Episode 424 - Hunting for atmospheres on other planets

Mars was once covered with water, so where did all the water on Mars go? What happened to the water in the Martian atmosphere? Why isn't there an abundance of heavy water in the Martian atmosphere? Water can get trapped inside rocks and minerals without volcanoes to cycle them. Volcanoes and tectonics help sequester, cycle and release water, so what happens on a planet without them? How can we hunt for signs of water atmospheres on exoplanets? On hot rocky exoplanets with oceans of magma, what happens to their hydrogen rich atmospheres?  An atmosphere of of hydrogen can slowly turn and change into water with the help of a magma ocean. 
References:

1. E. L. Scheller, B. L. Ehlmann, Renyu Hu, D. J. Adams, Y. L. Yung. Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust. Science, 2021; eabc7717 DOI: 10.1126/science.abc7717
2. Edwin S. Kite, Laura Schaefer. Water on Hot Rocky Exoplanets. The Astrophysical Journal Letters, 2021; 909 (2): L22 DOI: 10.3847/2041-8213/abe7dc