Will Pluto Surprise Scientists?

First-time encounters have a long history of shocking planetary scientists with unexpected features. It’s about time for Pluto.

Since New Horizons was launched in 2006, Pluto went from planet to minor planet to plutoid to dwarf planet. The Plutonians surely don’t care what the International Astronomical Union calls their home, but Earthlings are on the verge of a historic first: the completion of visits to the 9 “classic” planets that every student memorized after Clyde Tombaugh’s discovery of Pluto in 1930. This remarkable achievement was fulfilled in half a century since Mariner 4 radioed back pictures of Mars in 1964—within the lifetime of many seniors today. From now on, space exploration of the solar system will consist of follow-up observations of the planets, moons, comets, and asteroids. Since Pluto is a member of the Trans-Neptunian Objects (TNOs) and Kuiper Belt, and chief among the “Plutinos,” the other members won’t be quite as special if we ever visit them.

After 9 years of flight, New Horizons reaches Pluto on July 14. Not having brakes, it will zip past in just 14 hours, taking as many pictures and scientific measurements as it can. Nature‘s set of infographics provides a good overview of the mission. Pluto is unusual in having a very large moon (Charon) that is over half the diameter of Pluto itself, making the pair almost a binary planet. Four other moons, Styx, Kerberos, Nix and Hydra, are all much smaller. A pre-encounter paper on Icarus predicts that crater counts on Pluto will be able to fix its absolute age. The Conversation makes a big deal of the taxonomy; “NASA mission brings Pluto into sharp focus – but it’s still not a planet.”

Already, fuzzy images have been revealing surprises. The early man shows a “whale” of a dark feature. A large crater is visible. As of July 2, New Scientist called some pepperoni-like features on the surface “baffling.” The BBC News is puzzling over the difference in hue and brightness between Pluto and Charon.  In Space.com‘s “Expert Voices” series, Sophia Nasr commented on the counter-intuitive fact that Pluto has a dynamic atmosphere. She’s already gearing up to be surprised:

You might guess that a small and distant world almost 40 times farther from the sun than the Earth is from the sun would not have an atmosphere, but in the case of Pluto, you’d be wrong. In fact, Pluto is a complex world, particularly when it comes to weather patterns. Gusty winds, clouds, haze, micro snowflakes and even ice volcanoes — cryovolcanism — could all be part of Pluto’s dynamic weather system. While such observations have come from Earth-based telescopes, many more surprises might be revealed as NASA’s New Horizons spacecraft makes its nearest approach to Pluto on July 14, 2015.

But no matter how you classify it, this icy and remote dwarf planet is an odd little world.

The encounter will test how right or wrong the leading planetary scientists were in 1999, when The New Solar System: 4th Ed. was published. In the chapter on Triton, Pluto and Charon, Dale Cruikshank knew that Charon was tidally locked to Pluto, that Charon’s orbit was slightly eccentric, and that Pluto had more methane in its atmosphere than Triton, which he considered a similar body. Hubble images showed light and dark patches on the surface. He knew that Charon was darker, and noted that “Surprisingly, Charon’s surface is covered in large part by ordinary water ice, making it quite different from that of Pluto.”

An occultation in 1988 provided an opportunity to observe Pluto’s atmosphere. Planetary scientists suspected a photochemical haze exists with hydrogen cyanide, acetylene, ethane “and other complex molecules that should gradually fall to the surface.” Those molecules were not detected to date. How will the atmosphere react as Pluto moves farther from the sun after its 1990 perihelion? More frost should precipitate onto the surface, raising the reflectivity, he says. One of his sentences about this ends with a bang (!): “Maybe the entire planet will turn uniformly white as the entire, already pitifully thin, atmosphere collapses in a global freeze-out!

As for Pluto’s origin, Cruikshank entertained an ejection hypothesis from Neptune but found it implausible in light of the discovery of Charon. He also found “drawbacks” in the theory that Charon was ejected from Pluto during its formation. That leaves him with the theory that Pluto accreted in place, but then later, “the powerful impact of a fairly large planetesimal with Pluto resulted in the formation of Charon.” He thinks a “hypothetical impact may also explain why Pluto’s rotational axis is tipped so extremely.” Whether this theory survives the more recent discovery of four small moons remains to be seen.

Much of what we say today about the early results will be overtaken by the realities to come next week. What seems certain is that if the scientists are already surprised, they are poised to be shocked. Oddly, they seem to enjoy that.

Let’s make some predictions. Pluto will show active geology, including ice volcanoes or geysers, and evidence of resurfacing (e.g., few craters or smeared craters) that will challenge the belief it is billions of years old. (We have a track record for this prediction: Enceladus and Triton are both active though cold and distant. Their vents remain puzzling years after they were discovered.) A second prediction: atmospheric escape rates of molecules will be too rapid to sustain for billions of years. Another prediction: the moons will challenge ideas that they were formed from a collision. Finally, reporters will use the L-word life in teasers that Pluto might be habitable, especially if a subsurface liquid ocean is inferred.

Stay tuned! Get the kids involved; let them know they are witnessing a historic event. In a few days, Pluto will no longer be a mere speck of light, but a place we know something about. Enjoy the thrill of discovery!