How did the "spider" on Mars come into being? A new explanation for the strange landform

Written by Xiao Long

Spider-shaped landforms are unique to Mars. Scientists have proposed many hypotheses about how these landforms were formed and why they only appear in certain places on Mars. Recently, scientists may have found the answer to this mystery.

The spider-like landforms on the surface of Mars are unique terrains observed by Mars rovers. In the early 2000s, the European Space Agency's Mars Express probe took the first high-resolution photos of this terrain.

These spider-like terrains can be clearly seen in the photos taken. These spider-like structures are very unique in shape. They usually start from a center and extend outward in multiple slender radial branches, resembling spiders. Each branch structure can extend from hundreds of meters to more than a thousand meters from beginning to end, and will "extend" hundreds of slender "legs". In terms of color, these landforms usually contrast with the surrounding red or brown Martian surface because they may have bright frost materials accumulated on their surfaces after evaporation, so they sometimes appear lighter in color.

Moreover, these "spider" landforms usually appear in "groups".

Interestingly, this type of landform is not found on Earth and is unique to Mars. Therefore, the formation of this special landform must be related to the unique environment of Mars. In response to this mystery, scientists have proposed a variety of hypotheses, including:

One is dry ice sublimation: It is believed that they are caused by dry ice (that is, solid carbon dioxide) covering the surface of Mars in winter, and then as the temperature rises, the dry ice sublimates in the spring (directly turns from solid to gas). The gas pressure accumulated in this process is released through the cracks, thus forming this radial groove.

The second is melting of ice inclusions: Another hypothesis is that these landforms may be caused by the melting of ice inclusions beneath the surface of Mars and flowing out through surface cracks or channels, causing these radial patterns.

The third is water erosion: Some scientists have suggested that the spider-shaped landforms may be related to the flow of liquid water on Mars in the past, although this theory remains controversial.

Among them, dry ice sublimation is a generally accepted theory, but it has never been verified experimentally.

Recently, a team of scientists verified the dry ice sublimation hypothesis of spider landforms through simulation experiments.

How did they do it specifically?

In a new paper published in the Planetary Science Journal, scientists at NASA's Jet Propulsion Laboratory detailed their experimental research, which for the first time simulated the temperature and pressure of Mars in a ground-based laboratory, trying to reproduce the formation process of spider structures under such conditions.

We know that the surface pressure of Mars is extremely low, and the temperature is as low as minus 100 degrees Celsius. In order to simulate the conditions on Mars, scientists used the "Dirty Simulation Test Bench in Vacuum Ice Environment" (DUSTIE). This is a wine barrel-sized, liquid nitrogen-cooled test chamber that simulates the Martian environment. We will refer to it as the "test chamber" below.

They first put the Martian soil simulant into a container immersed in liquid nitrogen to cool it, then put it into the test chamber, and then flowed carbon dioxide gas into the test chamber. The carbon dioxide will condense from the gaseous state into dry ice within 3-5 hours. When the ice layer with suitable properties is generated, they put a heater in the test chamber under the simulant to heat and crack the simulated soil. After waiting, these black powdery substances finally spewed out carbon dioxide gas.

Why was the experiment designed this way? This is to take into account the temperature changes on the surface of Mars and the fact that the atmosphere is mainly composed of carbon dioxide. Temperature changes are mainly caused by seasonal changes. In summer, sunlight heats the ice layer, causing the dry ice to sublime.

So, how do we explain the formation process of the Martian "spider"?

Scientists have discovered that when sunlight penetrates the transparent carbon dioxide ice that accumulates on the surface of Mars each winter, the light passes directly through the transparent dry ice layer and heats the soil below. Why is this so? This is because the soil is darker in color than the ice above it, so it absorbs heat, causing the ice closest to it to turn directly into carbon dioxide gas without first becoming liquid, a process called sublimation (a process that changes directly from a solid to a gas, the same as dry ice produces "smoke" clouds). In this experiment, the soil was heated from below instead of being heated by sunlight.

As the dry ice continues to sublime, the gas pressure beneath it builds up, eventually cracking the ice above and allowing the gas to escape. As the gas seeps upward, it picks up a plume of black dust and sand from the soil that settles on the surface of the ice.

According to the theory, when the remaining ice sublimated as winter turned to spring, these small eruptions left behind spider-like trails. If it was windy or there was a dust storm, dark dust would fall to the leeward side, which is also common in some spider landforms.

Overall, this experiment preliminarily verified that dry ice sublimation can form spider landforms.

However, there are still many questions about the formation of the Martian "spider" that cannot be answered in the laboratory.

For example, why do they form in some places on Mars but not in others? If they are caused by seasonal changes, why do their number and size not seem to grow over time? Perhaps, with more research in the future, people can use Martian "spider" as a unique window to understand the red planet.