We present a dynamical study of 39 active Centaurs and 17 high-perihelion (
- Award ID(s):
- 1910275
- NSF-PAR ID:
- 10335803
- Date Published:
- Journal Name:
- The Planetary Science Journal
- Volume:
- 2
- Issue:
- 4
- ISSN:
- 2632-3338
- Page Range / eLocation ID:
- 155
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract q > 4.5 au) Jupiter-family comets (JFCs) with a focus on investigating recent orbital changes as potential triggers for comet-like activity. We have identified a common feature in the recent dynamical histories of all active Centaurs and JFCs in our sample that is not present in the history of the majority of inactive population members: a sharp decrease in semimajor axis and eccentricity occurring within the past several hundred years prior to observed activity. We define these rapid orbital changes as “a -jumps.” Our results indicate that these orbital reshaping events lead to shorter orbital periods and subsequently greater average per-orbit heating of Centaur nuclei. We suggest that thea -jumps could therefore be a major trigger of cometary activity on Centaurs and JFCs. Our results further imply that analyses of the recent dynamical histories could be used to identify objects that are currently active or may become active soon, where we have identified three such Centaurs with recenta -jumps that should be considered high-priority targets for observational monitoring to search for activity. -
Introduction: With the capture of the first high- resolution, in-situ images of Near-Earth Objects (NEOs) a couple of decades ago [1–4], the ubiquity of regolith and the granular nature of small objects in the Solar System became apparent. Benefiting from an increased access to high computing power, new numerical studies emerged, modeling granular structures forming and evolving as small bodies in the Solar System [5–7]. Now adding laboratory studies on granular material strength for asteroid and other small body applications [8,9], we are steadily progressing in our understanding of how regolith is shaping the interiors and surfaces of these worlds. In addition, our ever-more powerful observation capabilities are uncovering interesting dust-related phenomena in the outer skirts of our Solar System, in the form of activity at large heliocentric distances and rings [10–12]. We find that our recent progress in understanding the behavior of granular material in small body environments also has applications to the more distant worlds of Centaurs and Trans-Neptunian Objects (TNOs). Internal Strength: We currently deduce internal friction of rubble piles from the observation of large numbers of small asteroids and their rotation rates, combined with the associated numerical simulations [13,14]. In the laboratory, we study internal friction of simulant materials using shear strength measurements [8]. Combining observations, modeling, and laboratory work, the picture emerges of rubble pile interiors being composed of coarse grains in the mm to cm range. The irregular shapes of the grains lead to mechanical interlocking, thus generating the internal friction required to match observations of the asteroid population [8,9]. We find that the presence of a fine fraction in the confined interior of a rubble pile actually leads weaker internal strength [9]. Surface Strength: Deducing surface regolith strength for NEOs is usually performed via average slope measurements [15–17] or, most notably, observing the outcome of an impact of known energy [18]. In the laboratory, we measure the angle of repose of simulant material via pouring tests, as well as its bulk cohesion using shear strength measurements [8]. In some cases, this allows us to infer grain size ranges for various regions of the surface and subsurface of pictured NEOs, beyond the resolution of their in-situ images. Surface Activity: The Rosetta mission revealed that a number of activity events on comet 67P/Churyumov–Gerasimenko were linked to active surface geology, most notably avalanches and cliff collapses [19]. In addition, the role of regolith strength in asteroid disruption patterns has been inferred from numerical simulations of rotating rubble piles [20]. By studying strength differences in simulant samples, it becomes apparent that a difference in cohesion between a surface and its subsurface layer can lead to activity events with surface mass shedding, without the presence of volatiles sublimating as a driver [8]. We show that such differences in surface strength can be brought upon by a depletion in fine grains or a change in composition (e.g. depletion in water ice) and could account for regular activity patterns on small bodies, independently of their distance to the Sun. This is of particular interest to the study of Centaur activity and a potential mechanism for feeding ring systems.more » « less
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Abstract We report statistically significant detections of nonradial, nongravitational accelerations based on astrometric data in the photometrically inactive objects 1998 KY26, 2005 VL1, 2016 NJ33, 2010 VL65, 2016 RH120, and 2010 RF12. The magnitudes of the nongravitational accelerations are greater than those typically induced by the Yarkovsky effect, and there is no radiation-based, nonradial effect that can be so large. Therefore, we hypothesize that the accelerations are driven by outgassing and calculate implied H2O production rates for each object. We attempt to reconcile outgassing-induced acceleration with the lack of visible comae or photometric activity via the absence of surface dust and low levels of gas production. Although these objects are small, and some are rapidly rotating, the surface cohesive forces are stronger than the rotational forces, and rapid rotation alone cannot explain the lack of surface debris. It is possible that surface dust was removed previously, perhaps via outgassing activity that increased the rotation rates to their present-day value. We calculate dust production rates of order ∼10−4g s−1in each object, assuming that the nuclei are bare, within the upper limits of dust production from a sample stacked image of 1998 KY26of
g s−1. This production corresponds to brightness variations of order ∼0.0025%, which are undetectable in extant photometric data. We assess the future observability of each of these targets and find that the orbit of 1998 KY26—which is also the target of the extended Hayabusa2 mission—exhibits favorable viewing geometry before 2025. -
Abstract We report a statistically significant detection of nongravitational acceleration on the subkilometer near-Earth asteroid (523599) 2003 RM. Due to its orbit, 2003 RM experiences favorable observing apparitions every 5 yr. Thus, since its discovery, 2003 RM has been extensively tracked with ground-based optical facilities in 2003, 2008, 2013, and 2018. We find that the observed plane-of-sky positions cannot be explained with a purely gravity-driven trajectory. Including a transverse nongravitational acceleration allows us to match all observational data, but its magnitude is inconsistent with perturbations typical of asteroids such as the Yarkovsky effect or solar radiation pressure. After ruling out that the orbital deviations are due to a close approach or collision with another asteroid, we hypothesize that this anomalous acceleration is caused by unseen cometary outgassing. A detailed search for evidence of cometary activity with archival and deep observations from the Panoramic Survey Telescope and Rapid Response System and the Very Large Telescope does not reveal any detectable dust production. However, the best-fitting H2O sublimation model allows for brightening due to activity consistent with the scatter of the data. We estimate the production rate required for H2O outgassing to power the acceleration and find that, assuming a diameter of 300 m, 2003 RM would require Q(H2O) ∼ 1023molec s−1at perihelion. We investigate the recent dynamical history of 2003 RM and find that the object most likely originated in the mid-to-outer main belt (∼86% probability) as opposed to from the Jupiter-family comet region (∼11% probability). Further observations, especially in the infrared, could shed light on the nature of this anomalous acceleration.
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Abstract Centaurs are minor solar system bodies with orbits transitioning between those of trans-Neptunian scattered disk objects and Jupiter-family comets (JFCs). 39P/Oterma (39P) is a frequently active centaur that has recently held both centaur and JFC classifications and was observed with the JWST NIRSpec instrument on 2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO2gas emission was detected in a centaur, with a production rate of
= (5.96 ± 0.80) × 1023molecules s−1. This is the lowest detection of CO2of any centaur or comet. CO and H2O were not detected down to constraining upper limits. Derived mixing ratios ofQ CO/Q ≤ 2.03 and /Q ≥ 0.60 are consistent with CO2and/or CO outgassing playing large roles in driving the activity, but not water, and show a significant difference between the coma abundances of 29P/Schwassmann–Wachmann 1, another centaur at a similar heliocentric distance, which may be explained by thermal processing of 39P’s surface during its previous JFC orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in 2022 July (Gemini-North) and September (Lowell Discovery Telescope). Image analysis and photometry based on these data are consistent with a point-source detection and an estimated effective nucleus radius of 39P in the range ofR nuc= 2.21–2.49 km.