Yoshiyuki O. Takahashi

Ishiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) investigated multiple equilibrium solutions of a gray atmosphere for various values of solar constant, utilizing two types of models, namely, a one‐dimensional energy balance model and a general circulation model (GCM) with simplified physical processes, both of which permit existence of the runaway greenhouse state. The study was retracted, since there was a bug in their GCM that affected quantitative aspects of the study. Here, we revise the study with re‐performing all of the GCM experiments using an appropriately corrected model. The results of re‐experiments show that the main features of the climate regime diagram drawn in the solar constant ‐ ice line latitude plane are mostly unchanged, except that the ice‐free equilibrium state, which existed in Ishiwatari et al. (2007) (https://doi.org/10.1029/2006JD007368) now disappears. It is confirmed that there are the partially ice‐covered state, the globally ice‐covered state, and the runaway greenhouse state. These three states coexist for intermediate values of solar constant. The existence of the large ice cap instability is also confirmed, although the critical latitude of the partially ice‐covered state extends equatorward. Also same as before, the small ice cap instability does not seem to appear.

Direct-imaging techniques of exoplanets have made significant progress recently and will eventually enable monitoring of photometric and spectroscopic signals of Earth-like habitable planets. The presence of clouds, however, would remain as one of the most uncertain components in deciphering such direct-imaged signals of planets. We attempt to examine how the planetary obliquity produces different cloud patterns by performing a series of general circulation model simulation runs using a set of parameters relevant for our Earth. Then we use the simulated photometric lightcurves to compute their frequency modulation that is due to the planetary spin–orbit coupling over an entire orbital period, and we attempt to see to what extent one can estimate the obliquity of an Earth twin. We find that it is possible to estimate the obliquity of an Earth twin within the uncertainty of several degrees with a dedicated 4 m space telescope at 10 pc away from the system if the stellar flux is completely blocked. While our conclusion is based on several idealized assumptions, a frequency modulation of a directly imaged Earth-like planet offers a unique methodology to determine its obliquity.

The existence of liquid water within an oxidized environment on early Mars has been inferred by the Mn-rich rocks found during recent explorations on Mars. The oxidized atmosphere implied by the Mn-rich rocks would basically be comprised of CO₂ and H₂O without any reduced greenhouse gases such as H₂ and CH₄. So far, however, it has been thought that early Mars could not have been warm enough to sustain water in liquid form without the presence of reduced greenhouse gases. Here, we propose that H₂O₂ could have been the gas responsible for warming the surface of the oxidized early Mars. Our one-dimensional atmospheric model shows that only 1 ppm of H₂O₂ is enough to warm the planetary surface because of its strong absorption at far-infrared wavelengths, in which the surface temperature could have reached over 273 K for a CO₂ atmosphere with a pressure of 3 bar. A wet and oxidized atmosphere is expected to maintain sufficient quantities of H₂O₂ gas in its upper atmosphere due to its rapid photochemical production in slow condensation conditions. Our results demonstrate that a warm and wet environment could have been maintained on an oxidized early Mars, thereby suggesting that there may be connections between its ancient atmospheric redox state and possible aqueous environment.

In this work, we analyze 67 sprite events observed during the winter sprite campaign carried out in Japan from 1998 to 2003. Our aim is to clarify the relationship between winter sprites in Japan and the characteristics of their parent thunderstorm systems. Combining optical data of sprites, various meteorological data such as cloud maps obtained from the Geostationary Meteorological Satellite‐5, and precipitation maps obtained from Doppler radars as well as lightning detection network data, we classify sprite‐producing thunderstorm systems into four categories: (1) those associated with the cold front over the Sea of Japan, (2) those developing in the Japan Sea Polar Air Mass Convergence Zone, (3) those developing in local low‐pressure areas over the Pacific Ocean, and (4) those developing ∼200–900 km behind the cold front over the Pacific Ocean. These thunderstorm systems have average cloud top temperatures ranging from −25 to −40°C and average horizontal areas of 8500–40,500 km² at the −20°C isotherm. They are much smaller than thunderstorm systems over the U.S. high plains in the summertime. The average maximum occurrence rate of sprites is 7.7 per hour in category 1, while the occurrence rates in categories 2–4 are only 2–3 per hour. In each of the four categories of thunderstorm systems, winter sprites in Japan tend to be induced by positive cloud‐to‐ground lightning discharges occurring in the convective core region during the developing stage where and when convection becomes most active spatially and temporally.