Astronomers say that billions of years ago when the Sun was young, it shone with only 70 percent its current brightness, notes Robert Craddock (Smithsonian Institution). If that were true of today’s Sun, he explains, Earth’s surface would freeze over, and conditions on Mars would be much colder and drier than they are now.
Given a faint young Sun, “Earth and Mars should have been much too cold for liquid water to be present on the surface of either planet, creating a paradox,” he says. Yet on Earth, the geologic record from the early Precambrian era contains fluvial sediment, indicating surface conditions warm enough for flowing water.
Mars also presents the same paradox. The team says, “There’s a variety of geologic evidence indicating that early Mars supported rainfall as well as an advanced hydrologic cycle.”
Recent work shows that this “climatic optimum” occurred not throughout the Noachian period, the earliest in Martian geologic history, but rather came towards the end of the period. Nonetheless, the researchers say, “the scale of erosion represented by many valley network systems is immense, and can rival the Grand Canyon on Earth.” These systems likely operated over hundreds of thousands of years at a minimum.
In addition to fluvial valley networks, the scientists note, there are widespread areas of impact craters showing evidence of modification by erosion processes that softened and weathered landscape features on a global scale.
Sharpening the paradox, the researchers note that an atmosphere of carbon dioxide can’t stay warm enough to provide above-freezing temperatures, even if the surface pressure were 10 times greater than Earth’s.
Yet “on Mars, valley networks and modified impact craters both attest to the fact that Mars experienced periodic if not sustained rainfall during the Noachian and through the Hesperian,” the team says. “In addition, there are the outflow channels that also required catastrophic releases of liquid water, and there’s compelling evidence for an ocean in the northern hemisphere.”
If a pure CO2 atmosphere does not provide clement conditions regardless of its surface pressure, the scientists say, then some other combination of atmospheric conditions and gas constituents is needed. Here, more modeling of ancient climates should help.
Another possibility is that the earliest stages of sunlike stars run differently from what astrophysicists currently think. The team suggests that new data on extrasolar planetary systems from the Kepler Observatory may resolve the problem.
“Ultimately, reconciling the astronomy, geology, and meteorology rests with better climate models,” the team says. And the problem goes beyond just Mars, they add. “The implications are not only for understanding the history of water on Mars, but the origin of life on Earth as well as habitable zones and extrasolar planets.”