In the solar system, calamitous events generally do not spell the end of worlds. A planet or moon can take a hit from an asteroid or comet. Wrenched off its previous trajectory, it might falter for a time, or tilt on its axis, or experience a dramatic reorganization of its exterior. But things will eventually stabilize.
Titanic changes like these are happening at Pluto today, largely because of the iconic heart on its surface. The dwarf planet’s orientation in space is controlled by heavy ice in this heart, and by a massive global sea that astronomers now believe lies beneath it.
When the New Horizons probe zoomed past Pluto last year, the small world—the original ninth planet, demoted a decade ago to the biggest dwarf planet—revealed itself as a ball of rock wrapped in a shell sandy-colored ice, surrounded in a puffy nitrogen atmosphere. Astronomers think there is a water ocean between the rock and the icy crust, which is wrinkled with mountains that are dusted with methane snow. Much of the dwarf planet’s terrain looks like snakeskin, rippling with gray and reddish-brown creases and pits. But its distinguishing feature is an enormous tan heart, nicknamed Tombaugh Regio. The heart’s left lobe is a 1,000-km-wide basin called Sputnik Planitia. Many astronomers think this teardrop-shaped spot is a scar, left by a giant space rock that collided with Pluto eons ago.
Pluto and its moon, Charon, always show the same face to one another, the way the moon is locked in the same direction toward Earth. The bright Tombaugh Regio area always faces away from Charon. The alignment is so precise that it’s as if Charon floats over the area directly opposite Sputnik Planitia. This suggests there’s extra mass in Sputnik Planitia, and it forced Pluto to roll over to balance itself between its own mass and that of its sister moon. Astronomers spell out how this reorganization happened in a pair of papers published today in Nature.