Before going to Mars let me explain here the runaway greenhouse effect happening in Venus. It is a process in which a net positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its rivers, seas and oceans boil away.
To understand it more I give you an example, in Hot summer days when you open the door of a car park outside, it is hotter inside the car than outside. Why is it that? Sunlight has visible light and glass is transparent to it that’s why we can see both ways in & out. But sunlight has Ultra Violet light also.
So Uv when passing thru the glass of car losses its energy & becomes longer wavelength i.e Infrared which than trapped inside the car and makes it hotter than outside. The Same effect is happening in Venus that Uv light trapped as Infrared in the atmosphere. That’s why people on earth should be worried about climate changes and global warming on earth as because of pollution, factories, vehicles on road our atmosphere is also becoming polluted and heavier.