How many times have I heard people fretting over the Solar Wind blowing a planet’s atmosphere away when talking about the terraforming of Mars or the Moon. People… not going to happen in a hurry. The Solar Wind is on average 5 million particles (mostly protons, just like the Sun) per cubic metre travelling at 400 km/s at Earth’s distance from the Sun – thus the flux (number passing through per square metre) is 400,000 m/s*5,000,000 /m^3 = 2E+12 protons per square metre per second. Their collective energy flux is their kinetic energy times their number flux – i.e. 1/2*(400,000)^2*(2E+12)*m(p)… where m(p) is the proton mass (1.673E-27 kg)… so we’re talking 2.68E-4 J/s. Less than 0.3 milliWatts per square metre. The sunlight is 1365 W/sq.m some 5.1 million times stronger, but qualitatively the two are quite different in how that energy is distributed. A proton slamming into an oxygen atom at 400 km/s is quite a bump. If all the momentum went from proton to atom, the atom would fly away at 25 km/s. But ions and atoms tend to collide elastically and bounce off each other. To conserve momentum and energy, the proton reverses direction and slows a bit, while the atom is flung away at high speed. But when trillions of atoms and ions are involved it’s not just one interaction – many could occur and eventually the ion might share its momentum with quite a few oxygens before escaping to space. So there’s three outcomes – one interaction between ion and atom, an even share (on average) of ion momentum, and finally an even share of ion energy. Surprisingly the first is the least efficient – an upper Venusian atmosphere some 6,200 km in radius loses just 6.4 kg/s. The second case loses 2.5 atoms (assuming 10 km/s escape speed at altitude) per ion – thus 16 kg/s. The third case sends 100 atoms into the void per ion – 640 kg/s. Sounds respectable but Venus has 4.6E+20 kg of gas to space… meaning 2.3 trillion years, 0.911 trillion years and 0.023 trillion years respectively to lose Venus’s air via the solar wind. A long time.
So why is Mars described as losing all its air in the early days? Back when the Sun was young its Wind was up to 1,000 times stronger. Mars, being smaller than Venus, would lose its air very rapidly in those days (Venus would still take billions of years in scenario 1 &2), but not now the Sun is better behaved. There are complications to this picture too – magnetic fields and ionization of the impacted atoms – but the basic picture is pretty straightforward. Solar Wind erosion is SLOW…