Since the 1990s, scientists have been intrigued by unusual radar observations of the closest planet to the sun, Mercury. Ground-based telescopes found that patches near the planet’s poles that looked unusually “bright,” suggesting that either the planet harbored ice in its craters or the surface was very rough.

On Thursday, a large team of scientists announced that a NASA mission, MESSENGER, launched in 2004, has found strong evidence of frozen water and the hints of organic materials, carbon-based molecules that are the building blocks of life on Earth -- though the researchers emphasized the discovery does not mean there is, or ever was, life on inhospitable Mercury.