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

October 31, 2016
[ECS Interview] On the surface of Churyumov-Gerasimenko with Philae and Anthony
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

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

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

October 10, 2016
From Neptune to Planet Nine: finding planets with pen and paper
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.

Neptuneas observed by Voyager 2 during its flyby in August 1989

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.

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!