Venus atmosphere has staggeringly high pressure, but it also has vertical structure. See: Atmosphere of Venus - Wikipedia About 50 km up, the pressure is equal to Earth surface. On Venus, this is 96.5% CO2 and a temperature of 75 oC. Being closer to Sun, its solar irradiance (at top of atmosphere) is 1.9 times that to Earth. That temperature may or may not be what one could calculate with model based on infrared trapping, but it does not strike me as unreasonable. Mars atmosphere is 96% CO2 (% very similar to Venus!) but pressure at the surface is only 0.0063 of Earth. This leads to an interesting conclusion that there is about 15 times more CO2 in a cylinder of atmosphere between Mars surface and ‘space’, compared to Earth. Solar irradiance (at top of atmosphere) is 43% of that to Earth. With all that, I find it interesting that there seems to be no detectable greenhouse effect on Mars. I say this because average surface T is -63 oC, and on Phobos (a moon there) it’s -58 oC. We appreciate that those surface T values are based on sparse data (especially for Phobos), but there it is. Open for improvement by further exploration of Mars and its vicinity. This does not seem to be discussed in scientific literature, but the answer may be in the Beer-Lambert Law (of energy absorption by gas). In the first step, light energy (for example infrared) is absorbed if the gas can do it. CO2 can; and that molecule gains vibrational or rotational energy. It can lose this energy by collision with a non-absorbing (different) molecule. If it collides with another CO2 there is no net loss. The situation in Mars atmosphere is that other molecules are rare. Nitrogen and argon are both about 1.9%. Greenhouse effect is based on having non-absorbing molecules handy. Mars atmosphere has few; Earth atmosphere has lots. We can take a couple of lessons. First is that claiming Venus situation is all due to pressure and not at all to CO2 is misleading at best. If somebody did a thorough treatment of Venus atmosphere (absorption, scattering, and vertical mixing, such as has been done for Earth atmosphere) this could be better understood. Anybody sees a link for this, please post it. Second it that if one intends to ‘Terraform’ Mars, it will take more than just adding infrared absorbers (such as water vapor, CO2, or fluorochlorocarbons). Non-IR absorbers (nitrogen among others) would also be needed. Family portrait
Source: Planetary Fact Sheet planet/moon Avg. C Atmosphere C Equilibrium C 1 Mercury 167 164 2 Venus 464 <tdb> -45 3 Earth 15 d_35 C -17 4 Moon -20 0 5 Mars -65 -63(*) d_-24 C d_-15 C (*) -55 6 Phobos -41(**) -58(*) * - see #1 in thread ** - Wiki reports 233K (-41C) We have a difference of opinions about the mean temperatures from different sources. Time to check sources. However, it is reported that Phobos is descending ~1.8 to 2 m / 100 years. The mechanism for Phobos orbital decay isn't clear but one possibility might be tidal warming. Regardless loss of orbital energy as heat would be a possibility. Still, the thermodynamics of Phobos temperature is an interesting puzzle. Per Wiki, "Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071." That means it should be warmer from direct, solar heating. Bob Wilson
I understand that astrophysicists can calculate expected surface temperatures from photon flux (and spectra) and surface albedo. Not something I could do. Must be complex for Earth's Moon, as its two 'faces' are so different. Maybe that's why Moon is so strange in table @2 I don't see how it could be done for Venus though. Its surface albedo is ???. One can measure reflectivity of its atmosphere, but how is that relevant? I suppose they use an albedo appropriate for some type of rocky surface, local solar photon flux (about 1.9 times at Earth), and proceed to obtain -45 oC.
I don't think it is the surface as much as the metrics across the solar spectrum measured by telescope. As for the far side of the moon, I suspect there have been enough lunar orbiter metrics to nail it down. Bob Wilson
Yes I agree about knowledge of moon front/back albedo. But do not know how one puts those together in detail. Albedo (always) varies with surface illumination angle, and I have to suppose that is also suitably handled.. Finally, the expected T of a no-atmosphere body must have some uncertainty. This matters not at all for Venus (pressure and IR absorption discussion). It matters for Mars because I feel unsure whether there is any 'greenhouse effect there, with more atmospheric (column) CO2 than Earth and at 43% of Earths solar irradiance. Not important to talk about Earth with 1 'atmosphere' pressure. Potentially very important for efforts to terraform Mars in a century or two.
I don't have the source handy but I remember reading that the absence of a magnetic field leaves the Martian atmosphere exposed to the solar flux. The claim is the solar wind flux disassociates water at the top of the column allowing the hydrogen and oxygen to escape. Asking Mr. Google found: magnetic field - Would terraforming Mars be possible? - Space Exploration Stack Exchange Without a magnetic field any changes we do are seemingly temporary. As we make atmosphere it will be torn away by double solar winds. Double solar winds are the worst, occurring about 15% of the time this occurs when a faster solar wave catches a slower and rolls into one bigger wave. And these happen frequently, very frequently! . . . Terra forming without consideration of the nearest star ... well that would be incomplete. Bob Wilson
Our memory acuity seems similar I remember reading (somewhere) that no-magnet Mars lost its atmosphere in 100 million years. Anyway, yes Terraforming involves high tech and big tech not yet in human hands.
Dang it ... I wondered about Phobos: Phobos (moon) - Wikipedia The orbital motion of Phobos has been intensively studied, making it "the best studied natural satellite in the Solar System" in terms of orbits completed.[34] Its close orbit around Mars produces some unusual effects. With an altitude of 5,989 km (3,721 mi), Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates.[16] Therefore, from the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Because it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. Its orbit is so low that its angular diameter, as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at zenith it is 0.20°, one-third as wide as the full Moon as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's synodic period) to run their course, a mere 13 seconds longer than Phobos's sidereal period. As seen from Phobos, Mars would appear 6,400 times larger and 2,500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere. The Mars–Phobos Lagrangian L1 is 2.5 kilometers (1.6 mi) above Stickney, which is unusually close to the surface. Ok, so here's the deal. We get Phobos to descend faster and impact Mars. This puts a massive injection of heat, fractures the surface to release gasses, and spins up the planet. Hopefully enough to make a magnetic field and atmosphere. Probably need to reverse spin Phobos to minimize ejecta reaching Mars escape velocity. Bob Wilson
Phobos as impactor: ~20 km diameter (larger than Chicxulub impactor on Earth), mass 10^19 kg, orbital velocity 2 km/sec. If you want to have such a large effect on Mars, better find a way to accelerate it to 'proper impactor velocity', about 10x higher and 100 x kinetic energy before letting 'er rip. Otherwise I think you will just make a divot. Mars magnetic field: Had it 4 billion years ago, differentiated iron core, but why did it switch off? Nobody seems to say that the core 'froze' but that's my guess. So, giving the planet a wallop and making it spin fast will wake up the magnetic field? I don't see that. I don't see the need for magnetic field. Good, because I don't see any hope of instigating it either. With all the energy you'd have to add to 'prep for' the impact, I don't see the value of using Phobos either. Just go out to where the comets live and nudge a biggish one to slingshot past Jupiter or some other handy gravitational well. NASA folks have equations for that. After a while it impacts Mars at something like 20 km/sec. All that new water, plus whatever you've excavated, becomes atmospheric and absorbs IR. By my current thinking, you need to 'pad' that atmosphere with some non-IR absorbing gas as well. It would be good if I'm wrong about that because I haven't the faintest where you'd get enough (ex.) nitrogen. Perhaps a different impactor would be better suited. Outer large planets have many moons. You might find one with a nice greenhouse recipe. But all of them are stuck in host planet gravity wells. Pulling one out is going to be a large effort. However you construct Mars' new atmosphere, it will persist for millions of years. Long enough. We don't need no stinkin' magnets. We need an appropriate impactor. Concerning solar any 'cosmic' charged particle fluxes, you would be better off with a magnetic field. Either that or everybody lives in holes. Go with holes I guess.
