DECIPHERING THERMAL PHASE CURVES OF DRY, TIDALLY LOCKED TERRESTRIAL PLANETS

Abstract

Next-generation space telescopes will allow us to characterize terrestrial\nexoplanets. To do so effectively it will be crucial to make use of all\navailable data. We investigate which atmospheric properties can, and cannot, be\ninferred from the broadband thermal phase curve of a dry and tidally locked\nterrestrial planet. First, we use dimensional analysis to show that phase\ncurves are controlled by six nondimensional parameters. Second, we use an\nidealized general circulation model (GCM) to explore the relative sensitivity\nof phase curves to these parameters. We find that the feature of phase curves\nmost sensitive to atmospheric parameters is the peak-to-trough amplitude.\nMoreover, except for hot and rapidly rotating planets, the phase amplitude is\nprimarily sensitive to only two nondimensional parameters: 1) the ratio of\ndynamical to radiative timescales, and 2) the longwave optical depth at the\nsurface. As an application of this technique, we show how phase curve\nmeasurements can be combined with transit or emission spectroscopy to yield a\nnew constraint for the surface pressure and atmospheric mass of terrestrial\nplanets. We estimate that a single broadband phase curve, measured over half an\norbit with the James Webb Space Telescope, could meaningfully constrain the\natmospheric mass of a nearby super-Earth. Such constraints will be important\nfor studying the atmospheric evolution of terrestrial exoplanets as well as\ncharacterizing the surface conditions on potentially habitable planets.\n