Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et?al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et?al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67?AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70?AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200?K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T eff 5000?K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on ??, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

https://iopscience.iop.org/article/10.1088/0004-637X/765/2/131/pdf

Habitable Zones Around Main-Sequence Stars: New Estimates

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

/pdf/optical-gas-sensing-a-review-4uaf589xiq.pdf

Optical gas sensing: a review

We used ground-based near-infrared (NIR) observations of thermal emission from the Venus nightside to determine the temperature structure and water vapor distribution between the surface and the 6-km level. We show that emission from spectral windows near 1.0, 1.1, and 1.18 μm originates primarily from the surface and lowest scale height (∼16 km). These windows include absorption by weak H 2 O and CO 2 lines and by the far wings of lines in strong nearby CO 2 bands. Rayleigh scattering by the 90-bar CO 2 atmosphere and Mie scattering by the H 2 SO 4 clouds attenuate this emission, but add little to its spectral dependence. Surface topography also modulates this NIR thermal emission because high-elevation regions are substantially cooler and emit less thermal radiation than the surrounding plains. These contributions to the emission are clearly resolved in moderate-resolution (λ/Δλ ∼ 400) spectral image cubes of the Venus nightside acquired with the infrared imaging spectrometer (IRIS) on the Anglo-Australian Telescope (AAT) in 1991. To analyze these observations, we used a radiative transfer model that includes all of the radiative processes listed above. Synthetic spectra for several topographic elevations were combined with Pioneer Venus altimetry data to generate spatially resolved maps of the NIR thermal emission. Comparisons between these synthetic radiance maps and the IRIS observations indicate no near-infrared signature of the surface emissivity differences seen at microwave wavelengths by the Magellan orbiter. Assuming constant surface emissivity in the near-infrared, we derive nightside averaged temperature lapse rates of -7 to -7.5 K/km in the lowest 6 km. These lapse rates are smaller and indicate much greater static stability than those inferred from earlier measurements and greenhouse models (-8 to -8.5 K/km) [Seiff 1983]. An acceptable fit to the data was obtained with an H 2 O mixing ratio profile which increases from 20 ppmv at the cloud base to 45 ppmv at 30 km, and then remains constant between that altitude and the surface. There is no evidence for H 2 O mixing ratios that decrease with altitude, like those inferred from the Pioneer Venus large probe mass spectrometer [Donahue and Hodges, 1992a] or the Venera 11 and 12 Lander spectrophotometers [Moroz, 1983].

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/95JE03567%4010.1002/%28ISSN%292169-9100.COLLVEN1

Ground-based near-infrared observations of the Venus nightside: The thermal structure and water abundance near the surface

Water vapour continuum absorption is an important contributor to the Earth's radiative cooling and energy balance. Here, we describe the development and status of the MTCKD (MlawerTobinCloughKneizy...

https://royalsocietypublishing.org/doi/pdf/10.1098/rsta.2011.0295

Development and recent evaluation of the MT_CKD model of continuum absorption.

Spectroscopic constants are given for eight isotopic variants of carbon dioxide which provide energy levels for transitions required for terrestrial atmospheric i.r. absorption. A new tabulation is also furnished with bands considered for the latest HITRAN molecular database. This list provides improved band intensities and Herman-Wallis coefficients generated from recent high-resolution measurements and theoretical calculations. Rotationally-dependent air- and self-broadened halfwidths are provided from a survey of recent experiments.

Energy levels, intensities, and linewidths of atmospheric carbon dioxide bands

The shapes of the extreme wings of self-broadened CO2 lines have been investigated in three spectral regions near 7000, 3800, and 2400 cm−1. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO2 lines as well as CO2 lines broadened by N2, O2, Ar, He, and H2. Beyond a few cm−1 from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H2- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

Absorption of Infrared Radiant Energy by CO_2 and H_2O IV Shapes of Collision-Broadened CO_2 Lines*

The absorption by the oxygen A band near 13,100 cm−1 (7620 A) has been investigated Spectral curves with resolution between 06 cm−1 and 12 cm−1 have been obtained for several samples of O2 and O2 + N2 with path lengths from 8 m to 1185 m and pressures up to 136 atm The strength of the entire band is 409 ± 025 g−1 cm2 cm−1, and the relationship between the band strength and the strengths of the individual lines has been determined Half-widths of self-broadened lines at 1 atm pressure vary from approximately 0074 cm−1 at J = 2 to 0043 cm−1 for J = 25 The lines are approximately 5% wider for air at the same pressure since broadening by N2 is more efficient than self-broadening The wings of the lines absorb less than Lorentz-shaped lines beyond approximately 10 cm−1 from the centers

Strengths, Widths, and Shapes of the Oxygen Lines near 13,100 cm(-1) (7620 A).

The continuum absorption by laboratory samples of pure H 2 O and of H 2 0 + N 2 throughout the infrared and millimeter regions has been studied. The samples cover a wide range of pressures and temperatures from 296 K to 428 K. Measurements have been made in the well-known 4 μm and 8 to 12 μm windows as well as in many very narrow “windows” between rotation and vibration-rotation lines. An empirical continuum has been derived for each of several spectral regions to account for the “excess” absorption observed for pure H 2 O over that calculated on the basis of known line strengths and widths and simple line shapes. The results indicate that much of the continuum absorption previously attributed to dimers, or clusters of H 2 O molecules, is due to the extreme wings of self-broadened H 2 O lines. If this is true, the lines must have three important characteristics that are not predicted by simple line-shape theories : (1) self-broadened lines absorb much more in the extreme wings than do N 2 -broadened lines of the same intensity and width; (2) the absorption in the extreme wings decreases with increasing temperature faster than is predicted by simple theory; (3) characteristic (2) is more pronounced for self-broadened lines than for N 2 broadened lines.

Continuum absorption by h 2 o vapor in the infrared and millimeter regions

An ir analyzer employing gas-filter correlation techniques has been designed and constructed to measure the concentrations of CO, NO, SO(2), HCl, and HF in the stacks or ducts of stationary pollutant sources. Use of a retroreflector allows the stack to be double passed, and no sample is extracted. For each gas, small interchangeable fixed-position grating polychromators are used as narrow (~1.5-cm(-1)) multiband spectral filters with the bands corresponding to locations of selected absorption lines. The approximate useful ranges (in parts per million-meters) over which this analyzer operates are 10-4000 for NO, 10-1500 for CO, 50-40,000 for SO(2), 10-2000 for HC1, and 5-200 for HF. The discrimination against other gases and particulates is excellent. The analyzer has been tested in the laboratory and on a variety of pollutant sources.

Infrared gas-filter correlation instrument for in situ measurement of gaseous pollutant concentrations.

In a gas analyzer a beam of radiant energy, chopped at a suitable frequency, and limited by filter means to a suitable portion of the spectrum, is passed through a sample region containing an unknown quantity of gas to be analyzed. Selector cells are alternately inserted into the beam at a rate substantially lower than the chopping frequency. One selector cell contains a quantity of the gas species to be analyzed at high pressure and the other contains a quantity of the same gas at low pressure. A photo detector converts the radiant energy variations to electrical signals which can be analyzed to indicate the quantity of gas in the sample region.

David A. Gryvnak

Papers

Absorption of Infrared Radiant Energy by CO_2 and H_2O IV Shapes of Collision-Broadened CO_2 Lines*

Strengths, Widths, and Shapes of the Oxygen Lines near 13,100 cm(-1) (7620 A).

Continuum absorption by h 2 o vapor in the infrared and millimeter regions

Infrared gas-filter correlation instrument for in situ measurement of gaseous pollutant concentrations.

Dual cell non-dispersive gas analyzer