Pluto’s “icy heart” is a bright, two-lobed feature on its surface that has attracted researchers ever since its discovery by the NASA New Horizons team in 2015. Of particular interest is the heart’s western lobe, informally named Sputnik Planitia, a deep basin containing three kinds of ices–frozen nitrogen, methane and carbon monoxide–and appearing opposite Charon, Pluto’s tidally locked moon. Sputnik Planitia’s unique attributes have spurred a number of scenarios for its formation, all of which identify the feature as an impact basin, a depression created by a smaller body striking Pluto at extremely high speed.