Planetary and Solar System Sciences

A water ocean inside Saturn’s moon Dione

Lê Binh San PHAM December 20, 2016

An image of Saturn’s icy moon Dione, with giant Saturn and its rings in the background. New research suggests Dione harbors a subsurface ocean. Credit: NASA/JPL-Caltech/Space Science Institute

Deep inside Saturn’s moon Dione lies a global ocean of liquid water, according to scientists of the Royal Observatory of Belgium. Such discovery places Dione as the third Saturn’s moon, beside Enceladus and Titan, to have a subsurface ocean.

Previous models of planet interior, based on gravity and shape data of Cassini, predicted no ocean at all for Dione and a very thick ice crust for Enceladus. However, Cassini spacecraft’s measurement of Enceladus back-and-forth oscillations, called libration, showed that they are bigger and suggest a thinner crust than predicted by those models. Beuthe et al. from the Royal Observatory of Belgium design a new model of planet interior that fits libration value of Enceladus. The findings are explained in a study published online in Geophysical Research Letters (also here on arXiv).

The Belgian trio design icy shells of Enceladus and Dione as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel. But to make the crust two times thinner than in previous model, they minimize the stress in the icy crust so that it can only stand just enough tension and compression to maintain surface landforms and no more. This principle is called minimum stress isostasy. Following Mikael Beuthe, lead author of the study, more stress would break the crust down to pieces.

Although Dione (near) and Enceladus (far) are composed of nearly the same materials, Enceladus’ surface is much brighter. Credit: NASA/JPL-Caltech/Space Science Institute.

Using this model, they do Bayesian inversion from Cassini’s gravity and shape data to find the the interior structure of Enceladus and Dione. As a result, the authors found that Enceladus’ ocean is much closer to the surface (at about 23 km), especially near the south polar region where the moon’s geysers spurt through only a few kilometers (7±4 km) of crust.

As for Dione, they find it harbors a water ocean below its 100 kilometer-thick ice crust. This ocean is tens of kilometers deep and surrounds a large rocky core whose diameter is about 70% of the diameter of Dione. Predicted libration for Enceladus agrees better with Cassini’s measurements. Dione librates, too. However, as it is more spherical than Enceladus and has a thicker crust, Dione libration is predicted to be about ten times smaller than Enceladus libration. Hence, it is below the detection level of Cassini. The authors hope that a future Saturn’s moon orbiter could verify this prediction.

Schematic of the interior of Enceladus with icy crust, ocean and solid core. Royal Observatory of Belgium researchers think that Dione may also have a subsurface ocean (NASA/JPL-Caltech/Space Science Institute.

If there is an ocean in Dione, and so in Enceladus, maybe there is life inside, too. And this is even more possible as Dione’s ocean has probably existed since the formation of the moon, providing thus a long-lived habitable zone for microbial life. According to Attilio Rivoldini, co-author of the study, the contact between the rocky core and the ocean would provide key nutrients and energy, both being essential ingredients for life. Only sample of the moons’ water could confirm the life hypothesis. The ocean of Dione seems to be too deep for an easy access. However, moons such as Enceladus and Jupiter’s moon Europa are “generous” enough to eject water geyser samples in space, ready to be picked by a passing spacecraft.

The club of “ocean worlds” – icy moons or planets with subsurface oceans in common parlance – seems to have more members in the solar system than we think. Three ocean worlds turn around Jupiter, three orbit Saturn and Pluto could also belong to this club, according to recent observations of the New Horizons spacecraft. Following Dr. Beuthe, their approach to modeling planetary bodies is a promising tool to study these worlds if their shape and gravity field can be measured.

“Future missions will visit Jupiter’s moons, but we should also explore Uranus’ and Neptune’s systems,” he says.

—This post is adapted from a press release published on the Royal Observatory of Belgium website.

Biography:

Lê Binh San PHAM is a Belgian PhD holder in planetary science. Her thesis deals with Mars habitability. She works now as a communication officer at the Royal Observatory of Belgium.

[ECS Interview] On the surface of Churyumov-Gerasimenko with Philae and Anthony

Loïc Rossi October 31, 2016

Artist's impression of Rosetta shortly before hitting Comet 67P/Churyumov–Gerasimenko on 30 September 2016. Credit: ESA/ATG medialab

Rosetta recently made a breathtaking dive towards the surface, bringing a wealth of science close from the surface, but also bringing the mission to its end. The operations might be over, but the science is not as there is still a lot of data to analyse, especially for the next generation of cometary scientists.

To illustrate this new generation, we asked a few questions to an early career scientist: Anthony Lethuillier who recently defended is PhD thesis and is now a post-doc at the LATMOS laboratory near Paris in France.

What is your background?

I studied geology and geophysics during my bachelor and master courses and it was only during the last year of my masters that I specialized in planetary science. In October 2013 I started my thesis at the LATMOS lab. My thesis was dedicated to the data collected on the nucleus of the comet Churyumov-Gerasimenko by the SESAME-PP instrument on-board the Philae lander.

You were working on the Philae lander, could you tell us more about what you were doing?

My thesis was dedicated to the data acquired by the SESAME-PP instrument on-board the Philae lander. The objective of SESAME-PP was to measure the electrical properties of the close subsurface of the comet (down to 1m). To achieve this the instrument uses transmitting electrodes (located on one of the feet of the lander) to inject a signal into the subsurface, it then records this signal on two receiving electrodes (located on the two other feet of the lander). The difference in amplitude and phase of the transmitted and received signal allows us to derive the electrical properties of the subsurface. These properties are the dielectric constant and the electrical conductivity and they depend on the composition and temperature of the material located in between the electrode. Once these values are known we perform lab measurements on the electrical properties of potential analogues to try and determine the composition of the subsurface.

1:1 replica of Philae in tests in LATMOS, France (left) and in in-situ simulations in Dachstein ice caves in Austria. Credit: A. Lethuillier and CNES/LATMOS

1:1 replica of Philae in tests in LATMOS, France (left) and in in-situ simulations in Dachstein ice caves in Austria.
Credit: A. Lethuillier/CNES/LATMOS

To derive the electrical properties I built 3D numerical model of the lander and its close environment. We also used a replica (scale 1:1) of the instrument and the lander we built to validate our method. This replica was used during field tests in the Dachstein ice caves in Austria (the closest we could get to a surface similar to a cometary surface).

Philae had a bit bouncy landing, wasn’t that a problem for you?

Yes it was for two reasons, the first is that the Lander entered a backup mode in which our instrument only performed part of the measurements it was supposed to do. The second, more important, problem was that the environment was far from flat and the position of the lander was unknown. To properly derive the electrical properties we need to know as precisely as possible the topography of the environment and the instruments attitude with regards to the surface.

Despite these limitations, using the available information on the lander’s position, we were able to constrain the porosity of the subsurface using accurate 3D numerical models.

3D modeling of the position of Philae after it landed in the Abydos region on the comet.
Credit: A. Lethuillier/CNES/LATMOS

What are your main conclusions then ?

We combined measurements of electrical properties performed on carbonaceous chondrites and on water ice with the measurements performed on the nucleus and found that the first meter of the nucleus has a maximum porosity of 55 %. We are only able to give a upper limit to the porosity due to the limitations explained above and we are not able to provide any information on the dust/ice ratio.

This value can then be compared to the porosity measured by the CONSERT radar which determined that the bulk porosity of the comet was between 70-80%. This lead us to the hypothesis of a consolidated shell covering a more porous interior. This is supported by the results from other instruments.

This shell could be, for example, the result of ice cementing processes where ice from the deep interior sublimates and refreezes when closer to the surface (our measurements were performed during the cometary night when the temperature is not high enough for the ice in the first meter to sublimate).

After a quite long search, Philae has been found. How does that influence your results?

We compared the picture taken by Osiris to our model in a similar orientation and found that our model was quite correct. By correcting our model we will find a more accurate value of the porosity but our conclusions will stay the same (a nucleus more compact on the surface).

Comparison of the numerical models of the attitude of the lander (left) and the actual position of Philae as seen from Rosetta.
Credit: A. Lethuillier and ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

In general what Rosetta result stands out according to you?

The ratio of Deuterium with regard to Hydrogen (D/H) in water is an important indicator its origin (theoretical simulations show that its value is dependent on the distance from the Sun at which the water formation took place). The origin of Earth’s ocean water can be investigated with this method: previous measurements on asteroids have shown a good agreement with Earth water. The D/H ratio measured on 67P/C-G is 3 times the ratio measured in the Earth’s oceans, suggesting that Jupiter family comets are not the source of Earth ocean–like water.

The interpretation made by multiple instruments of a consolidated shell covering a more porous interior will probably have a great influence on cometary formation models. The detection the amino acid glycine (found in proteins) and phosphorus (an essential part of DNA) by the ROSINA mass spectrometer is also quite stunning. The presence of geological features (identified by the OSIRIS camera) reminiscent of those found on earth was quite unexpected. The characterization of the comet’s water ice cycle (by the Rosetta spectrometer VIRTIS) is also very important for understanding the evolution of comets in general.

What are your plans for the future?

For the next three months I will be working on the PWA-HASI instrument of the Huygens lander (an instrument similar to SESAME-PP) to try and constrain the porosity of the surface of Titan. After that who knows ? I am looking for a post-doc that would allow me to keep on working on space instrumentation that helps understand the subsurface of planetary objects

From Neptune to Planet Nine: finding planets with pen and paper

Loïc Rossi October 10, 2016

Artistic view of Planet Nine. Credit: Tomruen, nagualdesign; CC BY-SA 4.0

In 1781, William Herschel discovers a faint uncatalogued point in his telescope. He first thinks he has discovered a comet, but the orbit of the new object seems more of a planetary nature. This will be confirmed by subsequent observations: the planet Uranus has been discovered!

Many years later, in the early nineteenth century, questions remain: the calculated orbit of Uranus does not match the observations. A significant discrepancy exists between the predictions and the effective position of the planet. Some suspect that Newton’s laws of gravitation might be different in the far Solar System, but another hypothesis emerges: Uranus’ motion might be perturbed by an unknown planet.

Armed with the tools of the celestial mechanics, astronomers tried to infer the properties and the orbit of the unknown planet based on its influence on Uranus. Two of them found independently solutions: in Great Britain, John Couch Adams and in France, Urbain Le Verrier. Le Verrier was the first to present his results in August 1846 to the French Academy of Sciences. After seeking help from astronomers, he got a reply from German observer Johann Gottfried Galle at Berlin Observatory.

Galle pointed his telescope to the portion of the sky given by Le Verrier and the 23rd of September 1846, Galle found an object which was not on recent stellar maps, at 1° of the position predicted by Le Verrier. After two nights of follow-up observations, the planetary nature of the object was confirmed: Neptune had been discovered.

Neptune as observed by Voyager 2 during its flyby in August 1989

As the French astronomer François Arago (at the time head of Paris Observatory) would put it, Le Verrier had discovered Neptune with “the point of his pen”.

This huge success of celestial mechanics led to attempts to find an inner planet inside the orbit of Mercury which would explain its anomalous motion. The search was vain and the real explanation for the unaccounted motion of Mercury would be later given by Einstein’s theory of general relativity.

140 years after the discovery of Neptune, the potential of celestial mechanics to find planets is still great as shown by the publication from Konstantin Batygin and Michael Brown from Caltech Institute, in January 2016. Since a Nature paper published in 2004, it has been noticed that some objects beyond the orbit of Neptune (trans-neptunian objects or TNO) had quite similar orbits: i.e. their perihelion (closest point of their orbit to the Sun) are located in the same region of space, something that is very unlikely to happen by chance. Batygin and Brown ran computations to try to determine what kind of perturbing object could cause such alignments. They suggested that the observed orbits could be explained by a planet with 10 times the mass of the Earth on a very eccentric orbit with a perihelion at 200 astronomical units (AU) and an aphelion (maximal distance to the Sun) at 1 200 AU. Pluto’s farthest distance to the Sun is about 50 AU…

The orbit of hypothetical planet Nine shown with orbits of other trans-Neptunian objects. Credit: MagentaGreen, CC0.

It is important here to state that the object has not yet been found observationally, but new studies narrowed the possible region in space where the object could be, allowing for a more precise search. The object could be found in the next few years with a dedicated research program.

So after the great success of the discovery of Neptune, will celestial mechanics have another trophy to expose with a planet Nine? The future will tell us!

Upcoming space events!

Loïc Rossi September 7, 2016

Artist impression of ExoMars Trace Gas Orbiter firing its engine for its insertion into Mars orbit. Credit: ESA/ATG medialab

Normally, September is not people’s favorite month as it goes with the end of the summer holidays, the beginning of the academic year and that of autumn. Not much to be happy about.

Thankfully, space and planetary science is here to help you overcome this difficult period with lots of exciting events.

Fly me to Bennu

Asteroïds and comets were formed at the same time as the other planets, at the beginning of the Solar System. But unlike planets, they have not been altered by active geology, erosion or life. They are in pristine condition and can give precious information about the formation of our solar system.

But despite our efforts, studying an asteroid from the orbit is not as efficient as experiments that can be conducted in a lab with a better equipment. This is why the samples from the lunar soil brought by Apollo (USA) and Lunokhod (USSR) missions are so valuable. And there has been an old desire to have a mission that would bring back samples of an asteroid to Earth.

A first attempt was made by the Japanese probe Hayabusa launched in 2003 which flew to the asteroid Itokawa. The probe tried to take a sample by throwing a projectile to the asteroid while flying very close to it and then “suck” a few grams of ejected samples from the surface. Sadly, the experiment didn’t work fully and only a few micrograms of dust were taken and brought back to Earth in 2010.

Picture of the asteroid Itokawa when visited by the Hayabusa spacecraft. Credit; JAXA

NASA has now a similar plan with the mission Osiris REX, which will fly by asteroid Bennu and take a sample. This mission is launched on the 8th of September and will take a few years to reach Bennu in August 2018. Then, after an observation period, the probe will try to take a sample from the surface with an extended arm that will touch the surface for a few seconds while blowing nitrogen to lift materials from the surface and capture them. The spacecraft will then leave Bennu in 2021 and bring back the sample capsule to Earth in 2023.

If everything goes well, up to 2000 grams could be taken, allowing for a detailed study of the components of this alien soil in ground-based laboratories!

Rosetta’s Grand Finale

If you’re a space enthusiast, you’ve probably heard of the Rosetta mission to comet 67P Churyumov Gerasimenko. This European mission is designed to study the comet for a long period, in particular studying how the comet changes as it moves around the Sun. As the comet moves closer it gets warmer and the tail and coma start to form. One of the goals was to monitor this activity of the nucleus of the comet, before, during and after its closest approach to the Sun.

But Rosetta is also a famous mission because it carried the lander Philae which successfully (but not without some bumps) landed on the comet and carried some measurements. The adventures of Rosetta and Philae were followed with a lot of attention by Earthlings, including the recent retrieval of Philae, but ESA has one final trick up its sleeve. [Read More]

Welcome to the PS division blog!

Loïc Rossi July 11, 2016

The Solar System with planets and dwarf planets

The Solar System with its planets and dwarf planets (distances not to scale). Image IAU, updated by Farry, public domain.

Welcome on-board the blog of the PS division of the EGU !

You may not be familiar with the PS division and what it is about, so I’ll give you a tour.

Most of the EGU divisions look towards the Earth, whether it is deep down the interior or right at the surface. Some look up to the atmosphere and the climate. Finally, there are two which look way up: PS and the Solar Terrestrial Sciences division (ST).

PS stands for Planetary and Solar System Sciences and it does what’s written on the tin: it is all about planetary science. But what is planetary science you might ask?

Planetary scientists are interested in objects in the Solar System like giant planets such as Jupiter, not-so-big planets like Earth and even dwarf planets like Pluto! They are also interested in smaller objects like asteroids and comets. Because going to space is quite harder than their daily commute, planetary scientists usually observe these objects using Earth-based telescopes and sometimes send robots and probes to explore on their behalf. Some of those robots land on celestial bodies like the rover Curiosity did on Mars or the Philae lander on comet 67P. There they take pictures, dig holes and fire lasers at rocks. Just like any other tourist would do…

Planetary scientists also like to look back in time and investigate how planets form and evolve with particular focus on cases like early-Venus, early-Mars or early-Earth. The question of the “before” brings some of them to look in detail at asteroids and comets as they contain pristine material that hasn’t changed much since the formation of the solar system. A bit like those comics that are very precious because they are still in their original packaging…

The comet 67P/Churyumov-Gerasimenko observed by Rosetta on 2015-07-07 at a distance of 157 km.
Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Life is also studied: where does it come from? And if it is a reproducible mechanism: has it appeared somewhere else? The hunt for extraterrestrial life (present or past) is primarily focussed on Mars but the search is now extending to icy moons in the outer solar system that are thought to harbour liquid water oceans (like Europa around Jupiter and Enceladus around Saturn). This question gave birth to a sub-field of planetary science, between physics, chemistry and biology, called astrobiology.

But space goes beyond the Solar System and we love space rocks so much, we started looking even further. In 1995 a planet was discovered around the Sun-like star 51 Pegasi. This was the first of the so-called exoplanets. At first exoplanets were mostly studied by astronomers. But planetary scientists also wanted to characterize them and came into the game, starting a new kind of planetary science, namely exoplanetary science. What is their mass, do they have an atmosphere, if so of what kind, and more importantly: could they have liquid water on the surface so that they could harbour life?

In this blog we will cover recent advances in planetary science and present research from early career scientists (ECS) across the division. We also aim to bring the PS community together by sharing events and tips that can be of interest to researchers, ECS or not.

But the blog is not the only way you can keep up to date with the division: we’re also on social media so you can follow us on Twitter (@EGU_PS) and on Facebook (EGUPSDivision). There we also share job offers, so have a look!

If you still want to know more about what being a planetary scientist is like, check out this blog in the coming months for some posts from people sharing their experience about their work in planetary science. If you want to share your experience or some research highlight, contributions in the form of guest posts are welcomed!

Welcome on-board!