The habitability of the surface of any planet is determined by a complex evolution of its interior, surface, and atmosphere. The electromagnetic and particle radiation of stars drive thermal, ...chemical, and physical alteration of planetary atmospheres, including escape. Many known extrasolar planets experience vastly different stellar environments than those in our solar system: It is crucial to understand the broad range of processes that lead to atmospheric escape and evolution under a wide range of conditions if we are to assess the habitability of worlds around other stars. One problem encountered between the planetary and the astrophysics communities is a lack of common language for describing escape processes. Each community has customary approximations that may be questioned by the other, such as the hypothesis of H‐dominated thermosphere for astrophysicists or the Sun‐like nature of the stars for planetary scientists. Since exoplanets are becoming one of the main targets for the detection of life, a common set of definitions and hypotheses are required. We review the different escape mechanisms proposed for the evolution of planetary and exoplanetary atmospheres. We propose a common definition for the different escape mechanisms, and we show the important parameters to take into account when evaluating the escape at a planet in time. We show that the paradigm of the magnetic field as an atmospheric shield should be changed and that recent work on the history of Xenon in Earth's atmosphere gives an elegant explanation to its enrichment in heavier isotopes: the so‐called Xenon paradox.
Plain Language Summary
In addition to having the right surface temperature, a planet needs an atmosphere to keep surface liquid water stable. Although many planets have been found that may lie in the right temperature range, the existence of an atmosphere is not guaranteed. In particular, for planets that are kept warm by being close to dim stars, there are a number of ways that the star may remove a planetary atmosphere. These atmospheric escape processes depend on the behavior of the star as well as the nature of the planet, including the presence of a planetary magnetic field. Under certain conditions, a magnetic field can protect a planet's atmosphere from the loss due to the direct impact of the stellar wind, but it may actually enhance total atmospheric loss by connecting to the highly variable magnetic field of the stellar wind. These enhancements happen especially for planets close to dim stars. We review the complete range of atmospheric loss processes driven by interaction between a planet and a star to aid in the identification of planets that are both the correct temperature for liquid water and that have a chance of maintaining an atmosphere over long periods of time.
Key Points
The different escape processes at planets and exoplanets are reviewed along with their mathematical formulation
The major parameters for each escape processes are described; some escape processes negligible in the solar system may be major source at exoplanets, or for the early solar system
A magnetic field should not be a priori considered as a protection for the atmosphere
Four transits of the planet orbiting the star HD 209458 were observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The wavelength domain (1180-1710 Å) includes H ...I as well as C I, C II, C IV, N V, O I, S I, Si II, Si III, and Si IV lines. During the transits, absorptions are detected in H I, O I, and C II (5%+/-2%, 13%+/-4.5%, and 7.5%+/-3.5%, respectively). No absorptions are detected for other lines. The 5% mean absorption over the whole H I Lyα line is consistent with the previous detection completed in 2003 at higher resolution (Vidal-Madjar et al.). The absorption depths in O I and C II show that oxygen and carbon are present in the extended upper atmosphere of HD 209458b (nicknamed ``Osiris''). These species must be carried out up to the Roche lobe and beyond, most likely in a state of hydrodynamic escape.
Context. Atomic hydrogen escaping from the planet HD 209458b provides the largest observational signature ever detected for an extrasolar planet atmosphere. However, the Space Telescope Imaging ...Spectrograph (STIS) used in previous observational studies is no longer available, whereas additional observations are still needed to better constrain the mechanisms subtending the evaporation process and to determine the evaporation state of other “hot Jupiters”. Aims. Here, we aim to detect the extended hydrogen exosphere of HD 209458b with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) and to find evidence of a hydrogen comet-like tail trailing the planet, whose size would depend on the escape rate and the amount of ionizing radiation emitted by the star. These observations also provide a benchmark for other transiting planets, in the frame of a comparative study of the evaporation state of close-in giant planets. Methods. Eight HST orbits were used to observe two transits of HD 209458b. Transit light curves were obtained by performing photometry of the unresolved stellar Lyman-α (Lyα) emission line during both transits. Absorption signatures of exospheric hydrogen during the transit were compared to light curve models predicting a hydrogen tail. Results. Transit depths of $(9.6\pm7.0)\%$ and $(5.3\pm10.0)\%$ were measured on the whole Lyα line in visits 1 and 2, respectively. Averaging data from both visits, we find an absorption depth of $(8.0\pm5.7)\%$, in good agreement with previous studies. Conclusions. The extended size of the exosphere confirms that the planet is most likely losing hydrogen to space, yet, the photometric precision achieved does not allow us to better constrain the hydrogen mass-loss rate.
The Large Area Telescope on board the Fermi satellite observed a gamma-ray flare in the Crab Nebula lasting for approximately nine days in April of 2011. The source, which at optical wavelengths has ...a size of approx =11 lt-yr across, doubled its gamma-ray flux within eight hours. The peak photon flux was (186 + or - 6) x 10 super(-7) cm super(-2) s super(-1) above 100 MeV, which corresponds to a 30-fold increase compared to the average value. During the flare, a new component emerged in the spectral energy distribution, which peaked at an energy of (375 + or - 26) MeV at flare maximum. The observations imply that the emission region was likely relativistically beamed toward us and that variations in its motion are responsible for the observed spectral variability.
Monazite grains from Greater Himalayan Sequence gneisses, Langtang valley, Nepal, were chemically mapped and then dated in situ via Th–Pb ion‐microprobe analysis. Correlation of ages and chemistry ...reveals at least five different generations of monazite, ranging from c. 9 to >300 Ma. Petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks, yielding an age for the melting of Greater Himalayan rocks within the Main Central Thrust sheet (c. 16 Ma), and for the timing of thrust sheet emplacement that are younger than commonly viewed. Chemical characterization of monazite is vital prior to chronological microanalysis, and many ages previously reported for monazite from the Greater Himalayan Sequence are interpretationally ambiguous.
Convergent biological, epidemiological, and clinical data identified urate elevation as a candidate strategy for slowing disability progression in Parkinson disease (PD).
To determine the safety, ...tolerability, and urate-elevating capability of the urate precursor inosine in early PD and to assess its suitability and potential design features for a disease-modification trial.
The Safety of Urate Elevation in PD (SURE-PD) study, a randomized, double-blind, placebo-controlled, dose-ranging trial of inosine, enrolled participants from 2009 to 2011 and followed them for up to 25 months at outpatient visits to 17 credentialed clinical study sites of the Parkinson Study Group across the United States. Seventy-five consenting adults (mean age, 62 years; 55% women) with early PD not yet requiring symptomatic treatment and a serum urate concentration less than 6 mg/dL (the approximate population median) were enrolled.
Participants were randomized to 1 of 3 treatment arms: placebo or inosine titrated to produce mild (6.1-7.0 mg/dL) or moderate (7.1-8.0 mg/dL) serum urate elevation using 500-mg capsules taken orally up to 2 capsules 3 times per day. They were followed for up to 24 months (median, 18 months) while receiving the study drug plus 1 washout month.
The prespecified primary outcomes were absence of unacceptable serious adverse events (safety), continued treatment without adverse event requiring dose reduction (tolerability), and elevation of urate assessed serially in serum and once (at 3 months) in cerebrospinal fluid. RESULTS Serious adverse events (17), including infrequent cardiovascular events, occurred at the same or lower rates in the inosine groups relative to placebo. No participant developed gout and 3 receiving inosine developed symptomatic urolithiasis. Treatment was tolerated by 95% of participants at 6 months, and no participant withdrew because of an adverse event. Serum urate rose by 2.3 and 3.0 mg/dL in the 2 inosine groups (P < .001 for each) vs placebo, and cerebrospinal fluid urate level was greater in both inosine groups (P = .006 and <.001, respectively). Secondary analyses demonstrated nonfutility of inosine treatment for slowing disability.
Inosine was generally safe, tolerable, and effective in raising serum and cerebrospinal fluid urate levels in early PD. The findings support advancing to more definitive development of inosine as a potential disease-modifying therapy for PD.
clinicaltrials.gov Identifier: NCT00833690.
Solar energetic particles (SEPs) can precipitate directly into the atmospheres of weakly magnetized planets, causing increased ionization, heating, and altered neutral chemistry. However, strong ...localized crustal magnetism at Mars can deflect energetic charged particles and reduce precipitation. In order to quantify these effects, we have developed a model of proton transport and energy deposition in spatially varying magnetic fields, called Atmospheric Scattering of Protons and Energetic Neutrals. We benchmark the model's particle tracing algorithm, collisional physics, and heating rates, comparing against previously published work in the latter two cases. We find that energetic nonrelativistic protons precipitating in proximity to a crustal field anomaly will primarily deposit energy at either their stopping altitude or magnetic reflection altitude. We compared atmospheric ionization in the presence and absence of crustal magnetic fields at 50°S and 182°E during the peak flux of the 29 October 2003 “Halloween storm” SEP event. The presence of crustal magnetic fields reduced total ionization by ~30% but caused ionization to occur over a wider geographic area.
Key Points
Solar energetic particle precipitation can ionize and heat atmospheres of weakly magnetized planets
A simulation was developed to predict atmospheric effects due to solar energetic particle precipitation into the Martian atmosphere
Less atmospheric ionization occurs near crustal field anomalies than unmagnetized areas of Mars
•We calculated the Lyman alpha emission from two hot Jupiters.•We found that the contrast between the star and the planet can reach 1%.•We found that the population of the n=2 level of hydrogen ...cannot be neglected.•Detecting these emissions can be feasible with HST in the case of HD189733b.
In the Solar System giant planets, H Lyman α is the most intensive FUV thermospheric emission line. It is expected to be a very intense emission line in the FUV spectra of hot Jupiters. The detection and analysis of this line could be used to characterize the planetary environment. However, the emission of the star is much more intense and the contrast is not necessarily favorable for detecting the planetary signal. The aim of this study is to investigate conditions that would make the direct detection of a planetary Lyman α emission possible and to estimate the emission level. The cases of the two planets HD 209458b and HD 189733b are considered. The contrasts between the Lyman α emissions of the planets and their parent stars are calculated. Dayglow, thermal and auroral emissions are calculated. Scaling laws based on Solar System conditions are used to estimate the energy inputs into the atmospheres of the planets. A radiative transfer code is used, coupled with a kinetic code which describes the transport of electrons in the upper atmospheres of the planets. Absorption by the interstellar medium is taken into account in calculating the contrast between the stellar and planetary lines as observed from Earth. We find that the planetary emission is mainly due to dayside scattering of the stellar line: thermal emission contributes up to ∼6% of the total dayside planetary intensity and within our hypotheses, the auroral contribution is less than 10−3 of it. In the two planetary systems considered, the ratios between the emissions of the planets and their parent stars are ∼10−3. When the planets are at quadrature phase angle, the contrasts between the planetary and stellar fluxes reach values between ∼10−3 and 10−2 at Earth. The detectability of the emissions depends mainly on the signal to noise ratio and thus of both the total flux received from the system and the line profile.
Combination of geochemical zoning in metamorphic garnet and monazite plus in situ Th–Pb isotopic dating of monazite yields
P–
T conditions, ages and convergence rates for the Main Central Thrust ...(MCT) and affiliated faults in central Nepal. Inferred rates were 1.5±0.9 cm/yr (Langtang Thrust, ∼19 Ma), 2.2±0.7 cm/yr (Main Central Thrust, ∼15 Ma) and 7±3 cm/yr (Ramgarh Thrust, ∼10 Ma). The lower values are similar to modern convergence rates across the Himalaya, but the Ramgarh Thrust may have briefly absorbed all Indo–Asian convergence at ∼10 Ma, when foreland and marine sedimentation rates markedly increased, and at least one major strike slip fault in Tibet experienced a hiatus in movement. Variable rates of convergence across the Himalaya on Myr timescales imply Myr variations in strain rates throughout all components of the Indo–Asian orogen.
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