► We developed a new model to explain the FUV transit observations of HD209458b. ► We find a qualitative agreement between the model and the observed atmosphere. ► We constrain the mass loss rate and ...escape mechanism based on stellar heating. ► We present density profiles for the detected heavy atoms and ions. ► Diffusive separation of heavy species is prevented by the escape of H and protons.
The detections of atomic hydrogen, heavy atoms and ions surrounding the extrasolar giant planet (EGP) HD209458b constrain the composition, temperature and density profiles in its upper atmosphere. Thus the observations provide guidance for models that have so far predicted a range of possible conditions. We present the first hydrodynamic escape model for the upper atmosphere that includes all of the detected species in order to explain their presence at high altitudes, and to further constrain the temperature and velocity profiles. This model calculates the stellar heating rates based on recent estimates of photoelectron heating efficiencies, and includes the photochemistry of heavy atoms and ions in addition to hydrogen and helium. The composition at the lower boundary of the escape model is constrained by a full photochemical model of the lower atmosphere. We confirm that molecules dissociate near the 1μbar level, and find that complex molecular chemistry does not need to be included above this level. We also confirm that diffusive separation of the detected species does not occur because the heavy atoms and ions collide frequently with the rapidly escaping H and H+. This means that the abundance of the heavy atoms and ions in the thermosphere simply depends on the elemental abundances and ionization rates. We show that, as expected, H and O remain mostly neutral up to at least 3Rp, whereas both C and Si are mostly ionized at significantly lower altitudes. We also explore the temperature and velocity profiles, and find that the outflow speed and the temperature gradients depend strongly on the assumed heating efficiencies. Our models predict an upper limit of 8000K for the mean (pressure averaged) temperature below 3Rp, with a typical value of 7000K based on the average solar XUV flux at 0.047AU. We use these temperature limits and the observations to evaluate the role of stellar energy in heating the upper atmosphere.
The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) provides sensitive detections of neutral gas and ambient ion composition. NGIMS measurements of nine ...atomic and molecular neutral species, and their variation with altitude, latitude, and solar zenith angle are reported over several months of operation of the MAVEN mission. Sampling NGIMS signals from multiple neutral species every several seconds reveals persistent and unexpectedly large amplitude density structures. The scale height temperatures are mapped over the course of the first few months of the mission from high down to midlatitudes. NGIMS measurements near the homopause of 40Ar/N2 ratios agree with those reported by the Sample Analysis at Mars investigation and allow the altitude of the homopause for the most abundant gases to be established.
Key Points
Neutral density structure measured with high temporal resolution
Scale height temperature of the upper atmosphere reported
Homopause altitude identified
High-energy photons, electrons, and ions initiate ion–neutral chemistry in Titan's upper atmosphere by ionizing the major neutral species (nitrogen and methane). The Ion and Neutral Mass Spectrometer ...(INMS) onboard the Cassini spacecraft performed the first composition measurements of Titan's ionosphere. INMS revealed that Titan has the most compositionally complex ionosphere in the Solar System, with roughly 50 ions at or above the detection threshold. Modeling of the ionospheric composition constrains the density of minor neutral constituents, most of which cannot be measured with any other technique. The species identified with this approach include the most complex molecules identified so far on Titan. This confirms the long-thought idea that a very rich chemistry is actually taking place in this atmosphere. However, it appears that much of the interesting chemistry occurs in the upper atmosphere rather than at lower altitudes. The species observed by INMS are probably the first intermediates in the formation of even larger molecules. As a consequence, they affect the composition of the bulk atmosphere, the composition and optical properties of the aerosols and the flux of condensable material to the surface. In this paper, we discuss the production and loss reactions for the ions and how this affects the neutral densities. We compare our results to neutral densities measured in the stratosphere by other instruments, to production yields obtained in laboratory experiments simulating Titan's chemistry and to predictions of photochemical models. We suggest neutral formation mechanisms and highlight needs for new experimental and theoretical data.
We report the results of the observations of the ionosphere of Mars by the Neutral Gas and Ion Mass Spectrometer. These observations were conducted during the first 8 months of the Mars Atmosphere ...and Volatile EvolutioN mission (MAVEN). These observations revealed the spatial and temporal structures in the density distributions of 22 ions: H2+, H3+, He+, O2+, C+, CH+, N+, NH+, O+, OH+, H2O+, H3O+, N2+/CO+, HCO+/HOC+/N2H+, NO+, HNO+, O2+, HO2+, Ar+, ArH+, CO2+, and OCOH+. Dusk/dawn and day/night asymmetries in the density distributions were observed for nearly all ion species. Additionally, high‐density fluctuations were detected on the nightside and may reflect the effect of the partial screening of the atmosphere of Mars by the weak intrinsic magnetic field of the planet. The two first MAVEN “deep dip” campaigns were used to investigate the location of the primary ion peak. This peak was detected at 190 km near the terminator but was below the spacecraft altitude of 130 km near the subsolar point.
Key Points
Several ionospheric species were detected during the first 8 months of the MAVEN mission
The major ion density peak was detected near the terminator at 190 km altitude
The density profiles of all ions exhibit day/night and dusk/dawn asymmetries
We present a study of the formation and distribution of benzene (C6H6) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the ...benzene signal exhibits an unusual time dependence, peaking ∼20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C6H5) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C6H6 from the atmosphere and (2) C6H6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion‐molecule reactions is ∼107 cm−2 s−1, of the same order of magnitude as the production rate by neutral reactions of ∼4 × 106 cm−2 s−1. We show that benzene is quickly photolyzed in the thermosphere, and that C6H5 radicals, the main photodissociation products, are ∼3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of ∼10−15 g cm−2 s−1, corresponding to an accumulated surface layer of ∼3 m.
► We compare UV transits of HD209458b with empirical and hydrodynamic models. ► We constrain the mean temperature, densities and escape rates of different species. ► The detection of atomic oxygen ...implies a minimum mass loss rate of 6×106kgs−1. ► The results constrain the temperature, chemistry, and ionization of the atmosphere. ► Detection of Si2+ indicates that clouds of forsterite and enstatite do not form.
Transits in the H I 1216Å (Lyman α), O I 1334Å, C II 1335Å, and Si III 1206.5Å lines constrain the properties of the upper atmosphere of HD209458b. In addition to probing the temperature and density profiles in the thermosphere, they have implications for the properties of the lower atmosphere. Fits to the observations with a simple empirical model and a direct comparison with a more complex hydrodynamic model constrain the mean temperature and ionization state of the atmosphere, and imply that the optical depth of the extended thermosphere of the planet in the atomic resonance lines is significant. In particular, it is sufficient to explain the observed transit depths in the H I 1216Å line. The detection of O at high altitudes implies that the minimum mass loss rate from the planet is approximately 6×106kgs−1. The mass loss rate based on our hydrodynamic model is higher than this and implies that diffusive separation is prevented for neutral species with a mass lower than about 130amu by the escape of H. Heavy ions are transported to the upper atmosphere by Coulomb collisions with H+ and their presence does not provide as strong constraints on the mass loss rate as the detection of heavy neutral atoms. Models of the upper atmosphere with solar composition and heating based on the average solar X-ray and EUV flux agree broadly with the observations but tend to underestimate the transit depths in the O I, C II, and Si III lines. This suggests that the temperature and/or elemental abundances in the thermosphere may be higher than expected from such models. Observations of the escaping atmosphere can potentially be used to constrain the strength of the planetary magnetic field. We find that a magnetic moment of m≲0.04mJ, where mJ is the jovian magnetic moment, allows the ions to escape globally rather than only along open field lines. The detection of Si2+ in the thermosphere indicates that clouds of forsterite and enstatite do not form in the lower atmosphere. This has implications for the temperature and dynamics of the atmosphere that also affect the interpretation of transit and secondary eclipse observations in the visible and infrared wavelengths.
Combining the Mars Atmosphere and Volatile Evolution (MAVEN) measurements of atmospheric neutral and ion densities, electron temperature, and energetic electron intensity, we perform the first ...quantitative evaluation of local ionization balance in the nightside Martian upper atmosphere, a condition with the electron impact ionization (EI) of CO2 exactly balanced by the dissociative recombination (DR) of ambient ions. The data accumulated during two MAVEN Deep Dip (DD) campaigns are included: DD6 on the deep nightside with a periapsis solar zenith angle (SZA) of 165°, and DD3 close to the dawn terminator with a periapsis SZA of 110°. With the electron temperatures at low altitudes corrected for an instrumental effect pertaining to the MAVEN Langmuir Probe and Waves, a statistical agreement between the EI and DR rates is suggested by the data below 140 km during DD6 and below 180 km during DD3, implying that electron precipitation is responsible for the nightside Martian ionosphere under these circumstances and extra sources are not required. In contrast, a substantial enhancement in EI over DR is observed at higher altitudes during both campaigns, which we interpret as a signature of plasma escape down the tail.
We describe a detailed study on the properties of alkali atoms in extrasolar giant planets, and specifically focus on their role in generating the atmospheric free electron densities, as well as ...their impact on the transit depth observations. We focus our study on the case of HD209458b, and we show that photoionization produces a large electron density in the middle atmosphere that is about two orders of magnitude larger than the density anticipated from thermal ionization. Our purely photochemical calculations, though, result in a much larger transit depth for K than observed for this planet. This result does not change even if the roles of molecular chemistry and excited state chemistry are considered for the alkali atoms. In contrast, the model results for the case of exoplanet XO-2b are in good agreement with the available observations. Given these results we discuss other possible scenarios, such as changes in the elemental abundances, changes in the temperature profiles, and the possible presence of clouds, which could potentially explain the observed HD209458b alkali properties. We find that most of these scenarios cannot explain the observations, with the exception of a heterogeneous source (i.e., clouds or aerosols) under specific conditions, but we also note the discrepancies among the available observations.
We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have ...used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside.
•Temperature and density waves have been extracted from in situ spacecraft data in Mars' upper atmosphere.•Waves cause temperatures and densities to vary by about 10% around the background values.•Wave characteristics, especially amplitude, appear to be correlated with temperature.•Wave amplitudes appear to grow with altitude on the nightside and remain constant on the dayside.•There appears to be enhanced wave activity in the northern hemisphere which may be caused by topography.
We present dayside electron temperature (Te) and density altitude profiles at Mars from MAVEN satellite deep‐dip orbits. The data are after recalibration of the Langmuir Probe and Waves instrument ...that results in reduced uncertainties to as low as ±82°K. At MAVEN's lowest altitudes, (∼120–∼135 km), the measured values of Te are, after uncertainties, higher than those predicted by several modeling efforts. To better understand this discrepancy, we perform a basic heat‐transfer analysis for two specific dayside deep dips. The analysis supports that CO2 excitation/de‐excitation of its lowest‐energy vibrational states dominates energy transfer to and from electrons. We hypothesize that the discrepancy between the measured and modeled Te is due to (a) the coupling of Te to CO2 vibrational temperatures combined with a non‐LTE (local thermal equilibrium) excess of excited CO2 and/or (b) a non‐Maxwellian electron distribution that moderates CO2 cooling.
Plain Language Summary
The MAVEN satellite has measured electron temperatures with sufficient accuracy to test atmospheric and ionospheric models at Mars. The measured electron temperatures are found to be significantly higher than most models predict, which indicates that the electron‐CO2 energy exchange may not be fully understood or that an unidentified energy source is heating electrons. This article proposes that the elevated electron temperatures may be due to a high abundance of CO2 in an excited state. The implication is that the electron temperature may be an indication of the excitement of CO2 rather than its thermal (kinetic) temperature.
Key Points
In‐situ measurements of the electron temperature and density in Mars' dayside ionosphere
Electron temperatures are higher than predicted at low altitudes in Mars' ionosphere
Modeling electron temperatures require evaluation of electron‐CO2 vibrational excitement/de‐excitement and electron distributions
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