On the left: An electrical discharge to a negatively charged surface, recorded on a photographic plate. On the right: A Martian “spider.”
As explained in thunderbolts website:
The mysterious “spiders” on Mars show all of the expected features of an electric discharge in its classic form as a Lichtenberg figure.
A similar picture carries this caption:
Mars’ carbon dioxide atmosphere partially condenses every winter to form polar caps of dry ice. In the spring, the evaporation of the ice is a dynamic process and carves channels into the ground as it escapes back into the atmosphere.
Often these channels are radial in nature, and are colloquially refered to as “spiders,” although the prefered term for these radially-organized channels is “araneiform” which means spider-like.
In this subimage all the seasonal frost is gone, and we can use stereo images or shadow measurements to measure the depth of the channels carved into the ground, typically 1 – 2 meters deep.
Written by: Candy Hansen (30 September 2009)
The article On Martian Spiders, Ice Spiders, the Namibian Desert and Remaining Open-Minded also elaborates on the subject of landscape formations that look like Lichtenberg figures, both on Earth and Mars. It shows, for example, that similar (in shape) channels carved on ice have been observed on earth. Here’s some examples (note the the martian channels are carved on rock, not ice).