The term 'sphere opening' can be broken down into four crossword solutions: ATMO (4 letters), HEMI (4 letters), IONO (4 letters), and ECO (3 letters). ATMO stands for atmosphere, which is the layer of gases surrounding a planet or other celestial body. HEMI stands for hemisphere, which is half of a sphere or globe. IONO stands for ionosphere, which is a region of Earth's upper atmosphere that is ionized by solar radiation. Finally, ECO stands for ecology, which is the study of the relationships between living organisms and their environment.