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The Neptune-mass GJ 436b is one of the most-studied transiting exoplanetswith repeated measurements of both its thermal emission and transmissionspectra. We build on previous studies to answer outstanding questions aboutthis planet, including its potentially high metallicity and tidal heating ofits interior. We present new observations of GJ 436b's thermal emission at 3.6and 4.5 micron, which reduce uncertainties in estimates of GJ 436b's flux atthose wavelengths and demonstrate consistency between Spitzer observationsspanning more than 7 years. We analyze the Spitzer thermal emission photometryand Hubble WFC3 transmission spectrum in tandem. We use a powerful dual-prongedmodeling approach, comparing these data to both self-consistent and retrievalmodels. We vary the metallicity, intrinsic luminosity from tidal heating,disequilibrium chemistry, and heat redistribution. We also study the effect ofclouds and photochemical hazes on the spectra, but do not find strong evidencefor either. The self-consistent and retrieval modeling combine to suggest thatGJ 436b has a high atmospheric metallicity, with best fits at or above severalhundred times solar metallicity, tidal heating warming its interior withbest-fit intrinsic effective effective temperatures around 300--350 K, anddisequilibrium chemistry. High metal-enrichments (>600x solar) can only occurfrom the accretion of rocky, rather than icy, material. Assuming Tint~300--350K, we find that Q'~2x10^5--10^6, larger than Neptune's Q', and implying a longtidal circularization timescale for the planet's orbit. We suggest thatNeptune-mass planets may be a more diverse class than previously imagined, withmetal-enhancements potentially spanning several orders of magnitude, to perhapsover 1000x solar metallicity. High fidelity observations with instruments likeJWST will be critical for characterizing this diversity.

Forward and Inverse Modeling of the Emission and Transmission Spectrum of GJ 436b: Investigating Metal Enrichment, Tidal Heating, and Clouds