What Melted the Ice Planet?

Categories: Outer solar system
The doppler effect
Quaoar is about 4 billion miles away from Earth, well over a billion miles farther away than Pluto. Earth as seen by the departing Voyager spacecraft as it departed the solar system: a tiny, pale blue dot.
Credit: NASA

A planetoid beyond Pluto was discovered using the Hubble Space Telescope in October 2002, and given the name, Quaoar. Like Pluto, Quaoar dwells in the Kuiper belt, an icy debris field of comet-like bodies extending 7 billion miles beyond Neptune’s orbit. Over the past decade more than 500 icy worlds have been found in the Kuiper belt. With a few exceptions all have been significantly smaller than Pluto.

The Kuiper belt is the repository of the Solar System’s most primitive ices and organic molecules and is widely thought to be the source of the short-period (specifically "Jupiter Family") comets. For these reasons, there is great interest in determining the compositions of the Kuiper Belt Objects (KBOs) but, owing to the faintness of objects beyond Neptune, this has proved fantastically challenging even with the world’s largest telescopes.

In their December 9th Nature paper, David Jewitt (University of Hawaii) and Jane Luu (MIT Lincoln Lab) presented the first high quality spectrum of bright KBO (50000) Quaoar taken with the CISCO spectrometer at the Subaru 8-m telescope on Mauna Kea.

The crystallinity of the ice indicates something about the temperature history of the ice.

Artist's rendition of Kuiper belt
Artist’s rendition of Kuiper belt.
Credit: David Jewitt

The surface temperature of Quaoar is only 50 K (-220 C) and, at these low temperatures, the thermodynamically preferred form of ice is amorphous (meaning "structureless": the water molecules freeze where they stick in a jumbled pattern). The Subaru spectrum shows instead that the ice is crystalline (from the 1.65 micron absorption band, which is weak or absent in amorphous ice). Crystalline ice, in which the water molecules are arranged in a regular lattice-like pattern, indicates formation at temperatures in excess of 110 K (about -160 C). The data show that the ice on Quaoar has at some time been raised in temperature above 110 K, the critical temperature for transformation from amorphous to crystalline.

Two ways to heat the ice are 1) to form it at temperatures above 110 K, presumably beneath the frigid surface, and then somehow expose it to view from Earth. Warm ice could be excavated by impact from deeper layers, or blown onto the surface by low-level cryovolcanic outgassing through vents. 2) Ice on the surface could be heated above 110 K by micrometeorite impact.

The artist’s rendition shows "Quaoar" in relation to other bodies in the solar system, including Earth and its Moon; Pluto; and Sedna, a planetoid beyond Pluto that is the largest known object beyond Pluto.
Image Credit: NASA/JPL-Caltech

The extra wrinkle is that crystalline ice, however it came to be emplaced on the surface of Quaoar, is itself unstable. Bombardment by energetic particles from the solar wind and by cosmic rays breaks the bonds between molecules in the crystalline lattice producing amorphous ice.

The timescale for this "back-conversion" of crystalline to amorphous ice is uncertain but probably on the order of 10 Myr for the surface ice. 10 Myr is effectively "yesterday" compared to the 4500 Myr age of the solar system. This means that whatever process emplaces the crystalline ice (basically either impact gardening or cryovolcanic outgassing) has been active in the immediate past and, indeed, is probably still active.

A two-step scenario seems plausible. First, buried ice is heated by radiogenic decay in the interior of Quaoar. Ammonia in the ices depresses the melting temperature and crystalline water exudes onto the surface, much as we have seen in Voyager images of the icy (and crystalline) satellites of Uranus (e.g. 480 km diameter Miranda). This probably happened billions of years ago when radioactivity was a little stronger than it is now.

Orbit of new planet
Artist’s conception of orbit calculated relative to other planets in our solar system. Unlike Pluto, its orbit around the Sun is very circular, even more so than most of the planetary-class bodies in the solar system.
Credit: NASA and A. Feild (STScI)

Second, some resurfacing process (impact gardening or burps of gas with entrained ice crystals percolating to the surface) replenishes the optical surface on a much shorter (<10 Myr) timescale, erasing the destructive effects of energetic particle bombardment.

While the interpretation remains speculative, the good news is that the researchers are, for the first time, able to take useful spectra that reveal unexpected and intriguing properties of the surface of distant Quaoar.

Quaoar’s "icy dwarf" cousin, Pluto, was discovered in 1930 in the course of a 15-year search for trans-Neptunian planets. It wasn’t realized until much later that Pluto actually was the largest of the known Kuiper belt objects. The Kuiper belt wasn’t theorized until 1950, after comet orbits provided telltale evidence of a vast nesting ground for comets just beyond Neptune. The first recognized Kuiper belt objects were not discovered until the early 1990s.

This hard-to-pronounce planetoid was named after a creation god of the Tongva native American tribe, the original inhabitants of the Los Angeles basin. According to legend, Quaoar, "came down from heaven; and, after reducing chaos to order, laid out the world on the back of seven giants. He then created the lower animals, and then mankind."

Related Web Pages

Extrasolar Planets Encyclopedia
Fast Facts (Hubble Discovery)
Other Kuiper Belt Objects (KBO)
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Space Interferometry Mission
Voyager: Beyond the Great Beyond
Fire and Ice
Beyond Pluto: Ice Planet