New Horizons flew past Pluto in two thousand and fifteen and sent back a world with a pale heart-shaped region on one side, far brighter and smoother than the cratered ground around it. Pluto’s surface is geologically complex because its ices move on seasonal and longer timescales. Lewis and colleagues mapped which ice is where. What is the heart?
A reservoir of nitrogen ice. Mapping New Horizons spectral data globally, including methane diluted in nitrogen, the team found that a significant fraction of Pluto’s nitrogen is stored in , the heart’s western lobe. The ice sits at thirty-six point nine three kelvin, in vapor pressure equilibrium with the thin nitrogen atmosphere above it.
Pluto is not a static ball of rock. Its surface is geologically complex because of volatile ices that are mobile on seasonal and longer timescales, which means the bright and dark patterns are a weather map as much as a geology map. The study set out to learn about the seasonal processes redistributing those ices, to calculate the global energy balance, and to put a lower limit on how much nitrogen Pluto holds.
The method is spectroscopy with a twist. The team analyzed data from the New Horizons infrared spectrometer to map nitrogen ice globally. They included the shifted bands of methane in with nitrogen, so that nitrogen hiding inside other ice would still be counted. That produced the average latitudinal distribution of nitrogen and let them compare it against Pluto’s topography.
Two numbers anchor everything else. The global average came out at zero point eight three, similar to the value inferred for Triton. The nitrogen-dominated atmosphere stands at about eleven and a half microbars. Assuming it is in vapor pressure equilibrium with the ice, the ice temperature is thirty-six point nine three kelvin, as measured by New Horizons.
The rest follows from the energy budget. Combined with the measured albedo, it implies an average bolometric for the nitrogen ice of about zero point four seven to zero point seven two. That is in line with the low values estimated for Triton from Voyager data. At the time of the encounter the global pattern of volatile transport ran from north to south, and within Sputnik Planitia the boundary between condensation and sublimation tracks changes in grain size and methane concentration. The low emissivity suggests the atmosphere may stay at roughly constant pressure for an extended period even as Pluto moves away from the Sun.
wat. The heart is where Pluto’s air goes when it gets cold enough to lie down.
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The heart is not a marking. It is the planet’s cold trap, where the atmosphere condenses out, and the transport that fills it was running north to south at the time of the flyby.
How do we know?
An analysis of spectral data from the New Horizons infrared imaging spectrometer, mapping nitrogen ice globally and using the shifted methane bands to catch nitrogen in solid solution, published in Icarus.
What is still unknown
The temperature rests on the assumption that the atmosphere is in vapor pressure equilibrium with the ice. The emissivity comes out as a range rather than a value, between about zero point four seven and zero point seven two.
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