Given our intense scrutiny of planets around other stars, I find it interesting how little we know even now about the history of our own Sun, and its varying effects on habitability. A chapter in an upcoming (wildly overpriced) Elsevier title called The Archean Earth is informative on the matter, especially insofar as it illuminates which issues most affect habitability and how the values for these vary over time. It’s also a fascinating look at changing conditions on Venus, Earth and Mars.
We know a great deal about the three worlds from our local and planetary explorations, but all too little when it comes to explaining the evolution of their atmospheres and interior structures. But it’s important to dig into all this because as Stephen Kane, director of The Planetary Research Laboratory at UC-Riverside and colleagues point out, we seem to be looking at the end state of habitability on both Mars and Venus, meaning that our explorations of these worlds should yield insights into managing our own future. Powerful lessons are likely to emerge, some obvious, some obscure.
The chapter title is “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future” (citation below), from which this:
Resolving the numerous remaining questions regarding the conditions and properties of our local inventory of rocky worlds is… critical for informing planetary models that show how surface conditions can reach equilibrium states that are either temperate and habitable, or hostile with thick and/or eroded atmospheres.
Given the challenges of working at the Venusian surface, Mars is particularly valuable as a case study, a world that has evolved through a period of habitability (is this true of Venus?) and undergone interior changes including cooling, not to mention loss of atmosphere. The more we learn about the evolution of rocky worlds, the more we will be able to apply these discoveries to planets around other stars. We’re in search of models, in other words, that show how planets can reach equilibrium as habitable or hostile.
The Archean eon (roughly 4 to 2.5 billion years in extent) takes us through the formation of landmasses, Earth’s early ‘reducing’ atmosphere (light on oxygen but rich in methane, ammonia, hydrogen carbon dioxide) and all the way to the emergence of life. So we have plenty to work with, but for today I want to focus on one aspect of habitability that we haven’t discussed all that often, the changes in the Sun since Earth’s formation. Our G-class star had to progress through a protostar stage lasting, perhaps, 100,000 years or more, then a T-tauri phase likely lasting 10 million years. There’s a lot going on here, including shedding angular momentum as the young Sun expels mass, slowing its rotation rate and producing a powerful solar wind.
The Sun’s arrival on the main sequence follows, and we’re immediately involved in a singularly vexing astronomical puzzle. The consensus seems to be that the early main sequence Sun was only about 70 percent as bright as the star we see today. That being the case, how do we account for the fact that water appears early in Earth’s history in liquid form on the surface? The expectation of a frozen world is a natural one, yet liquid water at the surface seems to have been available for some four billion years.
Image: The landscape during the early Archean could have looked like this: small continents on a planet mostly covered by oceans, illuminated by the much fainter young Sun, and with a Moon that appears larger in the sky because of its smaller distance to Earth. Credit: Potsdam Institute for Climate Impact Research.
Infant stars in the T-tauri phase are quite bright, getting their energy primarily from gravitational contraction. When they reach the main sequence and hydrostatic equilibrium sets in, they are considerably dimmer. The Sun enters the main sequence undergoing stable hydrogen fusion, which drives its luminosity. The notion that the Sun was dimmer early in the main sequence derives from models showing a solar core that is less dense and contains less helium. As core density increases as hydrogen is converted to helium, the resultant heating accounts for the steady brightening since.
Carl Sagan was an early voice in exploring the ‘faint young Sun’ problem in relation to liquid water and the matter is still controversial, although a number of possibilities exist. The authors believe the logical way to explain why a fainter star could provide conditions for liquid water on the surface is through the presence of greenhouse gases like methane and carbon dioxide, although the influence of methane is problematic unless, as in recent papers, scientists invoke planetary impacts or a primitive photosynthetic biosphere that amplifies the methane cycle. Whatever the case, mechanisms to regulate habitability are clearly crucial. From the chapter:
It is remarkable that Earth seems to have become habitable within its first few hundreds of millions of years and maintained that state seemingly without interruption to the present. This persistence is a testimony to the power of feedbacks that drive and regulate climate evolution with notable success on Earth in comparison with our Solar System neighbors Venus and Mars. Most important is the silicate weathering negative feedback, whereby rising temperatures coinciding with a warming Sun are offset by increasing rates of CO2 consumption through continental weathering, which is enhanced under warmer, wetter conditions. And we can thank diverse plate tectonic processes and their roles in such carbon cycling for this sustained thermostatic capacity over much of our history.
But let’s return to that very early Sun emerging onto the main sequence. Factors affecting a young star’s luminosity include the rotation rate of the star, and here we have little information (the Sun’s rotational rate in this era is uncertain, although some evidence suggests that it was a slow rotator). But the earliest planets could have been pummeled by radiation in this early state for tens or even hundreds of millions of years, which would have had a dire effect on any atmospheres they had formed. Note this:
…the intensity of the emission depends on the rotation rates of the stars, which are highly variable (from observation of stars in <500 Myr old stellar clusters). “Fast rotating” young stars can reach about a hundred times the rotation rate of the modern Sun, whereas moderate rotators could rotate at about ten times that rate and slow rotators would reach a few times the Sun’s rotation rate (Tu et al., 2015). Corresponding luminosities for young stars in the EUV and X-ray range of the spectrum can reach on the order of a 1000 times present mean solar luminosity at similar wavelengths for fast rotators, a few 100 times for moderate rotators and between a few tens to a hundred times the reference for slow rotators.
We’ve looked before at both the solar wind and solar flares as factors influencing planetary atmospheres and interacting with their magnetic fields, perhaps stripping atmospheres entirely in the case of M-class planets orbiting tightly around their primary. Atmospheric escape takes two forms, the first being thermal loss, where atoms are accelerated to the point where they reach escape velocity. Intense radiation is one but not the only way of doing this, but the effect on an atmosphere can be dire.
Other processes that can adversely affect an atmosphere include ions being accelerated along magnetic field lines by the solar wind or photochemical reactions. You would think that these effects would vary with distance from the star, but as the authors note, the loss estimates for Mars, Venus and Earth are relatively similar, and this despite the major differences in the makeup of the atmospheres on these planets and their magnetic field situation. Mars lacks a protective magnetic field today but seems to have had one in the past. Venus likewise lacks an intrinsic magnetic field like that produced on Earth through its convective dynamo, although interactions between the solar wind and the CO2-rich atmosphere do produce a weak magnetic effect.
Image: Diagramming a solar flare encountering Earth’s magnetosphere. Credit: Jing Liu (Shandong University, China and National Center for Atmospheric Research in the U.S).
As to the Sun, variations in the solar magnetic field over time are assumed given that a faster spinning star would have produced higher levels of magnetic activity. Today’s solar cycle, eleven years long, is the result of the magnetic reversal cycle of the Sun. The twists and tangles of magnetic field lines during this cycle cause sunspot activity to peak and then decline as the north and south magnetic poles reverse. The Sun’s magnetic field strengthens and weakens during the cycle, with effects apparent in the solar wind itself and in spectacular events like coronal mass ejections.
If we back out to the largest timescales, the variations in the Sun’s magnetic field indicate a much stronger field early in the Sun’s evolution, coupled with a (presumed) faster stellar rotation rate. From the standpoint of habitability, that tells us that ultraviolet and X-ray flux along with the solar wind has been decreasing with time. We’re probably talking about orders of magnitude difference in UV and X-ray radiation, but the authors point out that the intensity of emission depends upon the rotation rate of stars, and there is much we still have to learn about both this and luminosity.
The high degree of uncertainty on the Sun’s rotation rate and temperature affect our estimates of the ultraviolet at X-ray emissions our star would have produced, and thus the impact of this flux on the atmospheres of the terrestrial planets. Enlarging our field of view to exoplanets, it’s clear that the more we can piece together about the Sun’s early history, the more we can learn about the atmospheres and habitability of planets around the various spectral types of stars. Kane and team see the Sun as a template for planetary evolution, which makes heliophysics crucial for the study of astrobiology.
This study, then, homes in on the factors that need to be clarified as both heliophysics and exoplanetary science move forward. Even the closest planet to us presents formidable challenges, and the authors’ discussion of Venus is well worth your time. Here we have a world whose young surface – relatively uniform in the range of 250 to 1000 million years old – makes digging into its more remote past all the more difficult. Evidence can only be indirect at this stage of planetary exploration. We’re lucky to have Mars within reach as we widen the study of planetary formation to worlds other than our own.
The chapter in the Elsevier book is Kane et al., “Our Solar System Neighborhood: Three Diverging Tales of Planetary Habitability and Windows to Earth’s Past and Future,” available as a preprint. Two Sagan papers on the faint young Sun paradox are notable: Sagan & Mullen, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures”, Science 177 (1972), 52 (abstract) and Sagan & Chyba, “The early faint sun paradox: Organic shielding of ultraviolet-labile greenhouse gases,” Science 276 (1997), 1217 (abstract). You might also want to look at Ozaki et al., “Effects of primitive photosynthesis on Earth’s early climate system,” Nature Geoscience 11 (2018), 55-59 (abstract), which offers up a methane solution for the faint young Sun.
From the post, it is not clear if the fainter young sun is also potentially emitting much more shorter radiation – UV and X-rays. If so, isn’t this a problem for the “warm surface pond” location for abiogenesis?
The assumption of a faint sun resulting in a frozen surface has been mitigated by assuming a larger H2 envelope to warm the planet. I am now wondering if the X-rays and UV increased photolysis, creating more H2 , but paradoxically oxidizing the CH4 to CO2, so both increasing and decreasing the GHG effect.
Lastly, given the Earth was very hot from accretion and bombardment, could a frozen surface overlay a liquid ocean kept warm by the heat of the rocks. with the surface ice acting as an insulating surface, even as the albedo increased?
Whatever the surface conditions, it seems to me that the case is strengthened for a deep ocean origin of life if it truly appears so quickly after the formation of the Earth.
Alternatively, could warmer Venus have been the site of abiogenesis, with its life migrating to Earth on the solar wind and taking hold once the surface was warm enough for exposed liquid oceans? This would not change the apparent genomic data for the age of LUCA. Unfortunately what hope is there for understanding Venus’s early state?
Will exoplanet data help resolve the problem as we could get data for a range of stellar ages and planetary conditions
As we collect data on the geological and atmospheric history of our planet – and its sun, it would certainly lead one to wonder how, by actually existing now, we have somehow rode atop a very unstable wave . Because, if we were baked like a cake, the 4.5 giga year preparation scheme must have been unseemly at the least with such elaborate preliminaries. Unless there is some other element in our kitchen of our birth that makes this a likely outcome.
In the American Association for Advancement of Science (AAAS) journal “Science”, the work of planetary scientist Robert Hazen was reviewed this week, based on the premise that evolution could be considered as a process beyond simply biology, but in inorganic processes such as planetary and even stellar evolution ( p. 608 8 NOVEMBER 2024 • VOL 386 ISSUE 6722). Indeed, in that last case, there are textbooks and classes with that name, though seldom bringing up Charles Darwin. But in this Hazen did. That Darwin did not apply his premise far enough.
Having listened to Hazen’s lectures over the past decade in the context of the early solar system and the settings for life’s origin, it does seem that he has a point. At the very least, he does highlight how the Moon, Mars and Earth’s chemical complexity contrasts in ascending complexity. Clearly the presence of life on Earth has shaped its surface and interior to some depth. With Mars it might have been more complex in this context eons before with some evidence eroded away. And Hazen a decade ago would have called its complexity intermediate. But the Moon has remained just a few dozen chemical compounds. We don’t have expectations of a Rover finding evidence of an ancient sea, save like Sea of Storms or Clouds smoothed by lava flows.
Hazen, as the article indicates, that in recent years, beside noting the solar system contrasts, contends that this rolling tendency toward chemical and atomic complexity might merit consideration as a fundamental law with an arrow indicator pointed by time from the big start. Otherwise, it is hard to explain why the infall of orbiting dust and gas just happened to result in you and me and our fair city. And likely nowhere else.
Perhaps this idea floats more easily if we do not mention the opposing concept of entropy – but I am sure that Hazen is well aware of its process role as well.
However, introducing this universal tendency to complication as an interpretation of what we see locally in the solar system with collapse from largely hydrogen clouds into stars, dust rings and planets. Now and then it is noted that a mineral wells back up due to plate tectonics, or an enrichment arrives on a comet or meteorite, the absence of which could have delayed another geological era.
It’s not necessarily just in time delivery for manufacture. But it does seem built into a larger system than a self absorbed Earth.
And with a planet as large as Neptune or Jupiter, we still have no idea what kind of things could be produced in such a cauldron, warmed enough and sustained at some stratum – a biochemical equilibrium of some sort? In those instances, it would seem that if life arose, the odds would be for it to remain in isolation owing to gravity and depth – unless it obtained remarkable features to allow its exit from that environment. Mars precursors for Earth have been argued. But if one were a martian, would not Jupiter and the asteroids have some grip on your notion of geological history? Likely more than the Earth. But attributing life on Earth to earlier life on Mars, shifts the whole argument back to one more planet, like placing a turtle under an elephant in some earlier cosmic display.
What is also persuasive about Hazen’s argument is that this growth of complexity is phased. If the sun or star and its surrounding cloud of gas, dust and proto planets is not fit for habitation for anything eukaryotic, then there are still a lot of precursor processes going on which could produce bio-favorable results
– later. For most of geological time, the Earth was uninhabitable for “you or me”.
Only about a tenth of its history appears to devoted to life with many cells
or specialized nuclei. Having developed to address the opportunities of an immediate environment…Then why didn’t the microbes just leave it at that?
The complication imperative?
So it leaves one to wonder about the structure of a certain domain for advance of complexity or the reverse of entropy …Where should we draw the line? The Earth? The solar system to its widest frontiers? The similar portions of the galaxy. Of many other galaxies? The Drake equation was focused largely on the communications task between us and radio broadcasting LGMs. But so many of those intermediate factors rest on the answer to the question that Hazen has come around to argue for in positive terms. At the very least, not everything about our origin had to be against nearly insurmountable odds, but perhaps due to pervasive features of the cosmos we live in.
And during our stay on this planet and its near environs we dream of complicating the cosmos just a bit further.