Answer to Question 1
Soon after Earth formed, it began to cool; once it cooled enough, oceans began to form, and carbon dioxide began to dissolve in the water. The removal of carbon dioxide is critical to Earth because an atmosphere rich in carbon dioxide can trap heat by a process called the greenhouse effect. When visible-wavelength sunlight shines through the glass roof of a greenhouse, it heats the interior. Infrared radiation from the warm interior can't get out through the glass, heat is trapped in the greenhouse, and the temperature climbs until the glass itself grows warm enough to radiate heat away as fast as sunlight enters.
Answer to Question 2
Two kinds of seismic waves show that the outer core is liquid. P waves travel like sound waves, and they can penetrate a liquid, but S waves travel as a side-to-side vibration that can travel along the surface of a liquid but not through it. Scientists can deduce the size of the liquid core by observing where S waves get through and where they don't.
Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by Mars Pathfind
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