Some comets were discovered by looking for their X-ray spectrum. About 2000 comets have been discovered in total. The X-ray spectrum of a comet arises from bremsstrahlung occurring in the thin corona boundary (bow shock) of the atmosphere (usually, called the coma) of the comet. The corona is oriented toward the sun. The spectrum corresponds to a temperature of 2.5 million degrees. The corresponding effect in the corona of the sun represents a temperature of 2 million degrees. This spectrum of the comet and sun is dominated by the ions, i.e., protons, alphas, oxygen ions, etc., emitted from the sun rather than the composition the corona.
Comets have two tails. The bluish one, mostly outside human visual range, is generated by the pressure of the solar wind on Comet's magnetosphere. The yellowish white one is generated from dust, carbon dioxide and other volatiles emitted from jets.
Comets may have exploded in the vicinity of the earth. Fullerenes containing noble gases including helium-3 and argon have been discovered in the debris layers arising from the major terrestrial explosions thought to have due to comets. The nonterrestrial isotopic composition of the debris is the reason that the debris is attributed to comets. Ten years ago, water was found trapped within salt crystals in a meteorite. There were bubbles trapped in the water. We are still waiting for someone for someone to analyze the gas in the bubbles. Amino acids have also been detected in meteorites.