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!!
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