Titan's organic chemistry : a planetary-scale laboratory to study primitive Earth

Saturn?s largest satellite, Titan, has been revealed by extended ground-based and space observations, and recently by the Cassini-Huygens mission. Titan?s atmosphere hosts a complex organic chemistry in the solar system starting with nitrogen and methane and leading to the formation of hydrocarbons and nitriles, including prebiotic molecules. The atmosphere also contains traces of oxygen compounds. This system is subject to seasonal variations and different physical, dynamic, and photochemical processes. Interactions between the atmosphere, the surface, and the interior also play an important role in the astrobiological potential of the satellite

New laboratory measurements of CH4 in Titan's conditions and a reanalysis of the DISR near-surface spectra at the Huygens landing site

International audienceLaboratory spectra of methane-nitrogen mixtures have been recorded in the near-infrared range (1.0 - 1.65 µm) in conditions similar to Titan's near surface, to facilitate the interpretation of the DISR/DLIS spectra taken during the last phase of the descent of the Huygens Probe, when the surface was illuminated by a surface science lamp. We used a 0.03 cm-1 spectral resolution, adequate to resolve the lines at high pressure (pN2 ~ 1.5 bar). By comparing the laboratory spectra with synthetic calculations in the well-studied ν2 + 2ν3 band (7515-7620 cm-1), we determine a methane absorption column density of 178±20 cm-am and a temperature of 118±10 K in our experiment. From this, we derive the methane absorption coefficients over 1.0-1.65 µm with a 0.03 cm-1 sampling, allowing for the extrapolation of the results to any other methane column density under the relevant pressure and temperature conditions. We then revisit the calibration and analysis of the Titan "lamp-on" DLIS spectra. We infer a 5.1±0.8 % methane mixing ratio in the first 25 m of Titan's atmosphere. The CH4 mixing ratio measured 90 sec after landing from a distance of 45 cm is found to be 0.92±0.25 times this value, thus showing no post-landing outgassing of methane in excess of ̴ 20 %. Finally, we determine the surface reflectivity as seen from 25 m and 45 cm and find that the 1500 nm absorption band is deeper in the post-landing spectrum as compared to pre-landing

Upper limits for undetected trace species in the stratosphere of Titan

In this paper we describe a first quantitative search for several molecules in Titan's stratosphere in Cassini CIRS infrared spectra. These are: ammonia (NH3), methanol (CH3OH), formaldehyde (H2CO), and acetonitrile (CH3CN), all of which are predicted by photochemical models but only the last of which observed, and not in the infrared. We find non-detections in all cases, but derive upper limits on the abundances from low-noise observations at 25{\deg}S and 75{\deg}N. Comparing these constraints to model predictions, we conclude that CIRS is highly unlikely to see NH3 or CH3OH emissions. However, CH3CN and H2CO are closer to CIRS detectability, and we suggest ways in which the sensitivity threshold may be lowered towards this goal.Comment: 11 pages plus 6 figure file

Titan's Far-Infrared 220 cm(exp -1) Cloud Seen for the First Time in the South

In 2012 an emission feature at 220 cm(exp -1) in Titan's far-infrared spectrum was seen for the first time in the south. Attributed to a stratosphere ice cloud formed at the winter pole, the 220 (exp -1) emission had previously been seen only at high northern latitudes where it bad been decreasing since the arrival of Cassini in 2004. Our far-infrared observations were performed With the Composite Infrared Spectrometer (CIRS) on Caasini. Although it bad been expected that the 220 cm(exp -1) emission would eventnal1y appear in the south, the emission appeared rather suddenly, increasing by a factor of at least four between February (when it was not detected) and July 2012. At the time of our observations, one Titan month after equinox, the 220 cm(exp -1) feature was present in both the north and south and showed a trend of continued slow decrease in the north and steep increase in the south. As has been the case in the north, the emission in the south was confined to high latitudes associated with winter polar shadowing. Our spectroscopic detection of the southern 220 cm(exp -1) ice cloud coincided with the rapid formation in 2012 of a haze hood and vortex at the south pole as seen in Cassini image. The 220 cm(exp -1) feature was first observed by the Infrared Interferometer Spectrometer (IRIS) on Voyager I and has been extensively studied in the north by CIRS. Until now the 220 cm(exp -1) emission, like the polar hood, has been associated solely with the north, owing to the fact that Voyager and Cassini have viewed Titan only during winter-spring. In 2012 we witnessed the start of a seasonal shift of this pattern to the south. The 220 cm(exp -1) emission arises from altitudes of 80-150 km and peaks sharply near 140 km. The material responsible for the spectral feature is not known, but indirect evidence hints at a condensate arising from complex nitriles, which also tend to be present only at high winter latitudes

EVOLUTION OF THE STRATOSPHERIC TEMPERATURE AND CHEMICAL COMPOSITION OVER ONE TITANIAN YEAR

Since the Voyager 1 (V1) flyby in 1980, Titans exploration from space and the ground has been ongoing for more than a full revolution of Saturn around the Sun (one Titan year or 29.5 Earth years was completed in May 2010). In this study we search for temporal variations affecting Titans atmospheric thermal and chemical structure within that year. We process Cassini CIRS data taken during the Titan flybys from 2006-2013 and compare them to the 1980 V1IRIS spectra (re-analyzed here). We also consider data from Earth-based and -orbiting observatories (such as from the ISO, re-visited). When we compare the CIRS 2010 and the IRIS data we find limited inter-annual variations, below the 25 or35 levels for the lower and middle, or the high latitudes, respectively. A return to the 1980 stratospheric temperatures and abundances is generally achieved from 50degN to 50degS, indicative of the solar radiation being the dominating energy source at 10 AU, as for the Earth, as predicted by GCM and photochemical models. However, some exceptions exist among the most complex hydrocarbons (C4H2 and C3H4), especially in the North. In the Southern latitudes, since 2012, we see a trend for an increase of several trace gases, possibly indicative of a seasonal atmospheric reversal. At the Northern latitudes we found enhanced abundances around the period of the northern spring equinox in mid-2009 (as in Bampasidis et al. 2012), which subsequently decreased (from 2010-2012) returning to values similar to those found in the V1 epoch a Titanian year before

Candidate regions on titan as promising landing sites for future in situ missions

The highly successful and still on-going Cassini-Huygens mission to the Saturnian system points to the need for a return mission, with both remote and in situ instrumentation. The surface of Saturn’s moon Titan, hosts a complex environment in which many processes occur shaping its landscape. Several of its geological features resemble terrestrial ones, albeit constructed from different material and reflecting the interiorsurface-atmosphere exchanges. The resulting observed morphotectonic features and cryovolcanic candidate regions could benefit from further extensive exploration by a return mission that would focus on these aspects with adapted state-of-the-art instrumentation affording higher spectral and spatial resolution and in situ capabilities. We suggest that some features on Titan are more promising candidate locations for future landing and we present the case for Tui Regio, Hotei Regio and Sotra Patera as to why they could provide a wealth of new scientific results

Titan's temporal evolution in stratospheric trace gases near the poles

International audienceWe analyze spectra acquired by the Cassini/Composite Infrared Spectrometer (CIRS) at high resolution from October 2010 until September 2014 in nadir mode. Up until mid 2012, Titan’s Northern atmosphere exhibited the enriched chemical content found since the Voyager days (November 1980), with a peak around the Northern Spring Equinox (NSE) in 2009. Since then, we have observed the appearance at Titan’s south pole of several trace species for the first time, such as HC3N and C6H6, observed only at high northern latitudes before equinox. We investigate here latitudes poleward of 50°S and 50°N from 2010 (after the Southern Autumnal Equinox) until 2014. For some of the most abundant and longest-lived hydrocarbons (C2H2, C2H6 and C3H8) and CO2, the evolution in the past 4 years at a given latitude is not very significant within error bars especially until mid-2013. In more recent dates, these molecules show a trend for increase in the south. This trend is dramatically more pronounced for the other trace species, especially in 2013–2014, and at 70°S relative to 50°S. These two regions then demonstrate that they are subject to different dynamical processes in and out of the polar vortex region. For most species, we find higher abundances at 50°N compared to 50°S, with the exception of C3H8, CO2, C6H6 and HC3N, which arrive at similar mixing ratios after mid-2013. While the 70°N data show generally no change with a trend rather to a small decrease for most species within 2014, the 70°S results indicate a strong enhancement in trace stratospheric gases after 2012. The 663 cm−1 HC3N and the C6H6 674 cm−1 emission bands appeared in late 2011/early 2012 in the south polar regions and have since then exhibited a dramatic increase in their abundances. At 70°S HC3N, HCN and C6H6 have increased by 3 orders of magnitude over the past 3–4 years while other molecules, including C2H4, C3H4 and C4H2, have increased less sharply (by 1–2 orders of magnitude). This is a strong indication of the rapid and sudden buildup of the gaseous inventory in the southern stratosphere during 2013–2014, as expected as the pole moves deeper into winter shadow. Subsidence gases that accumulate in the absence of ultraviolet sunlight, evidently increased quickly since 2012 and some of them may be responsible also for the reported haze decrease in the north and its appearance in the south at the same time

Planetary space weather: scientific aspects and future perspectives

International audienceIn this paper, we review the scientific aspects of planetary space weather at different regions of our Solar System, performing a comparative planetology analysis that includes a direct reference to the circum-terrestrial case. Through an interdisciplinary analysis of existing results based both on observational data and theoretical models, we review the nature of the interactions between the environment of a Solar System body other than the Earth and the impinging plasma/radiation, and we offer some considerations related to the planning of future space observations. We highlight the importance of such comparative studies for data interpretations in the context of future space missions (e.g. ESA JUICE; ESA/JAXA BEPI COLOMBO). Moreover, we discuss how the study of planetary space weather can provide feedback for better understanding the traditional circum-terrestrial space weather. Finally, a strategy for future global investigations related to this thematic is proposed

Active Upper-atmosphere Chemistry and Dynamics from Polar Circulation Reversal on Titan

Saturn's moon Titan has a nitrogen atmosphere comparable to Earth's, with a surface pressure of 1.4 bar. Numerical models reproduce the tropospheric conditions very well but have trouble explaining the observed middle-atmosphere temperatures, composition and winds. The top of the middle-atmosphere circulation has been thought to lie at an altitude of 450 to 500 kilometres, where there is a layer of haze that appears to be separated from the main haze deck. This 'detached' haze was previously explained as being due to the colocation of peak haze production and the limit of dynamical transport by the circulation's upper branch. Herewe report a build-up of trace gases over the south pole approximately two years after observing the 2009 post-equinox circulation reversal, from which we conclude that middle-atmosphere circulation must extend to an altitude of at least 600 kilometres. The primary drivers of this circulation are summer-hemisphere heating of haze by absorption of solar radiation and winter-hemisphere cooling due to infrared emission by haze and trace gases; our results therefore imply that these effects are important well into the thermosphere (altitudes higher than 500 kilometres). This requires both active upper-atmosphere chemistry, consistent with the detection of high-complexity molecules and ions at altitudes greater than 950 kilometres, and an alternative explanation for the detached haze, such as a transition in haze particle growth from monomers to fractal structures

The COSPAR Planetary Protection Requirements for Space Missions to Mars

The Committee on Space Research’s (COSPAR) Planetary Protection Policy (herein referred to as the Policy) has been developed through deliberation between the scientific community and the national space agencies to 1) ensure that scientific investigations of possible extra-terrestrial life forms, precursors, and remnants are not jeopardized; and 2) Earth is protected from the potential hazard posed by extra-terrestrial matter carried by a spacecraft returning from an interplanetary mission (COSPAR 2020).The COSPAR Panel on Planetary Protection (herein referred to as the Panel)regularly updates the Policy based on workshops and activities that are led by the community, or by national committees. For example, the requirements for the icy moons of the outer Solar System have been scrutinized as part of a European Commission’s H2020 Programme (Rettberg et al 2019) and a National Research Council report (NRC 2012), which led to recommendations being made to COSPAR, which resulted in an update to the regulations (COSPAR 2020). Another example is the recent update of the regulations relating to the moon. The Panel conducted a dedicated community consultation that led to an updated Policy (COSPAR, 2021). [...