How Pluto got its heart

Modelling study explains Pluto’s heart-shaped region.

A modelling study in Nature tries to get behind the heart-shaped structure you see on Pluto’s surface in pictures. The heart lies in the dwarf planet’s “Tombaugh Regio” region and the 1000-km-wide western lobe of the heart, made of nitrogen and other ices, is named “Sputnik Planum.” Researchers ran computer simulations of Pluto’s changing climate and topology over 50,000 earth years to understand how these features came about.

We spoke to the lead author Tanguy Bertrand, Université Pierre et Marie Curie, France about the study.

Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. Credit NASA.
Four images from New Horizons’ Long Range Reconnaissance Imager (LORRI) were combined with color data from the Ralph instrument to create this enhanced color global view of Pluto. Credit NASA.

ResearchGate: What inspired your study of Pluto’s heart shaped glacier?

Tanguy Bertrand: Pluto's surface is an amazing cocktail of different types of ice that do not exist naturally on Earth: nitrogen, methane and carbon monoxide. We developed a thermal model of Pluto’s surface to understand the mechanisms of condensation/sublimation of its ice at a global scale. This model also enabled us to explore the "climate" scenarios that could explain Pluto’s ice distribution.

Animated surface map during one Pluto year, obtained with the model after 50,000 years of simulation. Although most of the reservoir of methane ice is sequestred inside the Sputnik Planum-like 3-km deep basin with N2 and CO ices (blue), each year, frost of methane (red) appears in autumn and disappears in spring. Credit: T. Bertrand
Animated surface map during one Pluto year, obtained with the model after 50,000 years of simulation. Although most of the reservoir of methane ice is sequestered inside the Sputnik Planum-like 3-km deep basin with N2 and CO ices (blue), each year, frost of methane (red) appears in autumn and disappears in spring. Credit: T. Bertrand

RG: What did you discover? How did you uncover this?

Bertrand: The face of Pluto and its heart-shaped structure can be explained by its climate. When factoring the topographic basin located at Pluto’s Sputnik Planum region into our model, we discovered that the heart shape is to a large degree created by highly volatile nitrogen ice that unavoidably accumulates in the basin and forms a permanent reservoir of ice, as observed by New Horizons.

This happens because of nitrogen’s solid-gas equilibrium. At the bottom of the basin the pressure of the atmosphere - and therefore of gaseous nitrogen - is higher, thus the frost temperature is higher than the outside. As a result, nitrogen prefers to condense into ice there. Carbon monoxide ice, which is similarly volatile to nitrogen, was also found to be entirely sequestered with nitrogen in the basin.

Methane ice, which is much less volatile at Pluto’s temperatures, is not restricted to the Sputnik Planum glacier like nitrogen and carbon monoxide. Our model, like New Horizons, shows that pure methane frosts seasonally covers both hemispheres.

With our model, we also predict that atmospheric pressure is currently at its seasonal peak and will decrease in the next decades, while seasonal methane frosts will disappear.

This scenario shows that there is no need for an internal reservoir of nitrogen ice to explain the formation of the Sputnik Planum glacier, as suggested by previous studies. Instead, well-known physical principles are behind this.

New Horizons observations and model. Credit: T. Bertrand
New Horizons observations and model. Credit: T. Bertrand

RG: Is the ice on Pluto different to that of Earth? If so, why?

Bertrand: Pluto does have water ice that forms most of its crust and upper mantle. But at Pluto’s surface temperature (-235°C) water ice is rock solid and actually forms mountains. However, the surface temperature on Pluto is not too cold for nitrogen, which can exist in a gas and ice state. Nitrogen ice is not solid enough on Pluto to form mountains so it can flow similar to water ice glaciers on Earth.

RG: How much longer will this heart-shaped ice remain? How will you make future observations of Pluto?

Bertrand: The Sputnik Planum glacier is massive, and is not strongly impacted by the seasonal variations of sublimation/condensation on its surface. It is a permanent reservoir. In the future, we could validate or even challenge our model prediction by observing the disappearance of methane frost in the northern hemisphere and the decrease of surface pressure.

mars_vs_pluto
Mars and Pluto. Credit: T. Bertrand

RG: What does your study mean for our understanding of other planets and Pluto more generally?

Bertrand: We can compare Pluto and Mars: even though the mechanism of atmospheric condensation in the low altitude regions of Pluto is unknown on Earth, it does exist on Mars where the CO2 atmosphere can condense on the surface like nitrogen does on Pluto. In addition, by explaining the origin of the glacier in Sputnik Planum we can also explain the remarkable geology surrounding it, with the mountains eroded and shaped by the relentless glacial activity. All this activity was totally unexpected on such a small icy object like Pluto.

Featured image courtesy of NASA.