Pluto in the Rear-View Mirror

The New Horizons flyby of the Pluto system reveals a new world in stunning detail.  The progress made over the last few decades in understanding Pluto has suddenly undergone another round of explosive growth.  Perhaps this is a good time to review just how our knowledge of Pluto has evolved since I first became interested in this outpost of the Solar System.

My first source of information about space was Stars, a Golden Nature Guide, written by Herbert S. Zim in 1951.  This little pocket guide contained, on pages 104 and 105, a table of data on “The Planets”.  The table was populated with a lot of quaintly obsolete data and a liberal sprinkling of question marks.  The column devoted to Pluto, unabashedly listed as a planet, gave good information on its distance from the Sun, but all the rest seemed designed to pique the curiosity of a child.  The diameter, which could not be measured directly, is given conjecturally as “3600  (?)” miles, comparable to Mars—but New Horizons, viewing from up close, measured only 1473 miles.  The volume of Pluto is therefore only 6.85% of what was then accepted.  The mass of Pluto was based on the wildly wrong diameter and an equally wild guess about its density; its mass relative to Earth was given as “0.8 (?)” and its volume as “0.07 (?)” that of Earth.  One need not have mastered calculus to figure that the density of Pluto was 11.4 times that of Earth, or 63 grams per cubic centimeter.  Considering that the densest chemical elements (osmium and iridium) have densities of 22 g/cm³, this presented an obvious problem for a budding young chemist. 

Further paradoxes abounded.  The number of moons of Pluto is given as “0.1 (?)”.  I can perhaps be forgiven for wondering how a planet could have a tenth of a moon.  The length of the Pluto day was given simply as “(?)”, and its axial inclination is reported the same way.

Is it any wonder that I became intrigued with this mysterious planet?

In 1972, years before we had reliable data on the size and mass of Pluto, I published two papers on the structure of ice-plus-rock bodies, so-called “dirty snowballs”, with the large satellites of the Jovian planets in mind.  The densities of these bodies constrained their proportions of ice and rock; the rocky component, with its endowment of uranium, thorium, and potassium, contributed substantial heat to their deep interiors.  Considering the size of the heat source, the surface temperatures of the large icy satellites, and the melting behavior of ices, I predicted that bodies like Europa and Ganymede could have deep oceans covered by thin crusts of water ice.  Their surfaces would then be quite susceptible to resurfacing, and would be very poor at preserving evidence of impact cratering.  Later, in 1979, Stan Peale, Pat Cassen and Ray Reynolds (Science 203, 892) proposed another, stronger heating effect for the Galilean satellites, through flexing driven by the tidal interactions of the moons. This model became entrenched in the literature, even to the point that most scientists ignored the radioactive heating component. 

We had no way to measure the mass of Pluto until its big satellite Charon was discovered in 1978, finally letting us track the orbital motions of Charon and Pluto around their common center of mass.  Mutual eclipses of Pluto and Charon provided much-improved data on their sizes.  But the best guesses on their densities still relied on condensation theory, which could not be tested with the best Pluto data in hand.

All that changed when New Horizons flew by Pluto.  Since Pluto and Charon are locked into a 1:1:1 spin-spin-orbit resonance, heating by tidal flexing is ruled out.  The other satellites of Pluto are tiny and have almost no effect.  Yet impact craters are absent and the whole planet has been recently resurfaced.  Clearly the driving force must be radioactive decay.  But how does it work?  What could the fluid be that resurfaces so efficiently?  Pluto’s surface is far, far below the melting temperature of water ice.  Clearly this is not a place for silicate volcanism: the resurfacing must be connected with the ices that make up a third of the mass of Pluto.  But which ones?

It turns out that there is a significant difference between ices formed in the Solar Nebula and ices formed in the satellite systems around the giant planets.  The environment in a protoplanetary disk girdling Jupiter or Saturn generally has much higher gas pressure than in the nearby Solar Nebula.  The effect of pressure strongly influences the chemistry of both nitrogen and carbon because their reactions with hydrogen (the dominant gas in the Universe) are driven to the right by higher pressures:

            3H₂ + N₂ à 2NH₃         and       3H₂ + CO à CH₄ + H₂O.

Thus ammonia and methane are minor constituents of ices formed in the Solar Nebula, but can be major components of ices formed in sufficiently dense and cool protoplanetary disks, such as those surrounding Saturn, Uranus, and Neptune.  These are available as the raw materials out of which their satellite systems formed.   Each disk was warmer near its center and cooler near its outer edge; in the case of Jupiter, the region inside Europa’s orbit (including Io) was too warm for even water ice to condense, thus making rocky moons.  Europa, forming close to the “snow line” in Jupiter’s nebula, retained only a small proportion of water ice and essentially none of the other, more volatile ices.  Ganymede and Callisto, formed farther out, are much more ice-rich.

Ammonia and methane can enter solid ices at temperatures too high for direct condensation of solid ammonia or solid methane because both gases can react with water ice to make solid hydrates.  This is how Saturn’s largest moon, Titan, retained vast stores of ammonia and methane.  Heating of Titan’s interior, whether by radioactive decay or tidal flexing, caused early melting of ammonia hydrates: in fact, ammonia-water ices begin to melt at only 100 K, or -173 ^oC.  Once melting begins, separation of the ice component from the “dirt” proceeds to generate a muddy core and a deep water-rich ocean with an ice crust.  Interestingly, the average surface temperature of Titan is 94 K, just 6 K colder than the onset of ammonia/water melting.  One can easily imagine cold, viscous ammonia/water melt being extruded onto the surface as cryovolcanic eruptions.  At these temperatures, little ammonia is released as a gas, but methane is given off in large quantities.  Ammonia is also very vulnerable to destruction by solar ultraviolet light, producing nitrogen and hydrogen (which is so light it readily escapes from Titan).  Not surprisingly, Titan today has an atmosphere dominated by nitrogen and methane.  Neptune’s large satellite Triton should be regarded as a colder version of the same scheme.

But Pluto and other Kuiper-belt bodies, formed in the much less dense Solar Nebula, would have experienced much more limited conversion of CO and N₂ into methane and ammonia.  Both CO and N₂ gases readily form solid hydrates, permitting them to be important constituents of the ice.  Any heating of the interior (in Pluto’s case, by radioactive decay) will release CO and N₂ into the atmosphere.  Thus low-temperature resurfacing is not only possible, but very important—and the key to the process is contained in a theory that dates back to 1972.

Oh yeah, is Pluto really a planet?  I DON’T CARE!!