Tag Archives: Cassini

Planets

Thunderstorms on Saturn

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Hi there! You know the thunderstorms on our Earth. In fact, you can have thunderstorms once you have an atmosphere. And you have many atmospheres in our Solar System, particularly on the giant planets. Today we describe a thunderstorm on Saturn, which happened between November 2007 and July 2008, and was observed by Cassini. This thunderstorm is described in Analysis of a long-lived, two-cell lightning storm on Saturn, by G. Fischer et al. This study will be published soon in Astronomy and Astrophysics.

Outline

Physics of a thunderstorm
The atmosphere of Saturn
The detected episodes
Radio and optical observations
The storm F
The study and its authors

Physics of a thunderstorm

Basically, a thunderstorm results from the encounter between cool and hot air. For instance, after a hot summer day, you have hot air in the low atmosphere, while colder air is brought by the wind. Then the hot air, which is lighter, gains altitude. This convective motion induces displacements of electric charges, and so a difference of electrostatic potential between the ground and the top atmosphere. This difference in electrostatic potential creates electric lightning, which actually balances the charges between the sky and the ground. All this results in unstable weather conditions, in particular rain and strong wind. The rain is due to the moist contained in the hot air, which coalesced as clouds while gaining altitude. Thunderstorms are among the most dangerous natural phenomena.

As I said, you can have thunderstorms on any planet with an atmosphere. Today, we are on Saturn.

The atmosphere of Saturn

The radius of Saturn is about 60,000 km, which corresponds to the distance to the center, where the atmospheric pressure reaches 1 bar. At its center Saturn has probably a rocky core, which radius is about 25,000 km. This leaves room for a very thick atmosphere, i.e. what I would call the Saturnian air, mainly composed of molecular hydrogen and helium. Interestingly for us, there are clouds in the atmosphere of Saturn, which composition depend on the altitude, itself correlated with the pressure. The less dense clouds (up to 2 bars), in the upper atmosphere, mainly consist of ammonia ice, while denser clouds contain water ice. The densest clouds, which pressure exceeds 9.5 bars, contain water droplets with ammonia in aqueous solution.

The winds on Saturn are very strong, i.e. up to 1,800 km/h, or 1,120 mph, which of course facilitates the encounters between different air masses (with different temperatures). Moreover, the atmosphere of Saturn is organized into parallel bands, as is the atmosphere of Jupiter. These bands rotate at slightly different rates, which prompted the International Astronomical Union to define 3 reference systems for the rotation of Saturn:

  • System I: spin period of 10 h14 min for the equatorial bands,
  • System II: spin period of 10 h 39 min 24 s, at the other latitudes,
  • System III: spin period of 10 h 39 min 22.3 s, for the radio emissions.

The detected episodes

To be honest with you, I did not manage to get an exhaustive list of the detected events. By the way, if you have some information, I would be glad to get it. You can comment at the end of this article.

You can find below a list of thunderstorms, which have been detected by the Cassini spacecraft between 2004 and 2010. The study we discuss today is on the Storm F.

  • Storm 0: May 26–31, 2004
  • Storm A: July 13–27, 2004
  • Storm B: August 3–15, 2004
  • Storm C: Sept. 4–28, 2004
  • Storm D: June 8–15, 2005
  • Storm E: Jan. 23 – Feb. 23, 2006
  • Storm F: Nov. 27, 2007 – July 15, 2008
  • Storm G: Nov. 19 – Dec. 11, 2008
  • Storm H: Jan. 14 – Dec. 13, 2009
  • Storm I: Feb. 7 – July 14, 2010

These events were identified in detecting radio emissions, due to Saturn electrostatic discharges (SEDs for short). Before that, the Voyager spacecrafts have detected SEDs in 1980 and 1981, but attributed their origins to impacts in the rings. Since then, other events have been detected. In particular, Great White Spot events, i.e. huge disturbance encircling the planets, can be seen from the Earth. They seem to appear roughly every 30 years, which could be correlated with the duration of Saturn’s year (29.46 years). The last Great White Spot has been observed in 2010-11.

The Great White Spot observed by Cassini in February 2011. Credit: NASA/JPL-Caltech/Space Science Institute
The Great White Spot observed by Cassini in February 2011. Credit: NASA/JPL-Caltech/Space Science Institute

Radio and optical observations

As I said, these events are usually detected thanks to their radio emissions. For that, Cassini disposed of the Radio and Plasma Wave Science (RPWS) instrument, equipped with a High Frequency Receiver.
This receiver listened to Saturn in 3 different modes alternatively, allowing to cover a pretty wide range between 325 and 16025 kHz.
These radio measurements were supplemented by optical observations by the Cassini ISS (Imaging Science Subsystem), by optical observations from Earth, and even by Earth-based radiotelescopes, for the strongest discharges.

The detection of such events strongly depends on the location of the spacecraft with respect to the storm. When the spacecraft is opposite to the storm, you detect almost nothing. Almost, because measuring radio emissions permits over-the-horizon detection, especially when the SED storm is located on the night side (opposite the Sun) and Cassini on the day side. This could be due to a temporary trapping of the radio waves below Saturn’s ionosphere before they are released.

So, Cassini’s RPWS detects the discharges, ISS and the Earth-based telescopes see the storms… Let us see the results for the Storm F (November 2007 to July 2008).

The Storm F

RPWS detected about 277,000 SEDs related to this Storm F. But the analysis of the images revealed two phases.

One or two events?

From November 2007 to March 2008, ISS saw one convection cell, at the latitude of ~35° south. And in March 2008 a second cell appeared, at roughly the same latitude, and separated from the first cell by about 25° in longitude. These two cells drifted both of about 0.35° per day. The presence of these two cells with a correlated motion makes this event a very interesting one… and the authors also detected dark ovals.

Dark ovals

A storm appears as a a bright spot, while a dark oval is a dark one. Several dark ovals were seen, the largest one, nicknamed S3 drifted by 0.92° per day, i.e. much faster than the storms. These dark ovals have probably no SED activity. Several explanations have been proposed to explain these features. They could either be clouds of carbon soot particles, produced by the dissociation of methane in the lightning channels, or remnants of convection cells, in which the ammonia particles have fallen deeper into the atmosphere, leaving darker spots.

Features related to the Storm F. The rectangle focuses on the so-called Storm Alley. This image was taken by Cassini ISS on 23 April 2008. © NASA
Features related to the Storm F. The rectangle focuses on the so-called Storm Alley. This image was taken by Cassini ISS on 23 April 2008. © NASA

So, this paper describes the event. The physics behind still needs some clarification, so you can be sure that devoted papers will follow. Stay tuned!

The study and its authors

You can find the study here. And now, the 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.

Satellites of Saturn

Spatial variations of Enceladus’ plumes

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Hi there! I guess most of you have heard of Enceladus. This mid-sized icy satellite of Saturn arouses the interest of planetologists, because of its geological activity. Permanent eruptions of plumes, essentially made of water ice, have been detected at its South Pole, by the Cassini spacecraft. The study I present you today, Spatial variations in the dust-to-gas ratio of Enceladus’ plume, by M.M. Hedman, D. Dhingra, P.D. Nicholson, C.J. Hansen, G. Portyankina, S. Ye and Y. Dong, has recently been published in Icarus.

Outline

The South Pole of Enceladus
The instruments UVIS & VIMS of Cassini
Analyzing a Solar occultation by the plumes
Spatial variations detected
The study and its authors

The South Pole of Enceladus

Enceladus orbits around Saturn in one day and 9 hours, at a mean distance of 238,000 km. It is the second of the mid-sized satellites of Saturn by its distance from the planet, and is in an orbital 2:1 resonance with Dione, i.e. Dione makes exactly one revolution around Saturn while Enceladus makes 2. This results in a slight forcing of its orbital eccentricity, which remains anyway modest, i.e. 0.005. Like our Moon and many satellites of the giant planets, Enceladus rotates synchronously.

Interestingly, the Cassini spacecraft detected geysers at the South Pole of Enceladus, and fractures, which were nicknamed tiger stripes. They were named after 4 Middle East cities: Alexandria, Cairo, Baghdad, and Damascus.

The South Pole of Enceladus. We can see from left to right the famous tiger stripes, i.e. Alexandria, Cairo, Baghdad and Damascus sulci. © NASA/JPL/Space Science Institute/DLR
The South Pole of Enceladus. We can see from left to right the famous tiger stripes, i.e. Alexandria, Cairo, Baghdad and Damascus sulci. © NASA/JPL/Space Science Institute/DLR

These 4 fractures are 2km-large and 500m-deep depressions, which extend up to 130 km. The plumes emerge from them. Interior models suggest that the source of these geysers is a diapir of water, located at the South Pole.

Analysis of these plumes require them to be illuminated, and observed with spectroscopic devices. This is where the instruments UVIS and VIMS get involved.

The instruments UVIS & VIMS of Cassini

The study I present you today presents an analysis of VIMS data, before comparing the results of the same event given by UVIS.

UVIS and VIMS are two instruments of the Cassini mission, which completed a 13-years tour in the system of Saturn in September 2017 with its Grand Finale, crashing in the atmosphere of Saturn. It was accompanied by the lander Huygens, which landed on Titan in 2005, and had 12 instruments on board. Among them were UVIS and VIMS.

And then, you wonder, dear reader, whether I will introduce you UVIS and VIMS, since I mention them since the beginning without introducing them. Yes, this is now.

UVIS stands for Ultraviolet Imaging Spectrograph, and VIMS for Visible and Infrared Mapping Spectrometer. Their functions are in their names: both analyze the incoming light, UVIS in the ultraviolet spectrum, and VIMS in the visible and infrared ones. And the combination of these two spectra is relevant in this study: the analysis in the ultraviolet tells you one thing (quantity of gas), while the analysis in the infrared gives the quantity of dust. When you compare them, you have the dust-to-gas ratio. Of course, this is not that straightforward. First you have to collect the data.

Analyzing a Solar occultation by the plumes

As I said, the plume needs to be illuminated. And for that, you have to position the spacecraft where the plumes occult the Sun. So, this could happen only during a fly-by of Enceladus, which means that it was impossible to have a permanent monitoring of these plumes. Moreover, from the geometry of the configuration, i.e. location of the plume, of the Sun, of the spacecraft,… you had the data at a given altitude. It is easy to figure out that the water is more volatile than dust, is ejected faster, and higher… In other words, the higher is the observation, the lower the dust-to-gas ratio.

The studied occultation happened on May 18, 2010, and lasted approximately 70 seconds, during which the illuminated plumes originated from different tiger stripes. This means that a temporal variation of the composition of plumes during the event means a spatial variation in the subsurface of the South Pole. The altitude was 20-30 km.

But detecting a composition is a tough task. Actually the UVIS data, i.e. detection of water, were published in 2011, and the VIMS ones (detection of dust) only in 2018, probably because the signal is very weak. The authors observed a Solar spectrum in the infrared, and at the exact date of the occultation, a slight flux drop occurred, which was the signature of a dusty plume. For it to be exploitable, the authors had to treat the signal, i.e. de-noise it.

After this treatment, the resulting signal was an optical depth in 256 spectral channels between 0.85 an 5.2 microns. You then need to compare it with a theoretical model of diffraction by micrometric particles, the Mie diffraction, to have an idea of the particle-size distribution. Because the particles do not all have the same size, of course! Actually, the distribution is close to a power law of index 4.

Spatial variations detected

And here is the results: at an altitude of around 25 km, the authors have found that the material emerging from Baghdad and Damascus are up to one order of magnitude, i.e. 10 times, more particle-rich than the ones emerging from Alexandria and Cairo sulci.

It is not straightforward to draw conclusions from this single event. Once more, a permanent monitoring of the plumes was impossible. Spatial variations of the dust-to-gas ratio at a given altitude could either mean something on the variations of the dust-to-gas ratio in the subsurface diapir, and/or something on the spatial variations of the ejection velocities of dust and gas. Once more, the ratio is expected to decline with the altitude, since the water is more volatile.

We dispose of data from other events, for instance a fly-by, named E7, which occurred in November 2009, of the South Pole at an altitude of 100 km, during which the Ion and Neutral Mass Spectrometer (INMS) analyzed the plumes. The data are pretty consistent with the ones presented here, but the altitude is very different, so be careful.

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.

Rings of Saturn

A constantly renewed ring of Saturn

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Hi there! The outstanding Cassini mission ended last September with its Grand Finale, and it gave us invaluable data, which will still be studied for many years. Today I present you a study which has recently been published in The Astrophysical Journal: Particles co-orbital to Janus and Epimetheus: A firefly planetary ring, by a Brazilian team composed of Othon C. Winter, Alexandre P.S. Souza, Rafael Sfair, Silvia M. Giuliatti Winter, Daniela C. Mourão, and Dietmar W. Foryta. This study tells us how the authors characterized a dusty ring in the system of Saturn, studied its stability, and investigated its origin.

Outline

The rings of Saturn
Janus and Epimetheus
Images of a new ring
Dynamical stability
Renewing the ring
A firefly behavior
The study and its authors

The rings of Saturn

As you may know, Saturn is the ringed planet, its rings being visible from Earth-based amateur telescopes. Actually, the 4 major planets of our Solar System have rings, and some dwarf planets as well, i.e. Chariklo, Haumea, and possibly Chiron. But Saturn is the only one with so dense rings. I summarize below the main relevant structures and distances, from the center of Saturn:

Distance Structure
60,268 km The atmospheric pressure of Saturn reaches 1 bar.
This is considered as the equatorial radius of Saturn.
66,900 – 74,510 km D Ring
74,658 – 92,000 km C Ring
92,000 – 117,580 km B Ring
117,580 – 122,170 km Cassini Division
122,170 – 136,775 km A Ring
133,589 km Encke Gap
140,180 km F Ring
151,500 km Orbits of Janus and Epimetheus
189,000 km Orbit of Mimas
1,222,000 km Orbit of Titan

The A and B Rings are the densest ones. They are separated by the Cassini Division, which appears as a lack of material. It actually contains some, arranged as ringlets, but they are very faint. The Encke Gap is a depletion of material as well, in which the small satellite Pan confines the boundaries. Here we are interested in a dusty ring enshrouding the orbits of Janus and Epimetheus, i.e. outside the dense rings. The discovery of this ring had been announced in 2006, this study reveals its characteristics.

The rings of Saturn seen by Cassini. From right to left: the A Ring with the Encke Gap, the Cassini Division, the B Ring, the C Ring, and the D Ring. © NASA
The rings of Saturn seen by Cassini. From right to left: the A Ring with the Encke Gap, the Cassini Division, the B Ring, the C Ring, and the D Ring. © NASA

Janus and Epimetheus

The two coorbital satellites Janus and Epimetheus are a unique case in the Solar System, since these are two bodies with roughly the same size (diameters: ~180 and ~120 km, respectively), which share the same orbit around Saturn. More precisely, they both orbit Saturn in 16 hours, i.e. at the same mean orbital frequency. This is a case of 1:1 mean-motion resonance, involving peculiar mutual gravitational interactions, which prevent them from colliding. They swap their orbits every four years, i.e. the innermost of the two satellites becoming the outermost. The amplitudes of these swaps (26 km for Janus and 95 for Epimetheus) have permitted to know accurately the mass ratio between them, which is 3.56, Janus being the heaviest one.

Interestingly, Epimetheus is the first among the satellites of Saturn for which longitudinal librations have been detected. As many natural satellites, Janus and Epimetheus have a synchronous rotation, showing the same face to a fictitious observer at the surface of Saturn. For Epimetheus, large librations have been detected around this direction, which are a consequence of its elongated shape, and could reveal some mass inhomogeneities, maybe due to variations of porosity, and/or to its pretty irregular shape.

Janus and Epimetheus seen by Cassini (mosaic of 2 images). © NASA
Janus and Epimetheus seen by Cassini (mosaic of 2 images). © NASA

Images of a new ring

So, Cassini images have revealed a dusty ring in that zone. To characterize it, the authors have first extracted images likely to contain it. Such images are made publicly available on NASA’s Planetary Data System. Since that ring had been announced to have been observed on Sept 15th 2006 (see the original press release), the authors restricted to 2 days before and after that date. The data they used were acquired by the ISS (Imaging Science Subsystem) instrument of Cassini, more precisely the NAC and WAC (Narrow- and Wide-Angle-Camera). They finally found 17 images showing the ring.

The images are given as raw data. The authors needed to calibrate their luminosity with a tool (a software) provided by the Cassini team, and sometimes to smooth them, to remove cosmic rays. Moreover, they needed to consider the position of the spacecraft, to be able to precisely locate the structures they would see.

One of the Cassini images used by the authors. I have added red stars at the location of the ring. © NASA / Ciclops
One of the Cassini images used by the authors. I have added red stars at the location of the ring. © NASA / Ciclops

It appears that the ring presents no longitudinal brightness variation. In other words, not only this is a whole ring and not just an arc, but no density variation is obvious. However, it presents radial brightness variations, over a width of 7,500 km, which is wider than the 5,000 km announced in the 2006 press release.

The next step is to understand the dynamics of this ring, i.e. its stability, its origin, the properties of the particles constituting it… Let us start with the stability.

The ring is removed in a few decades

The authors ran N-body simulations, i.e. numerical integrations of the equations ruling the motion of a ring particle, which would be gravitationally perturbed by the surrounding bodies, i.e. Saturn, and the Janus, Epimetheus, Mimas, Enceladus, Tethys, Dione, and Titan. Moreover, for a reason that I will tell you at the end of this article, the authors knew that the particles were smaller than 13 μm. The motions of such small particles are affected by the radiation pressure of the Sun, in other words the Solar light pushes the particles outward.

The authors simulated 14 times the motion of 18,000 particles equally distributed in the rings. Why 14 times? To consider different particle sizes, i.e. one set with 100 μm-sized particles, and the other sets with sizes varying from 1μm to 13μm.
And it appears that these particles collide with something in a few decades, mostly Janus or Epimetheus. This leaves two possibilities: either we were very lucky to be able to take images of the ring while it existed, or a process constantly feeds the ring. The latter option is the most probable one. Let us now discuss this feeding process.

Renewing the ring

The likeliest sources of material for the rings are ejecta from Janus and Epimetheus. The question is: how were these ejecta produced? By impacts, probably. This study show that Janus and Epimetheus are impacted by the particles constituting the rings, but the impact velocities would not permit to produce ejecta. This is why the authors propose a model, in which interplanetary particles collide with the satellites, generating ejecta.

A firefly behavior

And let us finish with something funny: the ring seems to behave like a firefly, i.e. sometimes bright, and sometimes dark, which means undetectable while present.
To understand what happens, figure out how the light would cross a cloud of particles. If the cloud is dense enough, then it would reflect the light, and not be crossed. But for dust, the light would be refracted, i.e. change its direction. This depends on the incidence angle of the Solar light, i.e. on the geometrical configuration of the Sun-Saturn-ring system. The Solar incidence angle is also called phase. And this phase changes with the orbit of Saturn, which results in huge brightness variations of the ring. Sometimes it can be detected, but most of the time it cannot. This can be explained and numerically estimated by the Mie theory, which gives the diffusion of light by small particles. This theory also explains the creation of rainbows, the Solar light being diffracted by droplets of water.

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.

Satellites of Saturn

Water-ice boundary on Titan

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Hi there! Titan may be the most famous satellite in the Solar System, I realize that I never devoted a post to it. It is high time to make it so. I present you Does Titan’s long-wavelength topography contain information about subsurface ocean dynamics? by Jakub Kvorka, Ondřej Čadek, Gabriel Tobie & Gaël Choblet, which has recently been accepted for publication in Icarus. This paper tries to understand the mechanisms responsible for the location of the boundary between the icy crust and the subsurface ocean. This affects the thickness of the crust, which itself affects the topography of Titan.

Outline

Titan
The quest for the internal ocean
Modeling the ice-water boundary
Resolution by spectral decomposition
The role of the grain size
In the future
The study and its authors

Titan

The existence of Titan is known since 1655 thanks to the Dutch astronomer Christiaan Huygens. It was the only known satellite of Saturn until the discovery of Iapetus in 1671. It is the second largest natural satellite of the Solar System (mean radius: 2,575 km), and it orbits Saturn in almost 16 days, on a 3% eccentric and almost equatorial orbit (actually, a small tilt is due to the gravitational influence of the Sun).

It has interesting physical characteristics:

  • A thick atmosphere (pressure at the surface: 1.5 bar) mainly composed of nitrogen, with clouds of methane and ethane.
  • A complex surface. We can find hydrocarbon seas, i.e. lakes of methane and ethane (Kraken Mare, Ontario Lacus…), we also have a mountain chain, which features have been named after Tolkien’s Lords of the Rings (Gandalf Colles, Erebor Mons,…). There are some impact craters as well, but not that many, which suggests a geologically young surface. There is probably cryovolcanism on Titan, i.e. eruptions of volatile elements. The surface and the atmosphere interact, i.e. there are exchange between the liquid methane and ethane of the lakes and the gaseous ones present in the atmosphere, and the atmosphere is responsible for erosion of the surface, for winds which are likely to create dunes, and for heat exchanges.
  • A global subsurface ocean, lying under the icy crust.
Map of Titan.
Map of Titan.

The quest for the internal ocean

An internal, water ocean is considered to be of high interest for habitability, i.e. we cannot exclude the presence of bacteriological life in such an environment. This makes Titan one of the priority targets for future investigations.

The presence of the ocean was hinted long ago, from the consideration that, at some depth, the water ice covering the surface would be in such conditions of temperature and pressure that it should not be solid anymore, but liquid. The detection of this ocean has been hoped from the Cassini-Huygens mission, and this was a success. More precisely:

  • The rotation of the surface of Titan is synchronous, i.e. Titan shows on average the same face to Saturn, like our Moon, but with a significant obliquity (0.3°), which could reveal the presence of a global ocean which would decouple the rotation of the crust from the one of the core.
  • A Schumann resonance, i.e. an electromagnetic signal, has been detected by the lander Huygens in the atmosphere of Titan, during its fall. This could be due to an excitation of a magnetic field by a global conductive layer, i.e. a global subsurface ocean.
  • The gravitational Love number k2, which gives the amplitude of the response of the gravity field of Titan to the variations of the gravitational attraction of Saturn, is too large to be explained by a fully solid Titan.

All of these clues have convinced almost all of the scientific community that Titan has a global subsurface ocean. Determining its depth, thickness, composition,… is another story. In the study I present you today, the authors tried to elucidate the connection between its depth and the surface topography.

Modeling the ice-water boundary

The authors tried to determine the depth of the melting point of the water ice with respect to the latitude and longitude. This phase boundary is the thickness of the icy crust. For that, they wrote down the equations governing the viscoelastic deformation of the crust, its thermal evolution, and the motion of the boundary.

The viscoelastic deformation, i.e. deformation with dissipation, is due to the varying tidal action of Saturn, and the response depends on the properties of the material, i.e. rigidity, viscosity… The law ruling the behavior of the ice is here the Andrade law… basically it is a Maxwell rheology at low frequencies, i.e. elastic behavior for very slow deformations, viscoelastic behavior when the deformations gets faster… and for very fast excitation frequencies (tidal frequencies), the Maxwell model, which is based on one parameter (the Maxwell time, which gives an idea of the period of excitation at the transition between elastic and viscoelastic behavior), underestimates the dissipation. This is where the more complex Andrade model is useful. The excitation frequencies are taken in the variations of the distance Titan-Saturn, which are ruled by the gravitational perturbations of the Sun, of the rings, of the other satellites…

These deformations and excitations are responsible for variations of the temperature, which are also ruled by physical properties of the material (density, thermal conductivity), and which will determine whether the water should be solid or liquid. As a consequence, they will induce a motion of the phase change boundary.

Resolution by spectral decomposition

The equations ruling the variables of the problem are complex, in particular because they are coupled. Moreover, we should not forget that the density, thickness, temperature, resulting heat flows… not only depend on time, but also on where you are on the surface of Titan, i.e. the latitude and the longitude. To consider the couplings between the different surface elements, the authors did not use a finite-element modeling, but a spectral method instead.

The idea is to consider that the deformation of the crust is the sum of periodic deformations, with respect to the longitude and latitude. The basic shape is a sphere (order 0). If you want to be a little more accurate, you say that Titan is triaxial (order 2). And if you want to be more accurate, you introduce higher orders, which would induce bulges at non equatorial latitudes, north-south asymmetries for odd orders, etc. It is classical to decompose the tidal potential under a spectral form, and the authors succeeded to solve the equations of the problem in writing down the variables as sums of spherical harmonics.

The role of the grain size

And the main result is that the grain size of the ice plays a major role. In particular, the comparison between the resulting topography and the one measured by the Cassini mission up to the 3rd order shows that we need grains larger than 10 mm to be consistent with the observations. The authors reached an equilibrium in at the most 10 Myr, i.e. the crust is shaped in a few million years. They also addressed the influence of other parameters, like the rigidity of the ice, but with much less significant outcomes. Basically, the location of the melting / crystallization boundary is ruled by the grain size.

In the future

Every new study is another step forward. Others will follow. At least two directions can be expected.

Refinements of the theory

The authors honestly admit that the presence of other compounds in the ocean, like ammonia, is not considered here. Adding such compounds could affect the behavior of the ocean and the phase boundary. This would require at least one additional parameter, i.e. the fraction of ammonia. But the methodology presented here would still be valid, and additional studies would be incremental improvements of this one.
A possible implication of these results is the ocean dynamics, which is pretty difficult to model.

More data?

Another step forward could come from new data. Recently the mission proposal Dragonfly has been selected as a finalist by the NASA’s New Frontiers program. It would be a rotorcraft lander on Titan. Being selected as a finalist is a financial encouragement to refine and mature the concept within the year 2018, before final decision in July 2019 (see video below).

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.

Satellites of Saturn

The mountainous equator of Iapetus

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Hi there! You may have heard of Iapetus, a large satellite of Saturn which orbits very far. Today I will tell you about its equatorial ridge. This is the opportunity for me to present you Testing models for the formation of the equatorial ridge on Iapetus via crater counting, by Amanda L. Damptz, Andrew J. Dombard & Michelle R. Kirchoff, which has recently been accepted for publication in Icarus.

Outline

The satellite Iapetus
The equatorial ridge
Counting the craters
Results
The study and its authors

The satellite Iapetus

The satellite of Saturn Iapetus was discovered by Giovanni Domenico Cassini in October 1671. It orbited then on the western side of Saturn. During many years, i.e. until 1705, he was unable to observe it on the eastern side, since it was two magnitudes fainter. This has two implications:

  1. Iapetus has a two-tone coloration, i.e. a dark and a bright hemisphere,
  2. its rotation is synchronous. Like our Moon, it is locked in the synchronous 1:1 spin-orbit resonance, constantly showing the same face to Saturn.

Beside this, Iapetus is a large body (diameter: 1,470 km), which orbits at 3.5 millions km from Saturn (for comparison, Titan orbits almost thrice closer), with an orbital eccentricity of 0.028, very close to the one of Titan. It has an unexpectedly high orbital inclination, i.e. 15.47° with the equator of Saturn, and 8.13° with the Laplace Plane. We should imagine the proto-Saturn nebula, from which Iapetus has probably been formed. It was pretty much like a disc, but distorted by the Sun if you were far enough from Saturn, which is the case for Iapetus. What I mean is that the gravitational action of the Sun tends to shift the equilibrium orbital plane from the equatorial one, this is why we need to distinguish it, and we call it the Laplace Plane. In that case, the orbital inclination of Iapetus with respect to the Laplace Plane should be very small, but it is not. This probably contains an information on the history of Iapetus, but we do not know which one yet.

Let me go back to the rotation. Iapetus is so far from Saturn that it needs almost 80 days to complete one revolution, and 80 days for a rotation, since it is synchronous. This is the largest known spin period for a natural satellite in the Solar System.

As most of the satellites of Saturn, our knowledge of Iapetus made invaluable progress since 2004 thanks to the Cassini spacecraft, which imaged it. It confirmed the two-tone coloration, and detected a mountainous equatorial ridge.

Iapetus seen by the Cassini spacecraft. © NASA
Iapetus seen by the Cassini spacecraft. © NASA

The equatorial ridge

The Cassini images showed a 20 km-wide mountainous ridge, which is very close to the equator. So close that it is hard to believe it appeared there by chance. It is present on the dark hemisphere, while isolated equatorial mountains can be seen on the bright side. Some peaks reach 20 km.

Since its discovery late 2004, this ridge is a matter of investigation, and several competing explanations can be found in the literature:

  1. A signature of its past, fast rotation (the measured oblateness of Iapetus is consistent with a rotation period of 16 hours)
  2. A signature of a past critical spin state, i.e. close to provoke disruption of Iapetus,
  3. Upwarping of the lithosphere from below,
  4. Cryovolcanism,
  5. Planetary contraction,
  6. Material from an ancient ring system,
  7. Material from impact generated debris.

We can see that some of these scenarios propose an inner (endogenic) cause, while others propose an outer (exogenic) one. Almost all of them suggest an early formation of the ridge, except the last one.
One way to date a geological feature is to count its craters, and this is where this study intervenes. Its first product is a database of 7,748 craters ranging from 0.83 to 591 km in diameter.

Counting the craters

When an impactor reaches a planetary surface, it creates a crater. If one day geological processes are strong enough to create a tectonic feature, then it may at least alter the crater, or even hide it. If we see an uncraterized geological feature, that means that it is pretty young. We could even try to give it an age in estimating the evolution of the cratering rates over the evolution of the Solar System. By the way, the early Solar System was very intensively bombarded, with an episode of Late Heavy Bombardment occurring between 4.1 and 3.8 billions years ago. Bombardments still happen nowadays, but are much less frequent.

In this study the authors worked from Cassini and Voyager images of the surface of Iapetus, and considered different zones: central ridge, peripheral ridge, and off ridge. Moreover they classified the craters following their diameters, so as to estimate a distribution law: number of craters vs. size. They also catalogued the orientation of the deformations of the craters, since it could tell us something on the geological evolution of Iapetus (how did it alter the surface?)

This systematic search for craters was assisted by the commercial software Esri’s ArcGIS, supplemented by the dedicated add-on Crater Helper Tools.

Results

The first result is a database of 7,748 craters. But the main question is: what can we say about the ridge? The authors observe a depletion of large craters, i.e. with a diameter bigger than 16 km, in the ridge, which would be consistent with a pretty recent formation, and thus would favor the scenario of a ridge created by the debris of an impact. Nevertheless, the authors are prudent with this conclusion, they seem to suggest that the resolution of the images and the risk of saturation of small craters (when you are heavily bombarded, new craters destroyed ancient ones, and the overall number does not increase) do not permit to discard a scenario of early formation of the ridge. Further studies will probably be needed to reach an agreement on the origin of this mountainous equator.

The study and its authors

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