Pluto, once considered the ninth planet, is now classified as a dwarf planet located in the distant Kuiper Belt. In 2015, NASA’s New Horizons spacecraft conducted a historic flyby, revealing an unexpectedly complex and geologically active world. Among its most striking features is a large, bright, heart-shaped region prominently visible on its surface. This iconic feature, informally known as Pluto’s “heart,” immediately captured public attention and became a focal point for scientific inquiry.
Unveiling Pluto’s Icy Heart
Pluto’s prominent heart-shaped feature is officially named Tombaugh Regio, honoring Clyde Tombaugh, who discovered Pluto in 1930. The western and larger lobe of this heart is a vast, bright basin known as Sputnik Planitia. This expansive plain, measuring approximately 1,200 by 1,400 kilometers, is predominantly filled with frozen nitrogen, along with smaller amounts of carbon monoxide and methane ices. The bright appearance of Sputnik Planitia is attributed to this white nitrogen ice, which constantly resurfaces the area.
Sputnik Planitia is characterized by a surface of irregular polygons, typically around 33 kilometers across, separated by troughs. These polygonal patterns are evidence of thermal convection currents within the nitrogen ice. Much like boiling oatmeal, warmer, less dense nitrogen ice from below rises to the surface, spreads, cools, and then sinks, creating a continuous cycle that smooths out any impact craters and keeps the surface remarkably young, estimated to be less than 10 million years old. This process causes movements within the ice layer over timescales comparable to Earth’s plate tectonics. The distinct lobe shape of Sputnik Planitia is likely formed by the boundaries of this deep basin, which is about 3 to 4 kilometers lower in elevation than the surrounding terrain.
The Heart’s Deeper Meaning for Pluto
The existence and dynamic nature of Sputnik Planitia provide compelling evidence of ongoing geological activity on Pluto. Before the New Horizons mission, many scientists assumed Pluto was a geologically dormant body due to its small size and distant location. However, the constantly resurfaced, crater-free plains of Sputnik Planitia demonstrate Pluto is more active than previously thought. This activity suggests that internal heat sources are still at work within the dwarf planet.
One implication of Sputnik Planitia is the possibility of a subsurface liquid water ocean beneath Pluto’s icy crust. The substantial mass of the nitrogen ice in Sputnik Planitia, combined with its precise alignment opposite Pluto’s largest moon Charon, suggests this region acts as a “gravitational anomaly,” which is best explained by a dense, liquid layer beneath the surface, as liquid water is denser than ice. The ongoing freezing of this ocean could also explain the extensive network of fractures observed across Pluto’s surface. While some models propose alternative explanations for Sputnik Planitia’s formation, the evidence for a liquid layer remains a prominent area of study. This discovery indicates that even small, cold worlds can harbor internal activity and potentially subsurface oceans.