These can be interpreted as [desiccation cracks][1], filled with calcium- and/or magnesium sulfate (Ca-/Mg-SO<sub>4</sub>, seen as white material).

Methods used, for brevity citing from the below pop science link:

> "... The team took a close physical and chemical look at those polygons using Curiosity's Mastcam, Mars Hand Lens Imager, ChemCam Laser Induced Breakdown Spectrometer (LIBS), and Alpha-Particle X-Ray Spectrometer (APXS) ..."

I believe this setting has led to the hypothesis of a [once dried out lake][2] on Mars.


----------

https://sciencesources.eurekalert.org/pub_releases/2018-04/gsoa-cam041918.php

https://www.jpl.nasa.gov/news/mars-rover-curiosity-examines-possible-mud-cracks


  [1]: https://pubs.geoscienceworld.org/gsa/geology/article/46/6/515/530329/Desiccation-cracks-provide-evidence-of-lake-drying
  [2]: https://www.nature.com/articles/s41561-019-0458-8