How much oxygen is too much? Constraining Saturn's ring atmosphere

The discovery of a molecular oxygen atmosphere around Saturn's rings has important implications for the electrodynamics of the ring system. Its existence was inferred from the Cassini in situ detection of molecular oxygen ions above the rings during Saturn Orbit Insertion in 2004. Molecular oxygen is difficult to observe remotely, and theoretical estimates have yielded only a lower limit (Nn?¹⁰¹³ cm⁻²) to the O₂ column density. Comparison with observations has previously concerned matching ion densities at spacecraft altitudes far larger than the scale height of the neutral atmosphere. This is further complicated by charged particle propagation effects in Saturn's offset magnetic field. In this study we adopt a complementary approach, by focusing on bulk atmospheric properties and using additional aspects of the Cassini observations to place an upper limit on the column density. We develop a simple analytic model of the molecular atmosphere and its photo-ionization and dissociation products, with Nn a free parameter. Heating of the neutrals by viscous stirring, cooling by collisions with the rings, and torquing by collisions with pickup ions are all included in the model. We limit the neutral scale height to h?3000 km using the INMS neutral density nondetection over the A ring. A first upper limit to the neutral column is derived by using our model to reassess O₂ production and loss rates. Two further limits are then obtained from Cassini observations: corotation of the observed ions with the planet implies that the height-integrated conductivity of the ring atmosphere is less than that of Saturn's ionosphere; and the nondetection of fluorescent atomic oxygen over the rings constrains the molecular column from which it is produced via photo-dissociation. These latter limits are independent of production and loss rates and are only weakly dependent on temperature. From the three independent methods described, we obtain similar limits: Nn?2×¹⁰¹⁵ cm⁻². The mean free path for collisions between neutrals thus cannot be very much smaller than the scale height.     

Initial interpretation of Titan plasma interaction as observed by the Cassini plasma spectrometer: Comparisons with Voyager 1

The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H⁺and N⁺/O⁺, were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H⁺ and H₂⁺as the light constituents and O⁺/CH₄⁺ as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O⁺ ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H⁺, H₂⁺, N⁺, CH₄⁺ and N₂⁺. (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N₂⁺ is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.     

Ion and neutral sources and sinks within Saturn's inner magnetosphere: Cassini results

Using ion-electron fluid parameters derived from Cassini Plasma Spectrometer (CAPS) observations within Saturn's inner magnetosphere as presented in Sittler et al. [2006a. Cassini observations of Saturn's inner plasmasphere: Saturn orbit insertion results. Planet. Space Sci., 54, 1197-1210], one can estimate the ion total flux tube content, N_IONL², for protons, H⁺, and water group ions, W⁺, as a function of radial distance or dipole L shell. In Sittler et al. [2005. Preliminary results on Saturn's inner plasmasphere as observed by Cassini: comparison with Voyager. Geophys. Res. Lett. 32(14), L14S04), it was shown that protons and water group ions dominated the plasmasphere composition. Using the ion-electron fluid parameters as boundary condition for each L shell traversed by the Cassini spacecraft, we self-consistently solve for the ambipolar electric field and the ion distribution along each of those field lines. Temperature anisotropies from Voyager plasma observations are used with (T_⊥/T_∥)_W⁺∼5 and (T_⊥/T_∥)_H⁺∼2. The radio and plasma wave science (RPWS) electron density observations from previous publications are used to indirectly confirm usage of the above temperature anisotropies for water group ions and protons. In the case of electrons we assume they are isotropic due to their short scattering time scales. When the above is done, our calculation show N_IONL² for H⁺ and W⁺ peaking near Dione's L shell with values similar to that found from Voyager plasma observations. We are able to show that water molecules are the dominant source of ions within Saturn's inner magnetosphere. We estimate the ion production rate S_ION∼10²⁷ ions/s as function of dipole L using N_H⁺, N_W⁺ and the time scale for ion loss due to radial transport τ_D and ion-electron recombination τ_REC. The ion production shows localized peaks near the L shells of Tethys, Dione and Rhea, but not Enceladus. We then estimate the neutral production rate, S_W, from our ion production rate, S_ION, and the time scale for loss of neutrals by ionization, τ_ION, and charge exchange, τ_CH. The estimated source rate for water molecules shows a pronounced peak near Enceladus’ L shell L∼4, with a value S_W∼2×10²⁸ mol/s.     

Electrostatic ion-cyclotron waves in a plasma with heavy negative ions

Results of a laboratory study of electrostatic ion-cyclotron (EIC) waves in a plasma containing K⁺ (39 amu) positive ions, electrons and C₇F₁₄⁻ (350 amu) negative ions are presented. Excitation of the fundamental and higher harmonic light and heavy ion EIC modes was observed. The presence of heavy negative ions in the plasma has a significant effect on the excitation of the light ion EIC modes. The results may be relevant to the understanding of plasma wave properties in plasmas containing negative ions, such as those found in the Earth's ionosphere, the solar system, and, in particular, near Saturn's moon Titan, where an abundance of heavy negative ion species has recently been discovered [Coates, et al., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett. 34, L22103].     

Heavy ion formation in Titan's ionosphere: Magnetospheric introduction of free oxygen and a source of Titan's aerosols?

Discovery by Cassini's plasma instrument of heavy positive and negative ions within Titan's upper atmosphere and ionosphere has advanced our understanding of ion neutral chemistry within Titan's upper atmosphere, primarily composed of molecular nitrogen, with ~2.5% methane. The external energy flux transforms Titan's upper atmosphere and ionosphere into a medium rich in complex hydrocarbons, nitriles and haze particles extending from the surface to 1200 km altitudes. The energy sources are solar UV, solar X-rays, Saturn's magnetospheric ions and electrons, solar wind and shocked magnetosheath ions and electrons, galactic cosmic rays (GCR) and the ablation of incident meteoritic dust from Enceladus’ E-ring and interplanetary medium. Here it is proposed that the heavy atmospheric ions detected in situ by Cassini for heights >950 km, are the likely seed particles for aerosols detected by the Huygens probe for altitudes <100 km. These seed particles may be in the form of polycyclic aromatic hydrocarbons (PAH) containing both carbon and hydrogen atoms C_nH_x. There could also be hollow shells of carbon atoms, such as C₆₀, called fullerenes which contain no hydrogen. The fullerenes may compose a significant fraction of the seed particles with PAHs contributing the rest. As shown by Cassini, the upper atmosphere is bombarded by magnetospheric plasma composed of protons, H₂⁺ and water group ions. The latter provide keV oxygen, hydroxyl and water ions to Titan's upper atmosphere and can become trapped within the fullerene molecules and ions. Pickup keV N₂⁺, N⁺ and CH₄⁺ can also be implanted inside of fullerenes. Attachment of oxygen ions to PAH molecules is uncertain, but following thermalization O+ can interact with abundant CH₄ contributing to the CO and CO₂ observed in Titan's atmosphere. If an exogenic keV O+ ion is implanted into the haze particles, it could become free oxygen within those aerosols that eventually fall onto Titan's surface. The process of freeing oxygen within aerosols could be driven by cosmic ray interactions with aerosols at all heights. This process could drive pre-biotic chemistry within the descending aerosols. Cosmic ray interactions with grains at the surface, including water frost depositing on grains from cryovolcanism, would further add to abundance of trapped free oxygen. Pre-biotic chemistry could arise within surface microcosms of the composite organic-ice grains, in part driven by free oxygen in the presence of organics and any heat sources, thereby raising the astrobiological potential for microscopic equivalents of Darwin's “warm ponds” on Titan.     

Electronic Sputtering Analysis of Astrophysical Ices

Experimental results on fast ion collision with icy surfaces having astrophysical interest are presented. ²⁵²Cf fission fragments projectiles were used to induce ejection of ionized material from H₂O, CO₂, CO, NH₃, N₂, O₂ and Ar ices; the secondary ions were identified by time-of-flight mass spectrometry. It is observed that all the bombarded frozen gas targets emit cluster ions which have the structure X_nR^±, where X is the neutral ice molecule and R^± is either an atomic or a molecular ion. The shape of the positive or negative ion mass spectra is characterized by a decreasing yield as the emitted ion mass increases and is generally described by the sum of two exponential functions. The positive ion water ice spectrum is dominated by the series (H₂O)_nH₃O⁺ and the negative ion spectrum by the series (H₂O)_nOH⁻ and (H₂O)_nO⁻. The positive ion CO₂ ice spectrum is characterized by R⁺ = C⁺, O⁺, CO⁺, O₂⁺ or CO₂⁺ and the negative one by R⁻ = CO₃⁻. The dominant series for ammonia ice correspond to R⁺ = NH₄⁺ and to R⁻ = NH₂⁻. The oxygen series are better described by (O₃)_nO_m⁺ secondary ions where m = 1, 2 or 3. Two positive ion series exist for N₂ ice: (N₂)_nN₂⁺ and (N₂)_nN⁺. For argon positive secondary ions, only the (Ar)_nAr⁺ series was observed. Most of the detected molecular ions were formed by one-step reactions. Ice temperature was varied from ∼20 K to complete sublimation.     

Laboratory measurements of the association rate coefficients of NO+, O+2, N+, and N+2 ions with N2 and CO2 at temperatures between 100 k and 400 k

Rate coefficients for the association reactions of NO⁺ ions with N₂ and CO₂, O₂⁺ with N₂, and N⁺ and N₂⁺ with N₂ have been determined as a function of gas temperature in a laboratory experiment employing a variable-temperature drift-tube apparatus. The measured rate coefficients were fitted to power laws of the form $\text{k = C}\text{(}\text{T}\text{300}\text{)}{}^{\mathrm{x}}$ where the exponents x ranged from 2.2 to 4.3. The strong temperature dependence observed in the case of the reaction of NO⁺ with N₂ (x = 4.3) supports the thesis by Arnold et al. (1979) that the temperature variability of D-region ion densities is a result of this reaction step in the ion clustering sequence.     

Collisional vibrational quenching of O2+(v) and other molecular ions in planetary atmospheres

New experimental techniques have yielded several thermal energy vibrational quenching rate constants for O₂⁺(v). Rates for quenching of O₂⁺(v = 1) by O₂, N₂, Ar, CO₂, H₂, and CH₄ are 3(?10), 2(?12), 1(?12), 1(?10), 2.5(?12), and 6(?10) cm³s^?1 at 300 K. The quenching is somewhat faster for O₂⁺(v = 2). The triatomic ions CO₂⁺, NO₂⁺, N₂O⁺, SO₂⁺, and H₂O⁺ are all vibrationally deexcited with an efficiency greater than 10^?3 in Ar or Ne collisions. A theoretical rationalization of the experimental results leads to the prediction that vibrational quenching in planetary atmospheres will generally be efficient, k > 1(?12) cm³s^?1 for almost all ion and neutral gas pairs.     

A model of the terrestrial ionosphere in the altitude interval 50–4000 km II. Molecular ions (N2 +, NO+, O2 +) and electron density

Empirical models of molecular ion densities (N₂ ⁺, NO⁺, O₂ ⁺) and the electron density (N _e ) are presented in the altitude interval 50–4000 km as functions of time (diurnal, annual), space (position, altitude) and solar flux (F _10.7). Using observations of 6 satellites (AE-C, AE-D, AE-E, ALOUETTE-2, ISIS-1, ISIS-2), 4 incoherent scatter stations (Arecibo, Jicamarca, Millstone Hill, St Santin) and more than 700 D-region profiles, this model describes the global gross features of the ionosphere for quiet geophysical conditions (K _p 3).The molecular ion densities and the electron density increase with increasing altitude up to a maximum (or several maxima) - and decrease from thereon with increasing height. Between ~80 and 200 km, the main ionic constituents are NO⁺ and O₂ ⁺; below ~80 km cluster ions are predominating. During local summer conditions the molecular ions and N _e increase around polar latitudes and decrease correspondingly during local winter. The diurnal variations are intrinsically coupled to the individual plasma layers; in general, the molecular ion and electron densities are enhanced during daytime and depleted during nighttime (for details and exceptions, see text).     

The production of water-cluster positive ions in the quiet daytime D region

In the quiet daytime D region, the primary positive-ion species is thought to be NO⁺, produced by solar Lyman-alpha ionization of NO. Below the altitude of the mesopause, however, the dominant ambient species observed are water-cluster ions of the general type H⁺(H₂O)_n. No satisfactory reaction scheme for producing these cluster ions from NO⁺ has yet been proposed. Following earlier suggestions, a model calculation has been carried out in which successive hydrations of NO⁺ take place through clustering with N₂ and CO₂, followed by “switching” reactions with H₂O. The third hydrate of NO⁺ is then converted into the water-cluster species H⁺(H₂O)₃, and the other water-cluster species are produced by successive clustering and thermal breakup reactions. Many of the reactions involved have not been measured in the laboratory, but reasonable estimates of their rates can be made on the basis of existing measurements of other species. Since both temperature and water-vapor content are of major importance in the model, calculations were carried out for two temperature profiles and two water-vapor profiles. It is shown that the results are in reasonably good agreement with observations as far as the water-cluster species are concerned. Under low-temperature conditions, the model predicts relatively large concentrations of various clusters of NO⁺, in agreement with some observations but in disagreement with others. The importance of sampling breakup of these weakly bound clusters, and their relevance to the free electron concentrations are discussed.     

A steady state model of negative ions in the lower stratosphere

A chemical model of negative ions in the lower stratosphere is presented. Under the steady state condition at each height from 15 to 30 km, the fractional abundances of the individual negatively-charged constituents were calculated by simulating the chemical processes which start from electrons and reach the ion clusters having NO₃⁻and HSO₄⁻ cores. The computed result shows that while the NO₃⁻(HNO₃)₂ ions predominate at lower than 26 km in altitude, the HSO₄⁻(HNO₃)_m type of ions increase slowly with height and the HSO₄⁻(H₂SO₄)_n type of ions increase rapidly with height around 30 km. The result is compared with the experimental result which was obtained by the in situ balloon-borne mass spectrometric measurements (Viggiano et al., 1983). Both results have comparable height profile of respective ions with each other. As the “adjusted” reaction rates for the formation of cluster ions were assumed in the calculations, the result should be considered a preliminary one. Future works to be extended from the present one will be suggested.     

Computedn=2 level populations in helium-like ions from Cv to Nixxvii

The population densities of all levels with principal quantum numbern=2 in a number of helium-like ions with nuclear charge numberZ, in the range 6 to 28 have been evaluated as a function of various parameters, i.e., electron temperature,T _e, electron density,N _e, radiation temperature,T _r, dilution factor,W, and of the state of ionization. The spectral line fluxes from all possible radiative transitions from these levels have been calculated for an optically thin plasma. The effects of cascades following collisional excitation of higher levels or radiative and dielectronic recombination have been computed in detail. Innershell ionization of the lithium-like ion to form the helium-like ion in a 2³ S or 2¹ S state has been considered. It can have a strong influence on the forbidden line intensity in a non-equilibrium plasma. Collisional and radiative coupling of levels of the same multiplicity (e.g. 2³ S ₁ and 2³ P _2,1,0) have been considered as a function ofT _e, N_e orT _r, W, respectively. The computations were performed both for stationary and time-varying plasmas. In the latter case strong departures from a stationary ionization equilibrium can significantly alter the line fluxes. A few examples of the results are shown and discussed.     

On the production of positive molecular ions in cometary comas

Positively charged molecular ions, such as H₂O⁺, which have been observed in cometary. comas, may be efficiently produced by the evaporation of positively charged clathrate grains of radii in the range 10^–6–10^–5 cm. Such grains may be expelled from nuclei of comets, along with gaseous molecules. Grain charging occurs via interaction with solar ultraviolet photons and/or solar wind protons. Observational data on the total quantities as well as the distributions of H₂O and H₂O⁺ in cometary comas are shown to be in accord with detailed model calculations.On leave from: Tata Institute of Fundamental Research, Bombay, India.     

H2O + ions in comets: models and observations

An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H₂O ⁺ shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H₂O ⁺ ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H₂O ⁺ ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H₂O ⁺ ions. Finally, the two dimensional images of model column densities are compared with observations.     

Products of meteoric metal ion chemistry within planetary atmospheres. 1. Mg at Titan

We report results of quantum chemical calculations of Mg⁺/ligand bond dissociation energies involving ligands identified as major constituents of Titan's upper atmosphere. Trends identified in these results allow elucidation of the important bimolecular and termolecular reactions of Mg⁺, and of simple molecular ions containing Mg⁺, arising from meteoric infall into Titan's atmosphere. Our study highlights, and includes calculated rate coefficients for, crucial ligand-switching and ligand-stripping reactions which ensure that a dynamic equilibrium exists between atomic and molecular ions of Mg⁺. Neutralization of ionized meteoric Mg is expected to produce the radical MgNC in high yield. The highly polar MgNC radical should provide an excellent nucleation site for condensation of polar (e.g., HCN, CH₃CN, and HC₃N) and highly unsaturated (e.g., C₂H₂, C₄H₂, and C₂N₂) neutrals at comparatively high altitude, leading to precipitation of Mg-doped tholin-like material. The implications for Titan's prebiotic chemical evolution, of the surface deposition of such material (which may feasibly contain magnesium porphyrins, or other bioactive Mg-containing complexes) remain to be assessed.     

Prompt acceleration of ≳ 30 MeV per nucleon ions in solar flares

Observation of prompt γ-rays in solar flares requires that ions be accelerated to >30 MeV nucl^-1 in ? 2 s. A model for prompt acceleration is developed. The energy release is assumed to occur in a flaring loop with the energy release region being ? 10⁴ km in dimensions and with an Alfvén speed υ _A ? 3 × 10³ km s^-1. The acceleration is assumed to occur in two steps. The second-step acceleration from ? ? _T = 1/2m _p υ_A ² nucl^-1 to ? 30 MeV nucl^-1 is attributed to stochastic acceleration by hydromagnetic turbulence which is found to be fast enough under conditions which are not extreme. Main emphasis is placed on the first step, called preacceleration, to ? _T ? 100 keV nucl^-1. Preacceleration mechanisms which involve accelerating a small fraction of ions from the tail of a Maxwellian distribution are unacceptable because they would lead to enormous abundance anomalies. Preacceleration is attributed either to localized heating of ions to ? 10⁹ K or to acceleration by potential electric fields. The latter mechanism is favoured and some theoretical ideas are outlined based on observations of reconnection in the Earth's magnetotail. Whether energetic ions are prompt, delayed or unobservable depends only on the rate at which the stochastic acceleration proceeds. The second-step acceleration of electrons, invoked to account for a harder microwave component, is predicted to be slower by a factor ? 3 than for ? 30 MeV nucl^-1 ions.     

Ion-ion mutual neutralization and ion-neutral switching reactions of some stratospheric ions

Following the recent mass spectrometric observations of the ambient stratospheric positive and negative ions we have carried out co-ordinated laboratory experiments using a selected ion flow tube apparatus and a flowing afterglow apparatus for the following purposes: (i) to consider whether CH₃CN is a viable candidate molecule for the species X in the observed stratospheric ion series H⁺ (H₂O_n (X)_m and (ii) to determine the binary mutual neutralization rate coefficients α_i for the reactions ofH⁺ (H₂O₄ and H⁺(H₂O)(CH₃CN)₃ with several of the negative ion species observed in the stratosphere. We conclude from (i) that CH₃CN is indeed a viable candidate for X and from (ii) that the α_i for stratospheric ions are within the limited range (5–6) × 10^?8 cm³ s^?1.     

Enceladus: The likely dominant nitrogen source in Saturn's magnetosphere

The spatial distribution of N⁺ in Saturn's magnetosphere obtained from Cassini Plasma Spectrometer (CAPS) data can be used to determine the spatial distribution and relative importance of the nitrogen sources for Saturn's magnetosphere. We first summarize CAPS data from 15 orbits showing the spatial and energy distribution of the nitrogen component of the plasma. This analysis re-enforces our earlier discovery [Smith, H.T., Shappirio, M., Sittler, E.C., Reisenfeld, D., Johnson, R.E., Baragiola, R.A., Crary, F.J., McComas, D.J., Young, D.T., 2005. Geophys. Res. Lett. 32 (14). L14S03] that Enceladus is likely the dominant nitrogen source for Saturn's inner magnetosphere. We also find a sharp enhancement in the nitrogen ion to water ion ratio near the orbit of Enceladus which, we show, is consistent with the presence of a narrow Enceladus torus as described in [Johnson, R.E., Liu, M., Sittler Jr., E.C., 2005. Geophys. Res. Lett. 32. L24201]. The CAPS data and the model described below indicate that N⁺ ions are a significant fraction of the plasma in this narrow torus. We then simulated the combined Enceladus and Titan nitrogen sources using the CAPS data as a constraint. This simulation is an extension of the model we employed earlier to describe the neutral tori produced by the loss of nitrogen from Titan [Smith, H.T., Johnson, R.E., Shematovich, V.I., 2004. Geophys. Res. Lett. 31 (16). L16804]. We show that Enceladus is the principal nitrogen source in the inner magnetosphere but Titan might account for a fraction of the observed nitrogen ions at the largest distances discussed. We also show that the CAPS data is consistent with Enceladus being a molecular nitrogen source with a nitrogen to water ratio roughly consistent with INMS [Waite, J.H., and 13 colleagues, 2006. Science 311 (5766), 1419-1422], but out-gassing of other nitrogen-containing species, such as ammonia, cannot be ruled out.     

Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion-molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O₂, N₂, NO, CO, Ar and CO₂.This study shows the strong influence of electronic as well as vibrational internal energy on most ion-molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O₂⁺ and NO⁺ ions in their a⁴Π_u and a³Σ⁺ metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O₂⁺ with NO as well as the reactions of CO₂⁺ with NO, O₂, CO and to a less extent N₂ are driven by vibrational excitation. N₂⁺ and CO⁺ reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO⁺+M reactions as we expect that a significant proportion of these CO⁺ could be produced with translational energy by dissociation of doubly charged CO₂²⁺, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.