The phrase "nitrogen ice" refers to a solid form of nitrogen, which is a type of gas found in the air we breathe. It means that the nitrogen has become very cold and has turned into a frozen, icy substance. Full definition
The results of both calculations suggest a far deeper layer of nitrogen ice than the minimum 500 meters needed for convection to occur, he said. (sciencedaily.com)
Part of the answer is that, after it formed, the basin has been partially filled in by nitrogen ice. (sciencedaily.com)
However in this case, they are thought to have stranded in shallow nitrogen ice areas. (astrobiology.nz)