Tag Archives: Pluto

Asteroids: Trans-Neptunian Objects

Pluto-Charon is dynamically packed

Leave a comment

Hi there! Today, we leave the comets for the system of Pluto-Charon. Of course, you know Pluto. Formerly the 9th planet of our Solar System, until 2006, it remains an object of interest. So interesting that it has been visited by the spacecraft New Horizons in July 2015. You know, the same spacecraft which gave us these amazing images of Ultima Thule (also known as 2014 MU69).
Anyway, we are not done with Pluto. It has a large satellite, Charon, which makes Pluto-Charon a binary object, i.e. Pluto and Charon orbit about a common barycenter, which is significantly outside of Pluto. And around this binary, you have (at least) 4 small satellites, which are Styx, Nix, Kerberos and Hydra. I say at least, because the authors of the study I present today address the following question: could there be more? I mean, if you add a satellite somewhere, will it survive? If no, then you can say that the system is dynamically packed. This the opportunity for me to present A Pluto-Charon sonata: The dynamical architecture of the circumbinary satellite system, by Scott J. Kenyon and Benjamin C. Bromley. This study has recently been published in The Astronomical Journal.

Outline

The binary Pluto-Charon
Simulations with Orchestra
Probably nothing inside the orbit of Hydra
Could there be something outside?
The study and its authors

The binary Pluto-Charon

I guess you have already heard of the discovery of Pluto by Clyde Tombaugh in 1933 (see here). It appeared that Pluto had been observed at least 16 times before, the first of these precoveries dating back to 1909.
The launch of the spacecraft New Horizons in 2006 motivated the observations of the binary Pluto-Charon by the most efficient observing facilities, in particular the Hubble Space Telescope. This telescope permitted the discoveries of 4 moons of the binary: Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012. You can find some of their properties below.

Discovery Diameter Semimajor axis Orbital period Spin period
Pluto 1933 2376.6 km 39.48 AU 248 years 6.39 days
Charon 1978 1212 km 19591 km 6.39 days 6.39 days
Styx 2012 16x9x8 km 42656 km 20.16 days 3.24 days
Nix 2005 53x41x36 km 48694 km 24.85 days 1.83 d
Kerberos 2011 19x10x9 km 57783 km 32.17 days 5.31 days
Hydra 2005 65x45x25 km 64738 km 38.20 days 10.3 hours

As you can see, the binary Pluto-Charon is doubly synchronous, i.e. Pluto and Charon have the same spin (rotation) period, and Charon has that same orbital period around Pluto. It would be accurate to say that Pluto and Charon have both this orbital period around their common barycenter. It can be shown that this state corresponds to a dynamical equilibrium, which itself results from the dissipation of rotational and orbital energy by the tidal interaction between Pluto and Charon.

However, the four other moons are much smaller, and much further from Charon. They spin much faster than they orbit, which means that the tides were not efficient enough to despin them until synchronization. Hydra spins in hours, while the others ones, which are closer to the binary, spin in days. So, they may have despun a little after all, but not enough.

Hydra as seen from NASA’s New Horizons spacecraft. © NASA/JHUAPL/SwRI
Hydra as seen from NASA’s New Horizons spacecraft. © NASA/JHUAPL/SwRI

No additional moon has been discovered since, even by New Horizons. The authors wonder whether that would be possible or not. For that, they ran intensive numerical simulations.

Simulations with Orchestra

They disposed of the numerical code Orchestra, which they developed themselves. This code is composed of several modules, permitting

  • N-body simulations,
  • to simulating planetary formation, especially the growth of the accreting bodies.

For this specific study, the authors considered only the N-body simulations. For that, they added massless particles in the binary, i.e. these particles were perturbed by the gravitational action of Pluto, Charon, and their four small moons. The simulations were ran over several hundreds of Myr.

I would like the reader to be aware that the stability, i.e. survival, of such particles is not trivial at all. You can imagine that if you come too close to a satellite, then you might be ejected. But this is not the only possible cause for ejection.

In such a system, you have many mean motion resonances. Imagine, for instance, that you are a massless particle (happy to be massless, aren’t you? trust me, it is not that fun), and that you orbit around Pluto-Charon exactly twice faster than Hydra (this is just an example). Every two orbits, your closest distance with Hydra will be at the same place. This will result in cumulative effects of Hydra on you, and since you are massless, you are very sensitive to these effects (which are actually a gravitational perturbation). And the outcome is: you might be ejected. Let us see now the results of the simulations.

Probably nothing inside the orbit of Hydra

Yes, because of these resonances, most of the massless particles orbiting inner to Hydra are unstable. In fact, some of them may survive, but in specific locations: either inner to the orbit of Styx, which is the innermost of the small moons, or outside the orbit of Hydra, i.e. outside of the known boundaries of the binary. In-between, you may have some particles, which would be coorbital to the small moons. This phenomenon of 1:1 mean-motion resonances appears in several locations of the Solar System. For instance, Jupiter has its Trojan asteroids, with which it shares its orbit. This also happens among the satellites of Saturn. Why not around Pluto-Charon? Well, you have to see them to be convinced they exist. These simulations just give you a theoretical possibility, i.e. this is not impossible.
Anyway, the preferred locations for yet-undiscovered moons is outside the orbit of Hydra. The challenge would be to discover such objects. Inside, the system appears to be dynamically packed.

Could there be something outside?

The authors present a discussion on the future possibility to detect them. First, they mention the stellar occultations.
Imagine the system of Pluto-Charon gets aligned between a terrestrial observer and a distant star. Then you can hope that, if there is something which is still unknown in that system, then it may occultate the light of the star, at least to some terrestrial observers. Of course, this may vary on from where on Earth you observe. For such a discovery to happen, you must be very lucky. But remember that the rings of Chariklo and Haumea were discovered that way.

Another hope for discovery is in the future instruments. The authors mention the JWST (Jawes Webb Space Telescope), which should be launched in March 2021. A kind of upgrade to HST (Hubble), its primary having a diameter of 6.5 meters, instead of 2.4 for Hubble. Moreover, it will be more efficient in the infrared, but unable to observe in the ultraviolet.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter, Facebook, Instagram, and Pinterest.

Dwarf planets

The chemistry of Pluto

Leave a comment

Hi there! The famous dwarf planet Pluto is better known to us since the flyby of the spacecraft New Horizons in 2015. Today, I tell you about its chemistry. I present you Solid-phase equilibria on Pluto’s surface, by Sugata P. Tan & Jeffrey S. Kargel, which has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

Outline

The atmosphere of Pluto
The surface of Pluto
An Equation Of State
Results
The study and its authors

The atmosphere of Pluto

I do not want here to recall everything about Pluto. This is a dwarf planet, which has been discovered by Clyde Tombaugh in 1930. It orbits most of the time outside the orbit of Neptune, but with such an eccentricity that it is sometimes inside. It was discovered in 1978 that Pluto has a large satellite, Charon, so large that the system Pluto-Charon can be seen as a binary object. This binary has at least 4 small satellites, which were discovered thanks to the Hubble Space Telescope.

Pluto has a tenuous atmosphere. It was discovered from the Earth in 1985 in analyzing a stellar occultation: when a faint, atmosphereless object is aligned between a star and a observer, the observer does not see the star anymore. However, when the object has an atmosphere, the light emitted by the star is deviated, and can even be focused by the atmosphere, resulting in a peak of luminosity.

Several occultations have permitted to constrain the atmosphere. It has been calculated that its pressure is about 15 μbar (the one of the Earth being close to 1 bar, so it is very tenuous), and that it endured seasonal variations. By seasonal I mean the same as for the Earth: because of the variations of the Sun-Pluto distance and the obliquity of Pluto, which induces that every surface area has a time-dependent insolation, thermic effects affect the atmosphere. This can be direct effects, i.e. the Sun heats the atmosphere, but also indirect ones, in which the Sun heats the surface, triggering ice sublimation, which itself feeds the atmosphere. The seasonal cycle, i.e. the plutonian (or hadean) year lasts 248 years.

Observations have shown that this atmosphere is hotter at its top than at the surface, i.e. the temperature goes down from 110 K to about 45 K (very cold anyway). This atmosphere is mainly composed of nitrogen N2, methane CH4, and carbon monoxide CO.

The surface of Pluto

The surface is known to us thanks to New Horizons. Let me particularly focus on two regions:

  • Sputnik Planitia: this is the heart that can be seen on a map of Pluto. It is directed to Charon, and is covered by volatile ice, essentially made of nitrogen N2,
  • Cthulhu Regio: a large, dark reddish macula, on which the volatile ice is absent.
A map of Pluto (mosaic made with New Horizons data). © NASA
A map of Pluto (mosaic made with New Horizons data). © NASA

The reason why I particularly focus on these two regions is that they have two different albedos, i.e. the bright Sputnik Planitia is very efficient to reflect the incident Solar light, while Cthulhu Regio is much less efficient. This also affects the temperature: on Sputnik Planitia, the temperature never rises above 37 K, while it never goes below 42.5 K in Cthulhu Regio. We will see below that it affects the composition of the surface.

An Equation Of State

The three main components, i.e. nitrogen, methane, and carbon monoxide, have different sublimation temperatures at 11μbar, which are 36.9 K, 53 K, and 40.8 K, respectively (sublimation: direct transition from the solid to the gaseous state. No liquid phase.). A mixture of them will then be a coexistence of solid and gaseous phases, which depends on the temperature, the pressure, and the respective abundances of these 3 chemical components. The pressure is set to 11μbar, since it was the pressure measured by New Horizons, but several temperatures should be considered, since it is not homogeneous. The authors considered temperatures between 36.5 K and 41.5 K. Since the atmosphere has seasonal variations, a pressure of 11μbar should be considered as a snapshot at the closest encounter with New Horizons (July 14, 2015), but not as a mean value.

The goal of the authors is to build an Equation Of State giving the phases of a given mixture, under conditions of temperature and pressure relevant for Pluto. The surface is thus seen as a multicomponent solid solution. For that, they develop a model, CRYOCHEM for CRYOgenic CHEMistry, which aims at predicting the phase equilibrium under cryogenic conditions. The paper I present you today is part of this development. Any system is supposed to evolve to a minimum of energy, which is an equilibrium, and the composition of the surface of Pluto is assumed to be in thermodynamic equilibrium with the atmosphere. The energy which should be minimized, i.e. the Helmholtz energy, is related to the interactions between the molecules. A hard-sphere model is considered, i.e. a minimal distance between two adjacent particles should be maintained, and for that the geometry of the crystalline structure is considered. Finally, the results are compared with the observations by New Horizons.

Such a model requires many parameters. Not only the pressure and temperature, but also the relative fraction of the 3 components, and the parameters related to the energies involved. These parameters are deduced from extrapolations of lab experiments.

Results

The predicted coexistence of states predicted by this study is consistent with the observations. Moreover, it shows that the small fraction of carbon monoxide can be neglected, as the behavior of the ternary mixture of N2/CH4/CO is very close to the one of the binary N2/CH4. This results in either a nitrogren-rich solid phase, for the coolest regions (the bright Sputnik Planitia, e.g.), and a methane-rich solid phase for the warmest ones, like Cthulhu Regio.

Developing such a model has broad implications for predicting the composition of bodies’s surfaces, for which we lack of data. The authors give the example of the satellite of Neptune Triton, which size and distance to the Sun present some similarities with Pluto. They also invite the reader to stay tuned, as an application of CRYOCHEM to Titan, which is anyway very different from Pluto, is expected for publication pretty soon.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Dwarf planets

Matter exchange in Pluto’s backyard

2 Comments

Hi there! Today I will tell you about a recent study accepted for publication in The Monthly Notices of the Royal Astronomical Society. This study, by Rachel A. Smullen and Kaitlin M. Kratter, addresses The fate of debris in the Pluto-Charon system, and has been conducted at Steward Observatory, AZ (USA).

The Pluto-Charon system

Pluto has been discovered by Clyde Tombaugh in 1930 at Lowell Observatory in Flagstaff, Arizona. This was the first discovered object of the Kuiper Belt, and it has been considered as the 9th planet of the Solar System until 2006. Still in Flagstaff, its satellite Charon has been discovered in 1978. Later on, the next arrival of the New Horizons spacecraft motivated observing this system, and thanks to the Hubble Space Telescope, 4 other small moons were discovered: Nix and Hydra in 2005, and Styx and Kerberos in 2012. You can find below images of these 6 bodies taken by New Horizons, and some of their orbital and shape parameters.

Pluto (left) and Charon (right) seen by New Horizons. The white heart on Pluto’s surface is an ice-covered basin named Sputnik Planitia, while the dark spot on Charon’s north pole is named Mordor macula. Copyright: NASA
The small moons, seen by New Horizons. Copyright: NASA
Discovery Radius Distance
Pluto 1930 1187 km 0
Charon 1978 606 km 19571 km
Hydra 2005 23 km 64738 km
Nix 2005 18 km 48694 km
Kerberos 2012 5 km 57783 km
Styx 2012 5 km 42656 km

The pair Pluto-Charon is fascinating from a dynamical point of view, since they represent a case of double synchronous spin-orbit resonance. You know that the Moon is always showing the same face to the Earth, which is due to its synchronous rotation. This means that its orbital period around the Earth is exactly the same as its rotation period, this is a dynamical equilibrium which has been reached after tides had dissipated the rotational energy of the Moon. But the phenomenon goes further for Pluto-Charon, since not only Charon shows the same face to Pluto, but Pluto shows the same face to Charon! This is a consequence of the relative size of the two bodies, each of them being sufficiently large to affect the other one.
On the contrary, the small moons have a much more rapid rotation, which is less obvious to explain.

The system of Pluto has been visited in 2015 by the spacecraft New Horizons, which gave us invaluable data and the nice images I show you today.

The Plutinos

The orbit of this system around the Sun is interested as well. Not only it has a significant inclination (17.16° wrt ecliptic), but it is also in a 3:2 mean-motion resonance (MMR) with Neptune. This means that Pluto makes exactly two revolutions around the Sun while Neptune makes three. Moreover, this is a pretty stable dynamical zone. This is probably why Pluto and its satellites are not the only bodies in this zone. Beside the Pluto system, the first Plutino has been discovered in 1993 at the Mauna Kea Observatory, HI.
The following figure gives a repartition of the known Trans-Neptunian Objects with respect to their semimajor axis, the Plutinos represent a peak at 39 astronomical units.

Distribution of the Kuiper-Belt Objects, plotted from the data of the Minor Planet Center, consulted on January 28th 2017. We see the Plutinos as an accumulation of objects close to 39 AU, which corresponds to the 3:2 MMR with Neptune. The second peak, close to 44 AU, does not correspond to a known resonance. Copyright: The Planetary Mechanics Blog.

Formation of planetary debris disks

The last thing I would like to tell you before presenting the study itself is: how to make a debris disk around a pretty massive body? It is thought to come from an impact. An impactor impacts the target, is destroyed into very small parts, which coalesce into rings, before eventually reaccreting and / or being ejected. The most famous debris disk in the Solar System in the system of the Saturnian rings, but there are actually rings about the four giant planets of the Solar System, and the Centaurs (asteroids between the orbits of Jupiter and Neptune) Chariklo and possibly Chiron.
It is thought that the Moon is the consequence of such a process, i.e. there has been a debris disk around the Earth. And it is also thought that Charon has been created the same way.

This paper

This study aims at understanding the fate of the debris disk which has created Charon. Once enough debris accreted to create Charon, or a proto-Charon, debris remained, and have been ejected. There are at least two ways to model a disk: either you consider it as a gas, i.e. some fluid, or you see it as a cloud of many particles, which interact. These interactions are close encounters and collisions, with translate into viscosity if you model the disk as a gas.

A numerical study

In this study, the authors chose to model the debris disk as a cloud of particles, which is probably the only way to model the path of ejecta. They made several simulations involved 27060 test particles, over 27.3 kyr, i.e. 1.5 million orbits of Pluto and Charon about their common barycenter. Such a study requires high performance soft- and hardware. Their code was based on the integrator Mercury, which is a commonly used N-body code modeling the motion of N body which interact gravitationally and may collide. The test particles are massless, so they have no gravitational action, but they are under the action of Pluto and its 5 satellites. In some of the tests, a migration of Pluto, which is predicted by models of formation of the Solar System, has also been considered.
The hardware is the Super-computer El-Gato (Extremely LarGe Advanced TechnOlogy), based at the University of Arizona, and partly funded by the National Science Foundation.

Once the simulations have run, the authors got the results. And the results are… drum roll please…

Making craters on Charon

The New Horizons images show that Charon is craterized. In all of their simulations, the authors have collisions between Charon and the debris disk. They show that the impact rate is higher if Charon formed on a wide and eccentric orbit. Moreover, they have fewer impacts if secular migration of Pluto is considered.
An issue is: what could be the signature of such an impact now? We know from its synchronous rotation and from the ridges at its surface that Charon has been hot. Hot enough would mean that part of its surface could have been renewed, and then the older impacts would have no signature anymore. Moreover, it would be interesting, but I doubt the information is present in the New Horizons data, to map the impact on the whole surface of Charon. If Charon was synchronous during the most intense episode of impacts, then we would expect a hemispheric repartition of the craters.

Making Plutinos

The simulations show that the most probable destination of the ejected debris is the 3:2 MMR with Neptune. This means that the observed Plutinos could originate from the impact which created Charon. This would mean that the Plutinos are a collisional family, which could be test from their composition. It should be similar to Pluto’s.

And the small moons?

The simulations do not manage to form the small moons. So, the question of their origin is still open.

Some links…

And that’s it for today! New Horizons is en-route to the asteroid 2014 MU69, which would be the first object visited by a spacecraft which had been launched before its discovery. It should reach it either on December 31th, 2018, or January 1st 2019.
Please feel free to comment. You can also subscribe to the RSS feed, and follow me on Twitter.