Astronomical Observations of Volatiles on Asteroids

Abstract

We have long known that water and hydroxyl are important components in meteorites and asteroids. However, in the time since the publication of Asteroids III, evolution of astronomical instrumentation, laboratory capabilities, and theoretical models have led to great advances in our understanding of H 2 O/OH on small bodies, and spacecraft observations of the Moon and Vesta have important implications for our interpretations of the asteroidal population. We begin this chapter with the importance of water/OH in asteroids, after which we will discuss their spectral features throughout the visible and near-infrared. We continue with an overview of the findings in meteorites and asteroids, closing with a discussion of future opportunities, the results from which we can anticipate finding in Asteroids V. Because this topic is of broad importance to asteroids, we also point to relevant in-depth discussions elsewhere in this volume.1 Water ice sublimation and processes that create, incorporate, or deliver volatiles to the asteroid belt Accretion/Solar system formation 1.1The concept of the "snow line" (also known as "water-frost line") is often used in discussing the water inventory of small bodies in our solar system. The snow line is the heliocentric distance at which water ice is stable enough to be accreted into planetesimals. The placement of the snow line has varied in different models. A location just inside Jupiter helps explain the greater mass of the giant planets as they accrete a larger fraction of the mass in the solar nebula. Although the location of the snow line before and during planet formation is uncertain, some studies show it may have fallen within the asteroid belt (Lunine, 2006). The location of the snow line in our solar system coinciding with the asteroid belt is consistent with observations of disks around other stars (e.g., Su et al., 2013), where planetesimal rings tend to coincide with the snow line around those stars. However, it seems the relatively simple concept of a static snow line requires revisions in light of new results on several fronts. For instance, the snow line likely moved with time as the Sun's luminosity changed early in solar system history (e.g., Martin and Livio, 2012). Even though there is no agreement on the details of planetesimal formation and growth, some models favor the growth of ~100 km-scale planetesimals directly from cm-scale pieces Cuzzi et al., 2010; for a dissenting view see Weidenschilling, 2011). Hence, the timing of those assemblies as conditions change could have a strong influence on the amount of radioactive elements present in a planetesimal's interior and on its rock-to-ice ratio, further complicating the concept of a snow line. Recent observations also point to a more complex picture. For example, the concept of an asteroid-comet continuum, where there is no clear boundary between primitive asteroids and cometary nuclei (e.g., Gounelle, 2011) is gaining support and is consistent with the intermittent cometary behavior of some main belt and near-E...

2P/Encke, the Taurid complex NEOs and the Maribo and Sutter’s Mill meteorites

“…Aqueous alteration of metal and silicate phases resulted in the formation of phyllosilicates and iron alteration minerals. The altered phases are seen as absorption features in reflected light at 700 nm and 3 µm (Rivkin et al 2002). Since our observations cover the wavelength range 400 -900 nm, we should therefore be able to observe the 700 nm, but not the 3 µm feature.…”

2004 EW95: A Phyllosilicate-bearing Carbonaceous Asteroid in the Kuiper Belt

“…Specifically, 2004EW 95 resembles a hydrated C-type asteroid (Bus & Binzel 2002b;DeMeo et al 2009;Vernazza et al 2016;we compare 2004 EW 95 to the asteroid 38 Leda in Figure 2). The shallow optical absorption feature at ∼700nm defines the hydrated Ch/Cgh asteroid subclasses, being observed in ∼30% of C-types (Rivkin 2012), and has been associated with charge transfer in aqueously altered silicate material (Vilas & Gaffey 1989;Fornasier et al 2014;Rivkin et al 2015). To characterize the ∼700nm feature and test the significance of its detection, we first remove a linear continuum slope of 3.6% per 100nm that was determined from a linear fit to the 530-580nm and 850-900nm wavelength ranges.…”

The Main Belt Comets and ice in the Solar System

“…Searches for water and OH features in near-Earth objects (NEOs) were upended by the discovery of OH in the lunar regolith by three spacecraft Pieters et al 2009;Clark 2009). Detections of water and/or hydroxyl on large asteroids have been reported for (175706) 1996 FG3 (Rivkin et al 2015), (16) Psyche (Takir et al 2017), and (433) Eros and (1036) Ganymede . Although smaller (km-scale) asteroids are generally too faint for spectroscopy in the 3 µm region, the activity of MBCs offers evidence that water is also present in at least some smaller asteroids.…”

“…To date, only a few detections of surface ice in asteroids have been reported. On the other hand, hydrated minerals have been widely observed in asteroids (Rivkin et al 2015).…”

“…However, the 3 µm region is notoriously difficult for ground-based observations because of intense atmospheric absorptions and faint solar radiation in this wavelength regime. Nevertheless, spectral surveys of large asteroids in the 3 µm region have flowered, mostly using the SpeX instrument at the IRTF telescope (Takir and Emery 2012;Rivkin et al 2015).…”