In a European project—CoReflect—researchersin seven countries are developing, implementing andevaluating teaching sequences using a web-based platform(STOCHASMOS). The interactive web-based inquirymaterials support collaborative and reflective work. Thelearning environments will be iteratively tested and refined,during different phases of the project. All learning environmentsare focusing ‘‘socio-scientific issues’’. In thisarticle we report from the pilot implementation of theSwedish learning environment which has an Astrobiologycontext. The socio-scientific driving questions are ‘‘Shouldwe look for, and try to contact, extraterrestrial life?’’, and‘‘Should we transform Mars into a planet where humanscan live in the future?’’ The students were in their last yearof compulsory school (16 years old), and worked togetherin triads. We report from the groups’ decisions and thesupport used for their claims. On a group level a majorityof the student groups in their final statements expressreluctance towards both the search of extraterrestrial lifeand the terraforming of Mars. The support used by thestudents are reported and discussed. We also look moreclosely into the argumentation of one of the student groups.The results presented in this article, differ from earlierstudies on students’ argumentation and decision making onsocio-scientific issues (Aikenhead in Science education foreveryday life. Evidence-based practice. Teachers CollegePress, New York, (2006) for an overview), in that theysuggest that students do use science related arguments — both from ‘‘core’’ and ‘‘frontier’’ science — in their argumentationand decision making.