Planets Gullies on Mars Probably Not Carved by Water Analysis of images and chemical data collected from Mars orbit shifts focus back to seasonal carbon dioxide frost melts, not water flows, as the channels' sculptor.

Image: The gullies, three- to 33 feet in width, on a scarp in Mars' Hellas impact basin. Credit: Mars Reconnaissance Orbiter/NASA/JPL-Caltech/University of Arizona The hunt for what carved channels into polar-facing slopes on Mars -- a process that is ongoing today -- took a new twist on Friday with the release a study showing no chemical fingerprints of flowing water at more than 100 gully sites. Similar features on Earth are carved by flowing liquid water. Mars' cold temperatures and a low atmospheric pressure means that liquid water would be transient, if it exists at all. RELATED: No Water Required: Mars Gullies Caused by 'Dry Ice'? Nevertheless, telltale signs of water, in the form of hydrated salts, has been tied to another seasonal feature on Mars known as recurring slope lineae, or RSL. Scientists are beginning to take test images of potential RSL sites in Gale Crater with NASA's Mars rover Curiosity, project scientist Ashwin Vasavada told DNews. "These things really aren't understood yet," Vasavada said. WATCH VIDEO: Water Found On Mars

Scientists consider gullies to be features that have an alcove on top, a channel and an apron of material at the bottom. RSLs are characterized by seasonal darkening and fading, not how the ground is shaped. Most RSLs are found on equator-facing slopes, as opposed to the pole-facing locations of gullies, leading scientists to theorize that the features stem from different processes. RELATED: This Is an Ancient Meandering River... On Mars Gullies are found all over Mars, with most located between 30 degrees and 50 degrees latitude in the northern and southern hemispheres. In the new study, planetary scientist Jorge Nunez with Johns Hopkins University Applied Physics Laboratory, and colleagues correlated high-resolution images of more than 100 gullies taken by Mars Reconnaissance Orbiter with chemical data obtained by the spacecraft's Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, instrument. Several mechanisms have been proposed over the years to explain gully formation, including the melting of ground ice or the melting of a relict, regolith-covered snow pack. Another option is carbon dioxide frost activity that doesn't involve liquid water at all, the study said. To help narrow the options, scientists looked for minerals, such as clays, silica, zeolites, sulfates, carbonates, or chlorides in the gullies which could indicate past water activity. RELATED: So Liquid Water Flows on Mars -- Now What? "We might have expected to see spectral evidence for liquid water such as hydrated salts as observed at RSL sites if present or recent liquid water activity had played a role in gully formation and evolution," the study said. "We find no such evidence for brines in any of the gullies we have investigated," Nunez wrote. The scientists conclude that their observations indicate "a limited role for long-lived liquid water in the formation and modification of Martian gullies, and support a stronger role for carbon dioxide frost-related processes." The research is published in this week's Geophysical Research Letters. GALLERY: The Weirdest Craters of Mars:

The Martian surface is peppered with impact craters of all shapes, sizes and ages. However, many of the craters are just plain weird. But just how 'weird' is weird? Curious, Discovery News asked the High-Resolution Imaging Science Experiment (HiRISE) team which craters they considered to be the strangest. HiRISE is the most advanced camera to be put into Mars orbit. It is attached to NASA's Mars Reconnaissance Orbiter (MRO) and currently snapping features on the Red Planet's surface -- it has seen a ton of strange objects (sadly, it hasn't found a top secret military base yet, despite what you may have heard to the contrary). So, let's take a tour of some of the weirdest craters Mars has on offer... Special thanks to Ari Espinoza of the HiRISE team for helping to compile this list (originally published Dec. 30, 2011. Updated May 7, 2013)

Crater, Horst and Graben: Is that a piece of modern art? Actually, it's an impact crater that has been bent and twisted by tectonic processes. Along the fault line that crosses this crater, blocks of rock are forced upward (called "horst") and downward (called "graben"). This is interesting to planetary scientists as it shows that tectonic activity was occurring after the crater was formed.

Rolling Stones Logo? If you squint and use a little imagination, you may see the Rolling Stones' logo. Well, that's what the HiRISE team told us anyway. (I'm still squinting...) In reality, it's an impact crater on a sloping surface. Presumably, the "tongue" of material is slipping down the slope.

Bulls-Eye Impact? Did a small meteorite have the incredible fortune to slam into the center of a larger impact crater? Probably not. This is one of several examples of "terraced" craters where alternating layers of hard and soft material in the surface layers of the Martian surface have been hit by a single meteorite. The result is a concentric nesting of ridges inside the same crater. Pretty!

What the...? What's the weirdest kind of impact crater? The kind that may not be an impact crater at all (but looks like one). On the slopes of Pavonis Mons, one of Mars' shield volcanoes, this crater has a hole in the middle. The hole is a "skylight," or the collapsed roof of a subterranean lava tube. The loose material above the collapsed roof appears to have slumped into the skylight, creating a crater lookalike. But what caused the roof of the lava tube to collapse? Could a meteorite be to blame? No idea, but HiRISE will be taking some more photos of this little oddity to find out.

Two-for-One Crater Special: What could be worse than a meteorite hitting you? Two meteorites hitting you... at the same time! Yes, that's exactly what happened here. It seems highly likely that one object tumbled through the Martian atmosphere and split in two. In doing so, the two halves impacted in the same location. As can be seen from this example, both halves were likely the same size, producing a rather satisfying imprint.

Another Double-Whammy: Looks like double-impacts are becoming a trend! This time, in addition to the two co-joined impact craters, HiRISE has picked out the rays that are produced when space rocks slam into the Martian surface.

Hit Me Baby Three More Times? It may seem hard to believe, but Mars also has triple-impact craters! It stands to reason that after countless impacts, you might get the occasional meteorite that splits into three when blasting through the atmosphere. So here you have it, a triple-impact crater.

A Triple Ricochet Crater: Another three (likely simultaneous) impacts, only this time their craters are elongated. This suggests the meteorites hit the surface at an oblique angle.

A Simple Blemish: In an apparently featureless plain in the north polar region, a single, small crater appears as the only blemish. Looking closely, the crater seems to be filled with ice.

Bubbly Landscape: This cluster of impact craters in the northern plains of Utopia Planitia contain strange uplift features likely caused by ground ice upheaval.

Crater of Mud: The strange concentric rings inside this crater near the Martian volcano Elysium Mons are thought to be the ancient remnants of a mud flow. Therefore, it is believed this crater wasn't caused by an impact from space, but by material flowing away from under the surface. The crater was then formed as the material above slumped.

Cracked Cookie Crater? There's an odd pair of craters in Hrad Vallis that the HiRISE website describe as a "pair of odd craters." Why so... odd? Well, to me, the larger crater looks like a cracked cookie, probably crevasses and faults carved across its diameter.

The Crater with a Robot Visitor: What makes this crater weird? Well, it's not the crater, it's the little man-made robot that's parked on the crater's western rim that makes this scene weirdly awesome. It's even weirder to think that a robot in Mars orbit has taken a photo of another robot on the Martian surface a couple of hundred miles below. Robots looking out for robots on alien worlds... This is of course NASA's tenacious Mars Exploration Rover Opportunity that keeps exploring the Martian surface since exceeding its primary mission duration of 3 months back in 2004. Opportunity now has company on the Martian surface -- on Aug. 5, 2012, the nuclear-powered Curiosity landed inside Gale Crater to look for clues behind the habitability of the red planet.