When our sun was young, it was significantly less luminous and sent out significantly less warming energy than it does now. Scientists estimate that 4 million years ago, when the sun and our solar system were 500 million years old, the energy that the sun produced and dispersed was about 75 percent of what it is today.
The paradox arises because during this time of the faint young sun Earth had liquid water on its surface and — as has been conclusively proven in recent years — so did Mars, which is 61 million miles further into space. However difficult it is to explain the faint young sun problem as it relates to early Earth, it is far more difficult to explain for far more frigid Mars.
Yet many have tried. And because the data is both limited and innately puzzling, the subject has been vigorously debated from a variety of different perspectives. In 2018, the journal Nature Geoscience published an editorial on the state of that dispute titled “Mars at War.”
There are numerous point of (strenuous) disagreement, with the main ones involving whether early Mars was significantly more wet and warm than previously inferred, or whether it was essentially cold and arid with only brief interludes of warming. The differences in interpretation also require different models for how the warming occurred.
Was there a greenhouse warming effect produced by heat-retaining molecules in the atmosphere? Was long-term volcanic activity the cause? Or perhaps meteor strikes? Or heat from the interior of the planet?
All of these explanations are plausible and all may have played a role. But that begs the question that has so energized Mars scientists since Mars orbiters and the Curiosity rover conclusively proved that surface water created early rivers and valley networks, lakes and perhaps an ocean. To solve the “faint young sun” paradox as it played out on Mars, a climate driver (or drivers) that produces significant amounts of heat is required.
Could the necessary warming be the result of radioactive elements in the Martian crust and mantle that decay and give off impressive amounts of heat when they do?