Learning intervention based on a "Mars and Space" exhibition was designed according to STEAM-education (Science, Technology, Engineering, Art and Mathematics) principles and practices in order to bridge the gap between formal and informal learning. The cognitive learning of 12-year-old students in Finland ("N" = 306) showed a sustained level for a six months period. The results of this study provided evidence that situational motivation was enhanced by interest in school science in the STEAM science exhibition context. This led to better cognitive learning results in the post-knowledge test. Thus, interest and situational motivation were the first steps, and the superficial situational motivation seemed to successfully change into content-based intrinsic motivation with longer-lasting, deep learning outcomes. STEAM intervention apparently produced long-term learning, and this exhibition learning setting is shown to provide an appropriate platform to reach the deeper layers to successfully retain knowledge. Boys' scores rose even in the delayed test. Using structural equation modelling (SEM) to assess the effects on individual, motivational and situational interest in learning, situation motivation is shown to work as a catalyst and acting as a catalyst and also acted as a stepping stone for intrinsic motivation as part of relative autonomy (RAI) and a deep learning strategy.