A Cheaper, Faster, Better Way to Detect Water of Hydration on Solar System Bodies

Abstract

The 3.0-μm water of hydration absorption feature observed in the IR photometry of many low-albedo and some medium-albedo asteroids strongly correlates with the 0.7-μm Fe 2+ → Fe 3+ oxidized iron absorption feature observed in narrowband spectrophotometry of these asteroids. Using this relationship, an empirical algorithm for predicting the presence of water of hydration in the surface material of a Solar System body using photometry obtained through the Eight-Color Asteroid Survey v (0.550 μm), w (0.701 μm), and x (0.853 μm) filters was developed and applied to the ECAS photometry of asteroids and outer planet satellites. The percentage of objects in low-albedo, outer main-belt asteroid classes that test positively for water of hydration increases from P → B → C → G class and correlates linearly with the increasing mean albedos of those objects testing positively. The aqueous alteration sequence in the Solar System thus ranges from P-class asteroids, which represent the least-altered objects created at temperatures attained at the onset of aqueous alteration, to G-class asteroids, which represent the upper range of the alteration sequence. The medium-albedo M-class asteroids do not test positively in large number using this algorithm. These objects could represent a mixture of metals with phyllosilicates that have been significantly altered to the extent that iron has been leached out of the phyllosilicates and sequestered in opaque phases such as magnetite or iron sulfides. Spatial variation in the presence of water of hydration across the surface of some asteroids is suggested in the variability of the water of hydration test for observations of many asteroids on different nights, although an effort to correlate variation with rotational period is unsuccessful due to the small sample of eligible asteroids. Aqueously altered asteroids dominate the Solar System population between heliocentric distances of 2.6 to 3.5 AU, bracketing the Solar System region where the aqueous alteration mechanism operated most strongly. One jovian satellite, J VI Himalia, and one saturnian satellite. Phoebe, tested positively for water of hydration, supporting the hypothesis that these may be captured C-class asteroids from a postaccretional dispersion. The proposed testing technique could be applied to an Earth-based survey of asteroids or a space-probe study of an asteroid's surface characteristics in order to identify a potential water source.