“Water” abundance at the surface of C-complex main-belt asteroids

Abstract

Recently published space-based observations of main-belt asteroids with the AKARI telescope provide a full description of the 3-µm band, related to the presence of OH bearing minerals. Here, we use laboratory spectra of carbonaceous chondrites obtained under controlled atmosphere (CI,CM,CO,CV,CR Tagish Lake) to derive spectral metrics related to the water content in the samples. After testing several spectral metrics, we use a combination of band depth at 2.75 µm and 2.80 µm that shows a correlation with [H2O] in the sample determined by TGA, though with a high uncertainty (4 wt. % H2O). This relation is used to determine water content at the surface of large C-complex mainbelt asteroids and discuss the origin of the variability found. On average C-complex Main-Belt Asteroids (MBA) have water contents of 4.5 wt.% (volume average, (1) Ceres excluded), significantly lower than average CM chondrites. The estimated water content for the most hydrated asteroids are lower than those of the most hydrated meteorites, a difference that could be attributed to space-weathering. An anti-correlation is also present between water content and overall spectral slope, which is opposite to expectation from laboratory simulations of space weathering on dark carbonaceous chondrites. This suggests that part of the variability in the surface hydration among the different C-complex asteroids is not due to space-weathering, but to the composition of surface material. When applied to Ceres, the hygrometer presented in this work enables us to estimate that at least 1.22 wt. % of the hydrogen is present in the form of organics. This richness in organics strengthens the connection between Ceres and cometary materials.

Metal pollution of the solar white dwarf by solar system small bodies

“…Within 10 au, objects of tens of km will become devoid of both free water and that embedded in rocks; large 100 km bodies may be able to retain a fraction of the embedded water. However, even for the relatively water-rich C-type asteroids, only 10% of their mass is contained in water (see a recent discussion by Beck et al 2021). Thus the loss of water will not affect the accretion rate significantly.…”

Thermal alteration of CM carbonaceous chondrites: Mineralogical changes and metamorphic temperatures

“…The visible and near-infrared (IR) reflectance spectra of heated CM chondrites closely resemble some low albedo C-type asteroids (Hiroi et al 1993(Hiroi et al , 1996Cloutis et al 2012), although it has been suggested that rather than being dehydrated these bodies never experienced widespread aqueous alteration (Vernazza et al 2015;Rivkin et al 2019). Nevertheless, estimates of the surface water content of many C-type asteroids fall within the range of heated CM chondrites (Rivkin et al 2003;Beck et al 2021). These relationships suggest that materials mineralogically and compositionally similar to heated CM chondrites could be a major constituent of the regolith on primitive asteroids.…”

Probing the surface environment of large T-type asteroids

“…rich in C, H, N, and O (Miyamoto & Zolensky 1994;Burbine et al 2002) that could have been relevant to habitability (Kwok 2016). Among CCs, CI (Ivuna-type) and CM (Migheitype) chondrites are of particular interest since they contain a prominent 3-µm OH absorption band, indicating aqueous alteration on their parent bodies (Lebofsky et al 1981;Beck et al 2010;Cloutis et al 2011a,b;Takir et al 2013;McAdam et al 2015;Beck et al 2021b). It is hard to directly compare asteroids and meteorites primarily due to the possible difference in sampling regions (i.e., observed asteroidal surfaces susceptible to space weathering in contrast to meteorites originating from any parts of the parent bodies) and observing geometry (Hiroi et al 2001;Jedicke et al 2004).…”