Where did Martian gullies come from? High obliquity could hold the answer

Another hypothesis suggests that small amounts of liquid water created the gullies in earlier climate conditions. Furthermore, previous studies have shown that during periods of higher obliquity in Mars' past, water ice could have accumulated in locations that now contain gullies.

Now, to better understand the role liquid water may have played in forming these features, James Dickson and colleagues used a three-dimensional global circulation model of Mars to simulate how the planet's climate differed when its axis tilted by varying amounts over the last million years.

Dickson et al. found that at 35 degrees (°) obliquity, locations of current gullies – areas that have abundant water ice near the surface today and likely had tens of meters more within the last million years – reached pressures of more than 612 pascals and ice surface temperatures probably surpassed 273 Kelvin (the melting point of water ice). 

As a result, melting ice could have stayed liquid during periods of high obliquity, carving gullies in the high-altitude areas where they are currently found.

A brief history of martian gully exploration

Gullies, which are small, narrow channels resembling ravines or valleys, were first observed in images captured by the Mars Global Surveyor in the late 1990s. These peculiar features sparked scientific curiosity, as they appeared to have been formed by liquid water.

In the early 2000s, NASA's Mars Odyssey orbiter provided further evidence for the presence of water on Mars by detecting hydrogen-rich soil, suggesting the existence of ice. This finding intensified the interest in understanding the origin and formation of gullies.

In 2008, NASA's Phoenix lander touched down near Mars' north pole, confirming the presence of water ice in the Martian soil. The discovery of subsurface ice bolstered the hypothesis that the seasonal flows of liquid water could form gullies, although the exact mechanisms remain elusive.

Since then, various Mars missions, including the Mars Reconnaissance Orbiter and the Curiosity rover, NASA's Mars 2020 rover, and the European Space Agency's ExoMars, have captured high-resolution images and conducted detailed investigations of gullies. Undoubtedly, future missions will add more data and shed light on the complex geological processes involved in their formation.

The complete study was published in Science on June 29 and can be found here.

Study abstract:

Gullies on Mars resemble water-carved channels on Earth, but they are mostly at elevations where liquid water is not expected under current climate conditions. It has been suggested that sublimation of carbon dioxide ice alone could have formed Martian gullies. We used a general circulation model to show that the highest-elevation Martian gullies coincide with the boundary of terrain that experienced pressures above the triple point of water when Mars’ rotational axis tilt reached 35°. Those conditions have occurred repeatedly over the past several million years, most recently ~630,000 years ago. Surface water ice, if present at these locations, could have melted when temperatures rose >273 kelvin. We propose a dual gully formation scenario that is driven by melting of water ice followed by carbon dioxide ice sublimation.

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