Kristina M. Barkume

Photometric observations constraining the size, shape, and albedo of 2003 el61, a rapidly rotating, pluto-sized object in the kuiper belt

We present measurements at optical wavelengths of the spectral reflectance, rotational light curve, and solar phase curve of 2003 EL61. With apparent visual magnitude 17.5 at 51 AU from the Sun, this newly discovered member of the classical Kuiper Belt is now the third brightest KBO after Pluto and 2005 FY9. Our observations reveal an unambiguous, double-peaked rotational light curve with period 3.9154 ± 0.0002 hr and peak-to-peak amplitude 0.28 ± 0.04 mag. This is the fastest rotation period reliably determined for any body in the solar system larger than 100 km. Assuming the body has relaxed over time to the shape taken by a homogenous fluid body, our observations tightly constrain the shape and density. Given the mass we recently determined for 2003 EL61 from the orbit of a small satellite, we also constrain the size and albedo. We find a total length of 1960–2500 km, a mean density of 2600–3340 kg m^-3, and a visual albedo greater than 0.6. We also measure a neutral reflectance at visible wavelengths and a linear phase curve with slope varying from 0.09 mag deg^-1 in the B band to 0.13 mag deg^-1 in the I band. The absolute V-band magnitude is 0.444 ± 0.021.

A collisional family of icy objects in the Kuiper belt

The small bodies in the Solar System are thought to have been highly affected by collisions and erosion. In the asteroid belt, direct evidence of the effects of large collisions can be seen in the existence of separate families of asteroids—a family consists of many asteroids with similar orbits and, frequently, similar surface properties, with each family being the remnant of a single catastrophic impact. In the region beyond Neptune, in contrast, no collisionally created families have hitherto been found. The third largest known Kuiper belt object, 2003 EL61, however, is thought to have experienced a giant impact that created its multiple satellite system, stripped away much of an overlying ice mantle, and left it with a rapid rotation. Here we report the discovery of a family of Kuiper belt objects with surface properties and orbits that are nearly identical to those of 2003 EL61. This family appears to be fragments of the ejected ice mantle of 2003 EL61.

Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9

The spectrum of the bright Kuiper Belt object 2005 FY9 from 0.34 to 2.5μm is dominated by the red coloring of many outer solar system objects in the optical wavelength regime and by absorption due to methane in the near-infrared. The solid methane absorption lines are significantly broader on 2005 FY9 than on any other solar system body, indicating long optical path lengths through the methane. These long path lengths can be parameterized as a methane grain size of approximately 1 cm in a Hapke reflectance model. In addition to large-grained methane, the infrared spectrum also indicates the clear presence of ethane, an expected product of UV photolysis of methane. No evidence for N2 or CO, both known to be present on Pluto, is found. We suggest that the large differences between the spectrum of 2005 FY9 and that of Pluto and 2003 UB313 is due to a depletion of nitrogen on the surface of 2005 FY9 that leads to large methane grains, abundant sites for ethane formation through UV photolysis, and highly irradiated tholin-like material.

The Surface of 2003 EL61 in the Near-Infrared

We report the detection of crystalline water ice on the surface of 2003 EL61. Reflectance spectra were collected from the Gemini North telescope in the 1.0 to 2.4 μm wavelength range and from the Keck telescope across the 1.4-2.4 μm wavelength range. The signature of crystalline water ice is obvious in all data collected. Like the surfaces of many outer solar system bodies, the surface of 2003 EL61 is rich in crystalline water ice, which is energetically less favored than amorphous water ice at low temperatures, suggesting that resurfacing processes may be taking place. The near-infrared color of the object is much bluer than a pure water ice model. Adding a near-infrared blue component such as hydrogen cyanide or phyllosilicate clays improves the fit considerably, with hydrogen cyanide providing the greatest improvement. The addition of hydrated tholins and bitumens also improves the fit, but is inconsistent with the neutral V - J reflectance of 2003 EL61. A small decrease in reflectance beyond 2.3 μm may be attributable to cyanide salts. Overall, the reflected light from 2003 EL61 is best fit by a model of 2/3-4/5 pure crystalline water ice and 1/3-1/5 near-infrared blue component such as hydrogen cyanide or kaolinite. The surface of 2003 EL61 is unlikely to be covered by significant amounts of dark material such as carbon black, as our pure ice models reproduce published albedo estimates derived from the spin state of 2003 EL61.

NEAR-INFRARED SPECTRA OF CENTAURS AND KUIPER BELT OBJECTS

We present here an extensive survey of near-infrared (NIR) spectra of Kuiper belt objects (KBOs) and Centaurs taken with the Keck I Telescope. We find that most spectra in our sample are well characterized by a combination of water ice and a featureless continuum. A comparative analysis reveals that the NIR spectral properties have little correlation to the visible colors or albedo, with the exception of the fragment KBOs produced from the giant impact on 2003 EL61. The results suggest that the surface composition of KBOs is heterogeneous, though the exposure of water ice may be controlled by geophysical processes. The Centaurs also display diverse spectral properties, but the source of the variability remains unclear. The results for both the KBOs and the Centaurs point to inherent heterogeneity in either the processes acting on these objects or materials from which they formed.

Water Ice on the Satellite of Kuiper Belt Object 2003 EL61

We have obtained a near-infrared spectrum of the brightest satellite of the large Kuiper Belt object 2003 EL_(61). The spectrum has absorption features at 1.5 and 2.0 μm, indicating that water ice is present on the surface. We find that the satellites absorption lines are much deeper than water ice features typically found on Kuiper Belt objects. We argue that the unusual spectrum indicates that the satellite was likely formed by impact and not by capture.