Primordial Dramas and Present Easy-Breathing

Our Solar System has changed dramatically over the aeons since the planets accreted/collapsed out of the initial nebula. The Sun both got brighter in overall output, but has dimmed in its extreme UV brightness (EUV) and solar-wind levels, with dramatic consequences for the atmospheres of the terrestrial planets & giant planet-sized moons. A good review article:

Atmospheric Escape and Evolution of Terrestrial Planets
and Satellites
Space Sci Rev (2008) 139: 399–436, DOI 10.1007/s11214-008-9413-5

…available at one of the co-authors, R.E.Johnson’s, webpage. EUV absorption in the very uppermost atmosphere, the exosphere, can drive the temperatures there to extremely high levels. This is due to the nature of EUV interactions with the very widely separated atoms and ions of the exosphere. Most of the atmosphere is well mixed, due to interparticle collisions, but in the exosphere the atoms rarely collide. Instead they can be struck by highly energetic photons, like EUV, and retain that energy, equivalent to thousands of degrees. This puffs the exosphere up even further and makes it easier for the atmosphere to escape into space, causing potentially several times the present day atmospheric masses to be driven off.

Another suite of processes produce what’s called non-thermal escape basically via energy transfer from the solar-wind to the upper atmosphere. Such processes can be very complicated to model and simulate, and can often only be properly understood by direct measurement thanks to long-term space-probe missions. Finally another possible atmospheric escape process is via direct blow-off via impacts. This isn’t happening in the present day at appreciable levels – fortunately – but is believed to have been important on smaller bodies like Mars and the large Gas Giant moons. Volatiles can also be accreted via this mechanism – the difference lies in the speed of the incoming impactor. Too fast and the incoming material blows away into space, taking some of the surface with it.

Some surprising masses of primordial atmosphere can be lost via these mechanisms, reshaping the body in question irrevocably. Some plausible changes – Venus lost an ocean, Earth lost excess hydrogen, Mars lost its primordial warming blanket of CO2, the Galileans lost Titan-like atmospheres and Titan lost several times its present day nitrogen atmosphere. And, just possibly, Earth owes its ocean to accretion via comets. At least some of it, almost certainly.

The implications for exoplanetary systems are worth considering. A smaller planet in a red-dwarf habitable zone will experience a much higher impactor speed, due to its higher orbital velocity. But, contrariwise, will it experience more impactors? Our own impact flux depended heavily on the movements of the Gas Giants – a primordial Titanomachy, if you will, which pummelled the planets with gravitationally tossed proto-comets & asteroids. Red dwarf stars seem to produce fewer Gas Giants, at least of Jupiter/Saturn class sizes, and may well produce a less severe impact flux. Could that mean their terrestrial planets are deprived of cometary volatiles and thus desert planets?

That’s one possible example and no doubt more will be conceived as our understanding improves.