<Dd> If the water layer is thick enough, water at its base will be in solid phase (ice polymorphs) because of the high pressure . Ganymede and Callisto are likely examples of this class . Their oceans are thought to be enclosed between thick ice layers . In such conditions, the emergence of even simple life forms may be very difficult because the necessary ingredients for life will likely be completely diluted . </Dd> <P> Along with the characteristics of planets and their star systems, the wider galactic environment may also impact habitability . Scientists considered the possibility that particular areas of galaxies (galactic habitable zones) are better suited to life than others; the Solar System in which we live, in the Orion Spur, on the Milky Way galaxy's edge is considered to be in a life - favorable spot: </P> <Ul> <Li> It is not in a globular cluster where immense star densities are inimical to life, given excessive radiation and gravitational disturbance . Globular clusters are also primarily composed of older, probably metal - poor, stars . Furthermore, in globular clusters, the great ages of the stars would mean a large amount of stellar evolution by the host or other nearby stars, which due to their proximity may cause extreme harm to life on any planets, provided that they can form . </Li> <Li> It is not near an active gamma ray source . </Li> <Li> It is not near the galactic center where once again star densities increase the likelihood of ionizing radiation (e.g., from magnetars and supernovae). A supermassive black hole is also believed to lie at the middle of the galaxy which might prove a danger to any nearby bodies . </Li> <Li> The circular orbit of the Sun around the galactic center keeps it out of the way of the galaxy's spiral arms where intense radiation and gravitation may again lead to disruption . </Li> </Ul> <Li> It is not in a globular cluster where immense star densities are inimical to life, given excessive radiation and gravitational disturbance . Globular clusters are also primarily composed of older, probably metal - poor, stars . Furthermore, in globular clusters, the great ages of the stars would mean a large amount of stellar evolution by the host or other nearby stars, which due to their proximity may cause extreme harm to life on any planets, provided that they can form . </Li>

Describe two ways in which radiation from the sun supports life on earth