THE widely held idea that the Sun began life dimmer than it is today has been challenged by a pair of astrophysicists who say it may actually have been brighter. If they are right, it would solve a major planetary puzzle: how water could have run on Mars 3.8 billion years ago when, by rights, the planet鈥檚 surface should have been frozen solid.
There is now plenty of evidence that liquid water existed on Earth and on Mars soon after they formed. On Earth, greenhouse gases might just have warmed the planet enough for ice to melt even if the Sun was fainter. But on Mars, which is farther from the Sun, this doesn鈥檛 seem possible.
So Arnold Boothroyd of the University of Toronto in Canada and Juliana Sackmann of the California Institute of Technology in Pasadena have come up with a scenario that would solve this problem. In a paper to be published in The Astrophysical Journal, they suggest that the newborn Sun may have been up to 7 per cent more massive than it is today, making it 50 per cent brighter than most astronomers imagine it was at the time, and 5 per cent brighter than it is now.
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A more massive Sun would also have had stronger gravity and so would have pulled the planets closer in, further boosting the sunlight Earth and Mars received. 鈥淚n our scenario, it鈥檚 warm enough for water to run on Mars 3.8 billion years ago, the age of the river valleys we see on its surface,鈥� says Boothroyd.
The researchers calculate that to reach the size it is today, the newborn Sun would have had to shed mass early on a thousand times as fast it does today, losing the equivalent of the Earth鈥檚 mass every 10,000 years. Astronomers who have studied other young stars like the Sun say this isn鈥檛 unprecedented. 鈥淭he mass loss rate of the very young Sun could indeed have been about a thousand times higher,鈥� says Brian Wood of the University of Colorado. 鈥淓ven a 7 per cent larger mass is not unreasonable.鈥�
Although Sackmann and Boothroyd don鈥檛 have any direct evidence to support their theory, they say the idea may soon be testable. The internal composition of the Sun today should depend on how brightly it burnt when it was young. This would be reflected in the speed of sound through the Sun, an effect detectable with a technique known as helioseismology. At the moment, the difference between the two predicted compositions is too small to spot, but Boothroyd says this is about to change. 鈥淲ith improvements in the technique, we think it will be possible to prove or disprove our idea.鈥�
If the researchers are right, then the Sun would have faded for the first 1 to 2 billion years of its existence as it shed its initial extra mass, before steadily brightening until the present day. It鈥檚 hard to know how this alternative history would have affected conditions on Earth though, as terrestrial temperatures are also affected by the amounts of greenhouse gases in the atmosphere.