In the Mars-Shine... (credits: NASA)

This is a close-up and enhanced part of image PSP_007769_9015. In the original image, detail in the black, un-illuminated portion is not apparent. However, seen in enhanced detail here, craters are clearly visible.
This faint illumination is from reflected light off of Mars (“Marsshine”). This is directly analogous to “Earthshine,” where reflected sunlight from our planet illuminates the dark side of the Moon.
The ability to see features on Phobos illuminated by Marsshine demonstrates the high sensitivity of the HiRISE camera.