Mercury's moon. Artwork of the hypothetical moon of Mercury, with the planet in the sky. The moon's existence was suggested in 1974 when the Mariner 10 spacecraft detected bursts of Extreme Ultravio- let (EUV) light from an unidentified source as it neared Mercury. It was believed that the inter- stellar medium absorbed EUV, meaning that the emi- ssion source was nearby. The source was tracked, & it was found to be moving at 4 kilometres per second, a speed similar to that of a moon. The source was found to be star 31 Crateris. Although no moon was found, it had been discovered that EUV could penetrate the interstellar medium, giving astronomers a new way of observing the universe.