Diurnal Habitability of Frozen Worlds

In this work we discuss effects allowing local habitability of some extraterrestrial planets of low average surface temperatures. We analyze the problem of diurnal and seasonal changes of temperature and biological productivity at different locations on a hypothetical Earth-like planet. We have found, that under some circumstances the temperature may locally rise well above the average value, allowing periods of enhanced biological activity. In this way, bioproductivity can become periodically possible on a planet that has an average temperature clearly below 0°C. Such thermal conditions are encountered on Mars (Smith et al. in Science 306:1750–1753, 2004) generally considered as inhabitable. In reality, an appropriate temperature is not sufficient for habitability. The presence of liquid water at the considered location is also necessary. We discuss how temperature oscillations affect habitability in the framework of a conceptual model. We find that the considered effect of diurnal and seasonal temperature oscillations can extend the outer boundary of the habitable zone up to 2 AU, while global average temperatures are below 0°C for heliocentric distances R _h > 1.12 AU (dry atmosphere, low CO₂ pressure), or R _h > 1.66 AU (humid atmosphere, high CO₂ pressure).     

A new chlorite geothermometer for diagenetic to low-grade metamorphic conditions

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior Fe³⁺ knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350 °C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of ‘Al-free chlorite S’, sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-Fe²⁺ thermometer performs at least as well as the recent models, which require an estimate of Fe³⁺ content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets—provided it is used in the calibration range (T < 350 °C, P < 4 kbar).