Figure 13.5 Internal structure of Pluto.

unknown, but it is thought to be covered in patches of frozen nitrogen, water, methane and ethane. Pluto's spectrum shows absorption lines of various solid ices that cover the planet's surface, including nitrogen, methane and carbon monoxide.

The strength of gravity on Pluto is much less than Earth's gravity (0.67 N/ kg compared to 9.8 N/kg). This means that a 75 kg person weighing 735 N on Earth would weigh only 50 N on Pluto.

The surface

Astronomers know little about the surface of Pluto because the body is so far from Earth. Pluto's diameter is less than one-fifth that of Earth so it is difficult to observe anything on a surface that is so far away. The best views of Pluto show a brownish disc with bright and dark areas (Figure 13.3). The bright areas are probably covered with frozen nitrogen, with smaller amounts of methane, ethane and carbon monoxide. The composition of the darker areas on the surface is unknown but they may be caused by decaying methane or other carbon-rich material.

Pluto's interior is probably rich in ices with some frozen water. The central core probably contains solid iron and nickel and rocky silicate materials.

The atmosphere

Little is known about Pluto's atmosphere but it thought to contain about 98 per cent nitrogen with about 2 per cent methane and carbon monoxide. The composition was determined from observations made when the planet passed in front of a bright star (an occultation).

The atmosphere is tenuous and the pressure at the surface is only a few millionths of that of Earth. It is thought to extend about 600 kilometres above the surface. Because of Pluto's elliptical orbit, the atmosphere may be gaseous when Pluto is nearest the Sun and frozen when furthest from the Sun. NASA wants its New Horizon space probe to arrive at Pluto when its atmosphere is unfrozen.

Pluto's weak gravity means its atmosphere extends to a greater altitude than does Earth's atmosphere.

Temperature and seasons

The surface temperature on Pluto varies between about -235°C and -210°C. The warmer regions roughly correspond to the darker regions on the surface.

Because the orbit of Pluto is so elliptical, the amount of solar radiation it receives varies markedly between its extreme positions. Its 248-year orbital period means that any seasonal change is very slow to take place.

Magnetic field

Pluto may have a magnetic field, but it would not be strong. The New Horizons probe may be able to take measurements of any field it may have.

Moons of Pluto

In 1978 the American astronomer James Christy noticed that Pluto had an elongated shape in photographs. A search through previous images also showed a similar shape. This observation led to the discovery of a moon orbiting Pluto. The moon was named Charon, and it was found to orbit Pluto at a distance of 19 700 km over a period of 6.39 days. It turned out that Pluto and Charon rotate synchronously, each keeping the same face to the other. Astronomers were able to observe the two bodies rotating around each other during 1985 and 1990 when the two were edge-on to Earth. Such observations enabled astronomers to determine that Pluto's diameter was 2320 km and Charon's 1270 km. Because these sizes are close, some astronomers referred to the two bodies as a double planet.


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