The second line of evidence concerns a feature that Pluto does not possess. Some bodies, like Earth’s Moon and Saturn’s satellite Iapetus, appear noticeably fatter around the equator than expected. These equatorial bulges formed earlier in their history when the moons spun much faster; later on, these ancient bulges somehow froze in place. In effect, the Moon and Iapetus have retained a memory of an earlier, faster spin state.
Pluto seemed a likely candidate for such a fossil bulge because it must have spun down considerably over time due to the gravitational influence of its large moon, Charon. Yet New Horizons failed to detect any such bulge. Although scientists have come up with several possible explanations, one sure way to remove a bulge is by developing a subsurface ocean — the ice shell above is simply too weak to sustain the bulge, and it collapses.
The heart of the matter?
The last line of evidence is the most complicated, but also the most intriguing. It starts with the enormous, bright basin known as Sputnik Planitia. This region appears bright because nitrogen ice fills it, supplied by nitrogen glaciers that flow down from the surrounding highlands.
Another key fact about Sputnik Planitia is its location. It lies almost directly opposite the point on Pluto that continuously faces Charon. (Pluto always presents the same face to Charon, and vice versa.) If you could somehow place an extra mass, like a large mountain, on Pluto’s surface, it would cause the planet to roll over until the mountain reached Sputnik Planitia’s location. Scientists call this process true polar wander, or TPW.
One consequence of TPW is that Pluto’s surface gets distorted in response to the movement of the excess mass. This, combined with the surface expansion, produces fractures — and the observed fracture orientations match those predicted by computer models rather well.
So, Sputnik Planitia’s location makes perfect sense if it represents an area of excess mass. But how could the basin achieve this extra mass? After all, it is a hole in the ground. It helps that solid nitrogen is slightly denser than water ice, so filling the basin with nitrogen ice assists a bit. Except in the case of implausibly thick nitrogen layers, however, that contribution alone is not enough. One explanation points to a thinning of the ice shell beneath. A thinner shell means denser water has replaced lighter ice, causing an excess of mass. This combination of nitrogen loading from the top and a thinned ice shell beneath can easily produce a mass excess and cause TPW.