SETI [ASTRO] First Detection of Extraterrestrial Ethane Ice

Larry Klaes (
Tue, 20 Jul 1999 18:12:44 -0400

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>Date: Tue, 20 Jul 1999 20:10:26 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] First Detection of Extraterrestrial Ethane Ice
>Reply-To: Ron Baalke <>
>Subaru Telescope
>650 N. A'ohoku Place
>Hilo, Hawaii 96720
>July 19, 1999
>First Detection of Extraterrestrial Ethane Ice!
>Subaru Telescope turned its gaze upon the most distant planet in our solar
>system last month. Using its Cooled Infrared Spectrograph / Camera (CISCO),
>Subaru obtained excellent images that clearly separated the planet Pluto
>from its satellite Charon just 0.9 arcseconds away. Subaru then used CISCO
>to gather some of the most revealing spectra ever obtained for both Pluto and
>Charon. Initial analysis of the data confirms that the surface compositions
>for these two bodies are strikingly different: Pluto is covered in frozen
>nitrogen, methane, carbon monoxide and newly discovered ethane "ice" (all at
>a temperature colder than - 346 F or -210 C); Charon appears mostly covered
>in the more familiar water ice (also at an extremely cold temperature). The
>detection of ethane is particularly significant, as this material may be a
>remnant of the original interstellar gas cloud that collapsed to form our
>solar system 4.5 billion years ago, preserved over the eons by the extreme
>cold that exists out at the distance of Pluto's orbit.
>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 ("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
>system as a binary planet rather than as a planet/satellite pair. At an
>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
>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
>(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
>[Image caption:
>[Image 1: Pluto and Charon]
>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
>x 2 arcseconds, with north 13 degrees to the left of vertical (east to the
>[Image 2: Spectrum of Pluto]
>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
>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.
>[Image 3: Spectrum of Charon]
>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.

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