Observation, Evolution and Origin of Planetary Satellites
Friday May 10, 2013 10:00-15:30
Royal Astronomical Society
Burlington House
Piccadilly
LONDON
W1J 0BQ
Meeting Description
The last decade has seen an explosion in the number of known natural satellites in the solar system. Satellites are now recognised as ubiquitous among the giant, terrestrial, and dwarf planets. They may also be found orbiting asteroids and Kuiper belt objects and embedded in ring systems. Observing campaigns and robot spacecraft (e.g., Cassini-Huygens, New Horizons) continue to reveal the complex physical characteristics of moons and the intricate and coupled dynamical evolution of satellite systems.
This discussion meeting aims to review current observations and characteristics of natural satellites including the satellite and ring systems of the giant planets, the Moon, the Martian satellites, dwarf planet systems (e.g., Pluto-Charon), and minor planet satellites. The meeting will then examine the dynamical processes that affect satellite evolution (e.g., tides, internal structure, satellite-disk-ring interactions, collisions, cratering, YORP, gravitational dynamics), satellite formation, and mission concepts for the continuous exploration these bodies.
Invited Speakers
Dr. Mark Showalter (SETI Institute)Dr. Aurelien Crida (University Nice Sophia Antipolis, Observatoire de la Côte d’Azur, CNRS)
Organised by:
Dr. Craig B. Agnor (Queen Mary, University of London) FRASDr. Apostolos Christou (Armagh Observatory) FRAS
Prof. Carl D. Murray (Queen Mary, University of London) FRAS
Programme
MORNING SESSION:
Chair -- Craig Agnor10:00 - 10:30 Coffee
10:30 Mark Showalter (SETI Institute) [Invited] - The Planetary Ring-Moon Systems: An Observational Survey
11:15 Samuel Duddy (University of Kent) - Unbound Asteroid Pairs: The Decoupled Remnants of Binary Asteroids
11:30 Carl D. Murray and Nicholas J. Cooper (Queen Mary) - The small, inner satellites of Saturn
11:45 Mark J. Burchell, A.J.W. Morris, and A. Lightwing (University of Kent) - Catastrophic Disruption of Rocky, Icy and Mixed Ice:Rocky Minor Bodies - What We Learn in the Laboratory
12:00 Kevin Degiorgio, S. Rodriguez, C. Ferrari, and A. Brahic (Laboratoire AIM, Université Paris) - Morphology of Enceladus' craters by photometric studies with ISS/Cassini
12:15 Geraint Jones (MSSL/UCL/Birkbeck) - The Plume of Enceladus
12:30 Lunch
Please note that we are not able to offer sandwiches for purchase at Burlington House. However, there are many good sandwich shops nearby.
AFTERNOON SESSION:
Chair - Apostolos Christou13:30 Valery Lainey (IMCCE, Observatoire de Paris) - Quantification of tidal dissipation among giant planets from astrometry
13:45 Maryame El Moutamid (LESIA, Observatoire de Paris), B. Sicardy, and S. Renner - Dynamics of the small Saturnian moons in coupled resonances
14:00 R. Cave and Craig B. Agnor (Queen Mary) - Implications of Tidal Dissipation in the Inner Uranian Satellites
14:15 Sebastien Charnoz (Université Paris Diderot), A. Crida, V. Lainey, and J. Salmon - Accretion processes and regular satellite formation: the role of planetary rings
14:30 Aurelien Crida (Université de Nice Sophia–antipolis) and S. Charnoz [Invited] - Formation of regular satellites from ancient massive rings in the Solar System
15:15 Discussion
15:30 - 16:00 Tea
16:00 - 17:00 RAS AGM (Fellows only)
17:00 - 18:00 Open A&G Meeting including Presidential Address (all invited)
Abstracts
The Planetary Ring-Moon Systems: An Observational Survey [Invited]
Mark R. Showalter (SETI Institute)
The outer planets, Mars through Neptune, are each accompanied by a family of "regular" satellites, characterized by orbits that are relatively close to the central body, nearly circular, nearly equatorial, and usually prograde. Dwarf planets Pluto, Haumea and Eris also have satellites with orbits that could be described as regular. These are distinct from the more distant, "irregular" moons that are most likely captured objects.
As the Voyager and Cassini missions have demonstrated, spacecraft still provide our best opportunities to take a census of the regular satellites. However, new capabilities and new analysis techniques have enabled the Earth-based observatories to augment the latest tallies. The Hubble Space Telescope has, for example, revealed two Uranian moons that were two small to be imaged by Voyager; it has also brought to light four tiny moons orbiting Pluto and Charon.
As the latest discoveries take us beyond the quirks of small-number statistics, one might have expected the ring-moon systems to show a certain level of uniformity. The truth has been far different, with each one raising distinct dynamical puzzles about that system's origin and evolution. For example, co-orbitals are a common feature of the Saturn system, but have not been seen elsewhere. Uranus has a densely-nested set of nine inner moons, which raises unique concerns about chaotic evolution and long-term instability. Pluto's five moons seem to defy our conventional understanding of how orbital resonances operate.
This presentation will provide an overview of the ring-moon systems as we understand them today, and will highlight some of the observational and dynamical challenges that lay before us.
Unbound Asteroid Pairs: The Decoupled Remnants of Binary Asteroids
Samuel Duddy (University of Kent)
Radar, spacecraft flybys and lightcurve observations have shown that asteroids can have satellites and exist as binaries. Modelling has shown that binary asteroids can decouple shortly after their initial formation producing a pair of unbound asteroids with very similar but independent heliocentric orbits. A population of these Unbound Asteroid Pairs has recently been discovered in the main asteroid belt via backwards integration of their orbits. We will present results from an ongoing optical/NIR spectroscopic survey to examine the composition of the asteroids in each pair.
The small, inner satellites of Saturn
Carl D. Murray, Nicholas J. Cooper (Queen Mary, University of London)
We present a review of the properties of the population of small (mean radius less than 50km) Saturnian satellites orbiting at or inside the orbit of Dione. These include four Lagrangian point satellites (Helene, Polydeuces, Telesto and Calypso), the three members of the Alkyonides group (Pallene, Anthe and Methone), the G ring satellite (Aegaeon), the co-orbital pair (Janus and Epimetheus), the F ring shepherd satellites (Pandora and Prometheus), as well as three moons located just outside or embedded in the A ring (Atlas, Daphnis and Pan). The Cassini spacecraft has discovered six of these objects and has provided high resolution images and spectral information for most of the population of fifteen moons. In this talk we review our current understanding of the orbital and physical properties of this diverse group of objects.
Catastrophic Disruption of Rocky, Icy and Mixed Ice:Rocky Minor Bodies – What We Learn in the Laboratory
Burchell M.J., Morris A.J.W, Lightwing A. (Centre for Astrophysics and Planetary Science, School of Physical Sciences, Ingram Building, Univ. of Kent, Canterbury CT2 7NH, United Kingdom)
The small bodies in the solar system include comets, asteroids and natural satellites. Their compositions range from rocky to icy (and mixtures thereof). All are impacted by other bodies. If such an impact is relatively energtic enough this will disrupt the target body. The fragments may re-assemble under self-gravity, or if the impact is violent enough they will disperse. In the latter case this will in effect represent an end-of-life for the body. The critical parameter in determining the outcome of an impact on a finite body is Q*, the energy density (in J kg-1) which is the energy density needed to catastrophically disrupt a body.
Here we present data from the laboratory on the value of Q* for rocky, icy and mixed ice:rocky bodies at cm size scales. The data were taken using our own two-stage light gas gun. We give values for Q* in each case and show how the results can be extrapolated to different size bodies. Finally we discuss the implications for possible lifetimes of bodies such as Pluto against catastrophic disruption.
Morphology of Enceladus’s craters by photometric studies with ISS/Cassini
K.Degiorgio (1), S.Rodriguez (1), C.Ferrari (1), A.Brahic (1) (1) Laboratoire AIM, Université Paris 7, CNRS UMR-‐7158, CEA-‐Saclay/DSM/IRFU/SAp, France
The study of crater morphology (depth to diameter ratio) can bring valuable information on the structural properties of the surface and sub-surface of the impacted body. We report here on the analysis of images of the cratered terrains of the Saturn’s moon Enceladus recorded by the ISS instrument onboard the Cassini spacecraft. We used a 3D morphological model (Buratti and Veverka, 1985) coupled with a regolith scattering model (Hapke, 1993, 2001) in order to analyze the photometric behavior of a sample of 80 Enceladus’s craters. This method enables us to verify the general photometric properties of Enceladus’ regolith (single scattering albedo and asymmetry factor in excellent agreement with previous studies – e.g. Verbiscer and Veverka, 1994) and derive for the first time robust estimations of the craters’ depth. As previously done for other bodies (Schenk, 1989), we used this new information to plot the depth/diameter ratio as a function of the diameter for all the craters of our sample. Such a diagram allows us to determine the presence of a transition diameter, which usually defines the transition between two regimes of cratering (simple to complex craters) and is diagnostic of the structural properties of its crust. The value of the transition diameter for Enceladus is found to be in good accordance with the empirical law followed by icy satellites (inversely proportional to surface gravity, Schenk, 1989). We will discuss the possible implications of such a value for Enceladus’s geological history and crust properties. Then a more general discussion on the constraints inferred from this law on formation models for icy moons in the Solar System will be opened.
The Plume of Enceladus
Geraint H. Jones (Mullard Space Science Laboratory, University College London and The Centre for Planetary Sciences at UCL/Birkbeck)
In 2005, following the discovery by the Cassini magnetometer that Saturn's moon Enceladus was active, several instruments on the spacecraft revealed the nature of this activity: The body's south polar region is the source of several jets of material, the output of which combines to form a vast plume of material expanding into space to the south of the moon. A review is given of the state of knowledge of this plume, including its physical and chemical composition, and inferences regarding the nature of its source. The plume's complex interaction with Saturn's magnetospheric plasma is also presented.
Quantification of tidal dissipation among giant planets from astrometry
Valery Lainey (IMCCE, Observatoire de Paris)
The quantification of tidal dissipation among giant planets is a key point to a better knowledge of the long-term evolution of many natural satellites, as well as exo-planetary systems. So far, tidal dissipation among giant planets used to be estimated from the expected past evolution of the moons. But recently, Jupiter's and Saturn's tidal dissipation could be quantified from the astrometric monitoring of their main moon orbits (Lainey et al., 2009; Lainey et al. 2012). Such work has revealed that tidal dissipation in giant planets could potentially be much higher than usually expected, with strong consequences on formation scenarios. We will address these recent results and the coming perspectives in the context of the Cassini mission. We will introduce the first results for the Uranian system, also.
Dynamics of the small Saturn’s moons in coupled resonances
M. El Moutamid (1,2), B. Sicardy (1,3,4) and S. Renner (2,5)
(1) LESIA, Observatoire de Paris, UMR 8109 du CNRS, 5 place Jules Janssen, 92195 Meudon, France
(2) IMCCE, Observatoire de Paris, UMR 8028 du CNRS, 77 avenue Denfert-Rochereau, 75014 Paris, France
(3) Institut Universitaire de France, 103 boulevard Saint-Michel, 75005 Paris, France
(4) Universit ́ Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
(5) Universit ́ Lille 1, Laboratoire d’Astronomie de Lille, 1 impasse de l’observatoire, 59000 Lille, France
Many satellites in the Solar System are involved in mean motion resonances. The simplest case of all is that of two satellites, one of them with negligible mass (test particle), orbiting in the same plane and close to a mean motion first order resonance of the type m+1:m. In this situation, two critical resonant angles appear, respectively called the Corotation Eccentric Resonance (CER) and the Lindblad Eccentric Resonant (LER) arguments. Each of them has very different physical effects on the test particle, but surprisingly, no general treatment of the coupling between these two resonances has been presented so far in the literature. Here we present a generic dynamical study of this coupling, that we call the CoraLin model. It uses non-dimensional quantities, and describes all possible configurations between the satellites near horizontal first order mean motion resonances. We apply this model to several recently discovered small Saturnian satellites dynamically linked to Mimas through first mean motion resonances : Anthe, Methone and Aegaeon, all associated with ring arc material. The presence of these three structures are consistent with their confinement by CER with Mimas : Aegaeon is trapped in an inner 7:6 CER with Mimas, while Anthe and Methone are respectively near the outer 14:15 and a 10:11 CER resonances. All satellites are also perturbed by the associated LER’s, in a way described by the CoraLin model. Poincar ́ surfaces of section reveal the dynamical structure of each orbit, and for some of them, their proximity to chaotic regions. Those sections may reveal the dynamical origin of those bodies. In particular, we discuss the probability of capturing a satellite into one of the CER’s with Mimas as the orbit of the latter evolves through tidal effects. We will discuss the potential implications of this work, in particular the constraints it may provide on Mimas’ orbital evolution.
Implications of Tidal Dissipation in the Inner Uranian Satellites
R. Cave and C.B. Agnor (Queen Mary, University of London)
Uranus hosts a system of at least 13 small (R~10-80km) satellites orbiting near the planet (a ~ 2 - 3.5 1.9-3.7 Ru). Dynamical scaling arguments and orbital integrations have shown that mutual gravitational interactions drive some of the inner uranian satellites to crossing orbits on timescales of 1e5, to 1e7 years - timescales remarkably shorter than the age of the satellite solar system (Duncan & Lissauer 1997, French & Showalter 2012). Tides raised in satellites by the planet lead to damping of their eccentricities on a timescale that depends on their orbit and internal structure. Standard models of tidal dissipation that assume a satellite's interior is solid and monolithic suggest that the timescales to damp orbital eccentricity via tidal dissipation are much longer than the time required for these satellites to reach crossing orbits. However, if these satellites have interiors of rubble then their shapes may be more susceptible to deformation by tides and the global rate of tidal dissipation may be greatly enhanced by small scale granular friction. Using a simple model for tidal dissipation in rubble piles, scaling arguments and numerical simulations, we are investigating how tidal dissipation affects the orbital evolution of the inner uranian satellites. We will present initial results of this study and discuss the implications for the orbital history, structure and stability of the system, the interior structure of the moons and the nature of tidal dissipation within them.
Accretion processes and regular Satellites formation : the role of planetary rings.
Sebastien Charnoz, Aurélien Crida, Valery Lainey, and Julien Salmon
The origin of Solar System satellites is actively debated. We now understand that, despite the morphological analogy between a satellite systems and a planetary systems, the formation processes of satellites may be significantly different from planetary formation processes. In addition, satellites evolve quickly under the effects of tides. Different scenarios seem to be required for different types of planets (terrestrial, giant or ice giant). In this talk I will quickly review our current understanding of satellite formation and the different constraints. Based on Cassini images and numerical simulations, I will show that there is today on-going accretion processes at the edge of Saturn's rings, pointing to a new satellite formation process. Satellite formation may be deeply linked to the evolution of planetary rings, to the point that it is very probable that most of Solar System’s regular satellites may have born inside rings, either massive, like the protolunar disk, or light, like giant planet’s rings. The cases of terrestrial planets and in particular Mars will be also discussed.
Formation of regular satellites from ancient massive rings in the Solar System [Invited]
Aurélien Crida and Sebastien Charnoz
In rings like Saturn's, tidal forces from the planet prevent aggregation of the solids. But planetary rings spread. Beyond the Roche limit, the tides are too weak, and solids agglomerate into new moons. These moons are then repelled by the rings and migrate outwards. I this talk, I will describe this process, and show that it leads (if the spreading is slow) to the formation of a retinue a satellites, whose mass-distance distribution follows a particular law. This law turns out to match the observed distribution of the satellites of Saturn, Uranus, ans Neptune. Thus, we conclude that almost all Saturn's regular satellites were born from the rings, and that Uranus and Neptune used to have massive rings that gave birth to their satellites. In contrast, if the spreading of the rings is fast, only one massive moon forms ; this has been the case around the Earth after the Moon-forming impact. In the end, the spreading of massive rings beyond the Roche limit of a planet is a generic mechanism for satellite formation, that unifies giant and terrestrial planets. Most regular satellites in the Solar System formed this way.