What Little Is Known about Plutos Interior and Surface

Pluto's density is estimated at about 130 pounds per cubic foot (2,000 kg/m3), though because density is the ratio of mass to volume, the error in Pluto's density estimate necessarily contains the errors of the estimates of the mass of the planet and its size compounded. A density in this range implies that the planet is between half rock and half ices and about 70 percent rock and 30 percent ices, similar to Triton's rock-to-ice ratio.The ratio of rock to ice depends, of course, on the compositions and therefore the densities of the rocks and ice themselves.

Water ice is thought to be the most abundant ice making up the planet because oxygen is more abundant in the solar system than nitrogen or carbon, the other ice-making atoms, and Pluto is expected to follow the solar system abundances.Water ice metamorphoses into different crystal structures depending on pressure: Humankind is most familiar with ice I, but with greater pressure ice transforms into ice III, ice V, and ice VI (see figure on page 114). Simultaneous changes in temperature allow the formation of other ice phases.These are called polymorphs:They consist of the same materials but have different crystal structures. At the pressure and temperature ranges of Pluto and Charon, water ice in their interiors may be in the forms of ices I, II, III, V, and IV.

At low pressures the water molecules organize themselves according to charge.The two hydrogen atoms in a water molecule are slightly positively charged, and the oxygen is slightly negatively charged. A hydrogen from a neighboring water molecule will therefore weakly bond with

Water freezes into a variety of different crystal structures depending on its pressure and temperature conditions.

Ice Structure Phase Diagram

Ice Structure Phase Diagram

(-80) (-60) (-40) (-20) (0) (20) (40) (60) (80) (100) Temperature °F (°C)

the oxygen, and the hydrogens will themselves weakly bond with oxygens in other molecules. Because water molecules are shaped like boomerangs, with the oxygen at the bend in the boomerang and a hydrogen at each end of the boomerang making an angle of about 108 degrees, the water molecules make a honeycomb shape when they all weakly bond together to form ice I.

The honeycomb structure of ice I is weak, not only because the water molecules are not efficiently packed together, but also because the electrical bonds between molecules are weak. As pressure is increased on ice I beyond the strength of the weak intermolecular bonds, the molecules eventually are forced into more and more efficient packing schemes. Pressure also inhibits melting, and so the higher-pressure polymorphs of ice can exist at temperatures above 32°F (0°C).

Pluto's other ices are likely to be nitrogen ice, carbon monoxide, and methane.The composition of the rock component in Pluto's interior is unknown. There may also be organic compounds on the planets, based on evidence from comet Halley. Pluto may be internally differentiated, in which case it would consist of a layer of ice about 190 miles (300 km) thick, underlain by a rock core about 560 miles (900 km) in radius. If the temperatures during Pluto's formation did not allow it to differentiate, then its interior is likely to be a heterogeneous mix of rock and ice.

Pluto's surface shows the most contrast of any solar system object aside from Iapetus and the Earth. Its brightness varies by 30 percent over the period of its rotation (just over six days). Alan Stern of the Southwest Research Institute and Marc Buie of Lowell Observatory have mapped Pluto's surface for the first time. The results were released March 7, 1996.The scientists took images in blue light over a period of several days using the Hubble Space Telescope, from which they made blurry maps of Pluto's surface as shown in the figure on page 116. During this period Pluto was 3 million miles (4.8 million km) from Earth.

The two upper images in the figure on page 116 show the actual Hubble Space Telescope data, and the lower images show computer-processed images of each of Pluto's hemispheres. There are approximately 12 dark and light regions on the planet.

Pluto has a bright south pole and a dark equator, perhaps darkened by chemical reactions with the solar wind.The bright patches on Pluto's surface are thought to be methane (CH4), nitrogen (N2), and carbon monoxide (CO) ices, bright like snow on Earth.Water (H O) has also been detected in substantial amounts.The dark patches may be primordial organic matter, or they may be the result of millennia of cosmic ray bombardment turning simple hydrocarbons into more complex and darker molecules, or they may be something else entirely.

The color and density of Pluto are remarkably similar to those of Neptune's moon Triton, lending support to the theory that Triton was once a Kuiper belt object that was perturbed out of its orbit and captured by Neptune.

In June 1988 Jim Elliot and Leslie Young of the Southwest Research Institute detected an atmosphere on Pluto when it passed in front of a bright star, an event called a stellar occultation. During the stellar occultation the astronomers measured the intensity of the starlight.

When a star passes behind a planet with no atmosphere, the intensity of starlight drops off quickly and smoothly as the planet passes in front of the star, and rises again smoothly as the star reemerges on the other side of the planet. This curve of light intensity versus time is called a light curve.A typical light curve for a planet with no atmosphere is shown on the left in the figure on page 117. In Pluto's case, the light curve dropped more slowly at first as the star passed behind the planet, and then dropped rapidly, as shown in the accompanying figure on the right. The best explanation to date for the shape of the light curve is that Pluto had a thin atmosphere at distances farther from the planet than the break in slope in the light curve, and that closer to the planet it had a haze layer with a steep temperature gradient.

Little is known about Pluto's atmosphere, but it is thought to consist of nitrogen (N2), carbon monoxide (CO), and methane (CH4).

In this Hubble Space Telescope image, each square pixel is more than 100 miles (60 km) across, though this was the highest-resolution image of Pluto ever made. At this resolution, Hubble discerns roughly 12 major regions where the surface is either bright or dark. (Alan Stern [Southwest Research Institute], Marc Buie [Lowell Observatory], NASA and ESA)

Methane is the only molecule definitively measured from spectroscopy, but nitrogen and carbon monoxide would be expected to sublimate into the atmosphere at the temperatures that allow methane to do so. Nitrogen is the most volatile gas on Pluto and also the most abundant (though water is thought to be the most abundant ice on the planet; at Pluto's temperatures ice is hard as rock is on Earth and will not exchange with the atmosphere). Based on thermo-dynamic considerations, the atmosphere should consist of 99 percent

The left side of the figure shows a light curve for a planet with no atmosphere. On the right lies a light curve in the shape of Pluto's, showing the angle indicative of an atmosphere.

Light Curves

Planet moves in front of a star

Planet moves in front of a star

3 Ji

Atmosphere progressively blocks more starlight closer to the planet

Without an atmosphere, starlight disappears linearly as the planet passes in front of the star nitrogen, slightly less than 1 percent carbon monoxide, and about 1/10 of a percent methane. Pluto thus joins the Earth, Saturn's moon Titan, and Neptune's moon Triton as one of the few bodies in the solar system with an atmosphere dominated by nitrogen.

Pluto's atmosphere may only exist in any quantities near its perihelion, where temperatures on the planet are aided by additional solar heating and ices are more likely to sublime into gases. At perihelion the surface pressure is thought to be three to 160 microbars, about 1 millionth that of Earth's atmosphere. At aphelion, the atmosphere may well be completely frozen into surface ices. Pluto's atmosphere at perihelion extends to depths greater than Earth's atmosphere. Its atmosphere may even enclose Charon during the warmest periods. The atmosphere is thought to be actively escaping; Pluto is the only planet in the solar system actively losing its atmosphere now.

Pluto's atmosphere has been monitored since 1989, the planet's most recent perihelion. Throughout the 15 years since Pluto's perihelion in 1989, its atmosphere has continued to expand. In this period the planet's surface atmospheric pressure has almost doubled. Though this may seem counterintuitive, since Pluto's atmosphere is expected to collapse back into an ice phase after perihelion, Pluto is experiencing normal seasons. Just as the hottest part of summer on Earth occurs well after the equinox, Pluto is experiencing further warming, up to its peak summer temperature.As Pluto continues in its orbit its temperatures will eventually fall and the atmosphere lessen, a state expected in about 10 years. Pluto reaches its next summer solstice in 2029 and its aphelion in 2114.

The surface temperature on Pluto varies between —390 and —328°F (—235 and —200°C).Two methods for measuring surface temperature, infrared blackbody radiation measurements and measurements of the shape of the nitrogen absorption bands, indicate different surface temperatures at the same time on the day side of Pluto. These day-side temperatures vary from —405 to —355°F (—243 to —215°C). Though the measurement techniques have errors associated with them, the results indicate that the bright, icy areas of Pluto are measurably colder than the dark areas. This result is consistent with the colors of the regions, since dark material absorbs more heat than the reflective bright material. Pluto's surface is almost 1,300°F (700°C) colder than Venus's surface, as shown in the figure on page 118.

Surface Temperatures

900 800

Temperature F 600 500 400 300 200 100

o hj

450 400 350 300 250 200

150 100 50 Temperature °C

450 400 350 300 250 200

Object

Average Surface Temperature °F (°C)

a

Venus

896 (480)

b

Mercury

day 800 (430) night -274 (-170)

c

Earth

60 (15)

d

Mars

-80 (-62)

e

Jupiter

-238 (-150)

F

Saturn

-292 (-180)

9

Uranus

-346 (-210)

h

Neptune

-364 (-220)

1

Pluto

-382 (-230)

Pluto's atmosphere heats up rapidly with distance above the planet's surface.The gas giant planets all share the strange phenomenon of a hot uppermost atmosphere, but Pluto's hot atmosphere starts within a few kilometers of its surface. On Triton, for comparison, an equally high temperature is not reached before several hundreds of kilometers. Pluto's atmosphere reaches a high temperature of about —279°F (—173°C), as much as 126°F (70°C) hotter than the surface. Methane is particularly efficient at absorbing solar heat, and the high temperature may be explained by a gradient in methane abundance in the planet's outer atmosphere.

The surface temperature ranges of each of the objects graphed here show that Mercury has by far the widest range of surface temperatures; Venus has the hottest surface temperature, while Pluto, unsurprisingly, has the coldest.

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