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We used WIRCam on CFHT to observe four transits of the super-Earth GJ 1214bin the near-infrared. For each transit we observed in two bandsnearly-simultaneously by rapidly switching the WIRCam filter wheel back andforth for the duration of the observations. By combining all our J-band (~1.25microns) observations we find a transit depth in this band of 1.338\pm0.013% -a value consistent with the optical transit depth reported by Charbonneau andcollaborators. However, our best-fit combined Ks-band (~2.15 microns) transitdepth is deeper: 1.438\pm0.019%. Formally our Ks-band transits are deeper thanthe J-band transits observed simultaneously by a factor of 1.072\pm0.018 - a4-sigma discrepancy. The most straightforward explanation for our deeperKs-band depth is a spectral absorption feature from the limb of the atmosphereof the planet; for the spectral absorption feature to be this prominent theatmosphere of GJ 1214b must have a large scale height and a low mean molecularweight. That is, it would have to be hydrogen/helium dominated and this planetwould be better described as a mini-Neptune. However, recently publishedobservations from 0.78 - 1.0 microns, by Bean and collaborators, show a lack ofspectral features and transit depths consistent with those obtained byCharbonneau and collaborators. The most likely atmospheric composition for GJ1214b that arises from combining all these observations is less clear; if theatmosphere of GJ 1214b is hydrogen/helium dominated then it must have either ahaze layer that is obscuring transit depth differences at shorter wavelengths,or significantly different spectral features than current models predict. Ourobservations disfavour a water-world composition, but such a composition willremain a possibility until observations reconfirm our deeper Ks-band transitdepth or detect features at other wavelengths. [Abridged]

Broadband Transmission Spectroscopy of the super-Earth GJ 1214b suggests a Low Mean Molecular Weight Atmosphere