Two French scientists have reported that the gullies on Mars, thought to be caused by water and not by water. Gullies are landforms that look like ravines. They’re created when running water erodes into the soil. Gullies can look similar to small ditches and valleys. The discovery of gullies on Mars was a great moment. People imagined an ancient planet with rivers and flowing water. The Russell crater megadune is shown in this image. (Image from www.nature.com). Scientists Francois Forget of Sorbonne Universites and Cedric Piilorget of the Universite France-Sud explained that during Martian winters, a layer or carbon dioxide (CO2) frost develops on several areas of the Red Planet’s surface. The likely reason is pressure from trapped CO2. When dry ice melts, CO2 gas becomes trapped beneath the surface. The pressure builds up from this gas. According to the authors, this trapped carbon dioxide may be responsible for the formation of gullies at Mars’ surface. Many areas of Mars’s surface have valleys cut into hillsides and dunes. Many scientists think that these hillsides and dunes were created during an era in Mars’ history when there was water. However, many of these formations are young geologically. They are actually still being formed. Pilorget, Forget, and others wondered if there was another reason for the formation of the gullies on the sand dunes. The NASA image below shows several channels that range from 1 meter up to 10 m wide at a scarp located in the Hellas Impact Basin. These formations are called gullies on Earth. The larger channels that Mars has are often called “gullies” are large enough to be called ravines. Image: jpl.nasa.gov. This does not mean that Mars was dry. Scientists have collected a lot of information about Mars over the years from NASA satellites and rovers. Many of these data show that Mars once had lots of water. Curiosity, NASA’s robotic car-sized rover that explored Gale Crater, Mars, as part NASA’s Mars Science Laboratory mission found evidence of numerous lakes rising and falling in the crater for many thousands of years. Numerous studies suggest that large oceans existed once upon Mars. However, these huge oceans disappeared over time. A study suggested that an ocean covered 19% the planet once. There is evidence that Mars had some liquid water at one time. This supports the possibility of Mars having abundant water very long ago. A rendering of Mars as it may have appeared 4 billion years ago. It includes vast oceans. Image: www.eso.org. Scientists have done modeling simulation. In their latest study, Forget & Pilogret found that dry ice on Mars changes from being a solid to becoming a gaseous (sublimation), when winter ends. This is how small amounts of CO2 are trapped beneath the surface. The team performed a simulation of how this could occur on the Russel Crater, Mars. The trapped CO2 gas creates pressure, which causes a “gas-lubricated debris stream” to occur. Sand flows downhill, creating a gully. The journal’s Abstract stated that the researchers found that the subsurface CO2 Ice condensation, sublimation, and pressurization occur at the locations where gullies have been observed. “Martian gullies may result from geologic dry-ice processes without terrestrial analogs. They do not need liquid water. These dry ice processes could have shaped the evolution of landforms on Mars’ surface,” Colin Dundas (a research geologist at U.S. Geological Survey in Arizona) wrote: “Martian gullies can be seen as evidence of past surface water runoff. Numerical modelling suggests that modern gully formation may have been driven by accumulation and sublimation carbon dioxide ice. Citation: “Formation gullies Mars by debris flows triggered CO2 sublimation,” C. Pilorget & F. Forget. Nature Geoscience. 21 December 2015. DOI: doi: 10.1038/ngeo2619. NASA photos of Mars gullies – Video NASA image taken by the Mars Reconnaissance Orbiter in April. This photo was taken with NASA’s High Resolution Imaging Science Experiment camera. It shows frost accumulation in several gullies. There are frost-covered valleys and peaks on Mars that look like what you see on Earth.
Scientists say Mars’ gullies are caused by trapped gas.
