What are the implications of an active S-type object from the Oort cloud?

When Jan Oort (6) formulated his model of the Oort cloud, he inferred that too few comets were on their return passage through the solar system. He proposed that the comets faded as a result of physical evolution and volatile loss. However, it is expected that volatiles in ice-rich bodies should be able to sustain cometary activity for up to 1000 perihelion passages (3). Furthermore, models for the number of Oort cloud comets that enter the inner solar system and subsequently fade predict a large number of dormant isotropic long-period comets that are not seen (7). Thus, most Oort cloud comets must be lost because they physically disrupt, not because they become inactive because of volatile loss. Nevertheless, it cannot be excluded that some comets become inactive. The point is that inactive comets should have a D- or P-type spectrum, not an S-type reflectivity (8). Accordingly, we can conclude that C/2014 S3 is not an almost-extinct comet.

C/2014 S3 is not the first nearly inactive object on a long-period comet orbit to be found. The first, discovered by the Near-Earth Asteroid Tracking (NEAT) search, was 1996 PW (9). 1996 PW generated only moderate attention observationally, with observations indicating that it was not active, was red, had a radius of between 4 and 8 km, and was a slow rotator. An exploration of the dynamical history of 1996 PW to assess whether it was an extinct Oort cloud comet, an Oort cloud asteroid, or something more recently ejected outward, such as an extinct ecliptic comet or a main belt asteroid, showed that it was equally probable that 1996 PW was an extinct comet or an asteroid ejected into the Oort cloud during the early evolution of the solar system (3). More recently, other Manx candidates have been discovered. We have observed five of them, which also show comet-like red colors similar to 1996 PW. C/2014 S3 is the first and only Manx candidate to date with an S-type reflectivity spectrum.

What are the implications of seeing a low-level potentially volatile-driven activity from an object with an S-type spectrum on a returning long-period comet from the Oort cloud? Widespread evidence indicates aqueous alteration throughout primitive asteroids originating in the outer asteroid belt (10, 11). Evidence also suggests that water may still be present in the outer asteroid main belt—observable as outgassing from main belt comets (12) and dwarf planets (13) or as ice on the surfaces of asteroids (14).

S-type asteroids, which are dominant in the inner main asteroid belt today and formed from inner solar system material, are clearly inactive and are neither expected nor observed to have ice (15). Cosmochemical studies of meteorites have shown that many classes of meteorites underwent extensive aqueous processing in their parent bodies. The hydrated C- and D-type asteroids are commonly associated with carbonaceous chondrite meteorites (16), but the best match between meteorite classes and the S-type asteroids comes from the ordinary chondrites (OCs). This was confirmed with Hayabusa mission samples returned from the S-type asteroid Itokawa (17, 18).

The 1-μm and 2-μm absorption bands in the near-infrared (NIR) and the visible NIR spectral slope are used to interpret surface mineralogies of S-type asteroids. The relative band centers and depths can be used to assess the relative abundance of olivine-pyroxene and the Fe2+ and Ca2+ contents in these minerals. The spectral slope in the 1- to 2-μm region is related to the FeNi metal content and olivine abundance (19). The wide range of S-type mineralogical variations have been divided into subclasses, with the S(IV) class matching C/2014 S3 best (Fig. 1). The S(IV) class most likely represents the parent bodies of the OC meteorites. The silicates (olivine and pyroxene) inferred for the S(IV) asteroids are similar to unequilibrated OCs (UOCs) (19). The UOCs are the most primitive of the OCs, never reaching very high temperatures (15). The UOCs have olivine/(olivine + low-calcium pyroxene) ratios, which are manifested in a very shallow 1-μm band similar to that seen in (3) Juno and (7) Iris (20). The surface of C/2014 S3 appears to be consistent with more primitive S-type material.

Chondrite accretion ages and the conditions under which they were aqueously altered can be used to constrain where they accreted. Previously, because of the absence of carbonates in OCs and the lack of proper standards, there had been no reliable ages for aqueous activity for OC parent bodies. New work on an L3 chondrite (one of the most pristine OCs) has now shown the presence of fayalite, a secondary mineral that is a product of aqueous alteration (21). The mineralogy and thermodynamic analysis of the sample showed that the fayalite was consistent with formation at low temperatures and a low water/rock mass ratio (0.1–0.2)—much lower than measured values in comets (22). Accretion ages between 1.8 and 2.5 million years after the formation of the first solar system solids for the L-parent bodies, and the ages at which the aqueous secondary minerals formed, suggest that some water ice was incorporated into the accreting parent bodies and that they accreted close to the protoplanetary disk snowline (21). The snowline likely varied in position over time, but many models suggest that, toward the end of the protoplanetary disk phase, it could have been within the terrestrial planet–forming region (23). This is close to where the OC parent bodies were believed to be accreted. Although the minerals in the OCs are anhydrous and formed under dry conditions, it is possible that they could have acquired some water later.

These observations may tie together other reported observations that appeared to contradict the current understanding of solar system formation. There was clear evidence in the comet dust samples from the Stardust mission that there had been substantial radial migration in the protoplanetary disk, with comet dust having seen regions of high temperature (24). Fluid water inclusions have been found in the Monahans OC, and one possible explanation for this was that water was exogenously delivered after it formed (25). The Orgueil meteorite has long been considered a candidate for a possible cometary origin or from a body rich in volatiles (26). The discovery of C/2014 S3, an object on a cometary orbit that has the characteristics of an inner solar system asteroid, can offer new ideas about the relation between meteorites and their sources.

The discovery of C/2014 S3 on the orbit of an Oort cloud comet, made of minimally thermally processed rocky S-type material strongly suggests that this object is one of the interlopers predicted by the various dynamical evolution models of the early solar system. These models make predictions about the amount of inner solar system material that could reside in the Oort cloud as a result of scattering by the giant planets, and these predictions radically differ depending on the initial mass of the asteroid population that these models assume/imply. Assessing how many S-type objects exist will be a strong test of these models. To unambiguously select between dynamical models, we need to characterize 50 to 100 Manx objects; the number of S-types found will distinguish between the models (see section S6 for a statistical assessment of the number of objects to be observed).