The classical paradigm divides minor bodies in the Solar System into two groups – comets and asteroids which should be distinguishable due to the presence or lack of sublimative activity and shape of their orbits. However, this image has been blurred with the discovery of the first known active asteroid in 1996 and the distinction of a new group of Minor Belt Comets (MBC) – small objects that have comet-like tails, but asteroidal orbits. Revealing the process behind their activity may have some far-reaching consequences about the evolution of our Solar System.
In general, the gaseous contribution of comet activity is detected directly through emission lines originating from the fluorescence of numerous gas compounds interacting with solar radiation. Quantitative measurements of these gases are widely used in planetary science to derive the presence of various parent volatile ices in the comet nucleus. Due to strong emission in the visible spectrum (especially, usually very prominent band at 388 nm), by far the most extensively evaluated emission lines comes from CN radicals. However, despite their brightness, none of them have been observed in a MBC yet. Even with fast development of ground-based telescopes in recent years, there is a little chance to detect any but the strongest emission lines from Earth considering how faint MBCs are in comparison to other periodic comets with gaseous compounds measured to date. The significance of the CN emission is all the greater as its production rates can be used as an estimator of water vapor abundance, however it is based on the assumption that the ratio of H2O and CN production rate is typical for a given comet population. For instance, in the coma of an average Jupiter-family comet it is estimated around Q(H2O)/Q(CN)=350-360, even though it may vary from one comet to another and is not necessarily conserved throughout a comet’s orbit.
And still, despite the non-detection of spectroscopic traces supporting gas emission, water ice sublimation remains by far the most likely explanation for MBCs recurrent activity. As emission lines cannot be measured directly, we must rely on upper limits of the CN production rates (and the ensuing water production rates) derived for several such objects using one of the largest telescopes, such as: Keck, the Very Large Telescope (VLT), Gemini, and the Gran Telescopio Canarias (GTC).
Most of these objects have exhibited some signs of activity during more than one succeeding apparition. 313P displays a fan-shaped tail lasting at least 3 months, while 358P revealed antisolar tail and diffuse coma, resulting from a couple of months of activity. Corresponding upper limits for CN production rates are 1.8×1023 mol s-1 and 1.5×1023 mol s-1 respectively. Jewitt et al. (2009) set a very similar upper limit Q(CN)=1.4×1023 mol s-1 for 259P, which also exhibited dust tail and remained active for about a month. Dust trail of 288P was aligned with the object’s orbital plane, indicating a long-duration emission event, likely due to a sublimative activity. On the other hand, activity of P/2013 R3 is caused by the fragmentation taking place successively over a period of many months. Although rotational break-up is the most likely scenario, the dust activity of the fragments may have been a result of newly exposed ice sublimation. Note that these upper limits are in agreement with current common measures for comets near 1 au, where detectable Q(CN) reaches a couple times 1023 mol s-1. Hence estimated production rates from ‘typical’ cometary H2O/CN ratio are Q(H2O) = 1025 – 1026 mol s-1, which is significantly lower than for an average Jupiter-family comet at similar heliocentric distance.
Another challenge is to distinguish ongoing long-lasting activity from an impulsive event with dispersing dust remnants. For the latter case, these estimates should not be valid. Models of dust coma morphology can face the problem of differentiating between these phenomena. We shall consider that Q(H2O)/Q(CN) ratio for MBCs may vary significantly from values expected from ‘regular’ comets, as these estimates are merely a result of averaging production rates for well-studied bodies and, moreover, the emission of H2O and CN are chemically unrelated, thus limiting the applicability of this approximation. Direct observations of Q(H2O) seems to be an ideal solution to this problem. There were two attempts to directly detect water vapor with the Herschel Space Observatory, however, the mission of this telescope has already came to an end, leaving upper limits on two MBC’s water production rates: Q(H2O)=4×1025 mol s-1 for 176P and Q(H2O)=7.6×1025 mol s-1 for 358P. Independent direct limit on Q(H2O) from VLT’s non-detection of the photo-dissociation products of water (OH, O) is consistent with Herschel data – 8×1025 mol s-1. Both direct measures and indirect estimates sets MBC’s water upper limit on 1×1026 mol s-1 at most and several times 1025 mol s-1 on the average. That is still at least an order of magnitude less than the lowest water production rates determined from SOHO SWAN all-sky Lyman-α detector (~1027 mol s-1) and these were restricted only for bright comets very close to SOHO and to the Sun, which certainly is not the case of MBCs.