2023.02.03 11:24 Chas-- (10) Bringing It All Together In Astrobiology and Astrophysics: The Problems With Cosmic Rays (In Space), Muons (On Planets), Homelessness and Extinction [and Some House Music]
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The emergence of life awaited the development of habitable environments. Life famously requires energy, water, and nutrients present in sufficient quantities to allow efficient chemical reactions, a key to organic chemistry and metabolism. The existence of a region surrounding a star, usually called the habitable zone, was introduced by Strughold, to define a range of distances for which liquid water on an Earth-like planet’s surface is possible. The formation of a habitable planet is life’s prerequisite. A potentially habitable planet might lose its water, like Mars and Venus, never have water in the first place, or even have too much water.He also summarizes the things that habitability must persistently preclude:
Now we must consider that the planetary environment must survive the effects of its host star(s)’ development. Low mass stars, M-types, have prolonged magnetic activity and thus they have habitable zones subject to bursts of XUV radiation and high particle fluxes. On the other hand, stars with only a bit more mass than the Sun have lifetimes that are far shorter. If complex life generally needs billions of years to develop as on Earth, then complex life cannot be supported by main sequence stars that are about 20% more massive that the Sun. Solar type stars and lower mass K-types both have extended lifetimes and activity levels suitable for complex life. Because of the dangers in the central regions of galaxies and low metallicities in the outskirts, the galactic habitable zone and super-galactic habitable zone concepts were introduced.Come to me now and rest your head
Figure 5: The universe becomes habitable for life in stages. Simple life became possible roughly 9 billion years ago and complex life emerged about 5 billion years ago and then only within the moderate density, Super-Galactic Habitable ZoneI mostly agree, although even in the hot zone closer into the denser galactic nucleus, at the upper end of the density scale, it is possible that there are oceans and complex life in the depths, shielding life from muonic hurricanes by miles of water. And that would hold for the enhanced radiation field inside merging galaxies as well. Muons have a short half life of 1.56μs (~15 hundred feet at ~one foot per nanosecond), which is extended by a factor of 70, due to relativistic time dilation in their early path at 0.9999c at the top of the atmosphere, but death comes quickly for them as they slow down. Remembering the flat pancake thing from Lorentz contraction, the 10km of our thick atmosphere (and the earth itself) appears 70 times thinner to a 0.9999c muon. In the center of all spiral galaxies and in all other large galaxies, those living advanced civilizations will be trapped by that stiff radiation field in those oceans, until their main sequence G2 star ends its life as our sun will, by heating up and expanding into a red giant, encompassing the orbits where life is possible and consuming those planets, like earth will be consumed, before shedding that outer atmosphere as a planetary nebula and becoming a white dwarf.
The emergence of life as we know it relies on several factors. Over time, galactic disks not only allowed for the concentration of elements, but the magnetized galactic wind of disk galaxies also provided protection.Nothing like a strong magnetic field, produced by pesky GCRs, preventing the deadly EGCRs from penetrating our defenses: "They may be bastards, but they're our bastards." - Dwight Eisenhowe5-star General and 34th POTUS.
SEPs: [Solar Energy Particles, the wind from the sun] are responsible for mass loss of planetary atmospheres. However, planets may be magnetically protected. For example, the magnetic field of the Earth detects or traps SEPs, enhancing the habitability of Earth. Mars currently does not have a significant global magnetic field, and its mass is about 10% of the mass of the Earth. Mars still experiences significant atmospheric mass loss due to solar activity as measured by instruments aboard the MAVEN spacecraft. In the early life of the Sun it was less luminous than it is now, however it was also much more active.Such a cozy room the windows
GCRs: SNe [Supernova explosions] are probably not the source of all GCRs, but this detection [the Fermi gamma-ray telescope] suggests that shocks within SN remnants do accelerate particles. [Benyamin] considered the spiral arms of galaxies as the main location of GCR sources. [Aartsen] found a large-scale anisotropy in PeV emission and suggested that this result is consistent with a superposition of flux from a just few nearby sources, likely SN remnants. SN explosions of the most massive stars (above 25M [M: solar masses]) are more important in many aspects, but the standard SNe (stars between 8 and 25M) are the major contributors to CR-protons, and dominate the overall energetics.
EGCRs: Taken at face value, UHECRs from M82 [starburst galaxy] suggest that planets in galaxies with rapid star formation are subject to high fluxes of UHECRs from local sources, like GRBs [gamma ray bursters], microquasars, and relativistic SNe remnants. ... An AGN [active galactic nucleus] likely inhibits life or even eliminates habitability within its host galaxy. AGN might even adversely affect other nearby galaxies as UHECRs are able to break free from the magnetic confines of the host galaxy. ... Radio galaxies are effective particle accelerators.
SFR: [star formation rate] The SFR history plays an important role in the habitability of galaxies. The most massive stars do not last long and explode as SN-Type Ib/c, followed by the chemical enrichment of the ISM [interstellar medium]. However, nearby SNe and GRBs probably have quite adverse effects on life (Gehrels; Thomas), causing mass extinctions, temporary sterilization, and even atmosphere removal if they happen very close.
GRBs: Gamma-ray bursts are stellar explosions that generate enormous radiation beamed in two opposing directions. Local GRBs may occasionally sterilize large portions of the land-based life in the Galaxy. (Annis) argues that GRBs are so critical to astrobiology that the Universe is currently undergoing a phase transition from no intelligent life to intelligence as a result of the reduction of GRBs. While we generally agree that the reduction of the number and luminosity of GRBs is critical to habitability, the timing of this transition is likely not universal. Galaxies with active SMBHs, especially those resulting from mergers, will remain uninhabitable for many Gyr into the future.
SMBHs: Nearby super-massive black holes. As we observe SMBHs in the local Universe, it is clear that many of them remain quiet for a long time, but these inactive SMBH have been active in the past and the right kind of accretion event will turn an inactive SMBH into an active one. AGN [active galactic nuclei] activity as a function of redshift is shown in Figure 4 [also, the poster]. The peak around z = 2 indicates that AGN were most active 10 Gyr years ago. AGN activity dropped by an order of magnitude by the time the Earth formed, about 4.6 Gyr ago, corresponding to z = 0.45, and it has fallen by another order of magnitude since then. The density of EGCRs was dramatically higher in the past not only because the density of CRs sources was orders of magnitude higher, but also due to an important cosmological effect. The co-moving volume of the Universe was smaller by a factor of (1 + z)3 in the past, exposing planets to more direct UHECRs from SMBHs as well as from extragalactic SNe. ... In our cosmic neighborhood we have several SMBH. These are in M31, M32, M81, M94, NGC5128 (Cen A), and in our own Galaxy [Sagittarius A*], all within about 5 Mpc today.
SMBH Mergers from Galaxy Mergers: Galaxy mergers amplify the SN and GRB rates and SMBH activity. During a merger, giant molecular clouds collide, resulting in a considerable increase in the SFR [star formation rate] and its associated SNType II rate. Considering the starburst galaxy M82 ... The SFR in that region is about 10 times that of a normal star forming galaxy like the Milky Way. ... When two big galaxies merge and both contain SMBHs, then their SMBHs also merge. Gergely and Biermann (2009) showed that the spin of the final merged black hole will be aligned with the direction of the angular momentum of the SMBH binary orbit, and so it is in a totally different direction than the original spins of the two SMBHs. This means that while this happens the two jets sweep through the sky [of the new galaxy], cleaning out [killing life across] a large conical solid angle (Gopal-Krishna).
The Galactic center SMBH: The nucleus of the Milky Way emits high energy radiation and particles, potentially harmful to life. Extremely high energy gamma-rays have been detected using Cherenkov telescopes ... UHECRs [ultra-high energy cosmic rays] detected at Earth are probably from accreting SMBHs at the centers of galaxies. The Galactic magnetic field helps to protect us from these EGCRs. ... we see that the Galactic SMBH poses the greatest threat to global sterilization of planetary surfaces in the Galaxy. By analogy, the greatest threat to planets in external galaxies might arise from the presence of local SMBHs. Many if not most galaxies do not harbor SMBHs, so these may be more suitable galaxies for complex life.
The danger posed by individual objects is a strong function of distance and depends on the direction of beamed radiation source. Planetary habitability is especially inhibited when high local SFRs [star formation rates] are coupled with the growth of SMBHs [galactic center super massive black holes].Are illuminated by the evening
The worst threats are those that cause complete planetary sterilization; these are most often local threats as a planet's location is a key to habitability. Life on Earth has avoided the worst catastrophes, since evidence shows the continuous presence of life for 3.5 Gyr. As discussed in the introduction, mass extinctions were quite common on Earth, yet there is no evidence of a complete planetary sterilization. [Actually, the likely collision with Theia (Mars-sized) that created the moon was sterilizing 4.5 Gyr ago.] Mass extinction events on Earth, have paved the way for an increase of evolutionary diversity as new niches became open. A key factor of habitability is the severity and frequency of sterilization and mass extinction events and how readily life is able to recover from them. The expansion of the Universe and the local reduction in the SFR improves the habitability of planets protected by magnetic fields and thick atmospheres.
Life depends on water, and therefore, a catastrophic or gradual loss of atmospheric water can result in a loss of habitability. Here we propose that a planet sufficiently supplied with a protective atmosphere and magnetic field in the mid-disk of the Milky Way is currently more habitable than similar planets in many other galaxies in the contemporary Universe. We argue this because, first, the Sun stays relatively far away from the SMBH in the center of the Milky Way, a major threat. Second, the Galactic SMBH has a relatively low mass and has been unusually inactive for some time, since only minor mergers have taken place in the Milky Way over the last 10-12 Gyr (Gilmore). Third, the galactic disk currently forms new stars at a rate which sustains a global magnetic disk wind, providing an efficient shield against EGCRs without supplying too many GCRs. It is difficult to evaluate the danger of radiation and particle dosage as we do not know what complex life is capable of withstanding. However, we speculate that life, and especially metazoan life on the surface, can be harmed by large doses of particles and radiation. Within this context, we conclude that life as we know it was rare or impossible in the disk of the Milky Way earlier than about 5-6 Gyr ago. An earlier limit, at about 8 Gyr ago, also exists for only microbial life. These temporal limits are illustrated in Figure 4 [also, in the poster]. Habitable conditions probably exist somewhat earlier in some disk galaxies, i.e. those without recent mergers or SMBH activity, like the Milky Way, compared to other more recently active galaxies.
... many additional factors also affect so called habitable zone planets. Planetary system dynamics, host star activity, asteroid and comet impacts are important habitability factors. A planet's geomagnetic, atmospheric, hydrological, and biological history shapes its ability to support life. ... Life on planets have several lines of protection from high energy particles. First, planets in the disk of a star forming galaxy are protected from all but the highest energy EGCRs because of shielding by the galactic magnetic field. Lower energy CRs accelerated from galactic sources, especially SN remnants, become deflected by magnetic fields to be contained in the Galactic magnetosphere. Those, mostly very high energy, EGCRs penetrate the galactic magnetosphere. Both magnetically confined GCRs and the dangerous high energy GCRs with direct trajectories impact the astrosphere [volume of lower density gas around a solar system created by the solar wind]... The astrosphere protects planets from the lower energy (E < 300 MeV) GCRs. A global planetary magnetic field provides the next layer of protection, not only from GCRs but also from the SEPs emitted by the host star(s). The last line of protection is the planetary atmosphere.Sunshine through them fiery gems for you, only for you
Elements: Atmospheric ozone O3 provides a significant UV shield against CRs. It is thought that the complexity and diversity of life on Earth increased as a result of the increase of oxygen in the atmosphere produced by photosynthetic life. This increase of oxygen coincided with the Cambrian explosion in the geological record, 541 million years ago. CRs indirectly cause harm to potentially habitable planets by removing ozone. With the removal of ozone, XUV radiation from the host star may desiccate the planet's atmosphere even if it is in the traditional habitable zone by definition. ... The high abundance of Fe is necessary to establish the planetary magnetic field. Planets without a sufficient Fe core probably cannot maintain a dynamo of sufficient strength to supply magnetic protection against CRs from a variety of sources over biologically relevant timescales. Planets with weak magnetic moments subject to high and intermittently intense fluxes of CRs are not expected to support life on the surface.
Galactic Magnetic Fields: life on planets in the disks of star forming galaxies are protected from EGCRs by a magnetic galactopause generated by the galactic wind. This wind is analogous to the solar wind generating the heliosphere surrounding the solar system. This protective effect of the magnetic field in galaxies is closely connected to the spiral structure. It is driven by SNe explosions within the disk as well the shearing effect from differential rotation of the disk ... Later after CNO abundances increased locally, the SFR slowed, and the associated threats to life decreased. ... Locally, we are just barely above the protective threshold of a strong galactic wind, and thus it is conceivable that potential life in a region not too far from the Galactic plane might not have enough protection from CRs, or suffer from a temporary weakening of that Galactic wind. The same holds true for a planet that is farther out in the disk [towards the edge], than about 10 kpc; out there the conditions to drive a galactic wind may no longer hold.
Astrospheres: The circumstellar environment is dominated by electromagnetic radiation from the host star(s) and stellar winds launched by magnetic fields in the hot corona of cool main sequence stars. ... As the wind escapes it fills an astrosphere - a bubble of outflowing plasma originating as a stellar wind and terminating at a dense hydrogen wall just outside the astropause. In the case of our Sun, the heliopause is about 120 au from the Sun. Protection from host star winds is a prerequisite to planetary habitability, however these winds also protect life on planets from GCRs. ... A strong stellar wind significantly reduces the flux of GCRs, but the higher energy CRs penetrate the astrosphere. ... The Voyager spacecrafts provided the first opportunity to directly measure the interstellar GCR flux. ... The interstellar proton number flux per kinetic energy is 15 times higher than it is at 1 au from the Sun. ... low-mass star winds are weaker and corresponding astrospheres are smaller and less effective at stopping GCRs than winds from higher mass stars. ... On the other hand, a planet in the circumbinary HZ (habitability zone) of a pair of solar like stars and with a binary period of say 25 days, will be exposed to reduced X-ray and UV radiation due to tidal deceleration of stellar rotation in binaries. In addition, a circumbinary astrosphere is produced from a combined wind, thereby reducing GCRs compared to the single star case.
Planetary magnetic fields: ... Venus is too close to the Sun to have surface water, but may have previously been in the HZ when the Sun was younger and fainter. However, Venus rotates very slowly, once every 243 days, and as a result it does not have a global magnetic field. Venus lost its habitability as the result of its proximity to the Sun and it suffered a runaway greenhouse effect. Mars previously had surface water, but its low gravity is unable to prevent atmospheric mass loss. Mars lost its habitability for complex life once it cooled, lost its dynamo, and its atmosphere lost its protection capacity from SEPs [solar energy particles].And 5 Gyr ago was when the Dark Energy acceleration kicked into high gear and the life of the cosmos was extended by earlier cosmic life than ours. But, go ahead, pat yourself on the back, nice work. ;)
... Larger planets of similar composition will generate stronger magnetic fields.
The Atmosphere: ... A team modeled atmospheric effects of the UV and SEP flux from a large flare of the active M dwarf AD Leonis. These effects on an Earth-like planet without a magnetic field in the HZ were simulated. They found that the UV emission from such a flare should not have a significant impact on the atmospheric ozone, however, NO2 produced by ionization from SEP protons may result in a greater than 90% ozone depletion about two years after the flare, with a predicted recovery time of 50 years.
... High energy CRs, especially UHECRs, hit the atmosphere and create an air-shower, as the enormous energy is converted into potentially millions of secondary particles, including muons, neutrinos, electrons, positrons, neutrons, and protons, with muons having the greatest penetration capacity. If the secondary particles are energetic enough, and their flux is sufficiently high, muons can impact even subsurface life. If the radiation dose is too high, the chances of sustaining life as we know it on the planet are very low. A team examined the dependence of the GCR-induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, and found that the latter is the decisive factor for the protection of a planetary biosphere. Thicker atmospheres provide longer path lengths for both primary and secondary CRs, especially lowering the flux of secondary particles at the surface. An atmosphere should be thick enough to provide surface pressure that is sufficient to maintain liquid water (or other solvent), which is required for surface life.
SGHZ: [super-galactic habitable zone] ... complex life is severely compromised on the surface of planets within the central region of large clusters of galaxies and superclusters: ... [Also,] 1) the ubiquity of galaxy mergers, 2) periods of intense star formation experienced by many galaxies either during natural spiral arm formation or as the result of a merger, and 3) activity associated with SMBH accretion and CR acceleration, especially as the result of a SMBH merger. The merger and star formation history is an important factor in the habitability of a galaxy. .We may find, that only relatively small galaxies are able to maintain habitable environments over long times.
... Our own galaxy is probably on the borderline. On the other hand, the metal abundance is correlated with galaxy size, and thus oxygen (e.g., in water) and other life-required elements may not be available in sufficient quantities in small galaxies. So between these two requirements, a) no central SMBH and no extreme central star density (prone to make GRBs), and b) sufficient amount of heavy elements, there may be only limited time in the Universe, and only a few galaxies, to allow life, especially complex life to develop. ... These scenarios are illustrated schematically on the right side of Figure 4 [also, in the poster]. The limit estimated for minimal or microbial life depends greatly on local conditions, but is estimated here to be about 8 Gyr ago or when the Universe was about 6 Gyr old. ... About 5-6 Gyr ago, when the Universe was 8-9 Gyr old, metal abundances were comparable to present day in some locations and AGN and the SFR had decreased to about 10 times present day levels. Where habitable conditions were met, life probably transitioned from simple to complex about that time.
Many planets in the densest regions of the local Universe may remain uninhabitable due to intense particle fluxes and sterilizing events. We introduce the concept of a Super-Galactic Habitable Zone to address this possibility. For the future, all this means that if humanity survives long enough, then our descendants may need to keep moving, First into space or to another planet with a different star (as the Sun dies), and then certainly to another galaxy.That last paragraph says move it, for survival or I say, with a mission.