Resolving the surfaces of extrasolar planets with secondary eclipse light curves

We present a method that employs the secondary eclipse light curves of transiting extrasolar planets to probe the spatial variation of their thermal emission. This technique permits an observer to resolve the surface of the planet without the need to spatially isolate its light from that of the central star. We evaluate the feasibility of this technique for the HD 209458 system by simulating observations made with the Spitzer Infrared Array Camera (IRAC). We consider two representations of the planetary thermal emission: a simple model parameterized by a sinusoidal dependence on longitude and latitude, and the results of a three-dimensional dynamical simulation of the planetary atmosphere previously published by Cooper & Showman. We find that observations of the secondary eclipse light curve are most sensitive to a longitudinal asymmetry in the dayside planetary emission. To quantify this signal, we define a new parameter, the uniform time offset,'' which measures the time lag between the observed secondary eclipse and that predicted by a planet with spatially uniform emission. We compare the predicted amplitude of this parameter for HD 20948 with the precision with which it could be measured with IRAC. We find that IRAC observations at 3.6 mu m of a single secondary eclipse should permit sufficient precision to confirm or reject the Cooper & Showman model of the surface flux distribution for this planet. We quantify the signal-to-noise ratio for this offset in the remaining IRAC bands and find that a modest improvement in photometric precision should permit a similarly robust detection.