aren't caused by water.
The scientists did find high correlations of slope streaks with regions that have higher wind speeds and lots of dust deposition—Mars is largely covered with a very fine-grained dust that's high in iron oxide (rust), giving it its characteristic ochre coloring. These results point more toward a dry origin for the streaks. They are also found streaks near younger craters, where the ground is more disturbed and can trigger dust flows more easily. For instance, the researchers cataloged some streaks next to a fresh 140-meter crater that formed when a relatively small meteorite hit the surface of Mars just a few years ago. That location has steep slopes and quite a bit of dust; other craters with flatter slopes and less dust exhibit fewer streaks.
Interestingly, some fossae (Latin for 'trenches'), locations where there may be ongoing underground volcanic activity, had streaks. The scientists didn't find a correlation with marsquake activity, but in their paper they note that the data there are limited because of a lack of long-term seismometers on the planet.
Still, this implies streaks may be caused when a sharp energetic event happens on or near the surface, such as a quake or an impact that dislodges dust that then cascades down slopes.
While all this isn't necessarily conclusive, a dry origin for the streaks does currently seem like the better bet. While that's disappointing from the standpoint of looking for native life or supporting our own when we visit, it's still an interesting result. The total estimated annual flux of dust from slope streaks suggests that they may move an amount of material equivalent to several global dust storms on Mars per year! (Actual global dust storms occur every few years on Mars.) These streaks are important geological features of the planet—and we should understand more about them before setting up shop there.
As for life on Mars, whether extant or eons dead, we'll keep looking. Mars is dry now, but it was very likely once very wet, so hope springs eternal.
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Rivkin helped test one approach in September 2022 as the principal investigator of NASA's Double Asteroid Redirection Test, or DART, which intentionally slammed a spacecraft into the asteroid Dimorphos in September 2022. Dimorphos is a moonlet asteroid that orbits a larger parent asteroid known as Didymos. Neither poses a threat to Earth, but the double-asteroid system was a perfect target to test deflection technology because Dimorphos' size is comparable to asteroids that could harm our planet in the event of an impact. The DART mission crashed a spacecraft into the asteroid at 13,645 miles per hour (6 kilometers per second) to find out whether such a kinetic impact would be enough to change the motion of a celestial object in space. It worked. Since the day of the collision, data from ground-based telescopes has revealed that the DART spacecraft did alter Dimorphos' orbital period — or how long it takes to make a single revolution around Didymos — by about 32 or 33 minutes. And scientists have continued to observe additional changes to the pair, including how the direct hit likely deformed Dimorphos due to the asteroid's composition. Similarly, if YR4 strikes the moon and doesn't result in damaging effects for satellites, it could create a tremendous opportunity for researchers to learn how the lunar surface responds to impacts, Wiegert said. But whether it would make sense to send a DART-like mission to knock YR4 off a collision course with the moon remains to be seen. It will depend on future risk assessments by planetary defense groups when the asteroid comes back into view around 2028, de Wit said. Though defense plans for a potential moon impact still aren't clear, YR4's journey underscores the importance — and the challenges — of tracking objects that are often impossible to see. Hidden threats YR4 was detected by the Asteroid Terrestrial-impact Last Alert System, or ATLAS telescope, in Río Hurtado, Chile, two days after the asteroid had already made its closest pass by Earth, hidden by the bright glare of the sun as it approached our planet. The same thing occurred when an asteroid measuring roughly 20 meters (about 65 feet) across hit the atmosphere and exploded above Chelyabinsk, Russia, on February 15, 2013, damaging thousands of buildings, according to the European Space Agency. While no one died, about 1,500 people were injured when the windows in homes and businesses blew out due to the shock wave. Trying to observe asteroids is challenging for many reasons, Rivkin said. Asteroids are incredibly faint and hard to see because rather than emitting their own light, they only reflect sunlight. And because of their relatively tiny size, interpreting observations is not a clear-cut process like looking through a telescope at a planet such as Mars or Jupiter. 'For asteroids, we only see them as a point of light, and so by measuring how bright they are and measuring their temperature, basically we can get a size based on how big do they have to be in order to be this bright,' Rivkin said. For decades, astronomers have had to search for faint asteroids by night, which means missing any that may be on a path coming from the direction of the sun — creating the world's biggest blind spot for ground-based telescopes that can't block out our star's luminosity. But upcoming telescopes — including NASA's NEO Surveyor expected to launch by the end of 2027 and the European Space Agency's Near-Earth Object Mission in the InfraRed, or NEOMIR satellite, set for liftoff in the early 2030s — could shrink that blind spot, helping researchers detect asteroids much closer to the sun. 'NEOMIR would have detected asteroid 2024 YR4 about a month earlier than ground-based telescopes did,' said Richard Moissl, head of ESA's Planetary Defence Office, in a statement. 'This would have given astronomers more time to study the asteroid's trajectory and allowed them to much sooner rule out any chance of Earth impact in 2032.' NASA and other space agencies are constantly on the lookout for potentially hazardous asteroids, defined as such based on their distance from Earth and ability to cause significant damage should an impact occur. Asteroids that can't get any closer to our planet than one-twentieth of Earth's distance from the sun are not considered to be potentially hazardous asteroids, according to NASA. When the new Vera C. Rubin Observatory, located in the Andes in Chile, released its first stunning images of the cosmos in June, researchers revealed the discovery of more than 2,100 previously unknown asteroids after seven nights of observations. Of those newly detected space rocks, seven were near-Earth objects. A near-Earth object is an asteroid or comet on an orbit that brings it within 120 million miles (about 190 million kilometers) of the sun, which means it has the potential to pass near Earth, according to NASA. None of the new ones detected by Rubin were determined to pose a threat to our planet. Rubin will act as a great asteroid hunter, de Wit said, while telescopes such as Webb could be a tracker that follow up on Rubin's discoveries. A proposal by Rivkin and de Wit to use Webb to observe YR4 in the spring of 2026 has just been approved. Webb is the only telescope with a chance of glimpsing the asteroid before 2028. 'This newly approved program will buy decision makers two extra years to prepare — though most likely to relax, as there is an 80% chance of ruling out impact — while providing key experience-based lessons for handling future potential impactors to be discovered by Vera Rubin,' de Wit said. And because of the twists and turns of YR4's tale thus far, asteroids that have potential to affect the moon could become objects of even more intense study in the future. 'If this really is a thing that we only have to worry about every 5,000 years or something, then maybe that's less pressing,' Rivkin said. 'But even just asking what would we do if we did see something that was going to hit the moon is at least something that we can now start thinking about.'

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