Oxygen is thought to be a biomarker for extraterrestrial life, but there are at least three different ways that a lifeless planet can produce it.
- If an exoplanet houses life, it almost certainly will have gaseous oxygen.
- But a new study modeling the development of rocky planets identifies three scenarios in which oxygen can form abiotically.
- The notion that oxygenated exoplanets are all candidates to host life should be treated with skepticism.
Research that aims to identify exoplanets that might contain life usually use oxygen as a biomarker. But a new study published in AGU Advances explains that this can be very misleading: Oxygen can easily accumulate in an exoplanet's atmosphere without any biological origin.
Oxygen is considered a biomarker because photosynthesis — the process by which plants use sunlight to fix carbon dioxide into sugar — produces oxygen as a waste product. Thus, a planet with oxygen in its atmosphere is considered a strong candidate to host some kind of lifeform.
The team, led by Joshua Krissansen‐Totton of UC Santa Cruz, developed a model of planetary formation that allowed them to tinker with variables that could affect how an Earth-like planet develops. Using their model, the researchers were consistently able to produce three scenarios in which an Earth-like planet has levels of oxygen in its atmosphere similar to ours, but life was not part of the formula.
Three oxygenated worlds with no life
The planetary evolution model. Arrows show the flow of certain substances and heat energy between different layers of the Earth and its atmosphere.Krissansen-Totton et al./ AGU Advances
In the first scenario, an exoplanet has very high levels of carbon dioxide and water in the atmosphere. Under these conditions, a strong greenhouse gas effect means there will be no water on the exoplanet's surface. When hit by ultraviolet (UV) light, water vapor in the upper atmosphere can occasionally split into hydrogen and oxygen. The lighter hydrogen gas escapes into space, leaving the heavier oxygen gas behind.
In the second scenario, an exoplanet "waterworld" contains anywhere from 10 to 230 times as much water as the Earth has today. Under these conditions, the oxygen cycle — which involves the circulation of oxygen through the atmosphere, lifeforms, and rocks — essentially doesn't exist. Pressure from the massive oceans on the crust would shut down the geological activity necessary to recycle oxygen, leaving it in the atmosphere.
In the third scenario, an exoplanet "desertworld" has conditions exactly the opposite of those in the "waterworld." This type of exoplanet has very little water, no more than a third of what Earth has in its oceans. Under these conditions, the molten surface of a young exoplanet can freeze while the limited water supply is still found only as steam (vapor) in the atmosphere. This prevents oxygen from being absorbed by the crust. Then, as with the first scenario, UV light breaks up water into hydrogen and oxygen.
Implications for the hunt for E.T.
An infographic illustratingthe three planets described above and how they might form.Illustration by J. Krissansen-Totton
None of the three scenarios assures an oxygen-rich atmosphere; they simply allow for oxygen to occur abiotically. Professor Krissansen‐Totton described the utility of the model in a press release:
"This is useful because it shows there are ways to get oxygen in the atmosphere without life, but there are other observations you can make to help distinguish these false positives from the real deal. For each scenario, we try to say what your telescope would need to be able to do to distinguish this from biological oxygen."
Such telescopes should be in orbit by 2030. Now the scientists using them know what to look for.
Humans are more likely to have "first contact" with an advanced alien civilization, according to a recent NASA-funded paper.
- A new paper outlines some of the most promising ways scientists and space agencies can search for evidence of extraterrestrial civilizations.
- Because of a concept called "contact inequality," the researchers suggested it's relatively unlikely humans will discover evidence of alien civilizations that have similar levels of technology to us.
- However, near-future technology could soon allow scientists to search for both highly advanced and less advanced alien civilizations.
How will humans discover the existence of extraterrestrial civilizations?
Unless aliens decide to visit Earth, the most likely answer is by scanning the skies for "technosignatures," which are observational evidence of technological or industrial activity in the Universe.
In a recent paper published in the journal Acta Astronautica, a team of NASA-funded researchers outlined some of the most promising ways scientists and space agencies could search for technosignatures. The paper included a somewhat surprising proposition: Humanity's "first contact" with aliens is likely to be with a much more advanced civilization.
In other words, there could be many alien civilizations throughout the Universe, or even in our galaxy, but if they're similar to us in terms of technological advancement, we probably can't spot them yet. The same goes for those human-like civilizations spotting us.
That's because the "cosmic footprints" of our civilization and theirs would be relatively small, compared to highly advanced alien civilizations. The researchers call this concept "contact inequality."
"It seems unlikely that civilizations with a relatively low level of technological development would enter into contact with each other, since that would require either very high sensitivities or highly visible engineering," reads the paper. "Less advanced civilizations lack the sensitivity needed to detect other civilizations unless they have built very large or luminous structures."
So, unless near-future technology like the James Webb Space Telescope enables scientists to find biomarkers on other planets, we're far more likely to discover more sophisticated civilizations.
How? The paper outlines a series of strategies that are either currently being practiced, could be practiced in conjunction with other astronomical projects, or could be developed in the near future.
A few of those strategies include searching for:
- Dyson spheres — gigantic structures that, in theory, could orbit stars and generate vast amounts of energy for civilizations.
- Near-Earth objects — space agencies could search the Moon, Mars or other space bodies for evidence of extraterrestrial artifacts, such as crashed probes.
- Abnormal spectra in planetary atmospheres — if aliens are conducting industrial activity on another planet, that planet's atmosphere would likely contain evidence of such activity.
- Night-time illumination
- Radio and laser signals
When thinking about the best ways to search for alien life, it helps to reverse the perspective: How would aliens know humans exist? With this question in mind, researchers who deal with technosignatures consider all of the signals humans are sending into space.
Some of our technosignatures are transmitted intentionally, like the Arecibo message humans sent toward the globular star cluster M13 in 1974. Others are unintentional, like night-time illumination and pollution-driven atmospheric changes.
To put the concept of technosignatures into perspective, researchers have developed a framework called ichnoscale. Ichnoscale ranks the size of a technosignature relative to what human technology is currently capable of producing.
The scale also ranks the number of potential targets throughout the Universe. For example, searching for a crashed alien probe on the Moon would represent one target, while scanning the stars for Dyson spheres would have millions of targets.
Together, these measurements help scientists estimate the most likely ways to discover evidence of alien civilizations. Of course, there's no guarantee that any one strategy will work, or that aliens even exist.
That's one reason why efforts to search for technosignatures have received little funding. But the researchers propose that many of these strategies could be tacked onto other astronomical missions at little cost.
Socas-Navarro et al.
And even if the searches turn up nothing, the researchers said the results would still provide "enormous ancillary benefits on solar system research and advance our knowledge about the objects being scrutinized," and would "establish quantitative upper bounds on certain types of technologies or developmental stages of civilizations in the solar neighborhood."
"The search for TS deals with questions that have profound implications on the future of humanity," the researchers concluded. "Perhaps one the most important is whether technological civilizations are ephemeral or, on the contrary, can be long lasting. A closely related question is whether space faring civilizations are common, and if humankind will eventually become one of them."
"We do not yet have any answers for these and other important questions but if we can start to explore the search parameter space, even in the absence of any detection we may be able to gain some valuable insights."
How do you get usable phosphorus into a system? A new study suggests lightning can do the trick.
- A chance discovery in suburban Illinois may change how we understand the dawn of life.
- Among other things, life needs water-soluble phosphorus, which was hard to come by 3.5 billion years back.
- This finding may imply that life has more opportunities to begin on other worlds than previously supposed.
Even the youngest child often wonders where they came from. For many scientists, a group of people known for retaining their childlike wonder, the question naturally evolves to asking how life itself originated on Earth. As is often the case when working with questions about the Earth billions of years ago, those trying to answer this one have access to a limited amount of data.
Now, a chance finding from a lightning strike in Illinois may reshape how we understand the beginnings of life on this planetand worlds beyond.
In the beginning, there were a lot of meteorite impacts and lightning strikes
Phosphorous is an important chemical for life on Earth, cells use it to help build DNA and RNA and it is required for several other important functions. There is plenty of phosphorous on Earth, but not all of it is water-soluble. It is thought that much of the phosphorus on Earth three and a half billion years ago, about the time when life first appeared, was trapped in minerals that can not dissolve in water. Given how important water is for life on Earth, this was an obstacle to the rise of life.
Until very recently, the leading theory about where most of the soluble phosphorous came from credited meteorites, many of which have small amounts of the stuff. However, this theory always had problems. The number of meteorites hitting the early Earth, while high, is thought to have fallen drastically after the event which is theorized to have created the moon. The problem gets worse over time, with fewer and fewer expected impacts as the solar system stabilized.
Additionally, meteorite impacts are often catastrophic events more often known for ending life than helping to start it. The amount of phosphorous that could arrive this way is also limited, with the heat and trauma of impact potentially vaporizing much of the stuff and leaving a pittance readily accessible in the environment.
This is where the chance finding in Illinois comes in. In 2016, a hunk of fulgurite, a clump of fused sediment created by a lightning strike, was found in Glen Ellyn, a small Chicago suburb. The sample was given to the nearby Wheaton College.
A team of researchers from the University of Leeds examined the specimen as part of an investigation into the formation of fulgurite, but were surprised to discover that it contained a large amount of schreibersite, a water-soluble phosphate mineral.
Lead author and Ph.D. candidate Benjamin Hess explained how this find might alter theories on how water-soluble phosphates came into being billions of years ago:
"Most models for how life may have formed on Earth's surface invoke meteorites which carry small amounts of schreibersite. Our work finds a relatively large amount of schreibersite in the studied fulgurite. Lightning strikes Earth frequently, implying that the phosphorus needed for the origin of life on Earth's surface does not rely solely on meteorite hits."
Their findings were published in Nature Communications and can be read in their entirety here.
Okay, this is cool and all, but how can we possibly use this information?
In addition to shedding light on the Earth's past environment and how it changed over time, this finding might also aid the search for life on other planets.
Lead author Mr. Hess speculated that the finding "also means that the formation of life on other Earth-like planets remains possible long after meteorite impacts have become rare."
This is important because, as co-author Dr. Jason Harvey explains:
"The early bombardment is a once in a solar system event. As planets reach their mass, the delivery of more phosphorus from meteors becomes negligible. Lightning, on the other hand, is not such a one-off event. If atmospheric conditions are favourable for the generation of lightning, elements essential to the formation of life can be delivered to the surface of a planet. This could mean that life could emerge on Earth-like planets at any point in time."
While these speculations presume that alien life forms will require the same substances we do to exist, the discovery of a new source of usable phosphorus is an exciting find for those interested in alien worlds and in the early geology or biology of Earth. While we might never know precisely where the phosphorous used in the first life form came from, this discovery will help to make sense of where we came from and where we might find others like us out amongst the stars.
Sound waves behave quite differently on Mars than on Earth.
- NASA's Perseverance rover landed on Mars on February 18, and is currently preparing to begin its main mission of searching for signs of ancient life.
- The rover contains two microphone systems, one of which was recently used to capture sounds of the rover traveling at speeds below .01 mph.
- NASA hopes to return Perseverance's rock collection to Earth by 2031.
It's been over a month since Perseverance landed on Mars, where the rover will search for evidence of ancient life. Since the landing on February 18, Perseverance has returned images, conducted tests of its robotic arm and steering system, and recorded the sound of wind on the red planet.
This week, NASA released audio of the six-wheeled rover driving on the surface of Mars, captured by Perseverance's Entry Descent and Landing (EDL) microphones. The 16-minute recording features raw, unedited audio of the rover traveling 90 feet across the Martian surface at speeds approaching about .01 mph.
It's the first time a NASA rover has captured audio of itself driving.
It's also not the most pleasant recording.
"If I heard these sounds driving my car, I'd pull over and call for a tow," Dave Gruel, lead engineer for Mars 2020's EDL Camera and Microphone subsystem, told NASA's Jet Propulsion Laboratory. "But if you take a minute to consider what you're hearing and where it was recorded, it makes perfect sense."
It sounds like that partly because the rover's off-the-shelf EDL microphones weren't intended to capture sounds from the Martian terrain, but rather to record audio as the rover made its descent. And then there's the wheels.
"A lot of people, when they see the images, don't appreciate that the wheels are metal," Vandi Verma, a senior engineer and rover driver at NASA's Jet Propulsion Laboratory in Southern California, told NASA's Jet Propulsion Laboratory. "When you're driving with these wheels on rocks, it's actually very noisy."
Sound waves also behave differently on Mars. Compared to Earth, the red planet's atmosphere is colder, less dense and contains far more carbon dioxide. That means sound waves travel more slowly and quietly, and the atmosphere would absorb more higher-pitched sounds, an effect known as attenuation.
"The variations between Earth and Mars – we have a feeling for that visually," Verma said. "But sound is a whole different dimension: to see the differences between Earth and Mars, and experience that environment more closely."
NASA released an edited version of the audio that filters out some of the screeches and rattles.
Perseverance has a second microphone system included in its SuperCam instrument, which was designed to identify organic compounds on the Martian surface. SuperCam works by firing a laser at rocks and soil, and using a camera and spectrometers to study the composition of the materials.
"SuperCam's laser is uniquely capable of remotely clearing away surface dust, giving all of its instruments a clear view of the targets," Roger Wiens, the project's principal investigator, told NASA.
How do we fly a helicopter on Mars? It takes ingenuity and perseverance. Tune in on Thursday, March 11, 7pm PT (10p… https://t.co/FxHpBCMw8L— NASA JPL (@NASA JPL)1615339147.0
What's next for Perseverance? In April, NASA plans to conduct a test flight of the Ingenuity helicopter, which will fly near the rover to monitor the environment and provide imaging support. Soon after, Perseverance will spend one Mars year (687 Earth days) on its main mission: Collecting arguably the most scientifically significant rock collection in human history. NASA hopes the rocks will contain evidence that life once existed on Mars.
But it might take years to find out, considering that the ultimate goal is to send another spacecraft to Mars to return the rocks to Earth for closer inspection. For that retrieval mission, NASA and the European Space Agency have their sights on launching 2028 and returning in 2031.
Three lines of evidence point to the idea of complex, multicellular alien life being a wild goose chase. But are we clever enough to know?
- Everyone wants to know if there is alien life in the universe, but Earth may give us clues that if it exists it may not be the civilization-building kind.
- Most of Earth's history shows life that is single-celled. That doesn't mean it was simple, though. Stunning molecular machines were being evolved by those tiny critters.
- What's in a planet's atmosphere may also determine what evolution can produce. Is there a habitable zone for complex life that's much smaller than what's allowed for microbes?
"Do you think we are alone?" That question is, without fail, one of the first things people ask me when they learn I'm an astronomer. And I get why. It's also the question I most want an answer for. But that answer may depend a lot on what kind of life the universe favors (if it favors any at all). So, the question I want to briefly touch on today is how common will it be for any life that appears on any planet in the universe to start climbing up the evolutionary ladder of complexity?On Earth, the history of life is mainly a story of single cells. Earth's origin lies some 4.5 billion years ago, and the best fossil records put the emergence of life as single-celled creatures about a billion years later. After life's first appearance, almost two billion years go by during which all evolutionary activity was on those single-celled organisms. There was some really amazing biochemical machinery evolving within those little cells but if you are interested in multicellular creatures, they don't appear until sometime around 700 million years ago.
... if there is one thing we know is true, it's that nature is more clever than we are. That means it may know lots of ways to produce animals without oxygen around or even in the presence of buckets of CO2.
What are we to make of this incredibly long run of Earth as Planet Bacteria? (Note, there were actually other kinds of single-celled creatures too). Well, it certainly tells us that evolutionary success does not demand multicellularity. During these long eons, life invented the most amazing array of nano-machines for a jaw-dropping variety of purposes. For example, single-celled critters invented photosynthesis for turning sunlight into sugars, metabolisms for turning sugars into energy, and complex intracellular transport mechanisms to move stuff where it was needed and get rid of waste. Earth before plants and animals was already a fertile place full of life that had, in its way, become spectacularly complex at least on the level of biochemistry.
Given the long run of this version of Earth, it may be that there is no reason that more complex life should be expected to form in all or even most cases on other planets.
Protozoa—a term for a group of single-celled eukaryotes—and green algae in wastewater, viewed under the microscope.
Credit: sinhyu via Adobe Stock
Another way the story of life on Earth might not get repeated elsewhere in the cosmos relates to the composition of planetary atmospheres. Our world did not begin with its oxygen-rich air. Instead, oxygen didn't show up until almost two billion years after the planet formed and one billion years after life appeared. Earth's original atmosphere was, most likely, a mix of nitrogen and CO2. Remarkably it was life that pumped the oxygen into the air as a byproduct of a novel form of photosynthesis invented by a novel kind of single-celled organism, the nucleus-bearing eukaryotes. The appearance of oxygen in Earth's air was not just a curiosity for evolution. Life soon figured out how to use the newly abundant element and, it turns out, oxygen-based biochemistry was supercharged compared to what came before. With more energy available, evolution could build ever larger and more complex critters.
Oxygen may also be unique in allowing the kinds of metabolisms in multicellular life (especially ours) needed for making fast and fast-thinking animals. Astrobiologist David Catling has argued that only oxygen has the right kind of chemistry that would allow for animals to form on any world.
Atmospheres may play another role in what can and can't happen in the evolution of life. In 1959, Su-Shu Huang proposed that each star would be surrounded by a "habitable zone" of orbits where a planet would have temperatures neither too hot nor too cold to keep life from forming (i.e. liquid water could exist on the planet's surface). Since then, the habitable zone has become a staple of astrobiological studies. Astronomers now know that the outer part of the habitable zone will be dominated by worlds with lots of greenhouse gases like CO2. A planet in a location like Mars, for example, would require a thick CO2 blanket to keep its surface above freezing. But all that CO2 could present its own problems for life. Almost all forms of animal life on Earth, including sea creatures, die when placed in CO2-rich environments. This has led astronomer Eddie Schwieterman and colleagues to propose a habitable zone for complex life: A band of orbits where planets can stay warm without requiring heavy CO2 atmospheres. According to Schwieterman, animal life of the kind we know would only be able to form in this much thinner band of orbits.
So, we have three lines of evidence that may suggest multicellular life (including thinking animals) may not be the road most taken across the universe. If this were true, then the galaxy might be awash with life but be sparse in terms of tentacles, paws, or boots on the ground.
Now, before your shoulders sag in sadness, it's important to note some facts. First, there are likely 400 billion planets in our galaxy alone. This provides a lot of leeway for experimentation. Second, if there is one thing we know is true, it's that nature is more clever than we are. That means it may know lots of ways to produce animals without oxygen around or even in the presence of buckets of CO2.
We just won't know until we start looking. And here is the good news. We finally are ready to start looking.