  We report our spectroscopic investigation of the transiting ice giant GJ
3470b's atmospheric transmission, and the first results of extrasolar planet
observations from the new Keck/MOSFIRE spectrograph. We measure a planet/star
radius ratio of Rp/Rs = 0.0789 +/- 0.0020 in a bandpass from 2.09-2.36 micron
and in six narrower bands across this wavelength range. When combined with
existing broadband photometry, these measurements rule out cloud-free
atmospheres in chemical equilibrium assuming either solar abundances (5.4 sigma
confidence) or a moderate level of metal enrichment (50x solar abundances, 3.8
sigma), confirming previous results that such models are not representative for
cool, low-mass, externally irradiated extrasolar planets. Current measurements
are consistent with a flat transmission spectrum, which suggests that the
atmosphere is explained by high-altitude clouds and haze, disequilibrium
chemistry, unexpected abundance patterns, or the atmosphere is extremely
metal-rich (>200x solar). Because GJ 3470b's low bulk density sets an upper
limit on the planet's atmospheric enrichment of <300x solar, the atmospheric
mean molecular weight must be <9. Thus, if the atmosphere is cloud-free its
spectral features should be detectable with future observations. Transit
observations at shorter wavelengths will provide the best opportunity to
discriminate between plausible scenarios. We obtained optical spectroscopy with
the GMOS spectrograph, but these observations exhibit large systematic
uncertainties owing to thin, persistent cirrus conditions. Finally, we also
provide the first detailed look at the steps necessary for well-calibrated
MOSFIRE observations, and provide advice for future observations with this
instrument.
