Young Pluto Stuns Scientists with Astonishing Maps

New color maps score high on the Wow Factor. How can this lonely dwarf planet still be active? Its moon Charon looks young, too.

After two years of detailed work since the 2015 flyby of Pluto, the New Horizons team has released the most detailed maps ever of Pluto and Charon, and they are stunners. Mountains as high as Alaska’s Mt McKinley, and canyons deeper than the Grand Canyon have been revealed. Cliffs of razor-sharp ice dominate one region, and glaciers flow miles below the average elevation in another. A ridge-and-trough system wraps around the globe from pole to pole. And that’s just Pluto. Its moon Charon is equally fascinating, with its red north pole and cracks miles deep. Both show evidence of volcanic activity in the recent past. This is not what planetary scientists expected to see in the cold, remote regions of the outer solar system.

Portion of Hyecho Palus SW of Sputnik Planitia on Pluto, showing Wright Mons (center). Credit: Schenk et al, Icarus, 2018

Pluto’s Superlatives

First images of the two-day encounter in September 2015 generated an “overdose of awesomeness” (17 September 2015), but it would take almost another year for the stored images from the spacecraft to finish trickling in (29 Feb 2016). The first scientific papers were published in March (19 March 2016). By June, the science team was confirming reports of active geology (4 June 2016).

Now, after two years of painstaking work stitching together images, the true 3-D significance of Pluto’s features has come to light. The final global mosaic of the sunlit side of Pluto has been published in Icarus by Paul Schenk, Alan Stern, William McKinnon and other leading planetary scientists in a paper titled, “Basins, fractures and volcanoes: Global cartography and topography of Pluto from New Horizons.” It reads like an official diary of a voyage of discovery, and the photographs are really amazing. Here are some highlights:

• Pluto is very two-sided, with the south dominated by heart-shaped Sputnik Planitia plain.
• The tallest mountains rise 3.7 miles above the average elevation, comparable to Denali, “one of the highest mountains on earth base-to-peak.”
• The “bladed terrain” mountains east Sputnik Planitia are nearly as tall, from 2 to 3.5 miles high—the highest-standing terrain on Pluto. They may be younger than Sputnik Planitia.
• A global-scale ridge-trough system runs from north to south pole in a great circle, with several miles of relief. It is 150 to 250 miles in width.
• Sputnik Plantia, believed to be an impact scar filled with ice, is 2.5 miles deep relative to its rim. It has very few craters.
• Some pits outside Sputnik Planitia are over 2 miles deep; some are filled, “but not all!” They may show evidence of explosive volcanism.
• Glaciers from the highlands reveal that ice has flowed into Sputnik Planitia, breaking up blocks and moving them.
• Wright Mons has a total relief of 6.8 miles from peak to floor, and is probably volcanic. The surrounding plains are “essentially crater free,” indicating recent resurfacing.
• Domes in the Wright Mons area are reminiscent of the volcanic dome forming in the crater at Mt St Helens.

The rich variety of terrains on Pluto suggests different regions have different ages and different geological histories. Is the global ridge-trough system a ring collapse feature, as suggested for Iapetus at Saturn? Did a large impact fracture the planet, and if so, when? “The morphology and topography of Pluto is complex and can be divided into provinces of distinctive characteristics, the origins of which are only partly understood,” the authors say, adding that it will require years to unravel the history of this body. See Astrobiology Magazine for a few more images.

Charon’s Superlatives

Pluto has several small satellites, but Charon is the largest of any satellite in size relative to its parent body. It was discovered only 40 years ago, so it was a real treat for its discoverer, James Christy, to see it up close (Space.com). A separate paper in Icarus, titled “Breaking Up Is Hard to Do: Global Cartography and Topography of Pluto’s Mid-sized Icy Moon Charon from New Horizons,” lists some of its outstanding characteristics:

• 

  Charon from New Horizons, July 14, 2015, showing reddish Mordor Macula at top.

  Its largest crater, named Dorothy, is 155 miles across and 3.7 miles deep.

• “Charon has a topographic range over the observed hemisphere from lowest to highest of ~19 km [11.8 mi], the largest topographic amplitude of any mid-sized icy body (including Ceres) other than Iapetus.”
• “Unlike Saturn’s icy moons whose topographic signature is dominated by global relaxation of topography and subsequent impact cratering, large-scale tectonics and regional resurfacing dominate Charon’s topography.”
• Oz Terra on the northern hemisphere “is broken into large polygonal blocks by a network of wide troughs with typically 3-6 km relief; the deepest of these occur near the illuminated pole and are up to 13 km [8 miles] deep with respect to the global mean radius, the deepest known surfaces on Charon.”
• Vulcan Planitia to the south is made up of “rolling plains, locally fractured and pitted, that are depressed ~1 km below the mean elevation” with indications of flow from the north. It “could be due to volcanism of breakup and foundering of Charon’s crust.”
• The north pole is dominated by a large basin named Mordor Macula, 233 miles across, filled with reddish material.

Secular scientists are wedded to the latest version of the old Nebular Hypothesis—the idea that the planets condensed from a cloud of dust and gas—and they don’t want to divorce her, despite irreconcilable differences. We’ve discussed numerous conflicts over the years with physics. Planets don’t want to form from dust; they want to disperse. Even if you get planets, they want to migrate into the sun.

Let’s assume for the moment that you get our system of planets to form 4.6 billion years ago. To a first approximation, things should decay from there, based on the laws of thermodynamics. Bodies should cool down as their initial gravitational heat begins to dissipate into space. The smaller the body, the faster it should cool. Planets and moons should break up as they collide. What can keep them active?

There are several sources of energy to keep a body going:

• Atmospheres and oceans can absorb energy from the sun or from planets to drive currents, weather, and other kinds of flow.
• Radioactive isotopes in the core can add heat. Many of these are too short-lived, though, to last for billions of years.
• Tidal pumping can squeeze a body to generate heat, as is suspected for Jupiter’s volcanic moon Io. Whether it is enough heat for the current level of activity is another question.
• Gravitational perturbations with other bodies can add heat.
• Impacts from comets, asteroids and meteors can supply temporary heat, and cause fractures and flows.

Pluto has a highly eccentric orbit, which varies roughly from 30 to 49 AU (astronomical units, 93 million miles) over its orbit of 248 earth-years. Its orbit is also the most highly inclined of any of the classical 9 planets (though now it is dubbed a “dwarf planet”). If Pluto and Charon are really 4.6 billion years old, what made them active recently? None of these energy sources seems sufficient to build volcanoes as tall as Mt. McKinley, or to have glaciers flow across deep basins. The fact that so few craters exist on many parts of Pluto require that they be considered “young” according to the view that impactors come in at a roughly steady rate.

Yes, the moyboys will need “many years” to unravel the history of Pluto and Charon. Don’t expect any answers soon. Since the dawn of the space age, they still haven’t figured out Io, Ganymede, Callisto, Europa, Titan, Umbriel, Ariel, Triton, Iapetus, Enceladus, Tethys, Dione, Rhea, Hyperion, Neptune, Uranus, Saturn, Jupiter, Mars, Mercury, Venus, Earth…