Pluto's Geology and Surface-Atmosphere Interactions

A point frequently cited in support of Pluto’s planet status is that it is a complex world with geology and weather. On the fourth day of the conference, these subjects took center stage, as researchers described what is known about the surfaces of Pluto, Charon, and the four tiny moons and the interactions between these worlds’ surfaces and atmospheres.

Issues addressed regarding surface geology included predictions of whether Pluto and Charon have plate tectonics (processes in which a planet’s crust and upper mantle divide into plates which float and travel independently over the planet’s mantle), predictions on the overall geology of Pluto and Charon, impact craters and the dust particles they eject, and the effects of gravity on the surface processes of the system’s four tiny moons.

PhD student Marc Neveu presented a study questioning whether “exotic sodas” or gas exsolutions (the processes of separating or precipitating from a solid crystalline phase) could cause cryovolcanism on Pluto and Charon. His talk was followed by Dr. Lynnae Quick, who presented her predictions for cryovolcanic flows on Pluto’s surface.

Also discussed in depth were landforms and surface processes on Pluto and Charon, using the seasonal caps on Mars as an analogue for Pluto regarding jets, fans, and cold trapping, mapping coordinate systems used for Pluto, the use of geologic mapping to investigate the geologic history of Pluto and its moons, and speculation about what Pluto will look like.

I felt very grateful to Swinburne for their insistence that students use at least some scholarly journals in our research, in order to familiarize ourselves with the level and format of articles by professionals in the field. Even if one has difficulty following all the equations and theories presented in these studies, the key is to understand the major points being made, the conclusions the researchers reached and how and why they arrived at those conclusions.

Both Pluto and Charon are likely to have impact craters on their surfaces. Such craters are ubiquitous throughout the solar system, and researchers can use the wide variety of craters to learn about the impacting bodies that caused them.

Because Pluto has an icy surface, craters there will appear like those on other icy bodies. The craters will likely be small and shallow since the impacting bodies likely traveled at low velocities.

Pluto’s smaller moons are likely to have surfaces akin to those of asteroids and other small solar system bodies.

Pluto and Charon are tidally locked to one another, meaning they rotate synchronously, with the same side of Pluto always facing the same side of Charon and vice versa. This tidal locking provides unique constraints on the bodies’ interior structure, thermal history, and likely patterns of tectonic deformation.

Cryovolcanism is possible on Pluto, Charon, and other large Kuiper Belt Objects if there is liquid below their surfaces and there are cracks in ice on these objects’ surfaces. The gases able to exert enough pressure to get to their surfaces are hydrogen, nitrogen, argon, methane, and carbon dioxide.

Other places in the solar system where cryovolcanism occurs include Jupiter’s moon Europa, Saturn’s moons Titan and Enceladus, and Neptune’s moon Triton. Many speakers emphasized the similarities likely between Triton and Pluto. This is useful because we have data on Triton from the Voyager 2 flyby of Neptune in 1989. Triton likely originated in the Kuiper Belt and may have been a planet with its own orbit around the Sun before being captured by Neptune.

Pluto and Triton have similar densities, surfaces, and atmospheric compositions, so Pluto’s surface is likely to look a lot like Triton’s. However, albedo contrast is very different for the two bodies. Triton has only modest contrast in the brightness of its surface areas while Pluto shows enormous contrast, with some areas very bright and others dark.

Almost all presenters at the conference made reference to computer models used to determine the outcomes of the processes they are studying. Some described computer models they created either individually or through a team.

Three possibilities were discussed for Pluto’s surface. The first is that Pluto is geologically differentiated with a floating ice shell and a subsurface ocean. The second also has Pluto geologically differentiated, but frozen down to its core, with no ocean present. The third is that Pluto has uniform density globally with a viscous (thick, sticky consistency between that of a solid and that of a liquid) interior and a rigid outer shell.

Scarps, or cliff lines caused by erosion, are also ubiquitous in the solar system; they can be found on Earth, Mars, Titan, and Triton and are also expected on Pluto and Charon, where surface material is moved via condensation (change of physical matter from gas to liquid) and sublimation (change of physical matter from solid to gas without passing through a liquid phase). Another process that likely occurs on Pluto and Charon is sedimentation, or the transportation of eroded particles over long distances in thin atmospheres via plumes.

Geologic mapping is a tool in which scientists take data from many different observations and use it to understand an object as a whole. It was first used on Earth to map geologic features and their ages and characteristics, but has since been used for other solar system bodies and will be an ideal tool for astronomers to use New Horizons data for the purpose of defining and characterizing the brightness, texture, color, and morphology (form and structure) of Pluto and its moons and teasing out their natural history.

The next series of talks centered on surface-atmosphere interactions, addressing issues such as seasonal variations on Pluto’s surface, seasonal transport of volatile organic compounds and the use of computer models to illustrate these phenomena, seasonal light curves, computer modeling of Pluto’s climate, processes driving sublimation and deposition on Pluto, global surface-atmosphere interaction on the planet, chemistry in Pluto’s atmosphere, a comparison of Pluto’s photochemistry (the study of chemical reactions that proceed with the absorption of light by atoms or molecules) with that of Titan and Triton, and three dimensional modeling of the methane cycle on Pluto.

It may come as a surprise to some that Pluto does have seasons! Pluto’s seasons are affected by its highly eccentric orbit, which results in seasons of different lengths for its northern and southern hemispheres.

A good research project for students involves examining the history of Pluto’s light curves, then taking new ones, and comparing the information from all of them to determine whether volatiles have been or are being transported across the planet’s surface. Triton’s light curve has changed significantly since the Voyager 2 flyby. Past light curves of Pluto show no volatile transport; however, in recent years, it has been difficult to obtain accurate data since Pluto has been passing through the plane of the Milky Way. Starting in 2014, Pluto will move away from that plane and data collection will become easier.

Many scientists believe Pluto does experience significant seasonal transport of volatiles. If this is the case, light curve measurements will best confirm these processes, as these measurements are good at detecting changes in albedo (brightness) patterns.
Photometry is a technique that measures the intensity of an astronomical object’s electromagnetic radiation; it has been used to monitor Pluto’s brightest regions. The planet’s color appeared constant until 1992, but after that it changed significantly. Pluto’s south pole has become brighter, and its surface has become more red. The only explanation for these changes is that there have been actual changes on Pluto’s surface.

Three factors control Pluto’s climate. These are its obliquity (axial tilt) of 58 degrees, resulting in its poles receiving more sunlight than its equatorial regions; the eccentricity of its orbit, and the fact that the nitrogen in its atmosphere is in vapor pressure equilibrium with its surface ices (meaning the pressure of atmospheric nitrogen in gaseous form and surface nitrogen in ice form are the same).

In discussing Pluto’s atmosphere, Dr. Kevin Baines compared it to Titan and Triton, describing Pluto as “a really alive planet—really a small Titan.” Dr. Vladimir Krasnopolsky noted that the chemistries on Titan and Triton are very different from one another. Triton is a far better analogue for Pluto. Titan is much closer to the Sun than Pluto; its surface temperature is 94 Kelvin. If it were moved to Pluto’s orbit, Titan would be much more similar to both Pluto and Triton.

A computer model known as the LMD Pluto Climate Model has been used to simulate Pluto’s atmosphere, surface, and subsurface temperatures between 1988 and 2015. The model is being used to determine whether methane that never sublimated will disappear from the surface of Pluto’s poles by 2015. On most areas of Pluto, the atmosphere is much colder than the surface; however, this is not true in the areas where sublimating materials are carried by wind.

This model will be a useful tool to the New Horizons community and to researchers investigating climates on other planets.