Pluto's estimated surface temperature is about minus 387 degrees Fahrenheit (that's minus 233 degrees Celsius or about 40 Kelvin). Because it is so cold on the surface of Pluto nearly every material is frozen solid! Click here to learn more about the frigid conditions at Pluto.
Starting in the 1970s, technological advances in telescopes and other instruments brought tiny, faint Pluto within range for Earth-bound observers. In 1975, using the light-analysis technique called spectroscopy, astronomers Dale Cruikshank, David Morrison and Carl Pilcher measured a portion of Pluto's infrared spectrum using one of the then-largest telescopes on Earth.
They aimed the 4-meter Mayall Telescope and its powerful spectrometer (located at Kitt Peak National Observatory in Arizona) at Pluto and recorded the signature, or spectral fingerprint, of frozen methane. This discovery was the first indication that Pluto's surface is icy rather than rocky, and opened an era of investigations into the realm of small, icy, outer solar system objects that continues today.
Working with other colleagues, Cruikshank (now a New Horizons team member) later discovered the frozen nitrogen and carbon monoxide, as well as evidence for the colored organic chemicals that make up Pluto's surface.
In this photo, from left, Morrison, Pilcher and Cruikshank sit in the Cassegrain "cage" inside the giant telescope. The instrument with which they detected methane ice on Pluto hangs just above their heads. Since then, new generations of instruments have dramatically improved our knowledge of Pluto's (and Charon's) composition.
The densities of Pluto and Charon offer the strongest clues about their overall compositions. Pluto and Charon have densities about twice that of water, indicating that they are both composed of a mixture of ice and rocky material. The ice is most likely frozen water (H2O), which is common throughout the outer solar system. However, on the surface of Pluto we have detected three different kinds of ice so far. Much of the surface is covered with frozen nitrogen (N2) and methane (CH4), with small amounts of frozen carbon monoxide (CO). On Charon, only H2O ice has been found so far. These ices have been detected from measurements made on Earth (and with the Hubble Space Telescope) using the technique of spectroscopy. Light from Pluto and Charon collected with a large telescope is analyzed with a spectrograph that separates the component colors of the sunlight that is reflected from their surfaces, and distinguishes the characteristic "fingerprints" corresponding to the molecules of the four different ices mentioned.
Both Pluto and Charon have other materials mixed with the surface ices, and these materials may be minerals or solid materials made of complex organic chemicals, although scientists have not yet made specific identifications of their compositions. On Pluto these materials, which are darker in color and less reflective than the ices, are distributed in large patches around the planet. On Charon they appear to be more uniformly distributed over the surface.
This section of the infrared spectrum of Pluto shows the signatures of some of the ices making up its surface. The black dots indicate the "dips," or low points, in this spectrum tracing that are identified with specific ices, as shown. CH4 signifies methane ice, while N2 and CO indicate frozen nitrogen and carbon monoxide, respectively. Although this spectrum incorporates one entire hemisphere of Pluto, the Ralph instrument (and its LEISA component) on New Horizons is designed to collect similar data from regions as small as a few kilometers in size all over the planet and on Charon.
"Geometric Albedo" is a measure of Pluto's brightness, while "Wavelength" denotes the region of the spectrum. The wavelengths in this spectrum are two to five times longer than those that can be seen with the human eye. This spectrum was obtained with the 3.8-meter United Kingdom Infrared Telescope (UKIRT) at Mauna Kea Observatories, Hawaii, by Thomas R. Geballe, Tobias Owen, Dale Cruikshank, and Catherine de Bergh. An analysis of some parts of the spectrum was published by S. Doute, et al., in Icarus 142, 421, 1999.