Photochemical enrichment of deuterium in Titan's atmosphere: new insights from Cassini-Huygens

Cassini-Huygens data are used to re-examine the potential sources of the D/H enhancement over solar, measured in methane, in Titan's atmosphere. Assuming that the system is closed with respect to carbon, the use of constraints from the Huygens probe for the determination of the current mass of atmospheric methane and the most up-to-date determination of D/H from Cassini/CIRS infrared spectra allow us to show that photochemical enrichment of deuterium is not sufficient to be the sole mechanism yielding the measured D/H value. A possible fractionation between CH3D and CH4 during the escape process may slightly enhance the deuterium enrichment, but is not sufficient to explain the observed D/H value over the range of escape values proposed in the literature. Hence, alternative mechanisms such as a primordial deuterium enrichment must be combined with the photochemical enrichment in Titan's atmosphere in order to explain its current D/H value.Comment: 4 pages, 3 figures, accepted in ApJ

Triton Haze Analogs: The Role of Carbon Monoxide in Haze Formation

Triton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn\u27s moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% CO and 0.2% CH4 in N2 at 90 K and 1 mbar to generate Triton haze analogs. We then characterize the physical and chemical properties of these particles. We measure their production rate, their bulk composition with combustion analysis, their molecular composition with very high resolution mass spectrometry, and their transmission and reflectance from the optical to the near-infrared with Fourier Transform Infrared (FTIR) Spectroscopy. We compare these properties to existing measurements of Triton\u27s tenuous atmosphere and surface, as well as contextualize these results in view of all the small, hazy, nitrogen-rich worlds of our solar system. We find that carbon monoxide present at greater mixing ratios than methane in the atmosphere can lead to significantly oxygen- and nitrogen-rich haze materials. These Triton haze analogs have clear observable signatures in their near-infrared spectra, which may help us differentiate the mechanisms behind haze formation processes across diverse solar system bodies

The Detached Haze Layer in Titan's Mesosphere

Abstract P12A-04The Cassini observations reveal the presence of a detached haze layer in Titan's mesosphere at an altitude of 520 km, well above the stratosphere. Observations of scattered light made by the Imaging Science Subsystem (ISS) reveal a clearly defined layer encircling low and mid-latitude regions. The aerosol layer is also detected in stellar occultation measurements of UV extinction by the UltraViolet Imaging Spectrometer (UVIS). The haze is a global and permanent feature of Titan's atmosphere. Furthermore the location of the detached haze layer is coincident with and the likely cause of a local maximum in the temperature profile measured by the Huygens Atmospheric Structure Instrument (HASI). This temperature inversion is also permanent and global, having been detected in ground-based stellar occultations. The correlation between the extinction profile and the temperature maximum imply that the detached haze cannot be due to condensation, as previously suggested. Previously, Voyager high phase angle images at 500 nm revealed a detached haze layer near 350 km, more than 150 km lower than the Cassini layer. Close examination of the Voyager images suggests that the Cassini detached layer at 520 km is a separate phenomenon rather than a change in the Voyager detached layer. Analysis of the observed optical properties suggests that the average size of particles in the Cassini detached layer is < 45 nm, with an imaginary index k < 0.3 at 187.5 nm, while Non-LTE calculations of the temperature perturbation induced by the detached haze show that the average particle size must be greater than 35 nm for reproducing the heating rate implied by the HASI temperature profile. Calculation of the sedimentation velocity of the particles, coupled with the derived number density, imply a mass flux of 1.9-3.2 × 10-14 g cm-2 s-1. This is approximately equal to the mass flux required to explain the main haze layer and suggests that the stratospheric haze is formed primarily by sedimentation and coagulation of particles in the detached layer. This is further supported from the particle size range retrieved for the detached layer (35-45 nm), being approximately equal to the radii of ~50 nm for the monomers of the aggregate aerosols in the main haze layer. It follows that aerosols on Titan are formed primarily in the thermosphere, rather than the stratosphere as assumed in many pre-Cassini studies. This is consistent with the detection of negatively charged aerosols in the thermosphere by Cassini/CAPS. These conclusions are supported by microphysical aerosol models that couple the detached haze layer and the main haze layer and extend into the thermosphere. Our calculations suggest that the detached haze layer is due to the transition in the growth of particles from spherical shape to aggregates of fractal structure. The rapid increase in the size of the particles with the onset of fractal growth, in combination with the decrease of their settling velocity, casts them invisible in the transition region. This optical illusion process explains the well-defined and symmetric structure of the detached layer, something difficult to explain under a pure advection scenario. Further investigation of the processes defining the growth of the particles is required in order to understand why the transition takes place at this region and how the particles produced at higher altitudes by high energy radical and ion chemistry, are defining the vertical haze opacity in Titan's atmosphere

The detached haze layer in Titan's mesosphere: observations and analysis

abstract EPSC2008-A-00099International audienc

The detached haze layer in Titan's mesosphere: The formation process

abstract EPSC2008-A-00153International audienc

The Detached Haze Layer in Titan's Mesosphere

Abstract P12A-04The Cassini observations reveal the presence of a detached haze layer in Titan's mesosphere at an altitude of 520 km, well above the stratosphere. Observations of scattered light made by the Imaging Science Subsystem (ISS) reveal a clearly defined layer encircling low and mid-latitude regions. The aerosol layer is also detected in stellar occultation measurements of UV extinction by the UltraViolet Imaging Spectrometer (UVIS). The haze is a global and permanent feature of Titan's atmosphere. Furthermore the location of the detached haze layer is coincident with and the likely cause of a local maximum in the temperature profile measured by the Huygens Atmospheric Structure Instrument (HASI). This temperature inversion is also permanent and global, having been detected in ground-based stellar occultations. The correlation between the extinction profile and the temperature maximum imply that the detached haze cannot be due to condensation, as previously suggested. Previously, Voyager high phase angle images at 500 nm revealed a detached haze layer near 350 km, more than 150 km lower than the Cassini layer. Close examination of the Voyager images suggests that the Cassini detached layer at 520 km is a separate phenomenon rather than a change in the Voyager detached layer. Analysis of the observed optical properties suggests that the average size of particles in the Cassini detached layer is < 45 nm, with an imaginary index k < 0.3 at 187.5 nm, while Non-LTE calculations of the temperature perturbation induced by the detached haze show that the average particle size must be greater than 35 nm for reproducing the heating rate implied by the HASI temperature profile. Calculation of the sedimentation velocity of the particles, coupled with the derived number density, imply a mass flux of 1.9-3.2 × 10-14 g cm-2 s-1. This is approximately equal to the mass flux required to explain the main haze layer and suggests that the stratospheric haze is formed primarily by sedimentation and coagulation of particles in the detached layer. This is further supported from the particle size range retrieved for the detached layer (35-45 nm), being approximately equal to the radii of ~50 nm for the monomers of the aggregate aerosols in the main haze layer. It follows that aerosols on Titan are formed primarily in the thermosphere, rather than the stratosphere as assumed in many pre-Cassini studies. This is consistent with the detection of negatively charged aerosols in the thermosphere by Cassini/CAPS. These conclusions are supported by microphysical aerosol models that couple the detached haze layer and the main haze layer and extend into the thermosphere. Our calculations suggest that the detached haze layer is due to the transition in the growth of particles from spherical shape to aggregates of fractal structure. The rapid increase in the size of the particles with the onset of fractal growth, in combination with the decrease of their settling velocity, casts them invisible in the transition region. This optical illusion process explains the well-defined and symmetric structure of the detached layer, something difficult to explain under a pure advection scenario. Further investigation of the processes defining the growth of the particles is required in order to understand why the transition takes place at this region and how the particles produced at higher altitudes by high energy radical and ion chemistry, are defining the vertical haze opacity in Titan's atmosphere

Composition and chemistry of Titan's thermosphere and ionosphere

International audienceTitan has long been known to harbour the richest atmospheric chemistry in the Solar System. Until recently, it had been believed that complex hydrocarbons and nitriles were produced through neutral chemistry that would eventually lead to the formation of micrometre sized organic aerosols. However, recent measurements by the Cassini spacecraft are drastically changing our understanding of Titan's chemistry. The Ion and Neutral Mass Spectrometer (INMS) and the Cassini Plasma Spectrometer (CAPS) revealed an extraordinary complex ionospheric composition. INMS detected roughly 50 positive ions with m/z<100 and a density higher than 0.1 cm−3. CAPS provided evidence for heavy (up to 350 amu) positively and negatively charged (up to 4000 amu) ions. These observations all indicate that Titan's ionospheric chemistry is incredibly complex and that molecular growth starts in the upper atmosphere rather than at lower altitude. Here, we review the recent progress made on ionospheric chemistry. The presence of heavy neutrals in the upper atmosphere has been inferred as a direct consequence of the presence of complex positive ions. Benzene (C6H6) is created by ion chemistry at high altitudes and its main photolysis product, the phenyl radical (C6H5), is at the origin of the formation of aromatic species at lower altitude