How is ice on Mercury possible?
False-color image of Mercury. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Illustration of axial tilt being responsible for Earth's seasons.
The deep regions of Mercury's craters at high latitudes never receive direct sunlight, allowing water ice to persist there. If Mercury had an axial tilt, however, the picture would be quite different.
Infrared image showing dark material coating inner craters at high latitudes on Mercury, as seen with the MESSENGER space craft and published by Delitsky et al. (2017).