arXiv subjects

Luke Dones

Publications and source records attributed to Luke Dones.

At least 19 recordsLinked to original sources

arXiv2025

Formation of Planetesimals in the Outer Solar System

The Solar System hosts the most studied and best understood major and minor planetary bodies - and the only extraterrestrial bodies to have been visited by spacecraft. The Solar System therefore provides important constraints on both the initial stages of planetary growth, communicated to us by its surviving planetesimal populations, and for the final result of the planet formation process represented by the architecture of the system and properties of the individual planets. We review here models of planetesimal formation in the outer Solar System as well as the wealth of recent observational constraints that has been used to formulate and refine modern planetesimal formation theory.

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arXiv2025

NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)

We report on the observation and measurement of astrometry, photometry, morphology, and activityof the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS) with the NSF-DOE Vera C. Rubin Observatory. Comet 3I/ATLAS, the third known interstellar object, was discovered on UT 2025 July 1. Rubin Observatory had coincidentally collected images of the object's region of the sky during routine commissioning. Facilitated by Rubin's high resolution and large aperture, we successfully recovered object detections from Rubin observations spanning UT 2025 June 21 (10 days before discovery, when 3I/ATLAS was 4.5 au from the Sun) through the date of discovery, and we acquired additional images through UT 2025 July 20 as part of commissioning. We measure on-sky locations of 3I/ATLAS in Rubin ugrizy bands, with a typical precision of about 70 mas, and briefly describe the reason this is coarser than our measured static source astrometric precision of about 3 mas in Rubin images. We measure grizy magnitudes of 3I/ATLAS photometry at about 0.01 mag precision, detecting no short-term photometric variability above 0.01 mag. We derive an estimated near-nucleus dust-to-nucleus scattering cross-section ratio of eta >= 13 on UT 2025 July 2 based on Rubin photometry and an upper limit nucleus size computed from Hubble Space Telescope observations. We find Rubin colors of g - r = (0.657 +/- 0.013) mag, r - i = (0.235 +/- 0.018) mag, i - z = (0.147 +/- 0.042) mag, z - y = (0.047 +/- 0.052) mag. These data represent the earliest observations of this object by a large (>=8-meter class) telescope and illustrate the type of measurements (and discoveries) Rubin's Legacy Survey of Space and Time (LSST) will begin to provide after it begins in early 2026.

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arXiv2025

A Spiral Structure in the Inner Oort Cloud

As the Galactic tide acts to decouple bodies from the scattered disk it creates a spiral structure in physical space that is roughly 15,000 au in length. The spiral is long-lived and persists in the inner Oort cloud to the present time. Here we discuss dynamics underlying the Oort spiral and (feeble) prospects for its observational detection.

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arXiv2024

The Dynamical Origins of the Dark Comets and a Proposed Evolutionary Track

So-called 'dark comets' are small, morphologically inactive near-Earth objects (NEOs) that exhibit nongravitational accelerations inconsistent with radiative effects. These objects exhibit short rotational periods (minutes to hours), where measured. We find that the strengths required to prevent catastrophic disintegration are consistent with those measured in cometary nuclei and expected in rubble pile objects. We hypothesize that these dark comets are the end result of a rotational fragmentation cascade, which is consistent with their measured physical properties. We calculate the predicted size-frequency distribution for objects evolving under this model. Using dynamical simulations, we further demonstrate that the majority of these bodies originated from the $\nu_6$ resonance, implying the existence of volatiles in the current inner main belt. Moreover, one of the dark comets, (523599) 2003 RM, likely originated from the outer main belt, although a JFC origin is also plausible. These results provide strong evidence that volatiles from a reservoir in the inner main belt are present in the near-Earth environment.

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arXiv2023

Oort Cloud Comets Discovered Far from the Sun

Context: Increasingly, Oort Cloud comets are being discovered at great distances from the Sun and tracked over ever wider ranges of heliocentric distances as observational equipment improves. Aims: To investigate in detail how the original semimajor axis for near-parabolic comets depends on the selected data arc and the assumed form of the non-gravitational (NG) acceleration. Methods: Among currently known Oort Cloud comets with large perihelion distances ($q > 3$ au), we selected 32 objects observed over the widest ranges of heliocentric distances in orbital legs before and after perihelion. For each of them, we determined a series of orbits using at least three basic types of data sets selected from available positional data (pre- and post-perihelion data and the entire data set), and a few forms of NG acceleration representing water ice or CO sublimation. Results: We found that the motion of comets is often measurably affected by NG forces at heliocentric distances beyond 5 au from the Sun. The most spectacular example is C/2010 U3 (Boattini), whose perihelion distance is 8.44 au. NG effects are detectable for 19 of the 32 comets within the positional data. For five comets, we found asymmetric effects of NG forces - in three cases significantly greater before perihelion than afterward (C/2017 M4, C/2000 SV$_{75}$, and C/2015 O1), and in two others the opposite (C/1997 BA$_6$ and C/2006 S3). We also find that the well-known systematic effect of finding more tightly bound original orbits when including the NG acceleration than in purely gravitational solutions may be related to the specific form of the standard $g(r)$ function describing the sublimation of ices.

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arXiv2023

Impact Rates in the Outer Solar System

Previous studies of cometary impacts in the outer Solar System used the spatial distribution of ecliptic comets (ECs) from dynamical models that assumed ECs began on low-inclination orbits (<5 deg) in the Kuiper belt. In reality, the source population of ECs - the trans-Neptunian scattered disk - has orbital inclinations reaching up to ~30 deg. In Nesvorny et al. (2017), we developed a new dynamical model of ECs by following comets as they evolved from the scattered disk to the inner Solar System. The model was absolutely calibrated from the population of Centaurs and active ECs. Here we use our EC model to determine the steady-state impact flux of cometary/Centaur impactors on Jupiter, Saturn, Uranus, and their moons. Relative to previous work (Zahnle et al. 2003), we find slightly higher impact probabilities on the outer moons and lower impact probabilities on the inner moons. The impact probabilities are smaller when comet disruption is accounted for. The results provide a modern framework for the interpretation of the cratering record in the outer Solar System.

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arXiv2023

The Collisional Evolution of the Primordial Kuiper Belt, Its Destabilized Population, and the Trojan Asteroids

The tumultuous early era of outer solar system evolution culminated when Neptune migrated across the primordial Kuiper belt (PKB) and triggered a dynamical instability among the giant planets. This event led to the ejection of approximately 99.9\% of the PKB (here called the destabilized population), heavy bombardment of the giant planet satellites, and the capture of Jupiter's Trojans. While this scenario has been widely tested using dynamical models, there have been fewer investigations into how the PKB, its destabilized population, and the Trojans experienced collisional evolution. Here we examined this issue for all three populations with the code Boulder. Our constraints included the size-frequency distributions (SFDs) of the Trojan asteroids and craters on the giant planet satellites. Using this combination, we solved for the unknown disruption law affecting bodies in these populations. The weakest ones, from an impact energy per mass perspective, were 20 m in diameter. Overall, collisional evolution produces a power-law-like shape for multikilometer Trojans and a wavy-shaped SFD in the PKB and destabilized populations. The latter can explain (i) the shapes of the ancient and younger crater SFDs observed on the giant planet satellites, (ii) the shapes of the Jupiter family and long-period comet SFDs, which experienced different degrees of collision evolution, and (iii) the present-day impact frequency of superbolides on Jupiter and smaller projectiles on Saturn's rings. Our model results also indicate that many observed comets, most which are smaller than 10 km in diameter, are likely to be gravitational aggregates formed by large-scale collision events.

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arXiv2022

The Transition from the Kuiper Belt to the Jupiter-Family (Comets)

Kuiper Belt Objects, or more generally Trans-Neptunian Objects (TNOs), are planetesimals found beyond the orbit of Neptune. Some TNOs evolve onto Neptune-crossing orbits and become Centaurs. Many Centaurs, in turn, reach Jupiter-crossing orbits and become Jupiter-family comets (JFCs). TNOs are the main source of the JFCs. TNOs offer a different window than the JFCs, of more primordial bodies and over a different size and temperature range. It is in that context that this chapter is written. Here we discuss the dynamical pathways taken from the trans-Neptunian region to the JFCs, and the most important properties of TNOs that relate to the JFC population, including considerations of their origins, compositions, morphologies, and size distributions. We relate these properties to the JFCs whenever possible. We reflect on a few key outstanding issues regarding our incomplete knowledge of TNOs as they pertain to the Centaurs and JFC populations. We finish with a short discussion of notable new and upcoming facilities and the impacts they will have regarding these outstanding questions.

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arXiv2019

Origin and evolution of long-period comets

We develop an evolutionary model of the long-period comet (LPC) population, starting from their birthplace in a massive trans-Neptunian disk that was dispersed by migrating giant planets. Most comets that remain bound to the Solar system are stored in the Oort cloud. Galactic tides and passing stars make some of these bodies evolve into observable comets in the inner Solar system. Our approach models each step in a full-fledged numerical framework. Subsequent analysis consists of applying plausible fading models and computing the original orbits to compare with observations. Our results match the observed semimajor axis distribution of LPCs when Whipple's power-law fading scheme with an exponent $\kappa= 0.6^{+0.1}_{-0.2}$ is adopted. The cumulative perihelion ($q$) distribution is fit well by a linear increase plus a weak quadratic term. Beyond $q = 15$~au, however, the population increases steeply and the isotropy of LPC orbital planes breaks. We find tentative evidence from the perihelion distribution of LPCs that the returning comets are depleted in supervolatiles and become active due to water ice sublimation for $q\leq 3$ au. Using an independent calibration of the population of the initial disk, our predicted LPC flux is smaller than observations suggest by a factor of $\simeq 2$. Current data only characterize comets from the outer Oort cloud (semimajor axes $\gtrsim 10^4$~au). A true boost in understanding the Oort cloud's structure should result from future surveys when they detect LPCs with perihelia beyond $15$~au. Our results provide observational predictions of what can be expected from these new data.

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arXiv2018

Secular Resonance Between Iapetus and the Giant Planets

Using numerical integrations, we find that the orbital eccentricity of Saturn's moon Iapetus undergoes prominent multi-Myr oscillations. We identify the responsible resonant argument to be $\varpi-\varpi_{g5}+\Omega-\Omega_{eq}$, with the terms being the longitudes of pericenter of Iapetus and planetary secular mode $g_5$, Iapetus's longitude of the node and Saturn's equinox. We find that this argument currently (on a $10^7$~yr timescale) appears to librate with a very large amplitude. On longer timescales, the behavior of this resonant angle is strongly dependent on the resonant interaction between Saturn's spin axis and the planetary mode $f_8$, with long-term secular resonance being possible if Saturn's equinox is librating relative to the node of the $f_8$ eigenmode. We present analytical estimates of the dependence of the resonant argument on the orbital elements of Iapetus. We find that this Iapetus-$g_5$ secular resonance could have been established only after the passage of Iapetus through the 5:1 mean-motion resonance with Titan, possibly in the last Gyr. Using numerical simulations, we show that the capture into the secular resonance appears to be a low-probability event. While the Iapetus-$g_5$ secular resonance can potentially help us put new constraints on the past dynamics of the Saturnian system, uncertainties in both the spin axis dynamics of Saturn and the tidal evolution rate of Titan make it impossible to make any firm conclusions about the resonance's longevity and origin.

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arXiv2018

Solar System Ice Giants: Exoplanets in our Backyard

Future remote sensing of exoplanets will be enhanced by a thorough investigation of our solar system Ice Giants (Neptune-size planets). What can the configuration of the magnetic field tell us (remotely) about the interior, and what implications does that field have for the structure of the magnetosphere; energy input into the atmosphere, and surface geophysics (for example surface weathering of satellites that might harbour sub-surface oceans). How can monitoring of auroral emission help inform future remote observations of emission from exoplanets? Our Solar System provides the only laboratory in which we can perform in-situ experiments to understand exoplanet formation, dynamos, systems and magnetospheres.

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arXiv2016

Dynamical Evidence for a Late Formation of Saturn's Moons

We explore the past evolution of Saturn's moons using direct numerical integrations. We find that the past Tethys-Dione 3:2 orbital resonance predicted in standard models likely did not occur, implying that the system is less evolved than previously thought. On the other hand, the orbital inclinations of Tethys, Dione and Rhea suggest that the system did cross the Dione-Rhea 5:3 resonance, which is closely followed by a Tethys-Dione secular resonance. A clear implication is that either the moons are significantly younger than the planet, or that their tidal evolution must be extremely slow (Q > 80,000). As an extremely slow-evolving system is incompatible with intense tidal heating of Enceladus, we conclude that the moons interior to Titan are not primordial, and we present a plausible scenario for the system's recent formation. We propose that the mid-sized moons re-accreted from a disk about 100 Myr ago, during which time Titan acquired its significant orbital eccentricity. We speculate that this disk has formed through orbital instability and massive collisions involving the previous generation of Saturn's mid-sized moons. We identify the solar evection resonance perturbing a pair of mid-sized moons as the most likely trigger of such an instability. This scenario implies that most craters on the moons interior to Titan must have been formed by planetocentric impactors.

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arXiv2015

Reflected Light Curves, Spherical and Bond Albedos of Jupiter- and Saturn-like Exoplanets

Reflected light curves observed for exoplanets indicate bright clouds at some of them. We estimate how the light curve and total stellar heating of a planet depend on forward and backward scattering in the clouds based on Pioneer and Cassini spacecraft images of Jupiter and Saturn. We fit analytical functions to the local reflected brightnesses of Jupiter and Saturn depending on the planet's phase. These observations cover broad bands at 0.59-0.72 and 0.39-0.5 {\mu}m, and narrow bands at 0.938 (atmospheric window), 0.889 (CH4 absorption band), and 0.24-0.28 {\mu}m. We simulate the images of the planets with a ray-tracing model, and disk-integrate them to produce the full-orbit light curves. For Jupiter, we also fit the modeled light curves to the observed full-disk brightness. We derive spherical albedos for Jupiter, Saturn, and for planets with Lambertian and Rayleigh-scattering atmospheres. Jupiter-like atmospheres can produce light curves that are a factor of two fainter at half-phase than the Lambertian planet, given the same geometric albedo at transit. The spherical albedo is typically lower than for a Lambertian planet by up to a factor of 1.5. The Lambertian assumption will underestimate the absorption of the stellar light and the equilibrium temperature of the planetary atmosphere. We also compare our light curves with the light curves of solid bodies: the moons Enceladus and Callisto. Their strong backscattering peak within a few degrees of opposition (secondary eclipse) can lead to an even stronger underestimate of the stellar heating.

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arXiv2013

Titan-Hyperion Resonance and the Tidal Q of Saturn

Lainey et al. (2012), by re-analyzing long-baseline astrometry of Saturn's moons, have found that the moons' tidal evolution is much faster than previously thought, implying an order of magnitude stronger tidal dissipation within Saturn. This result is controversial and implies recent formation of at least some of the mid-sized icy moons of Saturn. Here we show that this more intensive tidal dissipation is in full agreement with the evolved state of the Titan-Hyperion resonance. This resonance was previously thought to be non-tidal in origin, as the amount of tidal evolution required for its assembly is beyond what is possible in models that assume that all the major moons are primordial. We find that the survival of the Titan-Hyperion resonance is in agreement with a past Titan-Iapetus 5:1 resonance, but not with unbroken tidal evolution of Rhea from the rings to its current distance.

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arXiv2011

Accretion of Saturn's mid-sized moons during the viscous spreading of young massive rings: solving the paradox of silicate-poor rings versus silicate-rich moons

The origin of Saturn's inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn's rings is debated. Charnoz et al. (2010) introduced the idea that the smallest inner moons could form from the spreading of the rings' edge while Salmon et al. (2010) showed that the rings could have been initially massive, and so was the ring's progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested in Canup (2010). However, the process driving mid-sized moons accretion from the icy debris disks has not been investigated in details. In particular, this process does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea's mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Qp<2000) would be required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites' cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process)while those still present in the rings are interpreted as today Saturn's rings' propellers and ring-moons (like Pan or Daphnis).

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arXiv2011

The Role of Ejecta in the Small Crater Populations on the Mid-Sized Saturnian Satellites

We find evidence that crater ejecta play an important role in the small crater populations on the Saturnian satellites, and more broadly, on cratered surfaces throughout the Solar System. We measure crater populations in Cassini images of Enceladus, Rhea, and Mimas, focusing on image data with scales less than 500 m/pixel. We use recent updates to crater scaling laws and their constants to estimate the amount of mass ejected in three different velocity ranges: (i) greater than escape velocity, (ii) less than escape velocity and faster than the minimum velocity required to make a secondary crater (v_min), and (iii) velocities less than v_min. Although the vast majority of mass on each satellite is ejected at speeds less than v_min, our calculations demonstrate that the differences in mass available in the other two categories should lead to observable differences in the small crater populations; the predictions are borne out by the measurements we have made to date. Rhea, Tethys, and Dione have sufficient surface gravities to retain ejecta moving fast enough to make secondary crater populations. The smaller satellites, such as Enceladus but especially Mimas, are expected to have little or no traditional secondary populations because their escape velocities are near the threshold velocity necessary to make a secondary crater. Our work clarifies why the Galilean satellites have extensive secondary crater populations relative to the Saturnian satellites. The presence, extent, and sizes of sesquinary craters (craters formed by ejecta that escape into temporary orbits around Saturn before re-impacting the surface) is not yet well understood. Finally, our work provides further evidence for a "shallow" size-frequency distribution (slope index of ~2 for a differential power-law) for comets a few km diameter and smaller. [slightly abbreviated]

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arXiv2009

Origin and Evolution of Saturn's Ring System

The origin and long-term evolution of Saturn's rings is still an unsolved problem in modern planetary science. In this chapter we review the current state of our knowledge on this long-standing question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn's rings are not older than 100 My. In addition, Saturn's large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn's satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the ''propellers'' in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume provide new constraints on evolutionary processes in Saturn's rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings's fine scale structure, and progress in our understanding of the formation of the Solar System provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn's rings. They strengthen the idea that Saturn's rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings's origin are reviewed in the light of t

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arXiv2009

Considerations on the magnitude distributions of the Kuiper belt and of the Jupiter Trojans

By examining the absolute magnitude (H) distributions (hereafter HD) of the cold and hot populations in the Kuiper belt and of the Trojans of Jupiter, we find evidence that the Trojans have been captured from the outer part of the primordial trans-Neptunian planetesimal disk. We develop a sketch model of the HDs in the inner and outer parts of the disk that is consistent with the observed distributions and with the dynamical evolution scenario known as the `Nice model'. This leads us to predict that the HD of hot population should have the same slope of the HD of the cold population for 6.5 < H < 9, both as steep as the slope of the Trojans' HD. Current data partially support this prediction, but future observations are needed to clarify this issue. Because the HD of the Trojans rolls over at H~9 to a collisional equilibrium slope that should have been acquired when the Trojans were still embedded in the primordial trans-Neptunian disk, our model implies that the same roll-over should characterize the HDs of the Kuiper belt populations, in agreement with the results of Bernstein et al. (2004) and Fuentes and Holman (2008). Finally, we show that the constraint on the total mass of the primordial trans-Neptunian disk imposed by the Nice model implies that it is unlikely that the cold population formed beyond 35 AU.

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