The original image is available at the link
NASA's Mars rover Curiosity acquired this image using its Mars Hand Lens Imager (MAHLI), located on the turret at the end of the rover's robotic arm, on January 7, 2018, Sol 1927 of the Mars Science Laboratory Mission, at 06:40:59 UTC.
Image Credit: NASA/JPL-Caltech/MSSS
Some processing of the image is giving this "shell".
