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Evidence for Crystalline Water and Ammonia Ices on Pluto's Satellite Charon

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Science  07 Jan 2000:
Vol. 287, Issue 5450, pp. 107-109
DOI: 10.1126/science.287.5450.107

Abstract

Observations have resolved the satellite Charon from its parent planet Pluto, giving separate spectra of the two objects from 1.0 to 2.5 micrometers. The spectrum of Charon is found to be different from that of Pluto, with water ice in crystalline form covering most of the surface of the satellite. In addition, an absorption feature in Charon's spectrum suggests the presence of ammonia ices. Ammonia ice–water ice mixtures have been proposed as the cause of flowlike features observed on the surfaces of many icy satellites. The existence of such ices on Charon may indicate geological activity in the satellite's past.

Pluto's satellite Charon orbits Pluto so closely that, even though it is only five times fainter than Pluto, Charon's existence was not discovered until 1978 (1). Even after discovery, the small separation between the two objects has made separate study of them difficult. Most of our current knowledge about the composition of Charon comes from observations of a series of mutual Pluto-Charon eclipses between 1985 and 1990. In two such series of eclipses, Pluto and Charon were observed together, and then Pluto was observed separately as it completely occulted Charon. Subtraction of the two observations then yielded the brightness of Charon. By performing these observations at a small number of wavelengths over the near-infrared region, a low-resolution spectrum of Charon was synthesized. These spectra showed evidence for a surface covered in water ice, much like the satellites of the giant planets and unlike the surface of Pluto (2). The possible existence of materials other than water ice was also considered, but the crudeness of the data did not permit any resolution of the issue (3).

Observing Pluto and Charon separately from the ground is difficult. The maximum separation between the two objects is currently 0.9 arc seconds, and typical atmosphere-induced blurring at the Keck telescope on Mauna Kea is ∼0.5 arc seconds in the infrared. On an exceptional night of 0.3–arc second atmospheric seeing, the two objects were distinct, and we obtained well-separated near-infrared images (Fig. 1) (4) and spectra of the two objects (Figs. 2 and 3) (5). The lack of any common spectral features between Pluto and Charon demonstrates that the final spectrum of Charon is separated from Pluto.

Figure 1

Images of Pluto (left) and its satellite Charon (right) at the K band (∼2 μm). The objects are spaced by 0.90 arc seconds and are separated in the 0.30–arc second seeing. The histogram above the image gives a trace of the intensity through the center of the system, showing how little of the light from Pluto contaminates the spectrum of Charon.

Figure 2

(left). The near-infrared spectrum of Pluto. The histogrammed points give the data, scaled to a value of unity at a wavelength of 1.5 μm, and the smooth line is a reflectance spectrum of pure methane ice from Fink and Sill (7). The surface of Pluto is also known to contain N2 and CO, but the weak spectral features due to these ices are not readily visible at this spectral resolution. Fig. 3 (right). The near-infrared geometric albedo spectrum of Charon. The histogram gives the data, which has been scaled to the geometric albedo of Charon derived by Roush (3). The dashed line shows a model consisting of only water ice and a dark neutral absorber. The solid line is a model in which ammonia and ammonia hydrate ices have been added to the water ice and neutral absorber.

The spectrum of Pluto has previously been studied in detail (6). The spectrum of Charon has been known previously to be dominated by the signature of water ice (2), but the crudeness of the previous spectrum prevented further analysis. As expected, our spectrum of Charon is also dominated by the 1.5- and 2.0-μm absorption bands of water ice. The additional appearance of the small 1.65-μm absorption feature redward of the main 1.5-μm absorption demonstrates that the surface water ice is unexpectedly crystalline, rather than amorphous, in form (7). At the ∼50 K temperature of Charon, crystalline water ice is turned into amorphous form under bombardment from solar ultraviolet radiation (8). The presence of crystalline ice on the surface of Charon suggests that continuous micrometeorite impact vaporization and the subsequent recondensation of crystalline ice on the surface of Charon proceed faster than the radiation-induced transformation to amorphous ice. The presence of crystalline water ice on all of the well-studied icy satellites in the outer solar system (9) confirms that a ubiquitous mechanism such as impacts must be responsible.

A model consisting of only crystalline water ice and a dark, spectrally neutral material (10) reproduces all of the major features of our spectrum of Charon except at the 2.2-μm peak and longward of 2.3 μm. Although the inclusion of larger grain sizes of water ice could contribute to a better fit beyond 2.3 μm, the suppression of the 2.2-μm peak in Charon cannot be attributed to water ice and requires additional absorbing materials on the surface. To identify the 2.2-μm absorption, we considered all of the ices previously observed on or proposed for solar system bodies and the interstellar medium. In addition, we considered all other simple combinations of H, C, N, and O, the four most abundant molecule-forming elements in the solar system, and more complex hydrocarbons such as tholins and kerogens, which have long been considered plausible causes for the low-albedo surfaces in the outer solar system. Given the recent reports of hydrated minerals on the icy satellites of Jupiter (11), we also considered all common terrestrial rock-forming minerals. Of all of these possible constituents, only HCN, NH3, NH3·2H2O (ammonia hydrate), and alumino silicate clays have absorption features in the 2.2-μm region (Fig. 4). We constructed spectral models using combinations of these compounds, water ice, and a dark neutral material (12) and compared them with the spectrum of Charon. These comparisons show that HCN cannot reproduce the abrupt flattening of the 2.2-μm peak that is observed on Charon and that the clays have band centers displaced to shorter wavelengths than observed on Charon. Ammonia and ammonia hydrate individually have absorption features too narrow to reproduce the Charon spectrum, but a combination of the two produces an absorption that matches both the depth and location of the observed Charon absorption. In addition, all other absorption features of ammonia and ammonia hydrate correspond to strong water ice absorptions, so the spectral signature of water ice is not disturbed elsewhere, as seen in a model consisting of water ice (44% surface coverage), ammonia (1%), ammonia hydrate (24%), and a dark neutral material (32%) (Fig. 3). Like crystalline water ice, ammonia hydrate ices are destroyed by radiation (13). Whatever process allows the crystalline ices to exist on the surface of Charon could also be responsible for the continued appearance of ammonia even in the presence of radiation. The remaining discrepancies between the model and the data and the use of a combination of pure ammonia and ammonia hydrate suggest that Charon may have a different hydrate structure (some inclusion of NH3·H2O). Unfortunately, a systematic spectroscopic study of ammonia hydrates has not been performed at these wavelengths. It is known, however, that hydration of ammonia will shift the 2.2-μm absorption feature to a shorter wavelength and broaden the feature (14). The model fit also has discrepancies at the bottom of the 2.0-μm trough and a higher albedo in the region longward of 2.3 μm, where additional absorbers may be needed. In addition, the model continuum level shortward of 1.5 μm is too high, suggesting that the dark surface component is optically red, as has been inferred for many low-albedo asteroids and other dark surfaces in the solar system (15).

Figure 4

Spectra of candidates for the 2.2-μm absorption seen on Charon (20). Montmorillonite and kaolinite are alumino silicate clays.

The existence of nitrogen in the form of ammonia ices on the surface of Charon is a marked contrast to the surface of Pluto, where the nitrogen exists in molecular form. This difference may be due to the high volatility of molecular nitrogen and the relative sizes of these outer solar system bodies. In the absence of gravity, bodies at the ∼50 K temperature of Pluto and Charon would sublime about 1000 km of molecular nitrogen over the age of the solar system (Fig. 5). Pluto, with a radius of 1195 km, a mass of 1.5 × 1022kg, and therefore an escape velocity of 1.3 km s−1, is massive enough to retain the nitrogen as an atmosphere and a frost surface. Charon, however, owing to its smaller size—a radius of 593 km and a mass of 3.3 × 1021 kg—and hence smaller escape velocity of 0.86 km s−1, is ∼2 million times less efficient at retaining an atmosphere against Jean's escape (16) so the volatile ices seen on Pluto (including CO and CH4) will be sublimed from Charon over time scales as short as millions of years. If this hypothesis for the origin of the difference in surface composition of Pluto and Charon is correct, a prediction would be that underneath a layer of condensed volatile frosts, the surface of Pluto is much like that of Charon.

Figure 5

Evaporation rates (in meters per billion years) of ices from the surface of an object in the outer solar system. The rates were calculated by assuming equilibrium between the temperature-dependent vapor pressure of the ice and the pressure of the escaping gas immediately above the surface of the object (21). AU, astronomical units.

Ammonia ices have been predicted to be an important component of icy satellites in the outer solar system (17). The discovery by Voyager that even relatively small satellites, with interiors too cold to melt pure ice, have had complex geologic histories led to the realization that ammonia-water mixtures, because of their lower melting temperatures and higher viscosities than pure water, may be responsible for the activity seen on these bodies (18). Charon is about the size of the uranian satellites Ariel (579 km) and Umbriel (586 km), both of which exhibit a variety of surface geologic units including cratered plains and fault valleys whose floors suggest infilling by erupted and flowing materials similar to terrestrial volcanic flows (19). Charon should have experienced similar amounts of accretional heating in the past, in addition to any extra heating from tidal evolution of the Pluto-Charon system, so similar geological activity on this body is expected. The detection of ammonia ices on Charon suggests that such ices may play an important role in geological activity on icy bodies in the outer solar system.

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