Astrobiology Hopes Diminish
Recent findings both near and far create challenges for beliefs that life is common in the universe.
This Place Has Atmosphere
Wham, bang! It’s not hard to imagine a big impact blowing off Earth’s atmosphere, but what about lots of smaller strikes? It’s likely numerous small impacts stripped away Earth’s primordial atmosphere more than once, astronomers say, raising new questions about how our gas envelope turned out so right. Astrobiology Magazine starts an article:
Today’s atmosphere likely bears little trace of its primordial self: Geochemical evidence suggests that Earth’s atmosphere may have been completely obliterated at least twice since its formation more than 4 billion years ago. However, it’s unclear what interplanetary forces could have driven such a dramatic loss.
Now researchers at MIT, Hebrew University, and Caltech have landed on a likely scenario: A relentless blitz of small space rocks, or planetesimals, may have bombarded Earth around the time the moon was formed, kicking up clouds of gas with enough force to permanently eject small portions of the atmosphere into space.
Tens of thousands of such small impacts, the researchers calculate, could efficiently jettison Earth’s entire primordial atmosphere. Such impacts may have also blasted other planets, and even peeled away the atmospheres of Venus and Mars.
In fact, the researchers found that small planetesimals may be much more effective than giant impactors in driving atmospheric loss. Based on their calculations, it would take a giant impact — almost as massive as the Earth slamming into itself — to disperse most of the atmosphere. But taken together, many small impacts would have the same effect, at a tiny fraction of the mass.
The research was published in Icarus. How do the researchers think Earth’s atmosphere survived? According to the list of Highlights in the paper’s Abstract, “The current atmosphere of the Earth could have resulted from an equilibrium between atmospheric erosion and volatile delivery by planetesimals.” This depends, though, on knowing the volatile content of the impactors, and represents a delicate balance that must have been maintained to keep Earth habitable. While the impactors “could have” brought in gas, the main conclusion is that impactors obliterate atmospheres: “We conclude that planetesimal impacts are likely to have played a major role in atmospheric mass loss over the formation history of the terrestrial planets.”
One of the authors, Hilke Schlichting, said in an MIT press release that their finding “sets a very different initial condition for what the early Earth’s atmosphere was most likely like.” It seems the mass loss would overtake any volatile gain. Impacts would cause ground motions and earthquakes “whose force would ripple out into the atmosphere, a process that could potentially eject a significant fraction, if not all, of the planet’s atmosphere.”
Because “the sum effect of small impacts may be too efficient at driving atmospheric loss,” it’s hard to understand how those planets have any atmosphere left, too.
Schlichting realized that if both planets were exposed to the same blitz of small impactors, Venus’ atmosphere should have been similarly depleted. She and her colleagues went back over the small-impactor scenario, examining the effects of atmospheric loss in more detail, to try and account for the difference between the two planets’ atmospheres.
Based on further calculations, the team identified an interesting effect: Once half a planet’s atmosphere has been lost, it becomes much easier for small impactors to eject the rest of the gas. The researchers calculated that Venus’ atmosphere would only have to start out slightly more massive than Earth’s in order for small impactors to erode the first half of the Earth’s atmosphere, while keeping Venus’ intact. From that point, Schlichting describes the phenomenon as a “runaway process — once you manage to get rid of the first half, the second half is even easier.”
The only reason Schlichting and her peers could think of for why Earth still has an atmosphere is that some of the small planetesimals brought in gas as well as removed it.
Exoplanet Starburn and Mirage Earths
A separate article made chances for life look dismal on planets orbiting the very common red dwarf stars. “‘Mirage Earth’ exoplanets may have burned away chances for life,” writes Peter Kelley in a press release from the University of Washington. The problem now is not just the tidal locking expected from low-mass stars, but also starburn: intense heat during a planet’s formative years. In computer models, assistant professor Rory Barnes found that exoplanets around M-type stars “likely had their water and atmospheres burned away when they were still forming.” Here’s the bad news in colleague Rodrigo Luger’s words:
“Planets around these stars can form within 10 million years, so they are around when the stars are still extremely bright. And that’s not good for habitability, since these planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam.”
Also boding ill for the atmospheres of these worlds is the fact that M dwarf stars emit a lot of X-ray and ultraviolet light, which heats the upper atmosphere to thousands of degrees and causes gas to expand so quickly it leaves the planet and is lost to space, Luger said.
“So, many of the planets in the habitable zones of M dwarfs could have been dried up by this process early on, severely decreasing their chance of actually being habitable.”
Interestingly, Luger and Barnes found, the UV light would break up water molecules, letting the light hydrogen escape while retaining some of the oxygen. This might give astrobiologists looking for oxygen as a biomarker a false impression. Luger calls such planets “mirage Earths,” picturing the appearance of water without any in reality. “Because of the oxygen they build up, they could look a lot like Earth from afar — but if you look more closely you’ll find that they’re really a mirage; there’s just no water there.”
Another blow to astrobiologists was published by Adrian Cho in Science Magazine: only 10% of galaxies may host complex life. That’s because energetic gamma ray bursts could sterilize every exoplanet in their vicinity.
The universe may be a lonelier place than previously thought. Of the estimated 100 billion galaxies in the observable universe, only one in 10 can support complex life like that on Earth, a pair of astrophysicists argues. Everywhere else, stellar explosions known as gamma ray bursts would regularly wipe out any life forms more elaborate than microbes. The detonations also kept the universe lifeless for billions of years after the big bang, the researchers say.
“It’s kind of surprising that we can have life only in 10% of galaxies and only after 5 billion years,” says Brian Thomas, a physicist at Washburn University in Topeka who was not involved in the work. But “my overall impression is that they are probably right” within the uncertainties in a key parameter in the analysis.
The paper by Piran and Jimenez concludes that only planets forming in the outer regions of galaxies could survive the intense gamma ray bursts. Earth, they say, had a 50% chance of getting blasted early in its history. While microbes might survive, a burst would hit the reset button on complex life, forcing it to start over. Live Science tells what happens to a planet hit by a burst, and examines the possibility that Earth did get zapped a billion years ago, causing a mass extinction.
The findings have implications for SETI, Cho concludes. Much of SETI research is looking at busy regions of the Milky Way near the galactic center—but that’s where gamma ray bursts would be more common. “We are saying maybe you should look in the exact opposite direction,” says Tzvi Piran from Hebrew University, co-author of the paper.
The active galaxy M82, visible in amateur telescopes, is not a friendly place for life. National Geographic reports that the streamers emanating from its center are deadly and have shut off star formation—assuming, that is, that astronomers know how stars form; why would US and Mexican astronomers need to develop a “new model of star formation,” according to PhysOrg? Even though M82 has been called a “starburst galaxy” for years, whatever exploded has driven out all the hydrogen, driving star formation to a halt:
“What we found was something surprising—a large fraction of the gas is being blasted out of the galaxy by the concentration of stars forming at the galaxy’s center,” says [James] Geach [U of Herfordshire].
“We are witnessing the aggressive termination of star formation, and the mechanism by which this is happening is an important new clue in our understanding of the evolution of galaxies.”
The article says that “The same mechanism may have halted an era of massive star formation in galaxies such as our own.” But as we just saw in the previous article, the centers of galaxies are fried by gamma ray bursts. Now, it appears the outer regions are losing their hydrogen by this blasting mechanism. Science Daily adds more perspective on this “blowout phase of galactic evolution.”
Construction Zones or War Zones?
Formation or destruction? Astrobiologists continue to hope against hope. The Hubble Site, for instance, cheerfully talks about “exoplanetary construction yards” when it finds dust disks around stars, assuming that dust is all that is needed. The dust that is observed, though, is “likely created by collisions between leftover objects from planet formation”—that is, if any planets formed at all. Some astronomers see dust and think life (PhysOrg). The truth is, astronomers cannot tell if the dust is spreading out from collisions, or forming planets. “Like a busy construction site, the process of building planets is messy,” the article teases, providing no solid evidence of construction over demolition. Yet a JPL scientist thinks “The presence of dust is a signpost for the planet formation process,” and if planets are forming, by implication, life must be sure to follow.
Good news, bad news: An optimistic press release from Cornell, “Finding infant earths and potential life just got easier,” appears overly optimistic about habitable zones. The good news is that habitable zones of common red dwarf stars must be farther out than expected (as explained above), meaning they will be easier to detect. The bad news, though, is that to keep from freezing, any life formed during the hot period would have to go under the ice when the star quiets down. Cornell astronomers modeled the conditions, and found that much of a planet’s water would be lost during the transition. To keep the planet habitable, they had to re-supply water from a “late, heavy bombardment of water-rich asteroids.” This not only sounds like an ad hoc theory rescue, but appears to rule out complex life on the surface that might communicate with us.
Entropy limit: Two astrobiologists are speculating about all they don’t know. In an article on PhysOrg, Adam Frank and Woodruff Sullivan speculate about how long an intelligent civilization could last (including if they force climate change). One thing they do know: the Second Law of Thermodynamics is inexorable, putting a time limit on survival. “If they use energy to produce work, they’re generating entropy,” Frank says. It’s a “bottleneck” for any civilization that dooms a civilization to not be able to remain sustainable forever.
Our Rare Bubble of Safety
It’s rad out there: Earth’s atmosphere and magnetic field protect complex life from damaging radiation. A press release from the University of New Hampshire reminds readers of the shooting gallery beyond our atmospheric bubble, stating that “Cosmic Rays Threaten Future Deep-Space Astronaut Missions.” One professor explains: “While these conditions are not necessarily a showstopper for long-duration missions to the moon, an asteroid, or even Mars, galactic cosmic ray radiation in particular remains a significant and worsening factor that limits mission durations.”
Another article on Science Daily reassures astronauts that “space travel is a bit safer than expected,” but the fine print reveals that “safer” applies to low earth orbit. The results come from an experiment on radiation levels on a mannequin in the International Space Station. Here’s the disclaimer: “We must remember that measurements within the MATROSHKA experiment were performed at low Earth orbit where the Earth’s magnetosphere significantly reduces the number of charged particles from cosmic radiation. In interplanetary space there is no such shielding.”
All these articles together strongly reduce the suitable locations for habitability. Chances for complex life are even more rare. The stakes in the cosmic lottery are higher than expected, and somehow, we won.
Sorry, sci-fi fans: that’s reality. The safest place to explore space is to let your imagination ride on Starship Enterprise in your living room at sea level.
It was common in the Carl Sagan era to assume that, because there are so many galaxies and stars, life must be common throughout the universe. Recent findings are challenging that assumption. If only 10% of galaxies are suitable, and in those, only the outermost portions that have held onto their hydrogen, and in those locations, only certain star types don’t fry their planets, and of those planets, there’s an atmosphere that didn’t get blasted away by millions of planetesimal impacts, and the ones remaining have a magnetic field to deflect cosmic rays… you get the picture. The number of suitable locales plummets drastically. It becomes more plausible to believe that Earth is really, really special, a designed world, the focus of God’s care, just as the Bible says.
We see a number of additional factors affecting habitability in these articles. Our list of factors last time was 12 (8/15/14). It’s getting hard to keep up. The shortcut is to change your religion from the cult of SETI to the Word of God. “In the beginning, God created the heavens and the earth” is just as plausible today as it was thousands of years ago. We live on a created Earth that God formed to be inhabited (Isaiah 45:18). With that knowledge, we can have confidence that Earth is within His sovereign plan, and will be protected till the time He has determined (II Peter 3), to create new heavens and a new earth.