One-dimensional aeronomical calculations of the atmospheric structure of extra-solar giant planets in orbits with semi-major axes from 0.01 to 0.1 AU show that the thermospheres are heated to over ...10,000 K by the EUV flux from the central star. The high temperatures cause the atmosphere to escape rapidly, implying that the upper thermosphere is cooled primarily by adiabatic expansion. The lower thermosphere is cooled primarily by radiative emissions from H
+
3, created by photoionization of H
2 and subsequent ion chemistry. Thermal decomposition of H
2 causes an abrupt change in the composition, from molecular to atomic, near the base of the thermosphere. The composition of the upper thermosphere is determined by the balance between photoionization, advection, and H
+ recombination. Molecular diffusion and thermal conduction are of minor importance, in part because of large atmospheric scale heights. The energy-limited atmospheric escape rate is approximately proportional to the stellar EUV flux. Although escape rates are large, the atmospheres are stable over time scales of billions of years.
The Imaging Ultraviolet Spectrograph (IUVS) is one of nine science instruments aboard the Mars Atmosphere and Volatile and EvolutioN (MAVEN) spacecraft. MAVEN, launched in November 18, 2013 and ...arriving at Mars in September 2014, is designed to explore the planet’s upper atmosphere and ionosphere and examine their interaction with the solar wind and solar ultraviolet radiation. IUVS is one of the most powerful spectrographs sent to another planet, with several key capabilities: (1) separate Far-UV & Mid-UV channels for stray light control, (2) a high resolution echelle mode to resolve deuterium and hydrogen emission, (3) internal instrument pointing and scanning capabilities to allow complete mapping and nearly-continuous operation, and (4) optimization for airglow studies.
We provide a comprehensive update of photochemical escape rates of atomic carbon from the present-day Martian atmosphere using a one-dimensional photochemical model and a Monte Carlo escape model. ...The photochemical model incorporates new results relevant to carbon photochemistry at Mars, including new cross sections for photodissociation of CO2 into C and O2 (Lu et al. 2014) and electron impact dissociation of CO (Ajello et al. 2019). We find the newly included channel of CO2 photodissociation to be the largest contributor to C escape, at 34%–58%. CO photodissociation and CO+ dissociative recombination, which have been discussed extensively in the literature, also show up as significant sources of hot C atoms, with respective contributions of 15%–23% and 7%–10%. Electron impact dissociation of CO2 (11%–15%) and photoionization of CO (6%–20%) are also important channels. Overall, escape rates vary over 3–11×1023 s−1, with an increase of 70% at perihelion compared to aphelion, and a much larger increase of 133% at solar maximum compared to solar minimum. While these present escape rates give a total integrated escape of only 1.3 mbar of CO2 when multiplied by 3.6 billion years, the better characterization of carbon photochemistry and escape from this study will enable us to more reliably extrapolate backwards in time to when conditions of the Martian atmosphere were significantly different from those of today.
•Carbon photochemical escape rate at Mars today is small at 1023 s−1.•New CO2 photodissociation channel makes up half of escape rate.•Escape rate varies significantly with seasons and solar activity.
Hydrogen escapes from Mars primarily by the Jeans mechanism but the rate is variable and the controlling factors complicated. One of the complications is that the temperature at the Martian exobase ...varies from ∼100K in the early morning hours to ∼300K in the afternoon. At the cold temperatures on the nightside of Mars, H escape rate is limited by Jeans escape, but on the warm dayside H escape is limited by the diffusion rate through the thermosphere. Nevertheless, the hot and cold regions are coupled by efficient ballistic transport through the exosphere. Because of this, H diffuses upward at the diffusion-limited rate even on the nightside and, once H reaches the exosphere, it is transported rapidly by ballistic flow to the warm dayside, where it escapes. As a result, escape is not at all limited by the cold regions of the exobase. The globally integrated escape flux is equal to the globally integrated diffusive limit. Because of this it is important to precisely calculate the diffusion-limited flux and we present a new formulation that is more accurate than the classical formula.
•A new, more accurate, formulation of the diffusion-limited flux for an escaping species is provided.•The exospheric temperature on Mars spans the range at which hydrogen escape is kinetically limited to diffusion limited.•Efficient ballistic transport through the exosphere results in a strong day-night flow of hydrogen.•Because of the ballistic flow, the upward transport of H on Mars is diffusion limited on the nightside despite the low temperatures.•The globally integrated H escape rate is equal to the globally integrated diffusion limit.
The Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission measures the structure and variability of the Martian upper atmosphere. We use ...NGIMS density profiles to derive upper atmospheric temperature profiles and investigate the thermal structure of this region. The thermal structure of the upper atmosphere is a critical component of understanding atmospheric loss to space, the main science objective of MAVEN, and measured temperatures serve as inputs to and constraints on photochemical and global circulation models. We describe proper treatment of the NGIMS data and correct for the horizontal motion of the spacecraft. Temperature profiles from week‐long, low‐altitude excursions executed by MAVEN, called Deep Dips, are used to investigate the diurnal variation of the temperature and the thermospheric gradient, which varies between 1.33 ± 0.16 and 2.69 ± 0.33 K km−1 on the dayside. NGIMS measurements acquired on nominal MAVEN orbits over more than a Martian year further elucidate the diurnal and latitudinal variations of the temperature. Diurnal variations of about a factor of 2, from 127 ± 8 to 260 ± 7 K, are observed high in the exosphere, and latitudinal variations of 39 ± 17 K are observed in this region. Comparisons indicate broad agreement between temperatures derived from MAVEN NGIMS with previous in situ and remote sensing observations of upper atmospheric temperatures. NGIMS temperatures are also shown to be consistent with predictions of a 1‐D model which includes solar UV and near IR heating, thermal conduction, and radiative cooling in the CO2 ν2 15‐μm band.
Key Points
Temperature profiles derived from MAVEN NGIMS data reveal the thermal structure of the Martian upper atmosphere
Proper treatment of instrumental effects is discussed, including impact on temperatures at the base of the thermosphere
Diurnal variations of 130 K and thermospheric gradients between 1.3 and 2.7 K km−1 are observed
Aerosol growth in Titan’s ionosphere Lavvas, Panayotis; Yelle, Roger V.; Koskinen, Tommi ...
Proceedings of the National Academy of Sciences - PNAS,
02/2013, Letnik:
110, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on ...atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan’s upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
This paper reports the first detections of two high‐altitude nighttime CO2 clouds on Mars during southern summer (Ls=264° and Ls=330°) with stellar occultation measurements by the Imaging Ultraviolet ...Spectrograph on board the Mars Atmosphere and Volatile Evolution spacecraft. Interpretation of the transmission spectra with Mie theory indicates particle radii of ∼90–110 nm assuming a monodisperse distribution. The altitude profile of extinction indicates that the cloud layers are confined horizontally to sizes less than ∼500–700 km. Examination of the CO2 density and temperature profiles reveals strong wave‐like perturbations. Supersaturated temperatures occur at the maximum negative extent of these wave‐like perturbations, which are organized in longitude with a dominant m=3 zonal harmonic. This suggests that tides are important in the formation of CO2 clouds.
Key Points
We report the first detections of two high‐altitude nighttime CO2 clouds on Mars during southern summer
The altitude profile of extinction indicates that the cloud layers are confined horizontally to sizes less than ∼500–700 km
Examination of the CO2 density and temperature profiles reveals that tides are important in the formation of Martian CO2 clouds
Atmospheric oscillations with daily periodicity are observed in in-situ near-surface pressure, temperature, and winds observations, and also in remotely sensed temperature and pressure observations ...of the Martian atmosphere. Such oscillations are interpreted as thermal tides driven by the diurnal cycle of solar radiation and occur at various frequencies, with the most prominent being the diurnal, semidiurnal, terdiurnal and quadiurnal tides. Mars global circulation models reproduce these tides with varying levels of success. Until recently, both the MarsWRF and newly developed MarsMPAS models were able to produce realistic diurnal and semidiurnal tide amplitudes but predicted higher-order mode amplitudes that were significantly weaker than observed. We use linear wave analysis to show that the divergence damping applied within both MarsWRF and MarsMPAS is responsible for suppressing the amplitude of thermal tides with frequency greater than 2 per sol, despite being designed to suppress only acoustic wave modes. Decreasing the strength of the divergence damping in MarsWRF and MarsMPAS allows for excellent prediction of the higher order tidal modes. This finding demonstrates that care must be taken when applying numerical dampers and filters that may eliminate some desired dynamical features in planetary atmospheres.
Using the Mars Atmospheric and Volatile EvolutioN mission (MAVEN) Imaging Ultraviolet Spectrograph (IUVS), we found periodic longitudinal variations in CO2 density in the Martian atmosphere. These ...density variations are derived from observations of the
CO2+ (
B2Σ+→X2Π) emission from limb scans in the 100–190 km altitude range. The variations exhibit significant structure with longitudinal wave numbers 1, 2, and 3 in an effectively constant local solar time frame, and we attribute this structure to nonmigrating tides. The wave‐2 component is dominated by the diurnal eastward moving DE1 tide at the equator and the semidiurnal stationary S0 tide at the midlatitudes. Wave‐3 is dominated by the diurnal eastward moving DE2 tide, with possibly the semidiurnal eastward moving SE1 tide causing an amplitude increase at the midlatitudes. Structure in the wave‐1 component can be explained by the semidiurnal westward moving SW1 tide.
Key Points
MAVEN/IUVS observed periodic longitudinal variations in CO2 density in the Martian atmosphere
We identify nonmigrating tides from the structure of density variations in constant LST frame
Wave‐2 is dominated by DE1 and S0, wave‐3 by DE2 and SE1, and wave‐1 by SW1
Thermal escape from extrasolar giant planets Koskinen, Tommi T.; Lavvas, Panayotis; Harris, Matthew J. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
04/2014, Letnik:
372, Številka:
2014
Journal Article
Recenzirano
Odprti dostop
The detection of hot atomic hydrogen and heavy atoms and ions at high altitudes around close-in extrasolar giant planets (EGPs) such as HD209458b implies that these planets have hot and rapidly ...escaping atmospheres that extend to several planetary radii. These characteristics, however, cannot be generalized to all close-in EGPs. The thermal escape mechanism and mass loss rate from EGPs depend on a complex interplay between photochemistry and radiative transfer driven by the stellar UV radiation. In this study, we explore how these processes change under different levels of irradiation on giant planets with different characteristics. We confirm that there are two distinct regimes of thermal escape from EGPs, and that the transition between these regimes is relatively sharp. Our results have implications for thermal mass loss rates from different EGPs that we discuss in the context of currently known planets and the detectability of their upper atmospheres.
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