Imagine the early days of our Solar System, going back billions of years. The Sun was cooler and less luminous, but there were (at least) two planets — Earth and Mars — with liquid water covering large portions of their surfaces. Neither world was completely frozen over owing to the substantial presence of greenhouse gases, including carbon dioxide. Both may have even had primitive life forms in their young oceans, paving the way for a bright, biology-friendly future.
Over the past few billion years, both planets have undergone dramatic changes. Yet, for some reason, while Earth became oxygen-rich, remained temperate, and saw life explode on its surface, Mars simply died. Its oceans disappeared; it lost its atmosphere; and no life signs have yet been found there. There must be a reason why Mars died while Earth survived. It took decades, but science has finally figured it out.