Density Of Kuiper Belt Objects Research Articles

Using the density of Kuiper Belt Objects to constrain their composition and formation history

Telescopic observations of Kuiper Belt objects have enabled bulk density determinations for 18 objects. These densities vary systematically with size, perhaps suggesting systematic variations in bulk composition. We find this trend can be explained instead by variations in porosity arising from the higher pressures and warmer temperatures in larger objects. We are able to match the density of 15 of 18 KBOs within their 2σ errors with a constant rock mass fraction of 70%, suggesting a compositionally homogeneous, rock-rich reservoir. Because early 26Al would have removed too much porosity in small (∼ 100 km) KBOs we find the minimum formation time to be 4 Myr after solar system formation. This suggests that coagulation, and not gravitational collapse, was the dominant mechanism for KBO formation, or the gas disk lingered in the outer solar system. We also use this model to make predictions for the density of Makemake, 2007 OR10, and MU69

THE ROLE OF PEBBLE FRAGMENTATION IN PLANETESIMAL FORMATION. II. NUMERICAL SIMULATIONS

ABSTRACT Some scenarios for planetesimal formation go through a phase of collapse of gravitationally bound clouds of millimeter- to centimeter-size pebbles. Such clouds can form, for example, through the streaming instability in protoplanetary disks. We model the collapse process with a statistical model to obtain the internal structure of planetesimals with solid radii between 10 and 1000 km. During the collapse, pebbles collide, and depending on their relative speeds, collisions have different outcomes. A mixture of particle sizes inside a planetesimal leads to better packing capabilities and higher densities. In this paper we apply results from new laboratory experiments of dust aggregate collisions (presented in a companion paper) to model collision outcomes. We find that the internal structure of a planetesimal is strongly dependent on both its mass and the applied fragmentation model. Low-mass planetesimals have no/few fragmenting pebble collisions in the collapse phase and end up as porous pebble piles. The number of fragmenting collisions increases with increasing cloud mass, resulting in wider particle size distributions and higher density. The collapse is nevertheless “cold” in the sense that collision speeds are damped by the high collision frequency. This ensures that a significant fraction of large pebbles survive the collapse in all but the most massive clouds. Our results are in broad agreement with the observed increase in density of Kuiper Belt objects with increasing size, as exemplified by the recent characterization of the highly porous comet 67P/Churyumov–Gerasimenko.

On the Use of Meteor Camera Systems in the Detection of Kuiper Belt Objects Through Serendipitous Stellar Occultations

The direct detection of Kuiper Belt Objects (KBOs) by telescopic imaging is not currently practical for objects much less than 100 km in diameter. However, indirect methods such as serendipitous stellar occultations might still be employed to detect these bodies. The method of serendipitous stellar occultations has been previously used with some success in detecting KBOs—Roques et al. (Astron J 132(2):819–822, 2006) detected three Trans-Neptunian objects; Schlichting et al. (Nature 462(7275):895–897, 2009) and Schlichting et al. (Astrophys J 761:150, 2012) each detected a single object in archival Hubble Space Telescope data. However, previous assessments of KBO occultation detection rates have been calculated only for telescopes—we extend this method to video camera systems, and we apply this derivation to the automated meteor camera systems currently in use at the University of Western Ontario. We find that in a typical scenario we can expect one occultation per month. However recent studies such as those of Shankman et al. (Astrophys. J. Lett. 764. doi:10.1088/2041-8205/764/1/L2, 2013) and Gladman et al. (AAS/Division for Planetary Sciences Meeting Abstracts, 2012) which indicate that the population of small KBOs may be smaller than has been assumed in the past may result in a sharp reduction of these rates. Nonetheless, a survey for KBO occultations using existing meteor camera systems may provide valuable information about the number density of KBOs.

THE DENSITY OF MID-SIZED KUIPER BELT OBJECT 2002 UX25 AND THE FORMATION OF THE DWARF PLANETS

The formation of the largest objects in the Kuiper belt, with measured densities of ~1.5 g cm-3 and higher, from the coagulation of small bodies, with measured densities below 1 g cm-3 is difficult to explain without invoking significant porosity in the smallest objects. If such porosity does occur, measured densities should begin to increase at the size at which significant porosity is no longer supported. Among the asteroids, this transition occurs for diameters larger than ~350 km. In the Kuiper belt, no density measurements have been made between ~350 km and ~850 km, the diameter range where porosities might first begin to drop. Objects in this range could provide key tests of the rock fraction of small Kuiper belt objects. Here we report the orbital characterization, mass, and density determination of the 2002 UX25 system in the Kuiper belt. For this object, with a diameter of ~650 km, we find a density of 0.82+/-0.11 g cm-3, making it the largest solid known object in the solar system with a measured density below that of pure water ice. We argue that the porosity of this object is unlikely to be above ~20%, suggesting a low rock fraction. If the currently measured densities of Kuiper belt objects are a fair representation of the sample as a whole, creating ~1000 km and larger Kuiper belt objects with rock mass fractions of 70% and higher from coagulation of small objects with rock fractions as low as those inferred from 2002 UX25 is difficult.

A single sub-kilometre Kuiper belt object from a stellar occultation in archival data

The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects. Small, sub-kilometre-sized, Kuiper belt objects elude direct detection, but the signature of their occultations of background stars should be detectable. Observations at both optical and X-ray wavelengths claim to have detected such occultations, but their implied abundances are inconsistent with each other and far exceed theoretical expectations. Here we report an analysis of archival data that reveals an occultation by a body with an approximately 500-metre radius at a distance of 45 astronomical units. The probability of this event arising from random statistical fluctuations within our data set is about two per cent. Our survey yields a surface density of Kuiper belt objects with radii exceeding 250 metres of 2.1(-1.7)(+4.8) x 10(7) deg(-2), ruling out inferred surface densities from previous claimed detections by more than 5sigma. The detection of only one event reveals a deficit of sub-kilometre-sized Kuiper belt objects compared to a population extrapolated from objects with radii exceeding 50 kilometres. This implies that sub-kilometre-sized objects are undergoing collisional erosion, just like debris disks observed around other stars.

Analysis of the Rotational Properties of Kuiper Belt Objects

We use optical data on 10 Kuiper Belt objects (KBOs) to investigate their rotational properties. Of the 10, three (30%) exhibit light variations with amplitude delta_m >= 0.15 mag, and 1 out of 10 (10%) has delta_m >= 0.40 mag, which is in good agreement with previous surveys. These data, in combination with the existing database, are used to discuss the rotational periods, shapes, and densities of Kuiper Belt objects. We find that, in the sampled size range, Kuiper Belt objects have a higher fraction of low amplitude lightcurves and rotate slower than main belt asteroids. The data also show that the rotational properties and the shapes of KBOs depend on size. If we split the database of KBO rotational properties into two size ranges with diameter larger and smaller than 400 km, we find that: (1) the mean lightcurve amplitudes of the two groups are different with 98.5% confidence, (2) the corresponding power-law shape distributions seem to be different, although the existing data are too sparse to render this difference significant, and (3) the two groups occupy different regions on a spin period vs. lightcurve amplitude diagram. These differences are interpreted in the context of KBO collisional evolution.

Deep Imaging of the Kuiper Belt with the Keck 10 Meter Telescope

We present a new, pencil-beam survey of the Kuiper Belt taken with the Keck 10 m telescope. The cumulative surface density of Kuiper Belt Objects measured to apparent red magnitude mR=26.1 is 31+ 12−14 deg-2, while to mR=26.6 it is 40±33 deg-2. These numbers are compatible with an extrapolation of the luminosity function deduced earlier from measurements in the 20≤mR≤25 range, and they confirm a Kuiper Belt differential size distribution index q~4.