Centauri Dreams
Imagining and Planning Interstellar Exploration
The Oort Cloud and Close Stellar Encounters
If we assume that the Oort Cloud, that enveloping shroud of comets that surrounds our Solar System and extends to 100,000 AU or beyond, is a common feature of stellar systems, then it’s conceivable that objects are interchanged between the Sun and Alpha Centauri where the two clouds approach each other. That makes for the ‘slow boat to Centauri’ concept I’ve written about before, where travel between the stars essentially mines resources along the way in migrations lasting thousands of years. The resulting society would not be planet-oriented.
When the Dutch astronomer Jan Hendrik Oort deduced the cloud’s existence, he theorized that there was an inner, disk-shaped component as well as an outer, spherical cloud, as shown in the image below. The outer cloud is only loosely bound to the Sun, making the interchange of cometary materials between stars a likely event over the aeons, while gravitational nudges from passing stars can dislodge comets in the other direction as well, causing them to move toward the inner system. Most long-period comets probably come from the Oort Cloud.
Image: Artist’s impression of the Oort cloud. The density has been hugely exaggerated. (c) Pablo Carlos Budassi [CC BY-SA 4.0] via Wikimedia.
I should also mention that we can find the Oort Cloud concept, though obviously not that name, discussed in the work of the Estonian astronomer Ernst Öpik in 1932, which is why you sometimes see the cometary cloud referred to as the Öpik–Oort Cloud. Compared to it, the Kuiper Belt is practically in our backyard at 30 to 50 AU. We’ve pushed probes into the Kuiper Belt, but it’s a matter of pure speculation when we’ll have the technologies to reach the Oort, though of course any future interstellar probes will need to pass through this region. Not to worry, though; comets out here are presumed to be tens of millions of kilometers apart.
At Leiden University in the Netherlands, Simon Portegies Zwart has run simulations mapping the growth of the Oort Cloud that are to be presented in a forthcoming paper in Astronomy & Astrophysics. The work confirms the idea that the Oort is a remnant of the protoplanetary disk from which the Solar System grew some 4.6 billion years ago. While much of it comes from comets ejected from the young Solar System by the gas- and ice-giant planets, the research team suggests that a second population of comets comes from other stars.
Bear in mind that at the Sun’s birth, numerous other stars would have been nearby, from which objects in their circumstellar disks could have been exchanged, along with free-floating debris in the parent star cluster and other interstellar objects. Indeed, a high percentage of the Oort Cloud’s material could have come from such sources, as the paper notes: “About half the inner Oort cloud, between 100 and 104 au, and a quarter of the material in the outer Oort cloud ? 104 au could be non-native to the Solar system but was captured from free-floating debris in the cluster or from the circumstellar disk of other stars in the birth cluster.”
Let’s look further into the paper on how mass moves around as the Cloud is formed:
According to Cai et al. (2019) 20–80% (with an average of 50%) of the circumstellar material survives the first 100 Myr of its evolution in the parent cluster. The majority of this mass is lost through encounters with other stars. The amount of material lost from the solar system in the simulation presented here falls in this range, meaning that the circumstellar disk has lost about half its mass due to interactions with other stars in the parent cluster, or about 100 M? to 3000 M?. Each of the other processes results in a mass loss of roughly 20%. A small fraction of the ejected asteroids acquire bound orbits in the Oort cloud.
The transport of material from one star to another is seen in the simulations to be “rather symmetric.” While the Solar System is what the authors call “a copious polluter of interstellar space,” so too is it receiving material from other systems. The authors argue that stars in the Sun’s birth cluster would have experienced numerous encounters with other stars, and that the Solar System shows evidence of both single strong encounters and a series of relatively weak encounters, based on the orbital parameters of Sedna and the complexity of the orbits found in the scattered Kuiper Belt beyond 45 AU.
These simulations demonstrate that the Oort Cloud evolved using materials from numerous sources. Here is how lead author Portegies Zwart puts the matter:
“With our new calculations, we show that the Oort cloud arose from a kind of cosmic conspiracy, in which nearby stars, planets and the Milky Way all play their part. Each of the individual processes alone would not be able to explain the Oort cloud. You really need the interplay and the right choreography of all the processes together. And that, by the way, can be explained quite naturally from the Sun’s birth environment. So although the Oort cloud is complicatedly formed, it is probably not unique.”
The paper is Zwart et al., “Oort cloud Ecology II: The chronology of the formation of the Oort cloud,” accepted at Astronomy & Astrophysics (preprint).
Analyzing White Dwarf Debris Disks
You can blame H. G. Wells’ The Time Machine for my interest in the Earth’s far future. That swollen red Sun at the end of the novel created vivid ‘end of the world’ scenarios for me as a boy, and later I would learn that outer planets or moons around a G-class star might turn habitable once it became a red giant. But it would only be in the last few years that I learned how robust the investigations into white dwarf systems — the fate of a red giant — have become, and now we’re finding out not only that such stars can retain planets, but can conceivably create new ones through an emerging disk packed with the pulverized dust of remnant materials like asteroids.
Image: This artist’s concept shows a white dwarf debris disk. Credit: NASA/JPL-Caltech.
Jordan Steckloff (Planetary Science Institute, Tucson) has just published a short paper on the matter, looking at how white dwarf debris disks emerge. The disks seem to form only after ten to twenty million years following the end of the red giant phase. And a turbulent phase it is, with stars like the Sun losing a large proportion of their mass before they turn into a white dwarf. The dwarf itself is Earth-sized and only about half as massive as the star that gave birth to it.
The process of mass loss and collapse into a white dwarf exerts a destabilizing effect on those planets that have not already been swallowed up by the red giant (Mercury, Venus, and most likely Earth will be lost this way). Whether or not planets beyond a few AU of the star survive, their disrupted orbits can result in turmoil within the asteroid population of the system, with some of them falling toward the dwarf star, to be pulverized into dust by these gravitational interactions. Steckloff and team noticed the fact that a process of debris disk formation which ought to occur quickly takes shape only after the delay mentioned above. What is it that is holding back the formation of a debris disk for millions of years? The scientist explains:
“We found that this delay is a result of these young white dwarfs being extremely hot. So hot that any dust that forms from a tidally disrupted asteroid rapidly vaporizes and dissipates. We found that this dust only stops vaporizing after the white dwarf has had time to sufficiently cool down, to a surface temperature of approximately 27,000 degrees kelvin (48,000 degrees Fahrenheit). This temperature agrees with observations of these white dwarf systems; all dusty debris disks are found around white dwarfs cooler than this critical temperature.”
Image: This is Figure 2 from the paper. Credit: Steckloff et al.
Thus our own Solar System, several billion years down the road, should see the Sun enter its red giant phase and ultimately be trasformed into a white dwarf, with the result, according to Steckloff, that Jupiter will migrate outward, thoroughly disrupting the asteroid belt and sending enough of the asteroids close to the Sun to begin the formation of a dusty debris disk. While it is true that the formation process of a debris disk like this should favor hot young stars, whose planetary systems are more unstable, the dwarf star must cool first to the needed temperatures.
Steckloff’s material sublimation model shows the interplay of thermal and tidal forces within the regions around a white dwarf where dusty debris disks can form. The temperatures involved vary somewhat depending on the size of the white dwarf, with more massive stars forming disks at somewhat warmer temperatures. There is also the interesting twist that extremely cool white dwarfs may have more than one debris disk. We see this is a system tagged LSPM J0207+3331, which may be hosting disks formed from the disruption of two planetesimals.
This is the coldest, oldest white dwarf known with a dusty debris disk, according to the paper, and there appears to be a gap within the disk which can also be explained another way:
The apparent gap in this disk is curious and unexpected, and, analogous to protoplanetary disks, may point to the presence of a dense planet clearing a gap along its orbit from within the disk (as proposed for the SDSS J122859.93+104032.9 system; Manser et al. 2019) or a planet sitting outside the Roche limit opening a gap via resonant dynamics. Similar dynamical processes may be at work in dusty debris disks around white dwarfs and the ~3 Gyr cooling age of this white dwarf provides ample time for such dynamical processes to occur.
Whichever possibility is true, this system demonstrates that the environment around white dwarf stars appears to be a lively one. The paper gives us insight into how the physical processes at work restrict the formation of dusty debris disks to white dwarfs that have sufficiently cooled.
The paper is Steckloff et al., “How Sublimation Delays the Onset of Dusty Debris Disk Formation around White Dwarf Stars,” Astrophysical Journal Letters Vol. 913, No. 2 (2021), L31. Abstract / Preprint.
Exotic Ice on Europa?
The first thing to say about the image below is that it fills me with anticipation for the imagery that Europa Clipper will acquire when it travels to the Jovian moon later this decade (arrival in 2030, according to current planning). This is a Galileo image taken in 1996, the subject of intense study, as have been all the Europa images, ever since. How much interaction does Europa’s subsurface ocean have with the icy crust? We can’t say for sure how much is going on now, but images like these show how much fracturing and re-formation there has been in the past. In any event, fresh data from Europa Clipper should give us entirely new insights.
Image: Enhanced image of a small region of the thin, disrupted ice crust on Jupiter’s moon Europa taken in 1996 by NASA’s Galileo spacecraft. Image Source: NASA.
Beyond that, though, there is another story with Europa implications that is being investigated by scientists at Oak Ridge National Laboratory, an installation under the aegis of the U.S. Department of Energy. Because while we can look at the above image and discuss plumes coming up from beneath (and there is some evidence for this), we also have to remember that this may be a different kind of ice than we find occurring on Earth. What we see on Europa and other icy objects in the outer system likely comes under the heading amorphous ice.
The distinction occurs because ice in a cold, hard vacuum shows an atomic structure that is not arranged into a tight crystalline lattice. Somewhere along the line water ice makes a transition between crystalline and amorphous ice phases. Just how this occurs is difficult to observe, but scientists at ORNL, working with colleagues at the Jet Propulsion Laboratory, have been able to lower the temperature of a crystal sapphire plate to 25 K, after which it was placed in a vacuum chamber, where a small number of heavy water molecules (D2O) were added.
The idea was to observe changes in the ice structure with temperature, with plans to simulate outer Solar System conditions by bombarding the sample with electron radiation and charting the effect on the ice structure. Chris Tulk is an ORNL physicist specializing in neutron scattering:
“The experiment produced a layer of amorphous ice similar to the ice that makes up most of the water throughout the universe. This is the same type of ice that could have formed on the extremely cold permanently shadowed regions of the Moon, on the polar regions of Jupiter’s moon Europa, and within the material between the stars in our galaxy, known as dense molecular clouds. Although much of the ice has by now probably crystallized on the warmer bodies, the fresh ice on colder bodies and in deep space is likely still amorphous.”
We know that Europa’s surface is constantly irradiated by charged particles coming off interactions with Jupiter’s intense magnetic field. Just how the ice is changed — and how much of the surface ice turns out to be amorphous — will be factors as we analyze data from Europa Clipper. The laboratory work is also applicable to likely amorphous ices in the Kuiper Belt.
The Spallation Neutrons and Pressure (SNAP) diffractometer at Oak Ridge’s Spallation Neutron Source is the site of these experiments. Here the researchers have configured the instrument to achieve conditions of extreme cold and high radiation like those around Europa. Ahead for the team is to study the changes in amorphous ice as it makes the transition from the crystalline structure, using an instrument called VISION, which performs vibrational spectroscopy, a method that can reveal molecular structure, chemical bonding, and intermolecular interactions.
Image: Scientists created this exotic “outer space” ice by freezing a stream of heavy water (D2O) molecules on a sapphire plate that is cooled to about -414° F in a vacuum chamber. Credit: ORNL/Genevieve Martin.
How much of Europa’s surface is made up of amorphous ice? We may soon have a good indication. Murthy Gudipati is a senior research scientist at JPL:
“This information could help us better interpret the science data from the Europa Clipper spacecraft and also provide some clues about how water ice evolves in various parts of the Universe. With a launch date planned for 2024, the goal of the Europa Clipper mission is to assess Europa’s habitability by studying its atmosphere, surface, and interior, including liquid water beneath the icy crust that could potentially support life.”
Long-term Survey Analyzes Gas Giant Distribution
Back in the 1990s, when the first exoplanet detections were made, the best possible targets for radial velocity searches were what we now call ‘hot Jupiters.’ Radial velocity looks at the Doppler shift of light as a star moves first towards us, then away, tugged by the invisible planet. A massive Jupiter in a tight orbit tugged maximally, and quite often, because its orbit could be measured in mere days or weeks. It was purely selection effect, but it seemed that such planets were common, until we began to discover just how many other kinds of worlds were out there.
Outer-system Jupiters like ours are a different problem. A gas giant in a multi-year orbit produces a radial velocity signature that is far smaller and dependent upon long analysis. Thus, early numbers on the existence of gas giants in the Jupiter or Saturn class and similarly far from their host star are just beginning to emerge as exoplanet science matures. We’ll be learning more — a lot more — but tentative findings from a major survey bear no real surprises.
Indeed, the longest-duration exoplanet survey to date has found that giant planets like Jupiter and Saturn are normally located between 1 and 10 AU from their host stars. From a statistical analysis of the data, about 14 cold gas giants occur per 100 stars in the galaxy, with the number detected around nearby stars indicating that such planets exist in the billions. The Solar System’s planetary configuration, though not the most common, is hardly unusual.
Dating back to the 1990s, the California Legacy Survey draws on data from the HIRES spectrometer at Keck Observatory (High-Resolution Echelle Spectrometer), as well as from the Shane and Automated Planet Finder telescopes at Lick Observatory, with two new papers on its findings now in process at the Astrophysical Journal Supplement. B J Fulton (Caltech), who is lead author on the second of the two papers cited below, is quick to point out the size limitations of planets used in the survey but finds the data on gas giants compelling:
“While we can’t detect smaller planets similar to Neptune and Uranus that are very distant from their stars, we can infer that the large gas giants like Jupiter and Saturn are extremely rare in the outermost regions of most exoplanetary systems.”
Image: This graph of data collected by the California Legacy Survey indicates that most giant planets in the galaxy tend to reside about 1 to 10 astronomical units (AU) from their host stars. An AU is defined as the distance from Earth to our sun, or about 150 million kilometers. This is similar to what we see in our own solar system: Earth orbits at 1 AU; Jupiter is situated at about 5 AU and Saturn at 9 AU. Credit: California Legacy Survey/T. Pyle (Caltech/IPAC).
The scientists performing the survey over the course of its lifetime observed 719 Sun-like stars (classes F, G, K and M), detecting 177 planets through radial velocity methods, 14 of which were new discoveries. The masses detected ranged between 1/100th and 20 times the mass of Jupiter; the catalog of planets is reported in the first of the papers cited below.
The second paper measured giant planet occurrence as a function of semi-major axis, using a hierarchical Bayesian technique. The paper notes that previous studies have likewise found “a strong enhancement in the occurrence rates of these planets around 1 au.” The occurrence measurements beyond 10 AU are consistent with similar findings from direct imaging surveys.
As the authors note, radial velocity methods are now beginning to discover planets in orbital periods comparable to Saturn’s. The finding is that giant planet occurrence is enhanced by a factor of four beyond 1 AU compared to within 1 AU. Planets more massive than 30 times Earth mass are 2-4 times more common at 1-3 AU than 0.1-0.3 AU.
Although the authors are interested in planet formation models that may be derived from this work, they’re looking to future data to widen the sampling of host stars:
Unfortunately, it is difficult to extract significant constraints on planet formation models from semi-major axis distributions alone. Future planet catalogs produced by Gaia and The Roman Space Telescope will help to measure the precise shape of the occurrence enhancement around 1 au with planet samples several orders of magnitude larger, but the stellar samples will be different from ours. We plan for future works in this series to analyze the host star metallicity, eccentricity, and multiplicity distributions of our sample, in the hopes of uncovering evidence that discriminates between different planet formation models.
So determining the distribution and orbital parameters of gas giants in the galaxy is very much a work in progress as we widen the range of radial velocity detections with longer runs of data. There will be more than one generation of work involved in tightening up such numbers.
The papers are Rosenthal et al., “The California Legacy Survey I. A Catalog of 177 Planets from Precision Radial Velocity Monitoring of 719 Nearby Stars Over Three Decades,” (preprint) and Fulton et al., “California Legacy Survey II. Occurrence of Giant Planets Beyond the Ice Line” (preprint), both in process at the Astrophysical Journal Supplement.
Are Planets with Continuous Surface Habitability Rare?
Put a rocky, Earth-sized planet in the habitable zone of a Sun-like star, and good things should happen. At least, that seems to be the consensus, and since there are evidently billions of such planets in the galaxy, the chances for complex life seem overwhelmingly favorable. But in today’s essay, Centauri Dreams associate editor Alex Tolley looks at a new paper that questions the notion, examining the numerous issues that can affect planetary outcomes. Just how long does a planetary surface remain habitable? Alex not only weighs the paper’s arguments but runs the code that author Toby Tyrrell used as he examined temperature feedbacks in his work. Read on for what may be a gut-check for astrobiological optimists.
by Alex Tolley
The usual course of the discussion about planet habitability assumes that the planet is in the habitable zone (HZ), probably in the continuously habitable zone (CHZ). The determination if the planet is inhabitable concerns the necessary composition and pressure of the atmosphere to maintain a surface temperature able to support liquid water. As stars usually increase their luminosity over time, we see charts like the one below for Earth showing the calculated range of average surface temperature. Atmospheric pressure and composition can be modified to determine the inner and outer edges of the HZ or CHZ.
Image: Life on an exoplanet in a globular cluster. Credit: David Hardy (Astroart.org).
However, these charts say nothing about the various factors that may upset the stability of the climate, especially the geological carbon cycle, where volcanic outgassing of carbon dioxide (CO2) is approximately balanced by weathering of rocks to ultimately sequester carbon as carbonate rocks such as limestone. Imbalances can create significant changes in greenhouse gas (GHG) composition of the atmosphere with temperature impacts [4].
Figure 1. Possible bounds of Earth surface temperature over 4.5 By of changing solar output and the impact of atmosphere. The early Earth would have required a different energy trapping atmosphere to maintain an inhabitable temperature during its early history to prevent freezing. Continued increase in solar luminosity will render the surface too hot for life even with no atmospheric trapping of the sun’s heat. Source: Kasting 1988 [3]
We have much evidence of widely fluctuating average surface temperatures for the Earth, from a possibly hot Archaean eon, several local temperature maximums including the end of the “Snowball Earth”, the Permian/Triassic extinction, and the Eocene thermal maximum. Earth has also cooled, most notably during the “Snowball Earth” period that lasted millions of years and several extreme glaciations over its history.
Before even considering the many other possible factors that may preclude an inhabitable planet, there is a question of just how stable are planetary surface temperatures, especially when subjected to shocks due to excess CO2 emissions from very active volcanism, or conversely from excess weathering depleting the atmospheric CO2 pressure.
The Journal Paper
A new paper [1] by Prof. Toby Tyrrell looks at this question in a very different way. He posits that for the many planetary types and conditions in the galaxy, we should assume a wide variety of possible temperature feedbacks and simulate the average surface temperature of a planet over a long period of geologic time (3.0 By) to simulate how frequently planets could continuously maintain an inhabitable surface temperature throughout this period.
Tyrrell on randomly configured feedbacks:
“It is assumed that there is no inherent bias in the climate systems of planets as a whole towards either negative (stabilising) or positive (destabilising) feedbacks. In other words, it is assumed here that the feedback systems of planets are the end result of a set of processes which do not in aggregate contain any overall inherent predisposition either towards or against habitability.”
His model is very simple. He assumes a randomly chosen number of feedback values (change in temperature over time for a specified temperature) within an inhabitable temperature range. Figure 2 below shows one example of the model showing random feedbacks, calculated temperature attractors, runaway temperature zones, and a time course of temperature impacted by random temperature perturbations. The values for temperatures between each feedback node are interpolated from the 2 surrounding nodes. If the feedback slope is negative and if the 2 points straddle a 0 feedback, the model calculates a temperature attractor at that point, so that temperatures between the 2 feedback nodes tend to stabilize the temperature at the attractor position.
If, however, the slope is positive, the temperature will be destabilized and driven towards the upper node if above the 0 feedback level, and conversely to the lower node if below the 0 level. He sets a minimum (-10C) and maximum(60C) surface temperature range that if the calculated temperature extends beyond those boundary temperatures or is in feedback that will lead to runaway feedback to either a very low or very high temperature, the model assumes the planet is no longer habitable on the surface. The model adds 2 other important elements. Firstly there is a long-term forcing (e.g. increased solar output), which for the Earth is a positive one as the sun continues to increase its output over time. The second is to introduce small, medium, and large temperature perturbations (i.e. shocks) that introduce noise into the model and can flip the climate between attractor temperatures and also into runaway temperature conditions where the feedbacks positively reinforce the temperature change. Figure 2 below is extracted from the paper to indicate an example. [Annotations added for clarity.]
Figure 2. Extracted from the Tyrrell paper and annotated. The left chart shows 9 randomly created feedbacks. Where 2 adjacent feedbacks are connected by a negative slope and cross the zero feedback line, a temperature attractor is created, in this example there are 2 attractors. At either end are zones where the temperature would cause a runaway increase or decrease in temperature and these are indicated by grayed areas. The chart also indicates that over the long term, there is a negative forcing that reduces the average temperature over time. The right-hand chart shows the temperature over time. The 2 attractors are shown, as is the starting temperature [blue square]. The various perturbations are indicated both in time and size by the red triangles below the chart axis. The gray bands show the runaway temperature conditions and the black bands the start of uninhabitable conditions. The 500 My display shows the temperature flipping between the 2 attractors, with each flip due to larger temperature perturbations. A few temperature perturbations approach but do not cross into the runaway temperature zones.
With the parameters he uses, the model demonstrates that with repeated runs, only a few percent of planet runs enjoy a 3 billion year period where surface temperatures stay within the inhabitable temperature range. Once the range is exited, surface life ends and the planet becomes lifeless on the surface.
Figure 3 shows how rarely planets can maintain inhabitable conditions over the entire 3 billion year time period.
Figure 3. The probability of a planet always surviving as inhabitable over 3 billion years over several runs with the same feedback conditions but with no temperature perturbations and with random temperature perturbations. The gray (H1 – chance alone) hypothesis is pure random perturbations withwout feedbacks and the red (H2 – mechanism alone) – feedbacks but without large perturbations – are compared with the simulation results. The most important result is that a planet that can maintain surface inhabitable conditions is quite rare.
Tyrrell:
“The initial prospects for Earth staying habitable could have been poor. If so, this suggests that elsewhere in the Universe there are Earth-like planets which had similar initial prospects but which, due to chance events, at one point became too hot or too cold and consequently lost the life upon them. As techniques to investigate exoplanets improve and what seem at first to be ‘twin Earths’ are discovered and analysed, it seems likely that most will be found to be uninhabitable.”
His conclusion is ominous for astrobiologists. Even if we discover many planets that are in the HZ, and confirm that their atmospheres could support an inhabitable surface, those planets are either frozen or too hot to allow life to exist on the surface. The vast majority of apparently suitable worlds will prove lifeless and appear as if abiogenesis (or even panspermia) has failed to ignite an evolutionary progression to complex life and even possibly technological civilization.
This conclusion is enough to dampen any astrobiologist’s day and suggests that the search for biosignatures may be as disappointing as the results of SETI.
While the paper shows the results using the values of the published model and code, the supplementary information includes a considerable analysis of the model, for example, extending the inhabitable range, and several other parameters. However, the broad conclusion remains robust. Maintaining an inhabitable temperature over 3 billion years is unlikely.
Tyrrell acknowledges the simplicity of his approach and suggested in a recent SETI Institute webinar that he hopes to apply his approach with a more sophisticated planetary climate model to determine if his findings hold up.
Given the proxy indications of Earth’s paleotemperatures (see figure 4 below) showing wide ranges and some close misses to survival shown by the mass extinctions, why did Earth life survive? Tyrrell argues that the anthropic principle has to be invoked. Just as the universe we live in needs the exact constants for life and we couldn’t be in any universe without those conditions, so we technological humans cannot investigate unless the Earth had maintained a continuous inhabitable surface temperature.
A Critique
Figure 4 below shows the estimated temperature fluctuations in the paleotemperature proxy data. Have we just been lucky that there do not appear to be any clear multiple attractor temperatures?
Figure 4. A chart of paleotemperature of Earth. For 3 billion years Earth’s average surface temperature has fluctuated in a range of less than 30C.
An obvious question is whether his model reflects reality. The random nature of the feedbacks coupled with the temperature perturbations might lead to many situations where even small temperature perturbations will tip the surface temperature beyond the acceptable range. As we can see from Figure 2, the upper-temperature attractor is within 10C of a runaway temperature increase, making habitability susceptible to even relatively small temperature perturbations.
Fortunately, the model code has been placed online and the source code available to experiment with.
Observing several runs it became clear that the model would quickly fail if the current temperature at an attractor temperature was near the boundary range so that even a modest temperature perturbation could push the temperature outside the range. How serious was this effect?
Figure 5. The most benign model. The 2 feedbacks are at the temperature range extremes and result in an attractor at 25C that is maintained across the temperature range. The long-term temperature forcing is set to 0. Only the infrequent large temperature perturbations, average size 32C are operative.
I created an experiment (also suggested at bottom of page 4 of the paper) where the planet would always have the most favorable conditions for a stable surface temperature. Just 2 feedbacks were created at each end of the range, with the calculated attractor in the middle of the range at 25 C, so that any perturbation would have to exceed 35C in either direction to exit the inhabitable conditions. I removed the long-term feedback too. I also removed both the small and medium-sized perturbations, leaving just the rare, large perturbations. The probability of timing and size of the perturbations was left as per the model. By starting the planet’s temperature at the attractor, the inhabitable conditions would be maintained at the attractor temperature unless a random large perturbation exceeded the 35C size.
The results for different perturbation probabilities are shown in figure 6. The average survival time of planetary runs and the %age survival plotted against perturbation probability demonstrate what might be intuitively guessed. Tyrrell’s purely mechanistic run (H2) with optimal feedback and no perturbations had all planet runs complete the 3 By survival. This is consistent with figure 6 where expected large perturbations = 0.
Figure 6. Survival times and %age survival of planets without an attractor and with a single attractor at 25C. With an attractor, survival times are greatly enhanced, especially as the expected number of perturbations increases.
Inspection of the model’s large temperature perturbation distribution indicated that the average size was 32C with a standard deviation of 16C. For the stable model planet I was testing, the temperature would be perturbed beyond either range boundary by a value of just 0.25 standard deviations, i.e. that about 40% of all randomly selected sample perturbations would trigger a surface temperature outside the inhabitable range. When that happened depended on the random timing and would dictate the survival time of inhabitable temperatures. As a control to determine the frequency of perturbations, a model world was created with no attractor temperature so that it sat on a temperature knife edge. Any perturbation would cause runaway heating or cooling. The impact of the lack of the stabilizing temperature attractor on survival time and average % of planets surviving for 3 By is evident.
Given the importance of the large perturbations, just how reasonable are the size of the perturbations and the maximum inhabitable temperature range.?
It is hypothesized that the “Snowball Earth” temperature ranged from deepest glaciation to a temperature maximum could have been as high as 100C (-50C to 50C). The chart of paleotemperature suggests for over the last few billion years an average surface temperature range of 26C (-10C to16C). That the Cryogenian glaciation period encompassing the “Snowball Earth” could have had a surface temperature of -50C, yet life quickly reemerged more vigorous than ever (the Cambrian “explosion”) after the glaciers melted, suggests that the lower temperature bound of -10C may be too conservative. As for the upper bound, it has been suggested that the Archaean eon may have had surface water temperatures of 70-80C. While most complex life has an effective upper limit of 60C, extremophiles have been found at 122C. For complex life, while the resilient tardigrades can withstand extreme temperatures for short periods, the inhabitable surface range is reasonable for complex life. However, we should bear in mind that ecological refugia can provide safety for complex life, for example around undersea vents to resist freezing, and migration to the poles to escape the equatorial temperatures and hence live in regimes that remain below the average surface temperature.
These points were acknowledged in the Tyrrell paper that discussed the limitations and caveats to the model.
Tyrrell:
Geographical variability implies that more extreme average global surface temperatures might be required to force extinction everywhere. Microbial life can potentially survive periods of inhospitable surface conditions within refuges, such as in subsurface rocks or deep in an ice-covered ocean at hydrothermal vents, emerging later to recolonise the surface; evidence from Neoproterozoic Snowball Earth events suggests however that eukaryotic photosynthetic algae persisted through the events and therefore that surface habitability was maintained at some locations. Other environmental conditions can affect habitability, but only temperature (and therefore water availability) are considered here.
Dynamic models are often unstable without tuning. The simplest example is Wolfram’s linear cellular automata with 3 cell states determining the next cell state. With just 8 possible rules for the 3 state combinations, there are 256 combinations of rules, yet just 6 (2.3%) do not converge on static states. The random feedback combinations may reflect a similar outcome, but where the majority of conditions will easily slip out of the inhabitable temperature range, rather than the benign experimental planet conditions I tested.
Conclusion
Dr. Malcolm (Jurassic Park):
“Life Finds a Way”
Given the results from my experiment with the optimal feedbacks for a stable climate, if feedbacks are more stabilizing on average than the hypothesized randomly assigned feedbacks, planets with inhabited surfaces possibly may not be quite as rare as the author’s model indicates. Tyrrell notes that average surface temperatures hide the variability of temperatures and exclude possible refugia, such as undersea hot vents, and lithosphere life warmed by the planet’s core. If we can accept that the Earth was populated by unicellular bacteria and eukaryotes for most of its history, and that the Earth’s complex biota may have even taken a major loss during the Cryogenian period, it seems likely that inhabitable worlds will have some sort of life assuming abiogenesis is easily achievable. While our climate history may be a lucky chance, history does not seem to indicate some attractor temperatures, but rather a single attractor that is subject to GHG source and sink imbalances that last for some time. The hypothesized extreme volcanism that ended the Permian resulted in the greatest extinction event in the fossil record and lasted for 2 million years. Our current fossil fuel burning that is increasing the atmospheric CO2 levels while very much like a temperature shock is not believed to be able to cause a runaway heating as happened on Venus. However, it is suggested that sometime in the next billion years, the Earth’s atmosphere will need to have no CO2 to stay habitable. Well before then, autotrophs will not be able to fix carbon and the complex life biosphere will collapse.
Once life starts, it is tenacious. A reset back to extremophiles may well be recoverable given time allowing new complex life forms to emerge under the right conditions and genetic “accidents”. However, there may be many more possible wrinkles to the sustainability of habitability, and eventually, surface life may be unable to survive. For subsurface life, the story may be very different. The lithospheric life might survive all other life until our sun destroys Earth billions of years in the future.
It would be interesting to modify the model so that rather than stopping when the temperature is outside the range, that the instances of these periods are recorded as possible reset conditions for refuge (e.g. lithosphillic) life to restart the evolutionary process rather than assuming the planet is “sterile”.
Time will tell when astrophysicists have cataloged and characterized a statistically useful sample of Earth-like worlds in the HZ that can test the model hypothesis of rare survival of surface inhabitablity over billions of years.
References
1. Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.
2. Ibid Supplementary Information
3. Kasting, James, et al “How Climate Evolved on the Terrestrial Planets”, Scientific American, (1988)
4. Berner, R. A. & Caldeira, K. “The need for mass balance and feedback in the geochemical carbon cycle”. Geology 25, 955-956 (1997).
Seafloor Volcanoes on Europa?
What’s going on on the floor of Europa’s ocean? It’s hard to imagine a place like this, crushed under the pressure of 100 kilometers or more of water, utterly dark, although I have to say that James Cambias does wonders with an ice moon ocean in his novel A Darkling Sea (Tor, 2014). Science fiction aside, Europa Clipper is in queue for a 2024 launch, and we can anticipate a flurry of new studies that feed into plans for the mission’s scientific investigations. The latest of these puts Clipper on volcano watch.
The work deploys computer modeling to show that volcanic activity seems to have occurred recently on Europa’s seafloor. The concept is that there may be enough internal heat to cause melting — at least in spots — of the rocky interior, which would produce the needed results.
How this heating affects the moon is deduced from the 3D modeling of heat production and transfer in the paper, which was recently published in Geophysical Research Letters. The lead author is Marie B?hounková (Charles University, Czech Republic), who describes the astrobiological import of the team’s results:
“Our findings provide additional evidence that Europa’s subsurface ocean may be an environment suitable for the emergence of life. Europa is one of the rare planetary bodies that might have maintained volcanic activity over billions of years, and possibly the only one beyond Earth that has large water reservoirs and a long-lived source of energy.”
Image: Scientists’ findings suggest that the interior of Jupiter’s moon Europa may consist of an iron core, surrounded by a rocky mantle in direct contact with an ocean under the icy crust. New research models how internal heat may fuel volcanoes on the seafloor. Credit: NASA/JPL-Caltech/Michael Carroll.
With massive Jupiter close at hand, it’s no surprise that gravitational interactions should account for heat production in Europa’s mantle, for the rocky interior flexes in the course of the moon’s orbit. The paper drills down into how this flexion operates, where the resulting heat dissipates, and how it results in melting in the mantle.
In Europa Clipper terms, it’s useful to learn that volcanic activity is most likely to occur near the poles, for this is where the most heat is produced from these effects. On top of this, we learn that the volcanic activity likely to be produced here is long-lived, giving life the opportunity to evolve. As an analogue, we can imagine hydrothermal systems like those at the bottom of Earth’s oceans, where seawater and magma interact.
In the absence of sunlight, the resultant chemical energy supports life on the seafloor and could conceivably do so on Europa. Europa Clipper will measure the moon’s gravity and magnetic field, looking for anomalies toward the poles that could confirm the presence of volcanic activity. Long-time Europa specialist and Europa Clipper project scientist Robert Pappalardo (JPL) sees all this as fodder for continuing investigation:
“The prospect for a hot, rocky interior and volcanoes on Europa’s seafloor increases the chance that Europa’s ocean could be a habitable environment. We may be able to test this with Europa Clipper’s planned gravity and compositional measurements, which is an exciting prospect.”
Europa Clipper should reach the Jupiter system in 2030, orbiting the giant planet and performing numerous close flybys of Europa as it surveys the surface, samples any gases that may have been emitted by the exchange of material from below the ocean, and possibly takes advantage of plumes of water vapor. If water is indeed welling up on occasion from below, we may be able to learn a good deal about the interior ocean without having the need to drill down through kilometers of ice.
The paper is B?hounková et al., “Tidally Induced Magmatic Pulses on the Oceanic Floor of Jupiter’s Moon Europa,” Geophysical Research Letters Vol. 48, Issue 3 (22 December 2020). Abstract.
Follow with RSS or E-Mail
