Curiosity Rover Sol 1947

The Curiosity Rover took this image on January 27th, 2018, or Sol 1947 Mars mission time
(Note that you can either click an image to enlarge, or right-click each image to open in a new window to zoom in better):

If you miss it the first time, scroll down for the annotated image.

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 27, 2018, Sol 1947 of the Mars Science Laboratory Mission, at 19:25:00 UTC.

Here's an annotated image, and another image (rotated) with context to help you see better what to look for. Note that you can right-click each image to open in a new window to zoom in better. The image is rotated to compensate for the crater-dome illusion.

The other strange thing, as seen from the above image, is the markings decrease in size as you move to the lower left.

All images are available here from NASA.

We don't know what these markings are. They could be a tooling mark from the rover, or a natural geological formation.

Another set of images the same day show very similar gray protrusions on another rock:

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 27, 2018, Sol 1947 of the Mars Science Laboratory Mission, at 19:03:44 UTC.

Subsequent pictures were taken of this rock more closely that revealed unusual 3-pronged indentations, as seen below (you can localize the zoomed-in images by looking at the lower-left image and comparing it to the above image):

The subtle gray indentations appear in almost a line at somewhat evenly spaced intervals but with variations. Typically from the rover's brush tool, the intervals are VERY evenly spaced and they are fewer in number, and also in a rectangular pattern. Below is the unmodified image that is depicted at the right of the preceding image. Note that the unmodified image is rotated 180 degrees, and may appear to be a protrusion instead of an indentation due to the crater dome illusion.

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 27, 2018, Sol 1947 of the Mars Science Laboratory Mission, at 19:12:25 UTC. When this image was obtained, the focus motor count position was 14641