The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5-20 M() range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the long- and short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first approximately 1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ...

The changing view of planets orbiting low mass stars, M stars, as potentially hospitable worlds for life and its remote detection was motivated by several factors, including the demonstration of viable atmospheres and oceans on tidally locked planets, normal incidence of dust disks, including debris disks, detection of planets with masses in the 5-20 M() range, and predictions of unusually strong spectral biosignatures. We present a critical discussion of M star properties that are relevant for the long- and short-term thermal, dynamical, geological, and environmental stability of conventional liquid water habitable zone (HZ) M star planets, and the advantages and disadvantages of M stars as targets in searches for terrestrial HZ planets using various detection techniques. Biological viability seems supported by unmatched very long-term stability conferred by tidal locking, small HZ size, an apparent short-fall of gas giant planet perturbers, immunity to large astrosphere compressions, and several other factors, assuming incidence and evolutionary rate of life benefit from lack of variability. Tectonic regulation of climate and dynamo generation of a protective magnetic field, especially for a planet in synchronous rotation, are important unresolved questions that must await improved geodynamic models, though they both probably impose constraints on the planet mass. M star HZ terrestrial planets must survive a number of early trials in order to enjoy their many Gyr of stability. Their formation may be jeopardized by an insufficient initial disk supply of solids, resulting in the formation of objects too small and/or dry for habitability. The small empirical gas giant fraction for M stars reduces the risk of formation suppression or orbit disruption from either migrating or nonmigrating giant planets, but effects of perturbations from lower mass planets in these systems are uncertain. During the first approximately 1 Gyr, atmospheric retention is at peril because of intense and frequent stellar flares and sporadic energetic particle events, and impact erosion, both enhanced, the former dramatically, for M star HZ semimajor axes. Loss of atmosphere by interactions with energetic particles is likely unless the planetary magnetic moment is sufficiently large. For the smallest stellar masses a period of high planetary surface temperature, while the parent star approaches the main sequence, must be endured. The formation and retention of a thick atmosphere and a strong magnetic field as buffers for a sufficiently massive planet emerge as prerequisites for an M star planet to enter a long period of stability with its habitability intact. However, the star will then be subjected to short-term fluctuations with consequences including frequent unpredictable variation in atmospheric chemistry and surficial radiation field. After a review of evidence concerning disks and planets associated with M stars, we evaluate M stars as targets for future HZ planet search programs. Strong advantages of M stars for most approaches to HZ...

A one-dimensional coupled chemistry and flow model of the earth's atmosphere is used to study the relationship between ozone content and oxygen level. The effects of the biogenic trace gases methane and nitrous oxide are considered, as well as the production of odd nitrogen in lightning discharges. The ozone column depth is found to increase monotonically with increasing oxygen content, in contrast with previous predictions that the column depth should peak at ---10 -• PAL (present atmospheric level) of 02. The critical 02 level at which an effective ultraviolet shield is produced is in the neighborhood of 10 -1 PAL. Implications of this result for the emergence of terrestrial land life are considered. INTRODUCTIONAtmospheric ozone plays an important role in shaping our modern terrestrial environment by shielding out solar ultraviolet light in the 2000 to 3000-3, range. This, together with the realization that the amount of oxygen in the earth's atmosphere must have been much lower in the past than it is today [Walker, 1977], has led several authors to investigate the evolution of ozone as a function of atmospheric oxygen content.A question of particular interest is whether or not the development of the ozone layer was responsible for the emergence of life on land during the Late Silurian period, about 420 m.y. (million years) ago.The first serious attempt to solve this problem can in a series of papers by Berkner and Marshall [1964, 1965, 1966, 1967]. They predicted that a biologically effective ultraviolet shield would have been provided by a total ozone column of 0.2 arm cm, which should have reduced the ultraviolet flux at the earth's surface to less than 1 erg cm -2 s -l (50/•)-l. Using a semiempirical method to relate ozone densities to atmospheric oxygen levels, they found that this 03 column depth should have been achieved at an 02 level of approximately 10 -l PAL (present atmospheric level). Since their scenario for the rise of atmospheric oxygen called for an 02 level of 10 -2 PAL at the dawn of the Cambrian period (-600 m.y.), they suggested that the subsequent increase in oxygen to 10 -l PAL and the accompanying development of the ozone screen was 4irectly responsible for the spread of life onto land during the Silurian.The estimation of ozone column depth versus oxygen content was repeated in a more realistic fashion by Ratnet and Walker [1972], who calculated ozone densities by assuming photochemical equilibrium in an oxygen-nitrogen atmosphere. Their model predicted that the ozone column depth should increase as the oxygen content decreased from I to 10 -l PAL, as a result of the speeding up of the three-body ozone formation reaction as the 03 peak moved downward in the atmosphere. Below 10 -l PAL of 02 the ozone column depth began to decline, but much more slowly than had been predicted by Berkner and Marshall. Thus in spite of their use of a stricter biological tolerance limit on ultraviolet flux, requiring a minimum 03 column depth of 7 x 1018 cm -2 (0.26 atm cm), they found that the critical l...

This study utilizes data from the (AE-D) satellite. It is noted that a number of visual airglow experiment (VAE) and the low similar studies have been previously undertaken energy electron experiment (LEE) on board the [cf. Gustafsson, 1970; Deehr et al., 1970, 1973; Atmosphere Explorer D satellite. The 4278-% N• Gustafsson et al., 1972; Gustafsson and Egeland, surfade brightness profile is measured as a func-1976]. In no case, however, have the authors tion of angle along the satellite track by using a method which corrects for reflection from the ground below. This intensity is compared with the incident electron energy flux between 0.2 and 25 keV and plotted as a function of mean and tota•l electron energy. The observed ratio of I4278/F E is 256+ 125 R/erg cm -2 s -1, which is slightly higher than the value of ~210 R/erg cm -2 s -1 predicted theoretically by Rees and Luckey [1974]. Excess 4278-A emission on the southern edge of the aurora is shown to be a result of high-energy (>10 keV) proton precipitation. A general increase in the scatter of the data toward low mean electron energies is a possible indication of the presence of parallel electric fields below the altitude of the satellite (150-800 km). erg cm -2 s -1 for energies of <10 keV. The purpose of this study is to measure this ratio in the nighttime auroral zone by using observations from the Atmosphere Explorer D 1Now at Particle and auroral observations from the Esro I/Aurorae satellite, J. Atmos. Terr. Phys., 35, 1979, 1973. Ioo Edgar, B. C., W. T. Miles, and A. E. S. Green, Energy deposition of protons in molecular nitrogen and applications to proton auroral phenomena, J. Geophys. Res., 78, 6596, 1973. Gustafsson, G. A., Auroral luminosity patterns over northern Scandinavia during a number of Esro 1A (Aurorae) passages, Phy s . Norv., 4, 113, 1970.

A zonally averaged numerical model of the thermosphere is used to examine the coupling between neutral composition, including N2, O2, and O, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating results in a summer‐to‐winter flow, with a winter enhancement in atomic oxygen density that is a factor of about 1.8 greater than the summer hemisphere at 160 km. The O2 and N2 densities are found to be higher in the summer hemisphere by factors of 2–3 at altitudes near 100 km. The O2 and N2 variations are associated with a latitudinal gradient in total number density, which is required to achieve pressure balance in the presence of large zonal jets. Latitudinal profiles of OI (5577 Å) green line emission intensity are calculated by using both Chapman and Barth mechanisms. Composition of the lower thermosphere is shown to be strongly influenced by circulation patterns initiated in the stratosphere and lower mesosphere, below the lower boundary used in the model.