Aerosols on the Giant Planets and Titan

On the giant planets and Titan, like on the terrestrial planets, aerosols play an important part in the physico-chemistry of the upper atmosphere (P ≤ 0.5 bar). Above all, aerosols significantly affect radiative transfer processes, mainly through light scattering, thus influencing the atmospheric energy budget and dynamics. Because there is usually significant coupling between atmospheric circulation and haze production, aerosols may constitute useful tracers of atmospheric dynamics.More generally, since their production is directly linked to some kind of energy deposition, their study may also provide clues to external sources of energy as well as their variability. Finally, aerosols indirectly influence other processes such as cloud formation and disequilibrium chemistry, by acting either as condensation nuclei or as reaction sites for surface chemistry. Here, I present a review of observational and modeling results based on remote sensing data, and also some insights derived from laboratory simulations. Despite our knowledge of the effects of aerosols in outer planetary atmospheres, however, relatively little is understood about the pathways which produce them, either endogenously (as end-products of gas-phase photochemical or shock reactions) or exogenously (as residues of meteroid ablation).     

The Changing Face of Titan’s Haze: Is it all Dynamics?

Titan’ atmosphere shows some similarities with that of the Earth, in terms of composition and surface pressure. Also, its seasonal cycle is similar, as Titan’ obliquity is about 27^°(23^°,5 for the Earth), although it is about 30 times as long. Titan’ haze exhibits an albedo contrast (NSA for North-South Asymmetry) that is changing seasonally. From the analysis of Voyager and Hubble Space Telescope data, we learned that at short visible wavelengths, the albedo of the winter hemisphere is lower by 10-20% than that of the summer hemisphere. This asymmetry peaks at 450 nm and reaches maximum amplitude around Titan’ equinoxes. It reverses in about five years, faster than a season which spans seven years. At longer wavelengths, longward of 700 nm, the asymmetry is inverted. The NSA reversal process in the red and in the UV seems to lead the reversal in the blue by 1 or 2 years. No valid explanation exists for this lag, at least in the red. The results from a recent model which couples atmospheric dynamics, haze microphysics and transport, as well as photochemistry, show that the NSA and its seasonal changes can be explained by an accumulation of haze particles at the winter pole. This is due to the pole-to-pole Hadley circulation pattern that is present during most of Titan’ year and rapidly disrupts at the time of the equinoxes. This model can also explain the observed cooler stratospheric temperatures and higher abundances of heavy hydrocarbons and nitriles in the winter polar region. In addition, it provides a mechanism for the formation of a detached haze layer around 300–400 km altitude, as well as the existence of a polar hood. Thus, it appears that the latitudinal contrasts we observe on Titan are conveniently tracing for us the dynamical behavior of its atmosphere.     

气球型深空探测器技术研究进展

气球型深空探测器能够大大提高深空探测的机动能力,它不仅可以获取区域范围内的高分辨率观测数据,而且还可实现不同高度大气的原位测量。文章针对气球型深空探测器按技术特征进行了分类,并简述了各类气球探测器的原理和特点,重点总结了各类气球探测器在金星、火星和土卫六上应用的研究现状。针对我国未来的气球型深空探测器技术发展,提出首先以火星热气球为发展方向,与地球临近空间浮空器技术的发展彼此借鉴,促进关键技术领域的技术突破等建议,可为我国未来深空探测提供参考。     

深空自主飞艇探测器技术发展

介绍了美国典型自主飞艇探测器的研究现状、总体技术方案和关键支撑技术。针对我国未来的深空飞艇技术发展,提出以火星应用为重点,促进在智能自主控制、新型材料、同位素热电等关键技术领域的技术发展,并与地球临近空间飞艇技术的发展彼此借鉴等建议,可为我国未来深空探测提供参考。     

The Cassini Ion and Neutral Mass Spectrometer (INMS) Investigation

The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan’s upper atmosphere and its interaction with Saturn’s magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon’s surface to form hydrocarbon–nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn’s ring system and icy moons and on the identification of positive ions and neutral species in Saturn’s inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings.The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N₂ and CH₄; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥10⁴ cm⁻³ will be detected (compared with ≥10⁵ cm⁻³ in the open source neutral mode). For ions the detection threshold is on the order of 10⁻² cm⁻³ at Titan relative velocity (6 km sec⁻¹). The INMS instrument has a mass range of 1–99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C₆H₆.The INMS instrument was built by a team of engineers and scientists working at NASA’s Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan).This revised version was published online in July 2005 with a corrected cover date.     

From Titan’s tholins to Titan’s aerosols: Isotopic study and chemical evolution at Titan’s surface

In the present work, we focused on the possible isotopic fractionation of carbon during the processes involved in the formation of Titan’s tholins. We present the first results obtained on the ¹²C/¹³C isotopic ratios measured on Titan’s tholins synthesized in laboratory with cold plasma discharges. Measurements of isotopic ratio ¹²C/¹³C, done both on tholins and on the initial gas mixture (N₂:CH₄ (98:2)) used to produce them, do not show any evident deficit or enrichment in ¹³C relatively to ¹²C in the synthesized tholins, compared to the initial gas mixture. This observation allows to go further in the analyses of the ACP experiment data, including part of the Cassini–Huygens mission.     

Unusually strong magnetic fields in Titan’s ionosphere: T42 case study

Observations of unusually large magnetic fields in the ionosphere indicate periods of maximum stress on Titan’s ionosphere and potentially of the strongest loss rates of ionospheric plasma. During Titan flyby T42, the observed magnetic field attained a maximum value of 37 nT between an altitude of 1200 and 1600 km, about 20 nT stronger than on any other Titan pass and close to five times greater in magnetic pressure. The strong fields occurred near the corotation-flow terminator rather than at the sub-flow point, suggesting that the flow which magnetized the ionosphere was from a direction far from corotation and possibly towards Saturn. Extrapolation of solar wind plasma conditions from Earth to Saturn using the University of Michigan MHD code predicts an enhanced solar wind dynamic pressure at Saturn close to this time. Cassini’s earlier exits from Saturn’s magnetosphere support this prediction because the Cassini Plasma Spectrometer instrument saw a magnetopause crossing three hours before the strong field observation. Thus it appears that Titan’s ionosphere was magnetized when the enhanced solar wind dynamic pressure compressed the Saturnian magnetosphere, and perhaps the magnetosheath magnetic field, against Titan. The solar wind pressure then decreased, leaving a strong fossil field in the ionosphere. When observed, this strong magnetic flux tube had begun to twist, further enhancing its strength.     

Cassini Plasma Spectrometer Investigation

The Cassini Plasma Spectrometer (CAPS) will make comprehensive three-dimensional mass-resolved measurements of the full variety of plasma phenomena found in Saturn’s magnetosphere. Our fundamental scientific goals are to understand the nature of saturnian plasmas primarily their sources of ionization, and the means by which they are accelerated, transported, and lost. In so doing the CAPS investigation will contribute to understanding Saturn’s magnetosphere and its complex interactions with Titan, the icy satellites and rings, Saturn’s ionosphere and aurora, and the solar wind. Our design approach meets these goals by emphasizing two complementary types of measurements: high-time resolution velocity distributions of electrons and all major ion species; and lower-time resolution, high-mass resolution spectra of all ion species. The CAPS instrument is made up of three sensors: the Electron Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion Mass Spectrometer (IMS). The ELS measures the velocity distribution of electrons from 0.6 eV to 28,250 keV, a range that permits coverage of thermal electrons found at Titan and near the ring plane as well as more energetic trapped electrons and auroral particles. The IBS measures ion velocity distributions with very high angular and energy resolution from 1 eV to 49,800 keV. It is specially designed to measure sharply defined ion beams expected in the solar wind at 9.5 AU, highly directional rammed ion fluxes encountered in Titan’s ionosphere, and anticipated field-aligned auroral fluxes. The IMS is designed to measure the composition of hot, diffuse magnetospheric plasmas and low-concentration ion species 1 eV to 50,280 eV with an atomic resolution M/ΔM ∼70 and, for certain molecules, (such asN ₂ ⁺ and CO⁺), effective resolution as high as ∼2500. The three sensors are mounted on a motor-driven actuator that rotates the entire instrument over approximately one-half of the sky every 3 min.This revised version was published online in July 2005 with a corrected cover date.     

Cassini Radio Science

Cassini radio science investigations will be conducted both during the cruise (gravitational wave and conjunction experiments) and the Saturnian tour of the mission (atmospheric and ionospheric occultations, ring occultations, determinations of masses and gravity fields). New technologies in the construction of the instrument, which consists of a portion on-board the spacecraft and another portion on the ground, including the use of the Ka-band signal in addition to that of the S- and X-bands, open opportunities for important discoveries in each of the above scientific areas, due to increased accuracy, resolution, sensitivity, and dynamic range.This revised version was published online in July 2005 with a corrected cover date.