Size Distribution Of Kuiper Belt Objects Research Articles

Craters of the Pluto-Charon system

NASA's New Horizons flyby mission of the Pluto-Charon binary system and its four moons provided humanity with its first spacecraft-based look at a large Kuiper Belt Object beyond Triton. Excluding this system, multiple Kuiper Belt Objects (KBOs) have been observed for only 20 years from Earth, and the KBO size distribution is unconstrained except among the largest objects. Because small KBOs will remain beyond the capabilities of ground-based observatories for the foreseeable future, one of the best ways to constrain the small KBO population is to examine the craters they have made on the Pluto-Charon system. The first step to understanding the crater population is to map it. In this work, we describe the steps undertaken to produce a robust crater database of impact features on Pluto, Charon, and their two largest moons, Nix and Hydra. These include an examination of different types of images and image processing, and we present an analysis of variability among the crater mapping team, where crater diameters were found to average ±10% uncertainty across all sizes measured (∼0.5–300km). We also present a few basic analyses of the crater databases, finding that Pluto's craters' differential size-frequency distribution across the encounter hemisphere has a power-law slope of approximately –3.1±0.1 over diameters D ≈ 15–200km, and Charon's has a slope of –3.0±0.2 over diameters D ≈ 10–120km; it is significantly shallower on both bodies at smaller diameters. We also better quantify evidence of resurfacing evidenced by Pluto's craters in contrast with Charon's. With this work, we are also releasing our database of potential and probable impact craters: 5287 on Pluto, 2287 on Charon, 35 on Nix, and 6 on Hydra.

Craters and ejecta on Pluto and Charon: Anticipated results from the New Horizons flyby

We examine the flux of bodies striking Pluto and Charon, and the nature of the crater populations that will form as a result of these impacts. Assuming impact speeds of 2km/s and an impact angle of 45°, a 1km impactor will form a 4.2km diameter transient crater on Pluto, and a ∼5.0km crater on Charon, as compared with 8–13km for several mid-sized saturnian satellites and 8–10km for the icy Galilean satellites. We predict that secondary craters will be present in the crater size–frequency distribution (SFD) for Pluto and Charon at sizes less than a few km, at spatial densities comparable to the range seen on the mid-sized saturnian satellites and distinctly less than seen on the icy Galilean satellites. Pluto should have more secondary craters formed per primary impact than Charon, so if neither crater population on these bodies is in saturation, Charon’s crater SFD should be the “cleanest” reflection of the primary, impacting SFD. Ejecta from Pluto and Charon escape more efficiently from the combined system, relative to ejecta from a satellite in orbit around a giant planet, due to the absence of a large central body. We estimate that Kuiper Belt Objects (KBOs) with diameters larger than 1km should strike Pluto and Charon on (nominal) timescales of 2.2 and 10 million years, respectively. These estimates are uncertain because the numbers of small KBOs are poorly constrained. Our estimated rates are smaller than earlier predictions of impact rates, primarily because we assume a KBO size distribution that is shallower overall than previous studies did. The impact rate, combined with the observed crater SFD, will enable estimates of relative and absolute age of different geologic units, should different geologic units exist. We explore two scenarios in regards to the crater population: (1) a shallow (differential power-law index of p∼2, i.e. for dN/dD∝D-p), based on the crater SFD observed on young terrains of Galilean and saturnian satellites; and (2) a slightly steeper SFD (p∼3), based on extrapolations of larger (∼100km) KBOs from ground-based surveys. If the observed primary crater SFD, at diameters less than a few tens of km, is consistent with a differential power-law index p∼2, that will confirm that KBOs are deficient in small bodies relative to extrapolations from known ∼100km KBOs, consistent with expectations derived from examination of crater populations in young terrains on the Galilean and saturnian satellites. If the crater SFD has p⩾3 over all observed sizes, then that power-law index applies across the KBO population over at least two orders of magnitude (1km to100km objects), and there must be some process that erodes the small KBOs when they migrate to the Jupiter–Saturn region of the Solar System. Whatever SFD is observed, the primary crater population on Pluto and Charon will provide the strongest constraint on the SFD of small KBOs, which will be beyond the observational reach of ground- and space-based telescopes for years to come. This, in turn, will provide a fundamental constraint for further understanding of the evolution of this distant and compelling population of bodies beyond Neptune.

THE TAOS PROJECT: RESULTS FROM SEVEN YEARS OF SURVEY DATA

The Taiwanese-American Occultation Survey (TAOS) aims to detect serendipitous occultations of stars by small (about 1 km diameter) objects in the Kuiper Belt and beyond. Such events are very rare (<0.001 events per star per year) and short in duration (about 200 ms), so many stars must be monitored at a high readout cadence. TAOS monitors typically around 500 stars simultaneously at a 5 Hz readout cadence with four telescopes located at Lulin Observatory in central Taiwan. In this paper, we report the results of the search for small Kuiper Belt Objects (KBOs) in seven years of data. No occultation events were found, resulting in a 95% c.l. upper limit on the slope of the faint end of the KBO size distribution of q = 3.34 to 3.82, depending on the surface density at the break in the size distribution at a diameter of about 90 km.

MEASURING THE ABUNDANCE OF SUB-KILOMETER-SIZED KUIPER BELT OBJECTS USING STELLAR OCCULTATIONS

We present here the analysis of about 19,500 new star hours of low ecliptic latitude observations (|b| < 20 deg) obtained by the Hubble Space Telescope's FGS over a time span of more than nine years; which is an addition to the 12,000 star hours previously analyzed by Schlichting et al. (2009). Our search for stellar occultations by small Kuiper belt objects (KBOs) yielded one new candidate event corresponding to a body with a 530 +/-70m radius at a distance of about 40AU. Using bootstrap simulations, we estimate a probability of approx 5%, that this event is due to random statistical fluctuations within the new data set. Combining this new event with the single KBO occultation reported by Schlichting et al. (2009) we arrive at the following results: 1) The ecliptic latitudes of 6.6 deg and 14.4 deg of the two events are consistent with the observed inclination distribution of larger, 100km-sized KBOs. 2) Assuming that small, sub-km sized KBOs have the same ecliptic latitude distribution as their larger counterparts, we find an ecliptic surface density of KBOs with radii larger than 250m of N(r>250m) = 1.1^{+1.5}_{-0.7} x 10^7 deg^{-2}; if sub-km sized KBOs have instead a uniform ecliptic latitude distribution for -20 deg < b< 20 deg then N(r>250m) = 4.4^{+5.8}_{-2.8} x 10^6 deg^{-2}. This is the best measurement of the surface density of sub-km sized KBOs to date. 3) Assuming the KBO size distribution can be well described by a single power law given by N(>r) \propto r^{1-q}, where N(>r) is the number of KBOs with radii greater than r, and q is the power law index, we find q=3.8+/-0.2 for a KBO ecliptic latitude distribution that follows the observed distribution for larger, 100-km sized KBOs. 4) Regardless of the exact power law, our results suggest that small KBOs are numerous enough to satisfy the required supply rate for the Jupiter family comets. (Abridged)

THE TAOS PROJECT: UPPER BOUNDS ON THE POPULATION OF SMALL KUIPER BELT OBJECTS AND TESTS OF MODELS OF FORMATION AND EVOLUTION OF THE OUTER SOLAR SYSTEM

We have analyzed the first 3.75 years of data from TAOS, the Taiwanese American Occultation Survey. TAOS monitors bright stars to search for occultations by Kuiper Belt Objects (KBOs). This dataset comprises 5e5 star-hours of multi-telescope photometric data taken at 4 or 5 Hz. No events consistent with KBO occultations were found in this dataset. We compute the number of events expected for the Kuiper Belt formation and evolution models of Pan & Sari (2005), Kenyon & Bromley (2004), Benavidez & Campo Bagatin (2009), and Fraser (2009). A comparison with the upper limits we derive from our data constrains the parameter space of these models. This is the first detailed comparison of models of the KBO size distribution with data from an occultation survey. Our results suggest that the KBO population is comprised of objects with low internal strength and that planetary migration played a role in the shaping of the size distribution.

First Results from the Taiwanese-American Occultation Survey (TAOS)

Results from the first 2 years of data from the Taiwanese-American Occultation Survey (TAOS) are presented. Stars have been monitored photometrically at 4 or 5 Hz to search for occultations by small (~3 km) Kuiper Belt objects (KBOs). No statistically significant events were found, allowing us to present an upper bound to the size distribution of KBOs with diameters 0.5 km < D < 28 km.

A Search for Small Kuiper Belt Objects by Stellar Occultations

We report the conditions and results of an observation campaign organized in 2000 September at the Pic du Midi Observatory, and dedicated for the first time to the study of Kuiper belt objects (KBOs) by stellar occultations. The observation consisted of recording the flux of a well-chosen star with a fast photometer (20 Hz) and counting occultations of this star by passing KBOs. The campaign provided 15 hr of good-quality signal (rms σ ~ 1.8%) and zero detections of KBOs at a 4 σ detection level. For a KBO differential size distribution assumed to vary as r-q, this first result suggests a slope q ≲ 4.5. A refined analysis of the data, studying diffraction patterns, allowed us to find an event at a 3 σ detection level compatible with a 150 m KBO. More generally, observation campaigns of stellar occultation by KBOs on ≥2 m class telescopes could statistically constrain the slope and the expected turnover radius due to collisional erosion of the subkilometer KBO size distribution.

Occultation Searches for Kuiper Belt Objects

The occultation of background stellar sources by foreground Kuiper belt objects (KBOs) can be used to survey physical properties of the KBO population. We discuss statistics related to a KBO occultation survey, such as the event duration distribution, and suggest that occultation searches can be effectively used to probe the KBO size distribution below 10 km. In particular, we suggest that occultation surveys may be best suited to search for a turnover radius in the KBO size distribution because of collisions between small-size objects. For occultation surveys that monitor stellar sources near the ecliptic over a few square degrees, with time sampling intervals of the order of 0.1 s and sensitivity to flux variations of a few percent or more, a turnover radius between 0.1 and 1.0 km can be probed. While occultation surveys will probe the low-radius limit and imaging surveys will detect KBOs of size 100 km or more, statistics of objects with sizes in the intermediate range of around 1-100 km will likely remain unattainable.

Color diversity among Kuiper belt objects: The collisional resurfacing model revisited

A re-evaluation of the collisional resurfacing model based on up-to-date Kuiper belt objects size distribution and a more precise treatment of the cosmic-ray environment at the outer Solar System is presented. The result of the irradiation due to cosmic-rays with different energies altered in a different way the material of the objects, producing, under certain conditions, a thick irradiation mantle. Since the collisional resurfacing model is based in the competition between darkening by cosmic-rays and resurfacing due to impacts, the color of objects in different regions of the belt could vary if the projectile populations in those regions are truncated at a different radius.

A Detection Method for Small Kuiper Belt Objects: The Search for Stellar Occultations

We explore the possibility of detecting small Kuiper belt objects (KBO) by serendipitous observation of stellar occultations: We show that such unpredictable occultations may allow us to detect a population of very small objects (typically of ∼100 m radius at 40 AU), invisible by any other observational method, as long as (i) the assumed population fills up a sufficient area on the sky plane, (ii) the instrumental sensitivity and acquisition frequency are high enough, and (iii) the observed star has a small angular radius. This result is basically due to the diffractive broadening of the geometric shadow of small (assumed numerous) occulting objects. This diffractive broadening is more pronounced for smaller stellar disks and better photometric precision. Assuming there exist about 10 11 objects of radius ρ>1 km, located between 30 and 50 AU near the Ecliptic, and that the differential size distribution varies as ρ − q with the index q=4 extending down to decameter-sized objects, we expect a number of valid occultations (i.e., a 4σ event) between a few to several tens per night, if we may obtain an rms signal fluctuation σ⋦1% and observe a star in the ecliptic with an angular radius ⋦0.01 mas. Since this occultation rate is very sensitive to the index slope q and plummets when q⋦3, a KBO occultation observation campaign could provide a decisive constraint on the actual slope of the KBO size distribution for subkilometer-sized objects. Blue O class stars are the best candidates for detecting KBOs since they have the smallest angular radius for a given visual magnitude. The occultation events are typically very brief (⋦1 s) and they are shorter but more numerous when observed in the antisolar direction, so rapid photometry (>1 Hz) is required and high-speed photometry (⋧20 Hz) is preferred. The French space mission Corot will provide an excellent opportunity to observe occultations by KBOs using high precision photometry.