The eirenesphere is the region in a planetary body with the right physical environment state to support microbial life as constrained from temperature and pressure alone (Méndez, 2011). Therefore, it does not consider all the requirements for life, such as the availability of water, but represent the maximum volume at planetary scales available for potential habitats. On Earth, the eirenespheres constrain the biosphere and includes parts of the atmosphere, lithosphere, and hydrosphere. The eirenesphere of Mars, Europa, Titan, and Enceladus are in their subsurface, while on Venus is in the upper atmosphere (Figure 1).
An eirenesphere can support a biosphere if an only all the other requirements of life are present under relatively stable conditions. The case of Venus is a good case of comparison, although Venus has a larger eirenesphere than Earth in its upper atmosphere, this region is very dry and unstable, so even if microbial life is injected in this "tolerable region," the atmospheric circulation will eventually sterilize it in the much hotter lower atmosphere. So Venus' eirenesphere just shows how much "wasted-for-life" space it has. On the contrary, the case of Europa and Enceladus are more interesting because we believe water and the other ingredients for life are present in their respective eirenespheres.
The eirenespheres were calculated from a vertical profile of thermal and pressure habitability functions derived from standard planetary atmospheric and subsurface models, and the known limits of microbial life (as we know it). Table 1 describes some of their properties. The name eirenesphere was inspired by a friend and colleague, Irene Schneider from Ihrenes Space Enterprises, who suggested that I should do the spheres for the rest of the planets too. Is Jupiter's eirenensphere even larger than Earth itself? I will later check this out. Eirene is also the goddess of peace in greek mythology.
Figure 1. Eirenesphere (green spheres) for Earth and Venus, and the astrobiological-relevant planetary bodies Mars, Europa, Titan, and Enceladus. They represent the maximum volume available for microbial life at planetary scales based on the planet mean surface or subsurface temperatures and pressures. Eirenesphere can become habitable in the other ingredients for life are present.
Table 1. Description of the eirenespeheres in terms of their volume, optimum depth habitability, mean habitability, and percent of the planet volume. The volume of the eirenesphere of Venus is higher than Earth, however, it is not a stable environment and lacks the other requirements for life. Enceladus shows the largest mean global habitability in the Solar System but also the smallest volume and very deep in its interior. Only Europa and Titan have a large eirenesphere to planet volume ratio. Titan has the lowest habitability because by the depth it has tolerable temperatures the pressures are too high.