From the Teams

Saturn’s Huge Moon Titan Drifting Away Faster Than Previously Thought

Spotlight From the Teams, Publications

Report from ISSI Team #411 The ENCELADE Team: Constraining the Dynamical Timescale and Internal Processes of the Saturn and Jupiter Systems from Astrometry led by V. Lainey

Did you know that the Earth’s moon distance is increasing at about 3.8 cm/year because of the tides the Moon raises on our own planet? Thanks to Newton and his law of gravity, entailed the proper explanation of tides: the side of the Earth that is closer to the Moon is more attracted than its opposite side. As a consequence, the Earth takes an elongated shape like a football. Distance increase comes then as a consequence of friction inside the oceans essentially. Both shape distortion and orbital variation rate are expected to get significantly lower with distance, since tides are a consequence of gravitation.

A giant of a moon appears before a giant of a planet undergoing seasonal changes in this natural color view of Titan and Saturn from NASA’s Cassini spacecraft. (Image Credit: NASA/JPL-Caltech/Space Science Institute)

Since tidal theory is universal, researchers have applied it over the last 50 years to predict the orbital evolution of many moons. From the evolution of the four big Galilean satellites of Jupiter to the small moons of Mars, Phobos and Deimos, the same theory was used underneath. Recently and in the context of the Cassini mission, the ISSI ENCELADE Team led by Valery Lainey in collaboration with a team from the University of Bologna tried to quantify from observations the orbital expansion of Titan, the largest Saturnian moon, under Saturn’s tides. Surprisingly, they found that Titan is escaping Saturn’s gravity at a large pace of 11 cm/year, more than a hundred times faster than expected from theoretical models. Even more surprising, such expansion rate is larger than for moons closer to Saturn, in complete contradiction with classical tidal theory! But the study demonstrates a perfect agreement with the prediction of a new tidal mechanism, suggested only four years ago by Jim Fuller (Caltech) and co-authors. Such so-called “tidal lock mechanism” suggests that Titan may have formed way closer to Saturn, than commonly believed. This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturnian system.

Like the classical tidal theory for terrestrial objects, tidal lock mechanism is a universal physical mechanism for giant planets. In principle, it could be at play in way other systems were giant planets are involved, starting with the Jupiter system itself.

More Information can be found here: News Release June 8, 2020, NASA JPL Caltech >>

Lainey, V., Casajus, L.G., Fuller, J. et al. Resonance locking in giant planets indicated by the rapid orbital expansion of Titan, Nature Astronomy, 2020. https://doi.org/10.1038/s41550-020-1120-5

Aliphatic Organics and Ammonium Salts on the Surface of Comet 67P/Churyumov-Gerasimenko

Spotlight From the Teams

Report from ISSI Team #397 Comet 67P/Churyumov-Gerasimenko Surface Composition as a Playground for Radiative Transfer Modeling and Laboratory Measurements” led by M. Ciarniello

A primary goal of ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (hereafter 67P) was the characterization of the nucleus material.

The Visible Infrared and Thermal Imaging Spectrometer (VIRTIS) revealed a mostly spectrally uniform nucleus surface, dominated by a low-albedo material exhibiting a broad infrared absorption centered at 3.2 µm. The surface composition of the comet was found compatible with a mixture of organics, minerals and minor amounts of ices, although the identification of the compounds responsible for the 3.2-µm feature has remained challenging so far.

Ammonium salts found on Rosetta’s comet (Image Credit: O. Poch, IPAG, UGA/CNES/CNRS (left); ESA/Rosetta/NavCam – CC BY-SA IGO 3.0 (right))

 

Here we report the results of a refined analysis of measurements acquired by the instrument’s infrared mapping channel (VIRTIS-M-IR).

In particular, spectral signatures of aliphatic organics within the broad absorption band were identified for the first time on a cometary nucleus. In addition, further investigations by means of laboratory experiments allowed the identification of ammonium salts as carriers of the 3.2-µm absorption feature. These salts may be a major reservoir of nitrogen in comets.

These studies highlight similarities between the infrared spectrum of 67P with those of other minor bodies in the Solar System, including some outer belt asteroids and Jupiter Trojans. As a consequence, these bodies may host organic materials and/or ammoniated salts, blurring the distinction between comets and other primitive objects. Aliphatic features in 67P appear similar to the typical aliphatic features in the interstellar medium (ISM) and are also compatible with those of the chondritic insoluble organic matter (IOM). This suggests that the organic material may be inherited from the ISM by comets and other minor bodies that delivered it to the inner Solar System. Similarly, ammoniated salts, potentially formed in the icy mantle of dust grains in the pre-stellar or protoplanetary phases, could have provided nitrogen to the inner planets. Such processes could have favored the emergence of a prebiotic chemistry.

These investigations took advantage, among other authors, of the collaboration of the ISSI Team “Comet 67P/Churyumov-Gerasimenko Surface Composition as a Playground for Radiative Transfer Modeling and Laboratory Measurements” led by M. Ciarniello and have been published in two separate papers:

“Infrared detection of aliphatic organics on a cometary nucleus” by A. Raponi et al., 2020, Nature Astronomy; DOI: https://doi.org/10.1038/s41550-019-0992-8

“Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids” by O. Poch et al., 2020,  Science 367, DOI: https://dx.doi.org/10.1126/science.aaw7462