Unravelling the Heart of Pluto to Reveal the Secrets Beneath

Foreign material delivered as a giant impact can dominate large portions of icy dwarf planets, according to impact simulations. This may explain the peculiar shape and location of the Sputnik Planitia region on Pluto, without the need for a present-day subsurface ocean.
Published in Astronomy and Earth & Environment
Unravelling the Heart of Pluto to Reveal the Secrets Beneath
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In the frigid expanse of the Kuiper Belt, where sunlight whispers rather than shouts, Pluto remains an enigmatic figure among celestial bodies. It was its heart-shaped region, Tombaugh Regio, that captured the imagination of the world when the New Horizons spacecraft beamed back its first images. The most prominent feature of this heart is Sputnik Planita, a bright, high-albedo basin, nestled within darker terrains.  Its discovery immediately posed a compelling mystery — how did this peculiar, pear-shaped basin form, and why is it located so close to Pluto’s equator?

Imagine a scenario billions of years ago, where the quiet in the outer Solar System is shattered by the cataclysmic dance of celestial bodies. In our latest study, we propose that Sputnik Planitia is not merely a surface anomaly but a scar from a monumental collision, where the foreign material of the impacting body still remains trapped beneath a veil of nitrogen ice.

Artistic representation of the collision that formed Sputnik Planitia. Credit: University of Bern, Illustration: Thibaut Roger

Using sophisticated Smoothed Particle Hydrodynamics (SPH) simulations, we recreated the conditions that might have led to the creation of this pear-shaped basin. These simulations are a window into the past, showing us not just the potential realities of Pluto’s violent encounters but also painting a picture of cosmic resilience. Pluto, with its icy crust and rocky core, absorbed the impact without melting down into chaos—a testament to the unique physical conditions prevailing in the outer Solar System.

Snapshots from SPH simulation showing the impact dynamics

Perhaps counter to one’s intuition, this resilience is a consequence of Pluto’s smaller dwarven stature, as the force of gravity does not entirely dominate over the material strength of the gelid ice and rock, and relatively slow, subsonic impacts are permitted. As the rocky core of the impacting body ploughs through Pluto’s icy shell, it resists deformation, giving rise to the distinctive pear shape of Sputnik Planitia before finally settling atop Pluto’s rocky core as a deeply buried splat.

Distribution of impactor material from the simulation (top) vs the observation elevation distribution of Sputnik Planitia (bottom)

This hypothesis sidesteps the need for a subsurface ocean beneath Sputnik Planitia, as proposed by previous studies. Instead, the simulations suggest that the impactor’s core, embedded as a splat within Pluto, is responsible for the positive gravity anomaly associated with this region, ultimately driving Sputnik Planitia to its present near-equatorial location through a process called true polar wander.

Proposed interior structure of Sputnik Planitia

The implications of this research stretch far beyond the icy plains of Pluto. They challenge our understanding of planet formation and surface evolution in the colder, outer regions of our Solar System. Unlike the larger, warmer inner planets, where geological features deep within their interior are a great challenge to maintain, the cold conditions of smaller Kuiper Belt objects far from the Sun may preserve these monumental impacts, harbouring huge reservoirs of foreign material that the parent body never quite digests.

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Planetary Science
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Planetary Science
Asteroids, Comets and Kuiper belt
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Planetary Science > Asteroids, Comets and Kuiper belt
Geophysics
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geophysics
Planet Formation
Physical Sciences > Physics and Astronomy > Astronomy, Cosmology and Space Sciences > Planetary Science > Planet Formation

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