How Pluto Got its Heart

A new study has finally resolved the question of how the dwarf planet Pluto got its heart!

In 2015, NASA’s New Horizon spacecraft sent back images of Pluto and the most impressive amongst these were the images of a giant heart-shaped structure (known as Tombaugh Regio) on the dwarf planet’s surface. Tombaugh Regio is covered in a high-albedo material that reflects more light than its surroundings, thus giving the structure its whiter color. Pluto’s surface is dominated by this huge, pear-shaped basin. This western teardrop-shaped part of Plutos heart feature is known as Sputnik Planitia.

Since then scientists have been mulling over the mystery, working to find out how Pluto got this feature on its surface. This mystery has finally been solved by an international team of astrophysicists led by the University of Bern and members of the National Center of Competence in Research (NCCR) PlanetS. This team became the very first to successfully reproduce the heart-shape with numerical simulations, and have attributed it to a giant and slow oblique-angle impact. Their findings have been shared in a new study.

According to the study: ” Using three-dimensional hydrodynamic simulations to model realistic collisions, we provide a hypothesis that does not rely upon a cold, stiff crust atop a contrarily liquid ocean where a differentiated ~730 km ice–rock impactor collides at low-velocity into a subsolidus Pluto-like target. The result is a new geologic region dominated by impactor material, namely a basin that (in a 30° collision) closely reproduces the morphology of Sputnik Planitia, and a captured rocky impactor core that has penetrated the ice to accrete as a substantial, strength-supported mascon. This provides an alternative explanation for Sputnik Planitia’s equatorial alignment and illustrates a regime in which strength effects, in low-velocity collisions between trans-Neptunian objects, lead to impactor-dominated regions on the surface and at depth.”

Basically, thier research suggests that a cataclysmic event formed Sputnik Planitia early in Pluto’s history, during a huge collision with another planetary body (of around 700km in diameter). Additionally, Pluto’s inner geology is different from what had been previously understood: the new results indicate a lack of a sub-surface ocean.

The heart is not composed of a single element: Sputnik Planitia is the western part, covering an area of 1,200 to 2,000 km. This area is three to four kilometres lower than the rest of Pluto’s surface.

“The bright appearance of Sputnik Planitia is due to it being predominantly filled with white nitrogen ice that moves and convects to constantly smooth out the surface. This nitrogen most likely accumulated quickly after the impact due to the lower altitude,” explains Dr. Harry Ballantyne from the University of Bern, lead author of the study.

Meanwhile, while the eastern part is also coved by nitrogen ice, that layer is a lot thinner and its origin is unclear. However, the researchers think it is related to Sputnik Planitia.

“The elongated shape of Sputnik Planitia strongly suggests that the impact was not a direct head-on collision but rather an oblique one,” points out Dr. Martin Jutzi of the University of Bern, who initiated the study.

These impacts were digitally recreated by the team using their Smoothed Particle Hydrodynamics (SPH) simulation software. The team varied the composition of pluto as well as its impactor and eventually revealed the oblique angle of impact and determined the composition of the impactor.

“Pluto’s core is so cold that the rocks remained very hard and did not melt despite the heat of the impact, and thanks to the angle of impact and the low velocity, the core of the impactor did not sink into Pluto’s core, but remained intact as a splat on it,” explains Ballantyne.

“Somewhere beneath Sputnik is the remnant core of another massive body, that Pluto never quite digested,” adds co-author Erik Asphaug from the University of Arizona. “This core strength and relatively low velocity were key to the success of these simulations: lower strength would result in a very symmetrical leftover surface feature that does not look like the teardrop shape observed by New Horizons.”

“We are used to thinking of planetary collisions as incredibly intense events where you can ignore the details except for things like energy, momentum and density. But in the distant solar system, velocities are so much slower, and solid ice is strong, so you have to be much more precise in your calculations. That’s where the fun starts,” says Asphaug.

The study further highlights a very important aspect of Pluto’s inner structure: that Pluto has a subsurface ocean. According to this previous theory, Pluto’s icy crust would be thinner in the Sputnik Planitia region, which would result in a bulging of the ocean in this region. As ice is less dense than liquid water, there would be a mass surplus inducing migration towards the equator. This is what can be seen on Pluto’s surface. Even though Sputnik Planitia should have migrated towards the pole over eons, it is near the equator.

However, accordint to the new study: “In our simulations, all of Pluto’s primordial mantle is excavated by the impact, and as the impactor’s core material splats onto Pluto’s core, it creates a local mass excess that can explain the migration toward the equator without a subsurface ocean, or at most a very thin one,” explains Martin Jutzi.

Dr. Adeene Denton from the University of Arizona, also co-author of the study, is currently conducting a new research project to estimate the speed of this migration. “This novel and inventive origin for Pluto’s heart-shaped feature may lead to a better understanding of Pluto’s origin,” she concludes.

Saima Baig

I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.