The term 'Rare Atmosphere' can be broken down into two crossword solutions: ETHER (5 letters) and Rare (4 letters). ETHER is a colorless, odorless, tasteless, non-flammable gas that is the second-lightest known gas and the second most abundant element in the universe. It is used as a refrigerant, anesthetic, and fuel. Rare is an adjective meaning not often occurring or found; unusual or uncommon. Together, the two words form the phrase 'Rare Atmosphere', which refers to an atmosphere that is not often found or encountered.