Emery, J. P.; Dalle Ore, C. M.; Licandro, J.; Pinilla-Alonso, N.; Lorenzi, V.
Bibliographical reference
Astronomy and Astrophysics, Volume 562, id.A85, 6 pp.
Description
Context. Models of the escape and retention of volatiles by minor icy
objects exclude any presence of volatile ices on the surface of
trans-Neptunian objects (TNOs) smaller than ~1000 km in diameter at the
typical temperature in this region of the solar system, whereas the same
models show that water ice is stable on the surface of objects over a
wide range of diameters. Collisions and cometary activity have been used
to explain the process of surface refreshing of TNOs and Centaurs. These
processes can produce surface heterogeneity that can be studied by
collecting information at different rotational phases. Aims: The
aims of this work are to study the surface composition of (20000)
Varuna, a TNO with a diameter 668+154-86 km and to
search for indications of rotational variability. Methods: We
observed (20000) Varuna during two consecutive nights in January 2011
with the near-infrared camera and spectrometer NICS at the Telescopio
Nazionale Galileo, La Palma, Spain. We used the low resolution mode with
the AMICI prism to obtain a set of spectra covering the whole rotation
period of the Varuna (Pr = 6.34 h). We fit the resulting relative
reflectance with radiative transfer models of the surface of
atmosphereless bodies. Results: After studying the spectra
corresponding to different rotational phases of Varuna, we did not find
any indication of surface variability at 2σ level. In all the
spectra, we detect an absorption at 2.0 μm, suggesting the presence
of water ice on the surface. We do not detect any other volatiles on the
surface, although the signal-to-noise ratio is not high enough to
discard their presence in small quantities. Based on scattering models,
we present two possible compositions compatible with our set of data and
discuss their implications in the framework of the collisional history
of the trans-Neptunian belt. Conclusions: We find that the most
probable composition for the surface of Varuna is a mixture of amorphous
silicates, complex organics, and water ice. This composition is
compatible with all the materials being primordial, so no replenishment
mechanism is needed in the equation. However, our data can also be
fitted by models containing up to a 10% of methane ice. For an object
with the characteristics of Varuna, this volatile could not be
primordial, so an event, such as an energetic impact, would be needed to
explain its presence on the surface.
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