In a European project--CoReflect--researchers in seven countries are developing, implementing and evaluating teaching sequences using a web-based platform (STOCHASMOS). The interactive web-based inquiry materials support collaborative and reflective work. The learning environments will be iteratively tested and refined, during different phases of the project. All learning environments are focusing "socio-scientific issues". In this article we report from the pilot implementation of the Swedish learning environment which has an Astrobiology context. The socio-scientific driving questions are "Should we look for, and try to contact, extraterrestrial life?", and "Should we transform Mars into a planet where humans can live in the future?" The students were in their last year of compulsory school (16 years old), and worked together in triads. We report from the groups' decisions and the support used for their claims. On a group level a majority of the student groups in their final statements express reluctance towards both the search of extraterrestrial life and the terraforming of Mars. The support used by the students are reported and discussed. We also look more closely into the argumentation of one of the student groups. The results presented in this article, differ from earlier studies on students' argumentation and decision making on socio-scientific issues (Aikenhead in "Science education for everyday life. Evidence-based practice." Teachers College Press, New York, (2006) for an overview), in that they suggest that students do use science related arguments--both from "core" and "frontier" science--in their argumentation and decision making.