The spider-like landforms are a distinctive terrain feature unique to Mars. How did this terrain form? Why do these “spiders” appear only in specific regions of Mars? Scientists have proposed various hypotheses to address these questions, and recently, they may have uncovered the answer to this mystery.
The spider-like landforms on Mars’ surface are special topographical features observed by Martian probes. In the early 2000s, the Mars Express probe from the European Space Agency captured high-definition images of this terrain for the first time.
The photographs clearly reveal these spider-shaped formations. These spider-like structures are uniquely morphed. They typically originate from a central point, extending multiple slender, radial branches outward, resembling spiders. Each branch structure can span hundreds of meters to over a kilometer, with hundreds of delicate “legs” protruding. In terms of color, these landforms often contrast with the surrounding red or brown Martian surface due to the accumulation of bright frosty substances after sublimation, sometimes appearing lighter in hue.
Moreover, these “spider” landforms tend to emerge in clusters.
Interestingly, such landforms have not been found on Earth, making them unique to Mars. Therefore, the formation of these special landforms must be related to the unique environment of Mars. Scientists have proposed several hypotheses for their formation, including:
First, dry ice sublimation: It is believed that they are formed when dry ice (solid carbon dioxide) covers the Martian surface during winter and then sublimates (changes directly from solid to gas) as temperatures rise in spring. During this process, the accumulated gas pressure is released through cracks, forming these radial grooves.
Second, melting of ice inclusions: Another hypothesis suggests that these landforms may be caused by the melting of ice inclusions beneath the Martian surface, which then flow out through surface cracks or channels, creating these radial patterns.
Third, water erosion: Some scientists propose that the spider-like landforms may be related to the flow of liquid water that existed on Mars in the past, although this theory remains controversial.
Among them, dry ice sublimation is a widely accepted theory, but it has not been experimentally verified until recently.
Recently, a team of scientists validated the dry ice sublimation hypothesis for spider landforms through simulation experiments.
How did they conduct the experiment?
In a new paper published in the Journal of Planetary Sciences, scientists from NASA’s Jet Propulsion Laboratory detailed their experimental research. They simulated Martian temperatures and pressures in a ground laboratory for the first time, attempting to recreate the formation process of spider structures under these conditions.
Mars’ surface has extremely low atmospheric pressure and temperatures dropping below -100 degrees Celsius. To simulate Martian conditions, scientists used the “Dusty Ice Vacuum Simulation Test Chamber” (DUSTIE), a wine barrel-sized simulation chamber cooled by liquid nitrogen, referred to as the “test chamber” hereinafter.
They first cooled Martian soil simulants in a container submerged in liquid nitrogen, placed them in the test chamber, and then flowed carbon dioxide gas into the chamber, which condensed into dry ice within 3-5 hours. Once an ice layer with suitable characteristics was formed, they placed heaters in the test chamber below the simulants to heat and crack the simulated soil. After waiting, these black powdery substances eventually ejected carbon dioxide gas.
Why was the experiment designed this way? It considered both the environmental temperature changes and the atmospheric composition dominated by carbon dioxide on Mars. Temperature changes are mainly caused by seasonal variations, with sunlight heating the icy layers in summer, leading to the sublimation of dry ice.
So, how can we explain the formation process of Martian “spiders”?
Scientists discovered that when sunlight penetrates the transparent carbon dioxide ice layer accumulated on the Martian surface each winter, it directly heats the soil below. Why does this happen? Because the soil is darker in color than the ice layer above it, it absorbs heat, causing the ice layer closest to it to directly turn into carbon dioxide gas without first becoming liquid, a process known as sublimation (similar to the “smoke” clouds produced by dry ice). In this experiment, heating from below was used to simulate sunlight heating.
As dry ice continues to sublimate, the gas pressure below the ice also increases, eventually causing the overlying ice layer to crack and allow gas to escape. During the upward infiltration of gas, it carries a stream of black dust and sand from the soil, depositing it on the ice layer’s surface.
According to this theory, when winter transitions to spring and the remaining ice layer sublimates, these small eruptions leave spider-like traces. If winds or dust storms occur at this time, dark dust will settle on the downwind side, which is also common in some spider landforms.
Overall, this experiment preliminarily verifies that dry ice sublimation can form spider landforms.
However, many questions about the formation of Martian “spiders” remain unanswered in the laboratory.
For example, why do they form in some places on Mars but not others? If they are caused by seasonal changes, why don’t their number and size seem to increase over time? Perhaps, future research will enable Martian “spiders” to serve as a unique window into understanding this red planet.