Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH₄ and N₂). Up to now, it has been ...assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (~1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH₄ and N₂) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (~8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.
A survey of the bulk plasma ion properties observed by the Cassini Plasma Spectrometer instrument over roughly the first 4.5 years of its mission in orbit around Saturn is presented. The moments ...(density, temperature, and flow velocity) of the plasma distributions below 50 keV have been computed by numerical integration of the observed counts in the “Singles” (non‐mass‐resolved) data, partitioned into species on the basis of concurrent determinations of the composition from the time‐of‐flight data. Moments are presented for three main species: H+, W+ (water group ions), and ions with m/q = 2, which are presumed to be H2+. While the survey extends to radial distances of 30 RS and thus includes some solar wind or magnetosheath values, our principal interest is the large‐scale spatial variation of the magnetospheric plasma properties, so we focus attention on radial distances inside of 17 RS. Principal findings include the following: (1) the densities of all three components are highly variable but are generally well organized by dipole L and magnetic latitude; (2) the density of ions with m/q = 2 varies from a few percentage of the H+ density in the inner magnetosphere to a maximum of several tens of percentage near the orbit of Titan, suggesting that Titan is an important source for H2+ in the outer magnetosphere; (3) water group ions are the dominant population in the inner magnetosphere, but only within ∼3 RS of the equatorial plane because of their strong centrifugal confinement; (4) derived latitudinal scale heights are largest for the light ions and generally increase with radial distance; (5) the L dependence of the calculated temperatures is not consistent with adiabatic transport but is in fair agreement with the expectations for plasma originating from ion pickup; (6) in agreement with the findings of Sergis et al. (2010), inside of L ∼ 11, the particle pressure is dominated by ions with energies below a few keV; (7) the derived flow velocities reveal the global circulation pattern of relatively dense populations in the magnetosphere, with no evidence for return circulation from the nightside to the dayside beyond ∼20 RS; and (8) the azimuthal flow speeds are typically less than full corotation over the entire L range examined, varying from ∼50% to 70% of full corotation.
Titan's ionosphere contains a rich positive ion population including organic molecules. Here, using CAPS electron spectrometer data from sixteen Titan encounters, we reveal the existence of negative ...ions. These ions, with densities up to ∼100 cm−3, are in mass groups of 10–30, 30–50, 50–80, 80–110, 110–200 and 200+ amu/charge. During one low encounter, negative ions with mass per charge as high as 10,000 amu/q are seen. Due to their unexpectedly high densities at ∼950 km altitude, these negative ions must play a key role in the ion chemistry and they may be important in the formation of organic‐rich aerosols (tholins) eventually falling to the surface.
The flyby measurements of the Cassini spacecraft at Saturn's moon Rhea reveal a tenuous oxygen (O₂)-carbon dioxide (CO₂) atmosphere. The atmosphere appears to be sustained by chemical decomposition ...of the surface water ice under irradiation from Saturn's magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter's moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O₂ locked within Rhea's ice. The presence of CO₂ suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO₂ endogenic to Rhea's ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.
Cassini discovered a plethora of neutral and ionized molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q−1. In this Letter, we ...forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950-1300 km. We report on detections consistently centered between 25.8 and 26.0 u q−1 and between 49.0-50.1 u q−1 which are identified as belonging to the carbon chain anions, CN−/C3N− and/or C2H−/C4H−, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73-74 u q−1 could be attributed to the further carbon chain anions C5N−/C6H− but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at >100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below <1100 km, the low-mass anions (<150 u q−1) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.
Magnetic reconnection is an important process that occurs at the magnetopause boundary of Earth's magnetosphere because it leads to transport of solar wind energy into the system, driving ...magnetospheric dynamics. However, the nature of magnetopause reconnection in the case of Saturn's magnetosphere is unclear. Based on a combination of Cassini spacecraft observations and simulations we propose that plasma βconditions adjacent to Saturn's magnetopause largely restrict reconnection to regions of the boundary where the adjacent magnetic fields are close to anti‐parallel, severely limiting the fraction of the magnetopause surface that can become open. Under relatively low magnetosheathβconditions we suggest that this restriction becomes less severe. Our results imply that the nature of solar wind‐magnetosphere coupling via reconnection can vary between planets, and we should not assume that the nature of this coupling is always Earth‐like. Studies of reconnection signatures at Saturn's magnetopause will test this hypothesis.
Key Points
Plasma betas at Saturn's magnetopause are generally higher than those at Earth's
These conditions should severely restrict where reconnection can occur
Nature of solar wind‐magnetosphere coupling at Saturn should not be Earth‐like
During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions ...characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 Rsubscript S (where Rsubscript S is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with approximately3% N⁺. Over the A and B rings, we found an ionosphere in which O₂⁺ and O⁺ are dominant, which suggests the possible existence of a layer of O₂ gas similar to the atmospheres of Europa and Ganymede.
The interaction of the solar wind with Earth's magnetosphere gives rise to the bright polar aurorae and to geomagnetic storms, but the relation between the solar wind and the dynamics of the outer ...planets' magnetospheres is poorly understood. Jupiter's magnetospheric dynamics and aurorae are dominated by processes internal to the jovian system, whereas Saturn's magnetosphere has generally been considered to have both internal and solar-wind-driven processes. This hypothesis, however, is tentative because of limited simultaneous solar wind and magnetospheric measurements. Here we report solar wind measurements, immediately upstream of Saturn, over a one-month period. When combined with simultaneous ultraviolet imaging we find that, unlike Jupiter, Saturn's aurorae respond strongly to solar wind conditions. But in contrast to Earth, the main controlling factor appears to be solar wind dynamic pressure and electric field, with the orientation of the interplanetary magnetic field playing a much more limited role. Saturn's magnetosphere is, therefore, strongly driven by the solar wind, but the solar wind conditions that drive it differ from those that drive the Earth's magnetosphere.
Radial convective transport of plasma in a rotation‐dominated magnetosphere implies alternating longitudinal sectors of cooler, denser plasma moving outward and hotter, more tenuous plasma moving ...inward. The Cassini Plasma Spectrometer (CAPS) has provided dramatic new evidence of this process operating in the magnetosphere of Saturn. The inward transport of hot plasma is accompanied by adiabatic gradient and curvature drift, producing a V‐shaped dispersion signature on a linear energy‐time plot. Of the many (∼100) such signatures evident during the first two Cassini orbits, we analyze a subset (48) that are sufficiently isolated to allow determination of their ages, widths, and injection locations. Ages are typically <10.8 hr (Saturn's rotation period) but range up to several rotation periods. Widths are typically <1 RS (Saturn's radius) but range up to several RS. Injection locations are randomly distributed in local time and in Saturnian longitude. The apex of the V sometimes coincides with a localized density cavity in the cooler background plasma, and usually coincides with a localized diamagnetic depression of the magnetic field strength. These signatures are fully consistent with the convective motions that are expected to result from the centrifugal interchange instability.
The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we ...combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan‐specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion‐ion (dusty) plasma below ~1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor ≥2 in the terminator and nightside ionosphere (ne/ni ≤ 0.1). We suggest that ion‐ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (ne/ni < 0.5 at 1000 km altitude). The average charge of the dust grains (≥1000 amu) is estimated to be between −2.5 and −1.5 elementary charges, increasing toward lower altitudes.
Key Points
Detection of electron‐depleted dusty ion‐ion plasma in lower ionosphere with enhanced densities
First empirical estimate of the negative ion and dust grain charge
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