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The Dark Red Spot on Dwarf Planet Haumea

Belfast, UK, September 2009 – A dark red area has been discovered on dwarf planet Haumea by measuring changes in its brightness as it rotates. This spot appears to be richer in minerals and organic compounds than the surrounding icy surface of Haumea. The origin of the spot is unknown but new clues are expected from observations planned to study its chemical composition.

Haumea orbits the Sun beyond Neptune, in a region known as the Kuiper belt. It is the 4th largest known Kuiper belt object (KBO) after Eris, Pluto and Makemake. These large KBOs, together with main-belt asteroid Ceres, are known as dwarf planets. One of the most surprising characteristics of Haumea is its very fast rotation, with one day lasting only 3.9 Earth hours. No other large object in the solar system spins as fast as Haumea. The rapid spin deforms Haumea into an elongated 2000×1600×1000 kilometre ellipsoid whose shape balances gravitational and rotational accelerations. It is believed that Haumea was spun up by a massive impact more than a billion years ago.

Because of its large distance from the Earth, Haumea is visible only as a rather uninformative point of light. Most of what we know about this object was derived from its brightness variations, or “light curve”. Because of its rotation and elongated shape, Haumea brightens and dims periodically as it reflects more and less sunlight. The extent of this variation tells us how elongated Haumea is, and the time between each brightening and dimming is a measure of the rotation period. The precise Haumea shape and spin period imply that it has a density 2.5 times that of water. Since we know from spectroscopic observations that Haumea is covered in water ice, this high density implies Haumea must have a rocky interior, in contrast with its bright icy surface.

But Haumea’s light curve has told us more. The two brightness maxima and the two minima are not exactly equal, as would be expected from a uniform surface. This indicates the presence of a dark spot on the otherwise bright surface. Furthermore, the light curve is not exactly the same shape in all wavelengths. Small but persistent differences tell us that the dark spot is slightly redder in visible light and slightly bluer at infrared wavelengths. “Our very first measurements of Haumea already told us there was a spot on the surface, but it was only when we got the infrared data that were we able to begin to understand what the spot might be” said Dr. Pedro Lacerda, Newton Fellow at Queen’s University Belfast.

Possible interpretations of these measurements are that the spot is richer in minerals and organic compounds, or that it contains a higher fraction of crystalline ice. If the spot is a scar of a recent impact onto Haumea then the spot material might resemble the composition of the impactor, perhaps mixed with material from the inner layers of Haumea.

New observations of this spot are planned for early 2010 using the ESO Very Large Telescope. “Now we will get detailed spectroscopy of the spot to hopefully identify its chemical composition and solve the puzzle of its origin” Lacerda concluded.

More information can be found here and the papers describing this discovery in more detail can be found here and here.

Contact information: Pedro Lacerda (+44 2890 973708).

Above — Artist's rendition of Haumea and its dark red spot.

Below — Lightcurve of dwarf planet Haumea in two wavelengths, blue and red. The regular, quasi-sinusoidal shape of the lightcurve, together with the rapid rotation (period P=3.9 hr) are strong indications that this object is elongated like a rugby ball. The two features on this lightcurve that reveal the existence of the dark red spot are the two different peak heights and trough depths, and the misalignment of the red and blue data points between rotational phases 0.7 and 1.0 hr.


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