Astronomy - Spiders Growing on the Surface of Mars Right Before Our Eyes!

For years, scientists have understood that in Mars’ polar regions, frozen carbon dioxide (aka. dry ice) covers much of the surface during the winter. During the spring, this ice sublimates in places, causing the ice to crack and jets of CO² to spew forth. This leads to the formation of dark fans and features known as “spiders”, both of which are unique to Mars’ southern polar region.

For the past decade, researchers have failed to see these features changing from year-to-year, where repeated thaws have led to their growth. However, using data from the Mars Reconnaissance Orbiter‘s (MRO) HiRISE camera, a research team from the University of Colorado, Boulder and the Planetary Science Institute in Arizona have managed to catch sight of the cumulative growth of a spider for the first time from one spring to the next.

Spiders are so-named because of their appearance, where multiple channels converge on a central pit. Dark fans, on the other hand, are low-albedo patches that are darker than the surrounding ice sheet. For some time, astronomers have been observed these features in the southern polar region of Mars, and multiple theories were advanced as to their origin.

HiRISE images of the Martian landscape, showing outgassing and the formation of dark fans and “spiders”. Credit: NASA/JPL

In 2007, Hugh Kieffer of the Space Science Institute in Boulder, Colorado theorized that the dark fans and spiders were linked, and that both features were the result of spring thaws. In short, during Mars’ spring season – when the southern polar region is exposed to more sunlight – the Sun’s rays penetrates the ice sheets and warm the ground underneath.

This causes gas flows to form beneath the ice that build up pressure, eventually causing the ice to crack and triggering geysers. These geysers deposit mineral dust and sand across the surface downwind from the eruption, while the cracks in the ice grow and become visible from orbit. While this explanation has been widely-accepted, scientists have been unable to observe this process in action.

By using data from the MRO’s High Resolution Imaging Science Experiment (HiRISE), the research team was able to spot a small-channeled troughs in the southern region which persisted and grew over a three year period. In addition to closely resembling spidery terrain, it was in proximity to dark fan sites. From this, they determined that they were witnessing a spider that was in the process of formation.

As Dr. Ganna Portyankina – a professor from the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder, and the lead author on the team’s research paper – explained to Universe Today via email,

“We have observed different changes in the surface caused by CO² jets before. However, they all were either seasonal changes in surface albedo, like dark fans, or they were only short-lived and were gone the next year, like furrows. This time, the troughs have stayed over several years and they develop dendritic-type of extension – right the way we expect the large spiders to develop.”  

Spiders trace a delicate pattern on top of the residual polar cap, after the seasonal carbon-dioxide ice slab has disappeared. Next spring, these will likely mark the sites of vents when the CO2 icecap returns. This MOC image is about 2 miles wide.