Author Bruce Dorminey
As seen in Forbes Science |
Our early Sun’s rate of rotation may be one reason we’re here to talk about it, astrobiologists now say. The key likely lies in the fact that between the first hundred million to the first billion years of its life, our G-dwarf star likely had a ‘Goldilocks’ rotation rate; neither too slow nor too fast.
Instead, its hypothetical ‘intermediate’ few days rate of rotation guaranteed our Sun was active enough to rid our newly-formed Earth of its inhospitable, hydrogen-rich primary atmosphere. This would have enabled a more habitable, secondary atmosphere composed of nitrogen, carbon dioxide, hydrogen and oxygen to eventually form.
If it had been a ‘fast’ (less than one day rotator), our Sun might have continually stripped our young planet of its secondary atmosphere as well. However, if it took more than 10 days to rotate, it might not have been active enough to strip Earth of its hypothetical primary atmosphere.
Such ideas were recently bandied about in oral presentations at last month’s the General Assembly of the International Astronomical Union (IAU) in Vienna.
Earth’s very first atmosphere would have been too hot and too thick, more like Venus’ present-day atmosphere, Theresa Luftinger, an astrophysicist at the University of Vienna, told me. No known organisms could have evolved under such an atmosphere, she says.
It’s the star’s magnetic dynamo that drives its magnetic fields. And these magnetic fields, in turn, interact with the star itself, creating an interplay of extreme stellar activity.
Faster rotation means higher extreme ultra-violet and x-ray activity, Helmut Lammer, an astrophysicist at Austria’s Space Science Institute in Graz, told me. This would lead to atmospheric stripping and water loss on earthlike planets around an active young star, he says.
Our Sun is now a very slow rotator at 27 days. But that wasn’t always the case. As for why some stars seem to inherently rotate faster than others?
Astrophysicists suspect that initial conditions within star-forming clouds cause newborn stars to have different rotation rates.
Researchers are able to roughly pinpoint the Sun’s early rotation rates by studying the isotopic ratios of neon, argon, potassium, and uranium here in Earth’s crust. That is, elements which have atoms that have the same numbers of protons in their atomic nucleus, but different numbers of neutrons. The researchers also considered such isotopic ratios from decades’-old Venus surface samples taken by Soviet Venus lander missions.
Isotopic ratios are changed by stellar activity, with less active and more active stars having different elemental effects.
When very early Venus and Earth accreted, potassium was also in the primordial atmosphere and could escape, says Lammer. In contrast, he says the much heavier element of uranium remained fixed in our planet’s crust.
“In our study, we can reproduce the present-day measurements of these elements only if the young Sun was slow to moderately rotating,” said Lammer.
There’s one big puzzle, however. Was our Sun truly a Goldilocks rotator, or did our Earth need the Moon-forming impact to get rid of our planet’s primordial atmosphere?
To explain our existence without our Moon-forming impact, we need to be a ‘Goldilocks’ intermediate rotator, says Luftinger.
That’s because a secondary atmosphere cannot evolve in the presence of a primordial atmosphere , says Luftinger.
Yet to better constrain the Sun’s young rotation rate, researchers need new isotope samples from Venus. Luftinger estimates that about a third of all sunlike stars are Goldilocks intermediate rotators.
Lammer says two upcoming Venus missions, the European Space Agency’s (ESA) Envision and Venera-D, a joint U.S.-Russia effort, will measure both argon and neon isotope ratios in our neighboring planet’s atmosphere.
New Venus measurements could better constrain the Sun’s early rotation rates, says Lammer.