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The purpose of this randomized experimental study is to apply two socio-cognitive models to understand possible ways to improve students' learning outcomes in an online introductory chemistry learning environment. Specifically, the social presence theory suggested that students' sense of relatedness and learning motivation can be increased by a real or imagined interaction with others. On the other hand, self-regulated learning theory and self-determination theory both suggested that students learn best when they direct their focus to self, rather than others. Using these two theoretical perspectives as the basis, two experimental conditions (social presence vs. self-regulated) were developed in the context of chemistry learning. Participants of the study were randomly assigned to one of the experimental conditions. Both groups were presented with a pre-test, an online micro-lecture on balancing equations, i.e., a subtopic of stoichiometry, and a post-test. Linear mixed model analysis revealed that participants in the self-regulated learning condition had higher improvement in quiz scores than had participants in the social presence condition. Implications of the study and future directions were discussed.

UV radiation on the surface of Mars is an important factor affecting the survivability of microorganisms on Mars; a habitable niche on Mars would be one that provides protection from harmful UV radiation. The possibility of Martian brines providing a habitable niche on Mars via attenuation of UV radiation is investigated. Minerals could have leached into bodies of water on early Mars and may remain present in the polar ice caps. Aqueous solutions of nine different salts found on the surface of Mars were prepared and UV absorption cross sections were derived in the spectral range 230-400 nm. The salts investigated were Fe[(N[O.sub.3]).sub.3], [Fe.sub.2][(S[O.sub.4]).sub.3], FeS[O.sub.4], Fe[Cl.sub.2], MnS[O.sub.4], CaS[O.sub.4], MgS[O.sub.4], Mg[Cl.sub.2], and NaCl. UV absorption cross sections found ranged from [10.sup.-24] to [10.sup.-17] [cm.sup.2][molecule.sup.-1], with the cation primarily affecting absorption strength. In particular, salts containing [Fe.sup.3+] were found to absorb significant amounts of UV radiation. Combining the derived absorption cross sections with the solar radiation at Mars, transmitted UV radiation was simulated for different combinations of depth and salt concentration. Greater than 95% UV absorption from 230 to 400 nm is possible with iron salts at depths and concentrations expected to be found on past and present-day Mars. The results of this work are promising for investigating the habitability of Mars and other rocky planets that lack an atmosphere capable of attenuating UV radiation. Key words: Mars, spectroscopy, UV, brines, habitability. La radiation UV a la surface de Mars est un facteur important qui affecte sa survivabilite: une niche habitable sur Mars en devrait offrir une protection contre les dangereux rayons UV. Nous examinons la possibilite que des mares d'eau salee fournissent une niche habitable grace a l'attenuation de la radiation UV. Des mineraux pourraient avoir migre dans des masses d'eau sur Mars jeune et etre toujours presents dans les glaces polaires. Nous avons prepare des solutions de neuf sels differents trouves sur la surface de Mars et nous avons obtenu leur section efficace d'absorption dans le domaine spectral 230-400 nm. Les sels utilises sont Fe[(N[O.sub.3]).sub.3], [Fe.sub.2][(S[O.sub.4]).sub.3], FeS[O.sub.4], Fe[Cl.sub.2], MnS[O.sub.4], MgS[O.sub.4], CaS[O.sub.4], Mg[Cl.sub.2] et NaCl. Les sections efficaces vont de 10-24 a [10.sup.-17] [cm.sup.2][molecule.sup.-1], avec le cation jouant un role majeur dans l'absorption. Les sels menant a des cations [Fe.sup.+3] sont particulierement efficaces pour l'absorption des UV. Combinant les sections efficaces obtenues avec la radiation UV incidente sur Mars, nous calculons la radiation transmise pour differentes profondeurs et differentes concentrations de sel. Une absorption de plus de 95% de la radiation UV entre 240 et 400 nm est possible avec des sels de fer, a des concentrations et des profondeurs attendues sur Mars d'hier et d'aujourd'hui. Les resultats sont prometteurs pour l'examen d'habilite de Mars et d'autre planetes rocheuses dont l'atmosphere est incapable d'attenuer suffisamment la radiation UV. [Traduit par la Redaction] Mots-cles : Mars, spectroscopie, eau salee/saumure, habitabilite., 1. Introduction Unlike the Earth, the atmosphere of Mars does not provide protection to the planet's surface from UV irradiation due to the low column abundance of ozone, but can [...]