HYDRATED MINERALS ON ASTEROIDS: WHAT WE DO
AND DO NOT KNOW
A. S. RIVKIN1, E. S. HOWELL2, F. VILAS3, L. A. LEBOFSKY4
1
2
3
4
MIT
Arecibo Obs.
JSC
U. Arizona
Minerals containing water and hydroxyl give rise to diagnostic absorption
features centered in the 2.7-3.0 micrometer spectral region. Iron-bearing
phyllosilicates have absorption features near 0.7 micrometers. The former
feature has been observed on asteroids for over 20 years, the latter
systematically studied for a decade. Taken together or singly, these features
tell us a great deal about the surface mineralogy of asteroids, appropriate
parent bodies for meteorites, the thermal history of the asteroid belt, and even
the interiors of asteroids. Hydrated mineral absorptions are present on some,
but not all asteroids. While this seems to be a trivial statement, it shows that
hydrated minerals can exist at the surface of asteroids without being
destroyed, in agreement with meteoritical observations of hydrated minerals
in regolith breccias. The lack of a feature on some asteroids suggests that
this feature is not due to hydrated minerals brought in by micrometeorite
impacts. The feature is preferentially found in C-class and related asteroids,
thought to be akin to the carbonaceous chondrite meteorites, which are the
meteorite types most likely to contain hydrated minerals. Conversely, the
asteroids associated with anhydrous meteorite types such as the HED
meteorites (4 Vesta) and iron meteorites (16 Psyche and 216 Kleopatra) show
no absorption feature, as expected. There have also been several surprises
associated with these observations, however. Outer-belt asteroids, expected
to be the most primitive, show no hydrated mineral absorptions. This has
been interpreted as due to the water ice presumed to be in their interiors
remaining frozen, with temperatures too low for aqueous alteration to occur.
This places strong constraints upon the thermal history of the outer asteroid
belt. The M-class of asteroids has hydrated members, indicating that they
cannot all be iron-nickel metal in composition. Most asteroid spectral classes
have both hydrated and anhydrous members, hinting at the great variety of
mineralogies present in the asteroid belt. And at least one asteroid (1 Ceres)
shows evidence of ammoniated phyllosilicates, suggesting that volatiles other
than water may be found among the asteroids. Much work remains to be
done, however. Because of constraints imposed by the Earth's atmosphere,
observations in the 3-micrometer region have been limited to relatively bright
objects. A new generation of ground-based instruments and telescopes will
allow NEAs to commonly be observed where only the very brightest were
reachable before, and facilitate observations of smaller and more distant
objects. Laboratory studies and mixture modelling will help determine what
effect "space weathering" has (if any) on observations at 3 micrometers.
Impact codes will show whether asteroidal regoliths are dehydrated relative
to their subsurface, and to what extent. We will summarize how far we have
come in 23 years, and how far we hope to go in the next decade.