Opposition Surge (mini post)
In SpaceEngine, you may notice a bright reflection on some objects in the opposite direction of the sun (or other light source). This is neither a bug nor an unrealistic feature, but a real phenomenon called the opposition effect or opposition surge. The header image above shows opposition surge on Saturn's rings and a procedural planet in SpaceEngine (contrast increased for clarity).
Opposition surge refers to the significant increase in brightness observed on many objects as their phase angle approaches zero (i.e. at opposition), when the observer is located directly between the object and the sun. Opposition surge can be observed on essentially all solid bodies in the Solar system, but is most prominent on rocky airless bodies and Saturn's rings. The effect is caused by a combination of shadow-hiding (particles on an object's surface blocking the view of their own shadows at zero phase), retroreflection (objects preferentially reflecting light back to the source due to their material properties and geometry) and coherent backscattering (constructive interference of reflected light waves back towards the light source). Shadow-hiding seems to be the main contributor in most cases.
Scroll down for real-life images of opposition surge.
Moon
Opposition surge on the lunar surface, centered on the shadow of Buzz Aldrin
Photo credit: NASA
In this video of an Earthrise from Japan's Kaguya lunar orbiter, opposition surge is visible during the first 18 seconds as a bright moving spot on the left side of the frame
Video credit: JAXA/NHK
Mars
Opposition surge is visible near the center of this low-orbit photo mosaic from the Mars Global Surveyor spacecraft
Photo credit: NASA/JPL/MSSS
Opposition surge is visible just above the center of this colorized image taken by the Perseverance rover during its descent to the martian surface
Photo credit: NASA/JPL-Caltech/Simeon Schmauß/AndreaLuck
Saturn
Saturn's rings display an especially strong opposition surge in this photo from the Cassini spacecraft
Photo credit: NASA/JPL/Space Science Institute
Asteroids and Comets
Opposition surge is clearly visible on this photo of comet 67P from the Rosetta spacecraft
Photo credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
In the above video from the Hayabusa 2 spacecraft, opposition surge can be seen surrounding the spacecraft's shadow on the surface of asteroid Ryugu
Video credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST
More Examples
Some great examples of opposition surge on Earth: [1] [2] [3] [4]
A few of the many great images processed by Thomas Appéré showing opposition surge on Mars: [1] [2] [3]
Opposition surge is visible in the descent videos captured by NASA's Curiosity and Perseverance Mars rovers