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We report our spectroscopic investigation of the transiting ice giant GJ3470b's atmospheric transmission, and the first results of extrasolar planetobservations from the new Keck/MOSFIRE spectrograph. We measure a planet/starradius ratio of Rp/Rs = 0.0789 +/- 0.0020 in a bandpass from 2.09-2.36 micronand in six narrower bands across this wavelength range. When combined withexisting broadband photometry, these measurements rule out cloud-freeatmospheres in chemical equilibrium assuming either solar abundances (5.4 sigmaconfidence) or a moderate level of metal enrichment (50x solar abundances, 3.8sigma), confirming previous results that such models are not representative forcool, low-mass, externally irradiated extrasolar planets. Current measurementsare consistent with a flat transmission spectrum, which suggests that theatmosphere is explained by high-altitude clouds and haze, disequilibriumchemistry, unexpected abundance patterns, or the atmosphere is extremelymetal-rich (>200x solar). Because GJ 3470b's low bulk density sets an upperlimit on the planet's atmospheric enrichment of <300x solar, the atmosphericmean molecular weight must be <9. Thus, if the atmosphere is cloud-free itsspectral features should be detectable with future observations. Transitobservations at shorter wavelengths will provide the best opportunity todiscriminate between plausible scenarios. We obtained optical spectroscopy withthe GMOS spectrograph, but these observations exhibit large systematicuncertainties owing to thin, persistent cirrus conditions. Finally, we alsoprovide the first detailed look at the steps necessary for well-calibratedMOSFIRE observations, and provide advice for future observations with thisinstrument.

Warm Ice Giant GJ 3470b. I. A Flat Transmission Spectrum Indicates a Hazy, Low-methane, and/or Metal-rich Atmosphere