Making sense of Pluto

Posted by David Eicher
on Monday, July 20, 2015

In the left-central region of Tombaugh Regio, the area on Pluto known as “the heart,” frozen plains appear to be incredibly young, no more than roughly 100 million years old. The lack of cratering suggests a dynamic process or processes working on this frozen world to resurface it and will challenge scientists to work through models to understand the mechanisms. This LORRI image was shot July 14, 2015, from a distance of 48,000 miles (77,000km) and shows features as small as half a mile (1km) across. // Credit: NASA/JHUAPL/SWRI
A week later, the incredible historic moment has passed. The flyby occurred. We have seen some spectacular images of Pluto and Charon. Let's take a breath to make sense of the Pluto we now know.

First, congratulations to Alan Stern and the entire New Horizons team. What a spectacular achievement, and the information will keep pouring in. Although this was a flyby, the mission design dictates that data will continue to be returned, processed, and released for another 16 months. So get ready to continue enjoying Pluto for a long time to come.

Now that we’re past the big moment, what should we think of Pluto?

1. We have been through planetary science history. This completes the “grand tour” of what was thought to be the principal solar system bodies when the Voyager spacecraft initiated the tour in the late 1970s.

2. Pluto is remarkably geologically active. At its current distance of 33 astronomical units — 33 times as far from the Sun as Earth — Pluto is extraordinarily cold. Its surface is about –387° F (–233° C). With such temperatures, planetary scientists expected to see a geologically inactive world. That is not the case: A young surface, betrayed by a relative lack of craters, means that Pluto is resurfacing within tens of millions of years. Scientists don’t yet know the mechanism that is producing enough warmth for planetary resurfacing.

3. We now know Pluto’s size accurately. New Horizons team members found Pluto to be 1,473 miles (2,370 kilometers) across, larger than some previous estimates.

4. The surface features of Pluto are more varied than scientists would have predicted. Bright areas such as the "heart," informally named Tombaugh Regio, are young and contain highly reflective ices and snows, whereas some adjacent older, darker regions reveal impact craters.

5. Pluto contains large water ice mountains. The equatorial region of Pluto ranging through portions of the “heart” show large mountain chains stretching some 11,000 feet (3,500m) tall, as large as spectacular mountain ranges on Earth. Under plutonian conditions, these could only be made principally of water ice.

6. Pluto’s largest moon, Charon, has a youthful and varied terrain. A lack of craters on Charon, along with a surprisingly complex surface containing troughs and cliffs, suggests a youthful surface. Charon’s crust has probably been repeatedly fractured, again suggesting an internal heat source. Whether this could be from radioactive decay, subsurface liquid water oceans, cryovolcanoes, or other sources remains to be seen. Dark markings on both Pluto and Charon suggest decay of organics, staining, and thin deposits of darker materials.

7. New Horizons examined the four small moons of Pluto. Nix, Hydra, Styx, and Kerberos all will reveal surprises of their own, mission planners believe. Measurements of Hydra reveal a size of 27 by 20 miles (43 by 33 km) and a veneer of water ice.

8. Pluto’s thin methane-rich atmosphere is extensive, stretching more than 1,000 miles (1,600km) from the surface. New Horizons’ Alice instrument studied the atmosphere as it passed in front of the Sun, allowing for extensive analysis.

9. As it passed Pluto and looked back, New Horizons discovered a huge cloud, a “tail,” of atmospheric ions in the wake of the planet. The region of cold, dense ionized gas consists of particles stripped from the planet’s atmosphere and blown away by radiation pressure from the solar wind.

10. The debate over Pluto’s planethood will go on. Is Pluto a planet or a dwarf planet? The International Astronomical Union ruled in 2006, removing Pluto’s planetary status some 76 years after its discovery. “Pluto killer” Mike Brown and anti-planethood activist Neil deGrasse Tyson have made their peace with Pluto, Brown attending some of the New Horizons festivities and Tyson tweeting a lovely picture of himself hugging the Disney character. In the eyes of the IAU, the story is over. But the flyby has stoked the fire of debate. More on this soon . . .

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