stages of terraformation. This primary stage of ecosystem creation is usually restricted to the initial seeding of microbial life. A 2019 opinion piece by Lopez, Peixoto and Rosado has reintroduced microbiology as a necessary component of any possible colonization strategy based on the principles of microbial symbiosis and their beneficial ecosystem services. As conditions approach that of Earth, plant life could be brought in, and this will accelerate the production of oxygen, theoretically making the planet eventually able to support animal life. Aspects and definitions In 1985, Martyn J. Fogg started publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1992. In his book Terraforming: Engineering Planetary Environments (1995), Fogg proposed the following definitions for different aspects related to terraforming: Planetary engineering: the application of technology for the purpose of influencing the global properties of a planet. Geoengineering: planetary engineering applied specifically to Earth. It includes only those macro engineering concepts that deal with the alteration of some global parameter, such as the greenhouse effect, atmospheric composition, insolation or impact flux. Terraforming: a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life as we know it. The ultimate achievement in terraforming would be to create an open planetary ecosystem emulating all the functions of the biosphere of Earth, one that would be fully habitable for human beings. Fogg also devised definitions for candidate planets of varying degrees of human compatibility: Habitable Planet (HP): A world with an environment sufficiently similar to Earth's as to allow comfortable and free human habitation. Biocompatible Planet (BP): A planet possessing the necessary physical parameters for life to flourish on its surface. If initially lifeless, then such a world could host a biosphere of considerable complexity without the need for terraforming. Easily Terraformable Planet (ETP): A planet that might be rendered biocompatible, or possibly habitable, and maintained so by modest planetary engineering techniques and with the limited resources of a starship or robot precursor mission. Fogg suggests that Mars was a biologically compatible planet in its youth, but is not now in any of these three categories, because it can only be terraformed with greater difficulty. Habitability requirements An absolute requirement for life is an energy source, but the notion of planetary habitability implies that many other geophysical, geochemical, and