Feng Tian

TRANSONIC HYDRODYNAMIC ESCAPE OF HYDROGEN FROM EXTRASOLAR PLANETARY ATMOSPHERES

Hydrodynamic escape is an important process in the formation and evolution of planetary atmospheres. Transonic steady state solutions of the time-independent hydrodynamic equations are difficult to find because of the existence of a singularity point. A numerical model is developed to study the hydrodynamic escape of neutral gas from planetary atmospheres by solving the time-dependent hydrodynamic equations. The model is validated against an analytical solution of the escape from an isothermal atmosphere. The model uses a two-dimensional energy deposition calculation instead of the single-layer heating assumption, which is not sufficiently accurate for hydrodynamic escapefrom ahydrogen-richplanetaryatmosphere.Whenapplied totheatmospheresofextrasolar planets, themodel results are in good agreement with observations of the transiting extrasolar planet HD 209458b. The model predicts that hydrogen is escaping from HD 209458b at a maximum rate of 6 ; 10 10 gs � 1 . The extrasolar planet is stable under the hydrodynamic escape of hydrogen. The rate of hydrogen hydrodynamic escape from other possible extrasolar planets is investigated using the model. The importance of hydrogen hydrodynamic escape for the long-term evolution of extrasolar planets is discussed. Simulation shows that through hydrodynamic escape of hydrogen, a planet at the orbit of Mercury (0.4 AU) and with 0.5 Uranus mass can lose about 10% of its mass within 850 million yr if the solar EUV radiation is 10 times the present level. This calculation provides an indication of how Mercury may have evolved during the early days of the solar system. Subject heading gs: planetary systems — planets and satellites: general

Thermal escape of carbon from the early Martian atmosphere

[1]xa0Observations suggest that Mars was wet and warm during the late Noachian, which probably requires a dense CO2 atmosphere. But would a dense CO2 early Martian atmosphere have been stable under the strong EUV flux from the young Sun? Here we show that thermal escape of carbon was so efficient during the early Noachian, 4.1 billion years ago (Ga), that a CO2-dominated Martian atmosphere could not have been maintained, and Mars should have begun its life cold. By the mid to late Noachian, however, the solar EUV flux would have become weak enough to allow a dense CO2 atmosphere to accumulate. Hence, a sustainable warm and wet period only appeared several hundred million years (Myrs) after Mars formed.

THERMAL ESCAPE FROM SUPER EARTH ATMOSPHERES IN THE HABITABLE ZONES OF M STARS

A fundamental question for exoplanet habitability is the long-term stability of the planets atmosphere. We numerically solve a one-dimensional multi-component hydrodynamic thermosphere/ionosphere model to examine the thermal and chemical responses of the primary CO2 atmospheres of heavy super Earths (6-10 Earth masses) in the habitable zones of typical low-mass M stars to the enhanced soft X-ray and ultraviolet (XUV) fluxes associated with the prolonged high-activity levels of M stars. The results show that such atmospheres are stable against thermal escape, even for M stars XUV enhancements as large as 1000 compared to the present Earth. It is possible that the CO2-dominant atmospheres of super Earths in the habitable zones of M stars could potentially contain modest amount of free oxygen as a result of more efficient atmosphere escape of carbon than oxygen instead of photosynthesis.

Time-Resolved Ultraviolet Spectroscopy of The M-Dwarf GJ 876 Exoplanetary System

Extrasolar planets orbiting M-stars may represent our best chance to discover habitable worlds in the coming decade. The ultraviolet spectrum incident upon both Earth-like and Jovian planets is critically important for proper modeling of their atmospheric heating and chemistry. In order to provide more realistic inputs for atmospheric models of planets orbiting low-mass stars, we present new near- and far-ultraviolet (NUV and FUV) spectroscopy of the M-dwarf exoplanet host GJ 876 (M4V). Using the COS and STIS spectrographs on board the Hubble Space Telescope, we have measured the 1150-3140 A spectrum of GJ 876. We have reconstructed the stellar H I Ly{alpha} emission line profile, and find that the integrated Ly{alpha} flux is roughly equal to the rest of the integrated flux (1150-1210 A + 1220-3140 A) in the entire ultraviolet bandpass (F(Ly{alpha})/F(FUV+NUV) Almost-Equal-To 0.7). This ratio is {approx}2500 Multiplication-Sign greater than the solar value. We describe the ultraviolet line spectrum and report surprisingly strong fluorescent emission from hot H{sub 2} (T(H{sub 2}) > 2000 K). We show the light curve of a chromospheric + transition region flare observed in several far-UV emission lines, with flare/quiescent flux ratios {>=}10. The strong FUV radiation field of an M-star (and specifically Ly{alpha})morexa0» is important for determining the abundance of O{sub 2}-and the formation of biomarkers-in the lower atmospheres of Earth-like planets in the habitable zones of low-mass stars.«xa0less

Hydrodynamic escape of nitrogen from Pluto

[1]xa0Hydrodynamic escape of nitrogen from Pluto is studied by solving time-dependent hydrodynamic escape equations that treat the spatial distribution of EUV energy deposition realistically. Simulations show that the N2 hydrodynamic escape rate is ∼1 × 1028 molecules s−1 when Pluto is at 40 AU (its average orbital location) and solar activity level is at minimum. The N2 hydrodynamic escape rate is ∼2 × 1028 molecules s−1 when Pluto is at its perihelion (30 AU) and solar activity level is at maximum. Through hydrodynamic escape, Pluto may have lost ∼0.5% of its total mass over the age of the solar system. Comet-like interactions may occur between Plutos atmosphere and the solar wind flow, which may be observed by the New Horizon mission.