Photographs and descriptions of many samples of the element Krypton in the Periodic Table.
So, if they see a pattern that matches krypton's, they know that there must be some krypton in that object. Photographs of Krypton Gas and Krypton Plasma:.
Noble gas, mass: 83.8 u, 5 stable isotopes (80, 82, 83, 84, 86), abundance rank ( earth/space): 82/32. Click image to magnify. Vial of glowing ultrapure krypton.