Compositional interpretation of the geometric albedo of asteroids
I. Solar phase effects
Observatório Nacional (COAA), rua Gal. José Cristino 77, São Cristóvão, CEP20921-400 Rio de Janeiro RJ, Brazil e-mail: carvano@on.br
Received:
23
January
2008
Accepted:
19
March
2008
Aims. In this first paper we investigate the dependence of the geometric albedo on the phase function of the particles that cover it, and derive the expected geometric albedo of bodies for a given mineralogy, taking into account the constraints imposed by the observed phase functions of the asteroids.
Methods. A genetic fitting algorithm is used to fit Hapke integral phase functions to Lumme-Bowell integral phase functions described by values of the slope parameter G of the IAU HG system. The resulting geometric albedo of laboratory samples are then compared to the observed values of asteroids with assumed similar mineralogy.
Results. Because of the weak dependence of 
on the integral phase functions it is not
possible to find a unique set of Hapke parameters that fit the Lumme-Bowell
function for a given value of G, at least for phase angles <60°. Instead,
unique solutions can be found if we leave as a free
parameter. It is shown that the laboratory derived scattering parameters in
general fail to match the geometric albedo and slope parameter of asteroids
of presumed equal mineralogy. It is also shown that a given value of the
single scatter albedo can lead to very different values of pv, depending on G and . The methodology developed is used to compare the
observed pv and G of the asteroids (4) Vesta and (21) Lutetia with
laboratory measurements of materials with suposedly similar compositions. As
expected, it is found that the albedo and slope parameter of Vesta are
compatible with measurements of unweathered terrestrial basalts with grain
sizes <= 250 μm. The albedo and slope parameter of Lutetia are found to be
compatible with samples of the Allende CV3 meteorite for grain sizes <500 μm. The routines that allow the conversion between w and pV (and
vice-versa) are available at http://funk.on.br/ carvano/albedo/albedo.html
Key words: minor planets, asteroids
© ESO, 2008