The first detection of extraterrestrial solid ethane
July 19, 1999
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Object Name: Pluto and Charon
Telescope: Subaru Telescope / Cassegrain Focus
Filter: J(1.25 micron), H(1.65 micron), K'(2.15 micron)
Color: Blue(J), Green (H), Red (K')
Date: UT 1999 June 9
Exposure: 2 sec for each band
Field of View: 3 arcsec x 2 arcsec
Orientation: North is 13 deg. left of vertical, east to the left
Position: RA (J2000.0)=16h35m55s, DEC (J2000.0)=-10d3m23s
Until only very recently, images taken by ground-based telescopes always showed Pluto and Charon blurred together since their maximum separation is only 0.9 arcsec as seen from the Earth. Thanks to the excellent quality of its 8.3-meter primary mirror and the stability of the atmosphere above Mauna Kea, Subaru Telescope has been able to provide clearly separated images of the two bodies using its Cooled Infrared Spectrograph / Camera (CISCO). With their light cleanly separated, subsequent infrared spectroscopy using CISCO reveals dramatically different surface compositions for Pluto and Charon.
This color image is produced from three 2-second exposures taken on June 9th, 1999 through infrared filters centered on 1.25, 1.65 and 2.13 microns (colored blue, green and red, respectively). The field of view is 3 arcseconds x 2 arcseconds, with north 13 degrees to the left of vertical (east to the left).
In addition to known absorption features of nitrogen (N2), methane (CH4) and carbon monoxide (CO) (all in solid form) on Pluto, Subaru Telescope has made the first discovery of narrow absorption features due to solid ethane (C2H6).
The ethane may be a component of the original primordial material from which our solar system was formed. Alternatively, it may have been created slowly (over the lifetime of the solar system) from the molecular fragments produced by the reaction of ultraviolet light with primordial methane on Pluto. Further analysis of the Subaru observations could help clarify which of these two theories is most correct.
Comparing the wavelengths of the observed ethane absorption features against recently published laboratory results, we can also learn something about the physical conditions on Pluto's surface. It seems the ethane does not exist as individual grains but rather, is dissolved within the much more plentiful molecular nitrogen ice believed to be covering much of Pluto's surface at a temperature of -233 C (-387 F).
The successful detection of ethane owes much to the large light-gathering power and excellent imaging capability of Subaru Telescope and the high wavelength resolution of CISCO.
The Subaru Telescope observation shown here is the first detailed spectrum of Charon between the infrared wavelengths of 2-2.5 microns and confirms the existence of Water (H2O) ice on its surface. Even more interestingly, the water ice signature is not apparent in the Pluto spectrum; and none of the molecular bands seen in the Pluto spectrum are apparent in the Charon spectrum. Clearly, the two bodies have very different surface compositions. This is similar to another famous binary planet, the Earth-Moon system whose formation was believed to be due do the impact of a large body when the Earth was young. This lends support to the theory that the Pluto-Charon system congealed out of the shattered remains of a single body following a (near) collision back when the solar system was in its infancy.
Pluto was discovered in 1930 and is the furthest known planet in our solar system, taking 249 years to go once around the Sun. It travels in a very elongated orbit which takes it from 30 AU ( = "Astronomical Unit", the distance between the Earth and the Sun = 150,000,000 km or 93,000,000 miles) to as far away as 50 AU from the Sun. It has a diameter of 2,274 km (1413 miles). In 1978, Pluto was found to have a companion in orbit about it, the satellite Charon (pronounced "KAIR-on"). With a diameter of 1,172 km (728 miles), Charon is about half the size of Pluto which is unusually large, relatively speaking, for a satellite. It may be more appropriate to regard the system as a binary planet rather than as a planet/satellite pair. At an average separation of 19,640 km (12,200 miles or just eight Pluto diameters), Charon orbits Pluto in 6.387 days. Charon travels in a synchronous orbit always keeping the same face pointed towards Pluto, just as the Moon does with respect to the Earth. But Pluto is unique among the planets in that it rotates at exactly the same speed that its companion orbits, always keeping the same face pointed towards Charon.
On June 9th, 1999, Pluto was 5.865 billion km (3.645 billion miles) from the Earth. At this distance, Pluto and Charon had apparent diameters of 0.08 and 0.04 arcseconds, respectively. Although these CISCO observations are among the best ever taken from the ground (with a resolution of about 0.35 arcseconds), they are not sharp enough to show the true disks of either Pluto or Charon. But in the near future, Subaru will begin using its Adaptive Optics (AO) unit with a potential maximum resolution of 0.06 arcseconds (equivalent to being able to read newspaper headlines twenty-five kilometers or fifteen miles away). This is sufficient to begin resolving Pluto's surface features directly.
- * There is an objection concerning "the first discovery of narrow absorption features due to solid ethane (C2H6)."
- Dale P. Cruikshank, Catherine de Bergh, Sylvain Dout, Thomas R. Geballe, Tobias C. Owen, Eric Quirico, Ted L. Roush, and Bernard Schmitt, 1999, Science, 285, 1355