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Probing Exoplanet Obliquity

It’s always a shock for me when the soft air and fecund smells of spring slam into a parched and baked July, but seasonal change is inevitable. At least it is on Earth. We get such seasonal changes because of Earth’s obliquity, the angle of its spin axis relative to the plane of its orbit. For Earth, the angle has stayed pretty close to 23 degrees for a long time, although the tilt’s direction wobbles over cycles of thousands of years. And this very constancy of obliquity turns up in exoplanet discussions at times because it affects conditions on a planetary surface.

Some have argued that without the gravitational effects of the Moon, the tilt of the Earth would be changed by the gravitational pull of the Sun and planets, producing a potentially high degree of obliquity. Contrast our situation with that of Uranus, where we find a 90-degree tilt that leaves one pole in sunlight for half the Uranian year as the other remains in darkness. Without knowing how long the Moon has been able to stabilize Earth’s axial tilt, we can’t say how apparent equatorial ice sheets some 800 million years ago fit into this view of the Moon’s effect.

But obliquity as a factor in habitability continues to energize exoplanetary researchers. At Georgia Tech, a team led by Gongjie Li, working with graduate student Yutong Shan (Harvard-Smithsonian Center for Astrophysics) has developed computer simulations to analyze the spin axis dynamics of two exoplanets, Kepler 186f and Kepler 62f, two planets considered to be in or close to the habitable zone of their stars. The paper argues that without our Moon, Earth’s obliquity variation would range from 0 to 45 degrees over billion-year timescales.

Thus obliquity is an interesting data point. Bear in mind that so far, we have no reliable values for exoplanet obliquity, although ways to infer it from light curves and from high-contrast direct imaging have been proposed in the literature. The authors make the assumption that in both exoplanet systems studied, all planets have been identified. They then go on to study the evolution of the two five-planet systems. The ‘secular analytical framework’ they arrive at allows them to factor in planetary rotation rates, additional planets and satellites, and regions where resonant interactions within the system can produce large obliquity variations. For various realizations of planetary systems, the paper thus describes an ‘obliquity evolution.’

We know that Mars and Earth interact strongly with each other, as do Mercury and Venus; other than Earth, none of these worlds has a large moon. The authors point out that the orientation angle of a planet’s orbit around its host star can be made to oscillate through gravitational interactions. If the orbit oscillates at the same pace as the precession of the planet’s spin axis, large obliquity variations can be induced, the kind of thing our Moon dampens out.

Image: An artist’s depiction of Kepler-62f. Credit: NASA Ames/JPL-Caltech/T.Pyle.

For these two exoplanet systems, we get an interesting result, for even without a stabilizing moon (if none is present), these two planets could be experiencing relatively low changes in their axial tilt:

“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li. “We don’t know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years. That’s not to say either exoplanet has water, let alone life. But both are relatively good candidates. Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds.”

As Li has just pointed out, we have no knowledge of surface conditions on either of these planets, making the lovely image above nothing more than a guess, and an optimistic one at that. The ‘super Earth’ Kepler 62f, about 40 percent larger and with a mass 2.8 times that of our planet, is in the constellation of Lyra, the outermost of the five planets orbiting a K2-class star some 1200 light years from Earth. Kepler-186f orbits a red dwarf about 550 light years out, part of a five-planet system in the constellation Cygnus. A stable axial tilt would make it likely that both worlds experience regular seasons and thus a stable climate.

But are large obliquity values necessarily inimical to life? Some recent work, considered by the authors, shows that variability in obliquity can keep a planet’s global temperature higher than it would otherwise have been, extending the outer edge of the habitable zone. But it does appear that obliquity variations can produce sharp transitions between climate states. From the paper:

Recently, Kilic et al. (2017) mapped out the various equilibrium climate states reached by an Earth-like planet as a function of stellar irradiance and obliquity. They find that, in this parameter space, the state boundaries (e.g. between cryo- and aqua-planets) are sharp and very sensitive to the climate history of the planet. This suggests that a variable obliquity can easily move the planet across state divisions, as well as alter the boundaries themselves, which would translate into a dramatic impact on instantaneous surface conditions and long-term climate evolution.

Planets with highly irregular seasons aren’t necessarily destined to be lifeless, but if we become capable of determining planetary obliquity, such a value could help us narrow the target list for future space telescopes. The authors also suggest that their framework can provide input parameters for existing global climate models as we analyze habitability in multi-planet systems.

The paper is Shan and Li, “Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f,” <em>Astronomical Journal</em> Vol. 155, No. 6 (17 May 2018). Abstract / preprint.


{ 34 comments… add one }
  • Michael Lorrey July 2, 2018, 12:37

    I would argue that without a large moon orbiting off the orbital plane of the planet to its primary, like ours does, we would not see much variation at all. We can look at all the other planets in our solar system other than Uranus as examples. Statistically, if planets were prone to wandering obliquity, there should be a much wider range of it among our planets in our solar system. Since there is not, there isn’t a benefit to having a large moon, and here is why: when a planet rotates, the planetary mass at the equator is drawn outward due to centrifugal forces. This creates an oblateness to the planetary sphere, such that the diameter pole to pole is significantly less than the diameter equator to equator. It is this difference, combined with the gyroscopic behavior of the rotating planet, and the overwhelming gravitational attraction of its star, that forces a spinning planet without moons to orient itself north/south along with its star. The greater mass at the equator of the oblate spheroidal planet due to planetary bulge imposes a torque that forces the planet to face its equator edge on to its star. It is only when there is a greater gravitational influence, like a large moon, or a nearby jovian, that the suns influence is overridden, and the planet trends toward obliquity.
    This also explains why Uranus orients its axis toward the sun: the greater gravitational influences upon it are not the sun, its own moons, which inhabit an accretion disk that is likewise, consistent with the Uranian equator, face on, rather than edge on, to the Sun, and result from some cataclysmic impact early in Uranian planetary formation. The combined masses of Uranian moons rivals the total mass of our own moon, and while it masses 14x that of Earth, this is apparently sufficient for the moons of uranus to exert a sufficient gravitational influence upon the planet to maintain its obliquity.

    • Alex Tolley July 2, 2018, 16:39

      For a world that has continental drift, the position of the continental masses on the surface move, resulting in changes to the mass distribution that is resolved by changes in obliquity. Even without CD, didn’t Mars change its obliquity in response to the formation of the Tharsis region with its massive volcanoes?

    • Ronald July 3, 2018, 14:25

      I agree, the gyroscopic effect of the Earth itself is probably much greater in stabilizing effect than the moon.
      There have been some studie indicating that the stabilizing effect of our moon was in fact so small, that even without it, the climatic changes resulting from it would only have been noticeable over the course of a hundred million years or so.

  • Alex Tolley July 2, 2018, 13:28

    For any position on the planet, a changing obliquity will change the environment of that position. While it might seem that a priori this makes ecosystems unstable, we do have both continental drift and glacial periods as proof that evolution can occur when environments change. The biogeography of these environments suggests ecosystem changes are quite possible over these timescales.

    Even with the extraordinary fast climate changes we are inducing, we see some species successfully migrating towards the poles, although we are also seeing a failure of ecosystems to adjust due to the too rapid changes. But over millions of years, even tens of thousands of years, ecosystems do seem to change successfully with the climate.

    There may even be benefits of climate changes induced by obliquity, as it could offer fresh niches for populations to invade as climate changes force migration of well-adapted species. This could help drive evolution, much like terrestrial extinction events have allowed new forms to dominate and radiate.

    • Rob Flores July 2, 2018, 18:34

      I concur, but without knowing if the distribution of
      continental landmass, of an earth like planet we cannot say for sure how much of a role obliquity play in providing extra variation or amount of desirable living space.

      Consider that if a low obliquity planet had large continents stradling the equator, the equatorial rain band would result in the eventual
      paths of the watersheds, to be quite intensive and long .

      I would guess that Low obliquity planets tend to have fewer ice ages, which considering the high number of extinct fauna and flora it may result in, is a positive for such a planet’s biosphere.

  • Robin Datta July 2, 2018, 16:15

    It would seem that the matter of habitability in the Drake equation is becoming increasingly nuanced with progress in knowledge and understanding. The flaring behavior of the host star, the circularity of the orbit, the presence of the needed life precursor materials (including an appropriate atmosphere) are amongst the variables that are fine-tuning “habitability”. Even stellar-scale engineering and (astro) archeology have been added to the mix in attempting an answer to the question that arose with the recognition of the spherical finitude of the Earth: “Are we alone?”

    Obliquity of axis of rotation becomes yet another variable to consider when evaluating habitability. The three imperatives of life as we know it – survival, growth and replication might not be thwarted by the effects of axial tilt alone, but when other conditions are borderline, tilt could become the critical variable.

  • Christopher Phoenix July 2, 2018, 16:16

    This is indeed a very interesting result, and suggests that we should be careful not to assume that an exoplanet needs to have the exact setup the Earth does in order to have a stable, habitable climate. Apparently exoplanetary systems can differ from what we see in our own solar system!

    @ Michael Lorry

    Statistically, if planets were prone to wandering obliquity, there should be a much wider range of it among our planets in our solar system.

    You should note, though, that the obliquity of Venus is downright weird. As listed in the ever helpful Planetary Fact Sheets, Venus has an obliquity of 177.4°, meaning it is almost flipped entirely over. Essentially, Venus rotates backwards relative to the other planets and its north pole points “down”. This suggests Venus might have experienced chaotic spin changes, like scientists suggest could happen to an Earth without a Moon, unless its odd rotation is the result of some cosmic catastrophe like an impact. Mars has a more “ordinary” obliquity of 25.2°, but Venus is arguably the closest planet to Earth in our solar system (in terms of mass, not surface conditions!) other than it has no large moons.

  • djlactin July 2, 2018, 16:25

    “snowball earth”

  • Geoffrey Hillend July 2, 2018, 16:39

    The Earth’s continental drift and carbon cycle explains the snowball Earth periods as well as the obliquity which was believed to be larger in the past but not by very much. These all play a role in those cold periods which were followed by the hottest periods. Mountain ranges absorb Co2 due to basalt and ultramafic rocks so plate tectonics plays a major role. Plants, mountain ranges and the carbon cycle removed the Co2 over the last hundred million years. We do know for sure that the Milankovitch cycles which are based on the Earth’s obliquity and eccentricity are responsible for the ice ages. Fifteen million years ago, there were no polar ice caps on Earth, due to a much large amount of Co2 in our atmosphere so the carbon cycle removed it over time. The obliquity was the same as now at this period of time. The Earth’s obliquity is believed to have been stabilized by our Moon over a large period of time. https://en.wikipedia.org/wiki/Axial_tilt
    The small variation in Earth’s obliquity 2.4 degrees is the result of the gravity of the both the Sun and the Moon on the Earth and the same is true of the ocean tides..

    Mars has a large obliquity and variation which does effect the climate over the past. The obliquity of Mars follows a large cycle of time. When one pole like the north pole is completely exposed to sunlight, then the polar caps of Mars melt so there is a more icy surface on Mars towards the middle latitudes. Mars does not have a large Moon to keep the obliquity from varying a large amount.

    The real problem is that if there is no large Moon, a planet might not have a strong magnetic field to deflect the solar wind which causes the loss of an atmosphere due to it’s stripping. Earth’s iron core is believed to have been made much larger by the it’s impact with Theia which gave up much of it’s iron core to the Earth in the formation of our Moon, the giant impact hypothesis.

    • Nicky July 3, 2018, 3:56

      Magnetic field does not necessary mean the protection of atmosphere from stellar wind. The observed atmosphere escape rates of Venus, Earth and Mars are very close. People might think the magnetospheres deflect charged particles to avoid the destruction of atmosphere. However, it also accelerates the escape rate at two polar caps and cusp to the equivalent rate seen on unmagnetized planets. Complete picture (considering all the escape mechanisms) of non-thermal escape shows that loss rate can be higher on magnetized planets in some cases.


  • Geoffrey Hillend July 2, 2018, 16:43

    The collision of Theia with the Earth also gave Earth a fast rotation and large angular momentum which is essential for a magnetic field since charged particles need to move in circles to create a magnetic field.

    • Rob Flores July 2, 2018, 22:49

      It would seem to be very usefull to get sideswiped to
      create potentially habitable world.

      But since the earth was rotating more than twice as fast early
      in its history, that implies a much stronger magnetic field.

      Would that mean that one reason for the inability of bacteria to
      evolve into prokaryotic cells for billions of years maybe due to
      a minimizing of mutations because energetic particles were mostly
      deflected by said field?

  • Harry R Ray July 3, 2018, 9:55

    UH-OH ON STEROIDS!!! The TRAPPIST telescope is being RE-NAMED SPECULOOS-1! Does that mean that the 7 planets will be re-named SPECULOOS-1–1b, SPECULOOS-1-1c, SPECULOOS-1-1d, SPECULOOS-1-1e, SPECULOOS-1-1f. SPECULOOS-1-1g, and SPECULOOS-1-1h? GAHHHHH!!!!!!!!!

    • ljk July 3, 2018, 13:19

      More information here:



      They really tortured that acronym out of the name, and after some kind of candy, no less. I hope they keep the name TRAPPIST for that exoworld system, as that is the telescope setup which discovered them.

      This makes me wonder: Will there be some kind of official naming system for all exoworlds? Or will it be as it is with the stars, going by several different catalog names?

      • Harry R Ray July 5, 2018, 9:59

        Luca Maltagliani tweeted THIS at 1:45 AM – 4 July, 2018. “Demory: an eight mars sized planet at the outskirts of the TRAPPIST-1 system? Answer’s maybe… there’s something in the data but must be checked by followup observations. If CONFIRMED, it would CLEARLY be Neptune-like(3.75-4 Re). A bit baffled as to why he chose “Mars sized” instead of “Earth-sized”.

        • Harry R Ray July 5, 2018, 12:45

          OOPS: Matagliati, NOT Maltagliani. Sorry about that. NOW: about that “Mars sized” versus “Earth sized connundrum: http://www.exoplanetary.cz interprets this as the EIGHTH TRAPPIST-1 planet with a size roughly that of Mars. We’ll just have to wait to see who is right.

      • Harry R Ray July 5, 2018, 11:51

        Rames Ramirez was right! NEW REVISED masses of TRAPPIST-1 planets make planets b, c, d, f, g, and h CONSIDERABLY LESS volatile rich(MUCH CLOSER to the ORIGINAL estimates than Grimm et al’s estimates). The ONLY EXCEPTION is e, which is now considerably MORE volatile rich! LIST BELOW: Dr Ramirez: Are you and your collegues okay with this now. Is this due to BETTER chi squared fits, or just more data? RSVP. LIST: TRAPPIST-1b 1.22 Earth masses, TRAPPIST-1c 1.24 Earth masses, TRAPPIST-1d o.37 Earth mass, TRAPPIST-1e 0.66 Earth mass, TRAPPIST-1f 0.97 Earth mass, TRAPPIST-1g 1.27 Earth masses, TRAPPIST-h 0.37 Earth mass.

  • Alex Tolley July 3, 2018, 10:19

    The issue of ecosystems adapting to local changes in climate due to changes in obliquity is similar to the issue of adapting to the slow increasing in solar output that the Earth has experienced. Lovelock’s Gaia hypothesis was a general solution where organisms drove much of the mitigating changes needed, for example changing albedo (Daisy World) and reducing the greenhouse gas CO2.
    How the Gaia hypothesis might work beyond toy models is the subject of a new paper. The PR release explains the idea. Once life has established itself on a world, it may be able to stabilize the effects of changing obliquity by a local version of the GH.

  • Harry R Ray July 3, 2018, 10:53

    Andrew LePage: Kepler 1652b has been FINALLY ADDED to HEC, BUT, its radius has been REVISED DOWNWARD from 1.6 Re to JUST 1.1 Re, and it is STILL in the habitable zone(now THIRD on the ESI list) at Teq: 229 Kelvins(formerly 268K), EXACTLY THE SAME AS Proxima b! WHAT GIVES?

    • Harry R Ray July 3, 2018, 12:18

      What’s even more bizarre is that previously, Torres et al estimated the MASS of Kepler 1652 to be roughly SEVEN TIMES the mass of the Earth, giving it a density of roughly 9gcm3, making it similar to LHS1140 b in composition, rather than a mini-Neptune. Has this now been revised downward TOO, or do we have our first super-dense CHTHONIAN planet in the habitable zone?

  • Geoffrey Hillend July 3, 2018, 17:25

    Quote by Nicky: “Magnetic field does not necessary mean the protection of atmosphere from stellar wind. The observed atmosphere escape rates of Venus, Earth and Mars are very close.” I looked at your reference and the paper does give any real scientific evidence to support for what the title claims so it is not correct. “Why an intrinsic magnetic field does not protect a planet against atmospheric escape.” It mentions Jupiter and Hydrogen but hydrogen always escapes from Earth sized or smaller bodies. Also it admits that any oxygen loss from Earth from the solar is not significant. Oxygen loss is one method how we know that Venus and Mars have lost much more water and oxygen than Earth through solar wind sputtering. It is called the DH20 ratio. The heavier deuterium, an isotope of hydrogen gets left behind in sputtering so Venus and Mars have more DH20 or a higher ratio of DH20 to H20. The ultra violet light splits the DH20 and H20 molecules into hydrogen and oxygen and the lighter hydrogen escapes and the heavier deuterium is left behind. The same thing happens with the C12 and C13 ratio. Co2 is split by ultra violet radiation into C and O2 and the lighter C12 escapes through sputtering and heavier C13 stays in the atmosphere. The paper is deceptive because it implies that a magnetic field does not protect us from solar wind sputtering which would violate what is accepted mainstream in planetary science by scientists.

    • Nicky July 3, 2018, 22:26

      I wonder if you have actually read it.
      The paper clearly says “a planetary magnetic field protects the atmosphere from sputtering and ion pickup” with models that predict high loss rate for unmagnetized planet.
      The paper is arguing for the enhancement of two other mechanisms—polar cap and cusp escape—by magnetic field, which can pull up the overall loss rate to the equivalent rate of unmagnetized planet.

      Many other scientists, including Roby Barnes, have noted the uncertainty of magnetic field (whether harmful or helpful).

      Also, D/H ratio is not a valid argument against this, for water can be lost through non-magnetic field-related mechanisms like Jeans escape or hydrodynamic escape. For Venus and Mars, high temperature and low gravity would both result higher water escape rate producing large D/H ratio, which has nothing to do with magnetic field. Mineral hydration can also affect D/H ratio.
      Earth might have lower carbon escape rate compared to Mars (depend on species), but their overall escape rates are comparable.

  • Geoffrey Hillend July 3, 2018, 17:47

    Quote by Rob Flores: “But since the earth was rotating more than twice as fast early in its history, that implies a much stronger magnetic field.” Absolutely. I like that idea. The magnetic field would not be that much stronger, but the important thing is that it is still strong enough to block the solar wind so sputtering is not a problem.

    As far as mutations are concerned, there are still high energy interstellar cosmic rays which collide with our upper atmosphere and create relativistic muons which can penetrate tens of meters into rocks and other matter.” Wiki, muon. Also there is harmful UVB and UVC and some radioactivity in foods which our magnetic field does not block.

  • Geoffrey Hillend July 3, 2018, 18:16

    C12 and C13 ratios show how Mars has lost much of it’s atmosphere to sputtering. https://www.nasa.gov/feature/jpl/msl/loss-of-carbon-in-martian-atmosphere-explained Consequently, Venus and Mars have lost much more atmosphere due to solar wind sputtering than Earth. There is a higher percentage of DH20 in Mars and Venus atmosphere than in Earth’s

  • Geoffrey Hillend July 4, 2018, 16:00

    Venus has a gravity and escape velocity comparable to Earths. Temperature plays a role, so one could argue Venus is hotter because it is closer to the Sun, so gas molecules will be propelled faster, but Venus has lost a lot of it’s atmosphere due to solar wind stripping(sputtering) due the the fact that Venus does not have a magnetic field. It is the solar wind stripping that causes the heavier hydrogen DH20 to be left behind on Venus. The lighter hydrogen gets accelerated away from Venus by the electric field of the solar wind. The same process happens on Mars. The deuterium is freed by the splitting of the DH20 molecule by ultra violet radiation into D and O2, and the same with the H20 for free hydrogen and oxygen.

    • Nicky July 5, 2018, 1:31

      Well, obviously now you are deviated from mainstream, because Venus’ water was lost through hydrodynamic escape not sputtering.

      Because of the close distance, Venus’ early magma ocean lasted for tens or even hundred million years, in which the steam atmosphere outgassed by the cooling magma must be totally driven off so the magma ocean can cool off. In this process, the solidification of Venus must be accompanied by rapid loss of water. It is natural to have high D/H ratio.

      Besides the hydrogen lost by hydrodynamic escape, Venus atmosphere really has not lost much, not like you said “a lot”. The 90 bars of CO2 is equivalent to the amount carbon stored in Earth’s mantle and carbonates. Magnetic field really doesn’t help that much.

  • Laz July 4, 2018, 21:17

    What could a Kardashev Type II Civilisation do to nudge a planet into a more favourable tilt?

    • ljk July 5, 2018, 12:54

      Without causing a major catastrophe such as flinging large space rocks at it?

      • Laz July 5, 2018, 21:04

        Though I do have a primal urge to solve problems by smashing large rocks I’m after more sophisticated options.

        • Mark Zambelli July 12, 2018, 11:21

          Asking it nicely won’t work, soooo….
          One (slow) idea… place a large moon/satellite (sprinkled liberally with degenerate matter to ‘up’ the mass) into a high inclination orbit. Keep it there under its own power much like a gravity-tractor and as the moon won’t be allowed to move down to the equatorial plane, the equator will slowly rotate up to the moons’ orbital plane.

          Can’t see any glaring probs but I never passed my KII Entrance Exams so what do I know!

  • Geoffrey Hillend July 5, 2018, 17:54

    Quote by Nicky: Well, obviously now you are deviated from mainstream, because Venus’ water was lost through hydrodynamic escape not sputtering.” Nicky, I think that hydrodynamic processes dominated the early atmosphere of Venus when the atmosphere had more water in it. So you are right about the hydrodynamic escape of water. D/H ratio depends on how faster the hydrogen escapes. In a hydrodynamic process, if hydrogen escapes fast enough, then there won’t be much deuterium, but after the oceans were gone, then we can have a D/H ratio.

    Venus actually had an atmosphere that was much more denser than 90 bars in the past and maybe three times that amount and it lost it’s atmosphere through solar wind stripping. In fact every day Venus losses a lot of atmosphere due to solar wind stripping, but I forget the amount. It has so much atmosphere that the loss it not noticed but it adds up over a long period of time. From the book A Scientific Exploration of Venus, by Taylor, Venus. I forget the page number. The hydrogen escapes easily and can take Deuterium with with, but that does not account for the loss of the other gases like oxygen which is left. The hydro in hydrodynamic means hydrogen. NASA thinks that the oxygen and loss of atmosphere over time is cased by the solar wind stripping of Venus atmosphere which has an electric field five times stronger than Earth’s. The solar wind has a magnetic field built into it due to charged particles in motion. As they pass the atmosphere of Venus, it creates an electric field and accelerates the charged particles of Venus’s uppers atmosphere into space at escape velocity. The process can be compared to a dynamo motor since a magnetic field will accelerate charged particles in wire like in a motor.
    NASA thnks that Venus and Mars have lost a lot of oxygen and atmosphere due to solar wind stripping. https://www.nasa.gov/feature/goddard/2016/electric-wind-can-strip-earth-like-planets-of-oceans-atmospheres And of course, oxygen loss from a planet shows how much water has been lost since H20 is split by ultra violet light into Hydrogen and oxygen and the D/H radio shows how much hydrogen has been lost and how much water has been lost.

    • Nicky July 6, 2018, 5:21

      You are saying hydrodynamic escape does not leave a significant mark in D/H because hydrogen escapes with the deuterium which makes the ratio the same?
      I see your point, and that is exactly why there is a fractionation factor to account the efficiency of D and H escape in the equation for Rayleigh fractionation of hydrogen. If fractionation factor = 1, D/H would not change, and if fractionation factor < 1, D/H would be elevated. Hydrodynamic escape could have enriched D/H on Venus as the escape drains out the water. The question is what is the initial D/H (we don't know, some calculations assume an Earth seawater value)? If we don't know the initial value, it is impossible to judge how much water was lost and how much D/H was elevated. Currently, hydrodynamic process and nonthermal (involving stellar wind and magnetic field) escape both can explain the Venus' present D/H. The mainstream prefers the first one for it is so well established.
      REF: doi.org/10.1016/0019-1035(83)90212-9
      REF: doi.org/10.1017/9781139020558.014

      Next is the 3 times higher pressure in the past. You say early Venus atmosphere had way higher pressure, but you didn't say what constituted it? Maybe a 270 bars of think high pressure steam/water-rich atmosphere derived naturally from the solidification of magma ocean, the steam was definitely not primarily lost by wind stripping or any other nonthermal escape.
      The hydrogen in the atmosphere was lost through hydrodynamic process for sure.
      So what happened to the oxygen that was left behind after hydrodynamic escape? Again, it was definitely not lost through wind interaction because the loss is insignificant in models (see doi.org/10.1038/nature06434). . In fact, the remained oxygen did not escape but was absorbed by a process called crustal oxidation. Crustal oxidation can consumed as much as 3 oceans of oxygen in 3 billion years.
      REF: doi.org/10.1017/9781139020558.014
      You cited NASA research to support that the oxygen was lost through wind interaction. That is true. But the article is about answering among all the escape mechanisms which one is the dominate one ("Understanding what processes govern atmospheric escape and the loss of planetary water")? Not about where all the oxygen went.
      REF: doi.org/10.1002/2016GL068327

      Simulations coupled with satellite-based measurements show nonthermal oxygen escape rate on Venus is 5-10*10^25/s; on Earth is 4*10^25/s; on Mars is 5*10^24/s. In term of nonthermal escape, Earth and Venus on similar on the same magnitude, and Mars is one magnitude lower though Jeans escape, a thermal escape not included, is the main contributor of Marian escape.
      REF: Origin and Evolution of Planetary Atmospheres
      REF: doi.org/10.1051/0004-6361/201832934

      Magnetic field helps deflecting charged particles to prevent sputtering, ion pickup, and detached plasma clouds, which would be important escape mechanisms on unmagnetized planets. However, two drawbacks of magnetic field should not ignored.
      In polar caps region where field is open, fast ascending electrons can set up an ambipolar electric field to accelerate the speed of atmospheric ions and greatly increase the overall loss rate of magnetized planets.
      REF: doi.org/10.3847/2041-8213/836/1/L3
      REF: doi.org/10.3847/2041-8213/aa7eca
      REF: doi.org/10.1051/0004-6361/201832934
      Wind plasma carried kinetic energy into cusps, the extension of polar caps to the dayside, and this energy heats ionosphere to cause ion escape. The escape rate is propositional to the size of cusps, which is propositional to magnetic field strength. The stronger the field is, the faster the escape rate is.
      REF: doi.org/10.1051/0004-6361/201832934

      These two drawbacks make up the slow escape rate of sputtering and ion pickup, so the total atmospheric loss rate of a magnetized planet (Earth) is not different from an unmagnetized planet (Venus/Mars).

  • Geoffrey Hillend July 5, 2018, 18:05

    And of course, it is the Earth’s magnetic field that protects our Earth from the solar wind so it’s particles, the protons and electons become deflected by the magnetosphere or trapped in the Van Allen belts. https://www.zmescience.com/space/mars-magnetic-tail-0423/


  • Harry R Ray July 9, 2018, 17:29

    FINALLY, an Update on LHS 1140b, and it MAY not be good. Toumei et al published ANOTHER SOLUTION to EXISTING rv data and ame up with a THREE PLANET solution, with the 25 day planet’s mass SIGNIFIGANTLY REDUCED!!!!! However, this is NOT a claim of a detection of ADDITIONAL PLANETS(orbital periods of ~4 and 90 days)because the 4 day planet has NOT been detected TRANSITING the planet, making the 3 planet solution MURE CONTRIVED than the one planet solution.

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