There is a new possibility for life on Titan. Scientists affiliated with Cornell University have created a blueprint for a cellular lifeform that wouldn’t need water to survive.
Water on Earth has been the principal ingredient of, as well as the catalyst for, the formation of life. The Cornell scientists believe water could be replaced by methane on Saturn’s moon Titan, where there are seas full of liquid methane.
The scientists’ work essentially lies in finding a suitable alternative for the phospholipid bilayer, a double-layer of fatty acids that constitutes every cell’s membrane on Earth. Because Titan’s atmosphere is rich in nitrogen and methane, their analysis suggested that a combination of the two molecules could create acrylonitrile azotosome, which when stacked together tends to assemble in membrane-like structures.
Acrylonitrile, it turns out, is already present in Titan’s atmosphere. What’s more is that the molecule could be reactive in the moon’s dreadfully cold environment (at -292 to -180 degrees Celsius). The next step is to see if cells with azotosome membranes can reproduce and metabolize in the methane- and nitrogen-rich environment. Their findings were published in Science Advances on February 27.
Incidentally, the formation of azotosomes also requires some hydrogen. This is interesting because astrobiologists have recently shown that the surface of liquid methane lakes on Titan could be host to microbes that metabolize acetylene, ethane and some other organic compounds, along with hydrogen.
Astrobiologists from the NASA Ames Research Center, who did this research, presumed that the microbes would need to consume hydrogen to make their metabolic reactions work. And because there is no other known process on Titan that could reduce the concentration of atmospheric hydrogen in the moon’s lower atmosphere, their calculations gave other astronomers an intriguing way to interpret anomalous deficiencies of hydrogen. Such deficiencies were recorded on Titan in 2010 by the NASA’s Cassini space probe.
There is an alternative explanation as well. Hydrogen may also be involved in chemical reactions in the atmosphere spurred by the bombardment of cosmic rays. Only continued observation will tell what is actually eating Titan’s hydrogen.
Yet another possibility for life on the moon was conceived in August 2014, when Dirk Schulze-Makuch, an astrobiologist from Washington State University, reported in Science that methane-digesting bacteria had been found in a lake of asphalt in Trinidad. The water content of the lake was only 13.5%. Schulze-Makuch suggested that, even if very little water was present on the moon, it would be enough to encourage the formation of these bacteria. What he couldn’t account for was the substantially lower temperature at which these reactions would need to occur on, say, Titan.
Slowly there has been a mounting number of possibilities, which suggest that life on Titan needn’t have to be fine-tuned or capitalize on one or two narrow windows of existential opportunity. Instead, there exist a variety of routes through which enterprising molecules could aspire for self-organization and awareness, even in an oxygen-deficient, methane-rich environment.
A TEXUS mission sounding rocket taking off in March 2011 from Kiruna, Sweden. Image: Adrian Mettauer
A team of European scientists have shown that DNA molecules can withstand the rough temperatures and pressures that rockets experience when they reenter Earth’s atmosphere from space. Their finding is important from the perspective of meteorites and other space rocks that crash on Earth. Many scientists think such objects could once have seeded our planet with the first molecules of life, billions of years ago.
The scientists had attached bits of plasmid DNA – the part physically separated from chromosomal DNA in biological cells and capable of reproducing independently – on 15 different parts of the outer shell of a TEXUS mission sounding rocket (powered by the Brazilian VSB-30 motor). On March 29, 2011, the rocket took off from the European Space and Sounding Rocket Range near Kiruna, Sweden, for a suborbital flight that exposed the DNA to the vacuum and low temperatures of space before shooting back toward Earth, exposing the samples to friction against the atmosphere.
The entire flight lasted 780 seconds and reached a height of 268 km. While going up, the acceleration maxed at 13.5 g and while coming down, 17.6 g. When outside Earth’s atmosphere, the rocket and samples also experienced about 378 seconds of microgravity. The maximum temperature experienced during atmospheric reentry was just below 130 degrees Celsius on the surface of the rocket; the gases in the air around the samples attached to the sides of the rocket could have reached 1,000 degrees Celsius.
A schematic showing the design of the TEXUS-49 payload and the various positions at which the DNA samples were attached. For full caption, see footnote. Image: Screenshot from paper
Promising results
In all, a maximum of 53% of the DNA could be recovered intact and 35% was fully biologically functional. Analysis also showed that “DNA applied to the bottom side of the payload had the highest degree of integrity followed by the samples applied in the grooves of the screw heads”, according to the study paper. It was published in PLOS ONE on November 26.
The ability of the DNA molecules to sustain life was then recorded by observing how many bacterial colonies each of the 15 samples could engender per nanogram. The 100% transformation efficiency was set at 1,955 colonies/nanogram, which was what an unaffected bit of plasmid DNA could achieve.
Curiously, for sample #1, which was attached on the side of the rocket where there was minimum shielding especially during atmospheric reentry, 69 colonies/nanogram were identified. The highest density of colonies was for sample #10, which was attached in the grooves of screw-heads on the rocket: 1,368/nanogram.
“We were totally surprised,” said Cora Thiel and Oliver Ullrich, coauthors of the study and biologists at the University of Zurich, in a statement. “Originally, we designed this experiment as a technology test for biomarker stability during spaceflight and reentry. We never expected to recover so many intact and functional active DNA.”
Last molecule standing
It’s clear that the damage inflicted on the DNA samples by the harsh conditions of acceleration, microgravity, temperature fluctuations, solar radiation and cosmic rays may not have been sufficient in deterring the molecules from retaining their biological functions. In fact, this study imposes new lower limits on the survivability of life: it may not be as fragile as we like to think it is.
Scientists have known temperature to be the most effective destroyer of DNA double-strands. Studies in the past have shown that the molecules weren’t able to withstand more than 95 degrees Celsius for more than five minutes without becoming denatured. During the TEXUS-49 mission, bacterial plasmid DNA temporarily withstood up to 130 degrees Celsius, maybe more.
By extension, it is not inconceivable that a fragment of a comet could have afforded any organic molecules on-board the same kind of physical shielding that a TEXUS-49 sounding rocket did. Studies dating from the mid-1970s have also shown that adding magnesium chloride or potassium chloride to the DNA further enhances its ability to withstand high temperatures without breaking down.
How big a hurdle is that out of the way? Pretty big. If DNA can put itself through as much torture and live to tell the tale, there’s no need for it to have been confined to Earth, trapped under the blanket of its atmosphere. In fact, in 2013, scientists from the Indian Center for Space Physics were able to show, through computer simulations, that biomolecules like DNA bases and amino acids are capable of being cooked up in the interstellar medium – the space between stars – where they could latch on to trespassing comets or asteroids and bring themselves into the Solar System.
According to the study, published in New Astronomy in April 2013, cosmic rays from stars can heat up particles in the interstellar medium and promote the formation of so-called precursor molecules – such as methyl isocyanate, cyanamide and cyanocarbene – which then go on to form amino acids. The only conditions his team presupposed were a particle density of 10,000-100,000 per cubic centimeter and an ambient temperature of 10 kelvin to say about 1 gram of amino acids could be present in 1014 kg of matter.
Compared to the mass density of the observable universe (9.9 × 10-27 kg/m3), that predicted density of amino acids, if true, is quite high. So, the question arises: Could we be aliens?
The first experiments
The first studies to entertain this possibility and send hapless living things to space and back began as far back as 1966, in the early days of the Space Age, alongside the Gemini IX and XII missions. Prominent missions since then include the Spacelab 1 launch (1983), the Foton 9, 11 and 12 rockets (1994-1999), the Foton M2 and M3 missions (2005-2007) and ISS EXPOSE-R mission (2009-2011). The Foton launches hosted the STONE and BIOPAN missions, which investigated if microbial lifeforms such as bacteria and fungi could survive conditions in space, such as a low temperature, solar radiation and microgravity.
Through most of these missions, scientists were able to find that the damage to lifeforms often extended down to the DNA-level. Now, we’re one step closer to understanding exactly what kind of damage is inflicted, and if there are simple ways for them to be fended off like with the addition of salts.
The STONE-5 mission (2005) was particularly interesting because it also tested how rocks would behave during atmospheric reentry, being a proxy for meteorites. It was found that the surface of a rock reached temperatures of more than 1,800 degrees Celsius. However, mission scientists concluded that if the rock layer had been thick enough (at least more than 5 mm as during the test, or 2 cm during STONE-6) to provide insulation, the innards could survive.
Fragment of the Murchison meteorite (at right) and isolated individual particles (shown in the test tube). Image: Wikimedia Commons
In the same vein, the ultimate experiments – though not performed by humans – could have been the Murchison meteorite that crashed near a town of the same name in Australia in 1969 and the Black Beauty, a rock of Martian origins, that splintered over the Sahara a thousand years ago. The Murchison meteorite was found to contain more than 70 different amino acids, only 19 of which are found on Earth. The Black Beauty was found to be 4.4 billion years old and made of sediments, signalling that a young Mars did have water.
Their arrivals’ prime contribution to humankind was that they turned our eyes skyward in the search of our origins. The experiments conducted with the TEXUS-49 mission keep them there.
Full caption for second image: a Scheme of the TEXUS 49 payload with DNA sample 1–12 application sites b Plasmid DNA samples 1–12 were applied on the outside of the TEM (TEXUS Experiment Module) EML 4 cI DNA samples 1–4 were applied circular at 0, 90, 180, 270 degree directly on the surface of the payload DNA samples 5–12 were also applied with a distance of 90 degree each in the screw heads of the payload c II DNA samples 13–15 were applied directly on the payload surface at the bottom side d DNA samples 1–4 were pipetted directly on the surface and locations were marked with a pen e DNA samples 5–12 were applied in the grooves of the screw heads f DNA samples 13–15 were applied directly on the payload surface on the bottom side and locations were marked with a pen.
A TEXUS mission sounding rocket taking off in March 2011 from Kiruna, Sweden. Image: Adrian Mettauer
A team of European scientists have shown that DNA molecules can withstand the rough temperatures and pressures that rockets experience when they reenter Earth’s atmosphere from space. Their finding is important from the perspective of meteorites and other space rocks that crash on Earth. Many scientists think such objects could once have seeded our planet with the first molecules of life, billions of years ago.
The scientists had attached bits of plasmid DNA – the part physically separated from chromosomal DNA in biological cells and capable of reproducing independently – on 15 different parts of the outer shell of a TEXUS mission sounding rocket (powered by the Brazilian VSB-30 motor). On March 29, 2011, the rocket took off from the European Space and Sounding Rocket Range near Kiruna, Sweden, for a suborbital flight that exposed the DNA to the vacuum and low temperatures of space before shooting back toward Earth, exposing the samples to friction against the atmosphere.
The entire flight lasted 780 seconds and reached a height of 268 km. While going up, the acceleration maxed at 13.5 g and while coming down, 17.6 g. When outside Earth’s atmosphere, the rocket and samples also experienced about 378 seconds of microgravity. The maximum temperature experienced during atmospheric reentry was just below 130 degrees Celsius on the surface of the rocket; the gases in the air around the samples attached to the sides of the rocket could have reached 1,000 degrees Celsius.
A schematic showing the design of the TEXUS-49 payload and the various positions at which the DNA samples were attached. For full caption, see footnote. Image: Screenshot from paper
Promising results
In all, a maximum of 53% of the DNA could be recovered intact and 35% was fully biologically functional. Analysis also showed that “DNA applied to the bottom side of the payload had the highest degree of integrity followed by the samples applied in the grooves of the screw heads”, according to the study paper. It was published in PLOS ONE on November 26.
The ability of the DNA molecules to sustain life was then recorded by observing how many bacterial colonies each of the 15 samples could engender per nanogram. The 100% transformation efficiency was set at 1,955 colonies/nanogram, which was what an unaffected bit of plasmid DNA could achieve.
Curiously, for sample #1, which was attached on the side of the rocket where there was minimum shielding especially during atmospheric reentry, 69 colonies/nanogram were identified. The highest density of colonies was for sample #10, which was attached in the grooves of screw-heads on the rocket: 1,368/nanogram.
“We were totally surprised,” said Cora Thiel and Oliver Ullrich, coauthors of the study and biologists at the University of Zurich, in a statement. “Originally, we designed this experiment as a technology test for biomarker stability during spaceflight and reentry. We never expected to recover so many intact and functional active DNA.”
Last molecule standing
It’s clear that the damage inflicted on the DNA samples by the harsh conditions of acceleration, microgravity, temperature fluctuations, solar radiation and cosmic rays may not have been sufficient in deterring the molecules from retaining their biological functions. In fact, this study imposes new lower limits on the survivability of life: it may not be as fragile as we like to think it is.
Scientists have known temperature to be the most effective destroyer of DNA double-strands. Studies in the past have shown that the molecules weren’t able to withstand more than 95 degrees Celsius for more than five minutes without becoming denatured. During the TEXUS-49 mission, bacterial plasmid DNA temporarily withstood up to 130 degrees Celsius, maybe more.
By extension, it is not inconceivable that a fragment of a comet could have afforded any organic molecules on-board the same kind of physical shielding that a TEXUS-49 sounding rocket did. Studies dating from the mid-1970s have also shown that adding magnesium chloride or potassium chloride to the DNA further enhances its ability to withstand high temperatures without breaking down.
How big a hurdle is that out of the way? Pretty big. If DNA can put itself through as much torture and live to tell the tale, there’s no need for it to have been confined to Earth, trapped under the blanket of its atmosphere. In fact, in 2013, scientists from the Indian Center for Space Physics were able to show, through computer simulations, that biomolecules like DNA bases and amino acids are capable of being cooked up in the interstellar medium – the space between stars – where they could latch on to trespassing comets or asteroids and bring themselves into the Solar System.
According to the study, published in New Astronomy in April 2013, cosmic rays from stars can heat up particles in the interstellar medium and promote the formation of so-called precursor molecules – such as methyl isocyanate, cyanamide and cyanocarbene – which then go on to form amino acids. The only conditions his team presupposed were a particle density of 10,000-100,000 per cubic centimeter and an ambient temperature of 10 kelvin to say about 1 gram of amino acids could be present in 1014 kg of matter.
Compared to the mass density of the observable universe (9.9 × 10-27 kg/m3), that predicted density of amino acids, if true, is quite high. So, the question arises: Could we be aliens?
The first experiments
The first studies to entertain this possibility and send hapless living things to space and back began as far back as 1966, in the early days of the Space Age, alongside the Gemini IX and XII missions. Prominent missions since then include the Spacelab 1 launch (1983), the Foton 9, 11 and 12 rockets (1994-1999), the Foton M2 and M3 missions (2005-2007) and ISS EXPOSE-R mission (2009-2011). The Foton launches hosted the STONE and BIOPAN missions, which investigated if microbial lifeforms such as bacteria and fungi could survive conditions in space, such as a low temperature, solar radiation and microgravity.
Through most of these missions, scientists were able to find that the damage to lifeforms often extended down to the DNA-level. Now, we’re one step closer to understanding exactly what kind of damage is inflicted, and if there are simple ways for them to be fended off like with the addition of salts.
The STONE-5 mission (2005) was particularly interesting because it also tested how rocks would behave during atmospheric reentry, being a proxy for meteorites. It was found that the surface of a rock reached temperatures of more than 1,800 degrees Celsius. However, mission scientists concluded that if the rock layer had been thick enough (at least more than 5 mm as during the test, or 2 cm during STONE-6) to provide insulation, the innards could survive.
Fragment of the Murchison meteorite (at right) and isolated individual particles (shown in the test tube). Image: Wikimedia Commons
In the same vein, the ultimate experiments – though not performed by humans – could have been the Murchison meteorite that crashed near a town of the same name in Australia in 1969 and the Black Beauty, a rock of Martian origins, that splintered over the Sahara a thousand years ago. The Murchison meteorite was found to contain more than 70 different amino acids, only 19 of which are found on Earth. The Black Beauty was found to be 4.4 billion years old and made of sediments, signalling that a young Mars did have water.
Their arrivals’ prime contribution to humankind was that they turned our eyes skyward in the search of our origins. The experiments conducted with the TEXUS-49 mission keep them there.
Full caption for second image: a Scheme of the TEXUS 49 payload with DNA sample 1–12 application sites b Plasmid DNA samples 1–12 were applied on the outside of the TEM (TEXUS Experiment Module) EML 4 cI DNA samples 1–4 were applied circular at 0, 90, 180, 270 degree directly on the surface of the payload DNA samples 5–12 were also applied with a distance of 90 degree each in the screw heads of the payload c II DNA samples 13–15 were applied directly on the payload surface at the bottom side d DNA samples 1–4 were pipetted directly on the surface and locations were marked with a pen e DNA samples 5–12 were applied in the grooves of the screw heads f DNA samples 13–15 were applied directly on the payload surface on the bottom side and locations were marked with a pen.
The dark spot of Venus crossed our parent star in 2012. Pictured above during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory. Image: NASA/SDO & the AIA, EVE, and HMI teams
A new study published in the online journal Life says a hotter, pressurized form of carbon dioxide could harbor life in a similar way water does on Earth. This is an interesting find, theoretical though it is, because it might obviate the need for water to be present for life to exist on other planets. In fact, of the more than 2,700 exoplanet candidates, more than 2,000 are massive enough to have such carbon dioxide present on their surface.
At about 305 kelvin and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. Its properties are very different from what they usually are in its common state – in the same way highly pressurized water is acidic but normal water isn’t. Supercritical carbon dioxide is often used as a sterilization agent because it can deactivate microorganisms quickly at low temperatures.
As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. Moreover, as stated in the paper,
… the surface tension in carbon dioxide is much lower than that of water, whereas the diffusivity of solutes in scCO2 is markedly higher [because of lower viscosity]. Thus, scCO2 can much easier penetrate [cell membranes] than subcritical fluids can.
The easiest way – no matter that it’s still difficult – to check if life could exist in supercritical carbon dioxide naturally is to check the oceans at about a kilometer’s depth, where pressures are sufficient to entertain pockets of supercritical fluids. As the authors write in their paper, supercritical carbon dioxide is less dense than water, so they could be trapped under rocky formations which in turn could be probed for signs of life.
A similarly accessible place to investigate would be at shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbor supercritical carbon dioxide. Dirk Schulze-Makuch, a coauthor of the paper and an astrobiologist at Washington State University, notes,
An interesting twist is that Venus was located in the habitable zone of our Solar System in its early history. [Him and his coworkers] suggested the presence of an early biosphere on the surface of this planet, before a run-away greenhouse effect made all life near the Venusian surface all but impossible.
The probability that Venus could once have harbored life is as strange as it is fascinating. In fact, if further studies indicate that supercritical carbon dioxide can play the role of a viable bio-organic solvent, the implications will stretch far out into anywhere that a super-Earth or gas-giant is found. Because its reactions with complex organic molecules such as amines will not be the same as water’s, the life-forms supercritical carbon dioxide could harbor will be different – perhaps more primitive and/or short-lived. We don’t know yet.
This study continues a persistent trend among astrobiologists since the 1980s to imagine, and then rationalize, if and how life could take root in environments considered extreme on Earth. After the NASA Kepler space telescope launched in 2009 and, in only four years of observation, yielded almost 4,100 exoplanet candidates (more than a thousand confirmed as of now), astrobiologists began to acquire a better picture of the natural laboratories their hypotheses had at their disposal, as well as which hypotheses seemed more viable.
In August this year, Schulze-Makuch himself had another paper, in Science, that discussed how a lake of asphalt in Trinidad harbored life despite a very low water content (13.5%), and what this said about the possibilities of life on Saturn’s moon Titan, which exhibits a similar chemistry on its surface. The Science paper had cited another study from 2004. Titled ‘Is there a common chemical model for life in the universe?‘, it contained a pertinent paragraph about why the search for alien life is important as well as likely endless:
The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.
On July 31, NASA announced the roster of instruments that would hitch a ride on board its planned rover to the red planet in 2020. John Grunsfeld, astronaut and associate administrator for the NASA Science Mission Directorate, Headquarters, Washington, said the instruments would extend the search for life in Mars’s past, conduct geological and environmental investigations to that end, equip it to cache martian material for future explorers to bring back to Earth, and conduct studies that will help the agency land humans on Mars.
Michael Meyer, lead scientist with the Mars Exploration Program, detailed the instruments that would go on board the rover. Going from mast to the body and then to the arm, he laid out seven major instruments developed by over 50 institutions from around the world. Meyer said their guiding principle is that no measurement will be done by only one instrument, that whether it was the chemistry, mineralogy or geology that was being studied, the instruments would overlap, provide multiple perspectives on readings and help constrain error.
The mast would hold the cameras called Mastcam Z and SuperCam. Mastcam Z will be a binocular with zoom capable of rapidly developing terrain models. According to Meyer, the Curiosity rover is slowed down by having to reassess its surroundings once every 10 m for rocky outcrops or surfaces that might threaten it. Mastcam Z will be equipped to plot out greater distances at once. SuperCam, with a significant contribution from France, is the 2020 rover’s counterpart of Curiosity’s ChemCam, which ionizes martian soil samples and studies the missions for their mineral composition. Additionally, SuperCam will also boast a visible and near-infrared spectrometer to make observations at those wavelengths. It will be a remote-sensing instrument to help make important decisions about soil composition and the presence of organic material.
The rover’s body will hold instruments called MOXIE, MEDA and RIMFAX, and MOXIE takes the cake for innovation. It will attempt to extract carbon dioxide from Mars’s atmosphere, break it down and produce pure oxygen. Meyer said that the oxygen could comprise rocket fuel for future human explorers. However, Bill Gerstenmaier, associate administrator for the NASA Human Exploration and Operations Directorate, implied that that claim was exaggerated, saying scientists would first study at what rate and efficiency oxygen could be produced and if its presence could pose any risks.
MEDA, from Spain, would be the on-board weather station, providing data on atmospheric conditions. RIMFAX will give the 2020 rover the ability to ‘see’ below Mars’s surface. It’s a ground-penetrating radar that can go up to 0.5 km downward and and help connect outcrops on the surface with geological formations beneath them.
Two instruments will ride the rover’s arm: PIXL, the interfacing instrument that tells scientists where the action is at smaller scales based on samples the other instruments have analysed, and SHERLOC, a deep-UV instrument adept at studying organic material.
Even though all instruments will be capable of performing multiple analyses, the flow of ‘work’ according to Meyer is roughly ordered as: mast instruments look around the landscape for interesting things, mineralogies that might be best at preserving biosignatures and recent outcrops; arm instruments study samples at finer scales and look at features that might’ve attracted microbial growth in the past; then, based on data, scientists decide whether they want to drill and cache that sample for posterity.
Mars-2020 is being envisaged as Curiosity’s next step with bifurcated goals: landing humans on Mars by studying local geological and radiological properties, and looking for life in its past and helping conduct more sophisticated studies on soil samples.
Like with MOXIE, Meyer explained that the caching of samples would also be proof-of-concept: NASA definitely intends to cache samples but isn’t yet sure what it will do with them. Grunsfeld quoted Carl Sagan to say that if they did find signs of life, they’d also have to muster extraordinary evidence to back up their claim – evidence that could only be established if the samples were subjected to tougher tests on Earth. Meyer concluded by adding that the one-metric-ton rover would be landed on Mars the same way Curiosity was – with the sky-crane.
Plumes of water seen erupting form the surface of Saturn’s moon Enceladus. NASA/JPL-Caltech and Space Science Institute
In 1950, the physicist Enrico Fermi asked a question not many could forget for a long time: “Where is everybody?” He was referring to the notion that, given the age and size of the universe, advanced civilizations ought to have arisen in many parts of it. But if they had, then where are their space probes and radio signals? In the 60 years since, we haven’t come any closer to answering Fermi, although many interesting explanations have cropped up. In this time, the the search for “Where” has encouraged with it a search for “What” as well.
What is life?
@AstroKatie@1amnerd@BBCOS everywhere we find a smidgen of liquid H20 on earth, regardless of conditions, we find life
Humankind’s search for extra-terrestrial life is centered on the assumption – rather hope – that life can exist in a variety of conditions, and displays a justified humility in acknowledging we really have no idea what those conditions could be or where. Based on what we’ve found on Earth, water seems pretty important. As @UrbanAstroNYC tweeted,
@AstroKatie@1amnerd@BBCOS there are parts of the Atacama Desert – driest place on earth – that are so arid NO LIFE survives.
And apart from water, pretty much everything else can vary. Temperatures could drop below the freezing point or cross to beyond the boiling point of water, the environment can be doused in ionizing radiation, the amount of light could dip to quasi-absolute darkness levels, acids and bases can run amok, and the concentration of gases may vary. We have reason to afford such existential glibness: consider this Wikipedia list of extremophiles, the living things that have adapted to extreme environments.
Yes. We only have 1 example. MT @1amnerd: So does this mean we don't know under what other conditions life could form?
Nonetheless, we can’t help but wonder if the qualities of life on Earth can tell us something about what life anywhere else needs to take root- even if that means extrapolating based on the assumption that we’re looking for something carbon-based, and dependent on liquid water, some light, and oxygen and nitrogen in the atmosphere. Interestingly, even such a leashed approach can throw open a variety of possibilities.
“If liquid water and biologically available nitrogen are present, then phosphorus, potassium, sodium, sulfur and calcium might come next on a requirements list, as these are the next most abundant elements in bacteria,” writes Christopher McKay of the NASA Ames Research Center, California, in his new paper ‘Requirements and limits for life in the context of exoplanets’. It was published in Proceedings of the National Academy of Sciences on June 9.
Stuff of stars
McKay, an astro-geophysicist, takes a stepped approach to understanding the conditions life needs to exist. He bases his argument on one inescapable fact: that we know little to nothing about how life originated, but a lot about how, once it exists, it can or can’t thrive on Earth. Starting from that, the first step he devotes to understanding the requirements for life. In the second step, he analyzes the various extreme conditions life can then adapt to. Finally, he extrapolates his findings to arrive at some guidelines.
It’s undeniable that these guidelines will be insular or play a limited role in our search for extraterrestrial life. But such criticism can be partly ablated if you consider Carl Sagan’s famous words from his 1980 book Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”
In 1991, RH Koch and RE Davies published a paper (titled ‘All the observed universe has contributed to life’) presenting evidence that “a standard 70 kg human is always making about 7 3He, 600 40Ca, and 3,000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively”. In other words, we’re not just made of starstuff, we’re also releasing starstuff! So it’s entirely plausible other forms of life out there – if they exist – could boast some if not many similarities to life on Earth.
To this end, McKay postulates a ‘checklist for habitability’on an exoplanet based on what we’ve found back home.
Temperature and state of water – Between -15° C and 122° C (at pressure greater than 0.01 atm)
Water availability – Few days per year of rain, fog or snow, or relative humidity more than 80%
Light and chemical energy sources
Ionizing radiation – As much as the bacterium Deinococcus radiodurans can withstand (this microbe is the world’s toughest extremophile according to the Guinness Book of World Records)
Nitrogen – Enough for fixation
Oxygen (as the molecule O2) – Over 0.01 atm needed to support complex life
McKay calls this list “a reasonable starting point in the search for life”. Its items show that together they make possible environmental conditions that sustain some forms of chemical bonding – and such a conclusion could inform our search for ‘exo-life’. Because we’re pretty clueless about the origins of life, it doesn’t mean we’ve to look for just these items on exoplanets but the sort of environment that these items’ counterparts could make possible. For example, despite the abundance of life-friendly ecosystems on Earth today, one way life could have originated in the first place is by meteorites having seeded the crust with the first microbes. And once seeded, the items on the checklist could have taken care of the rest.
Are you sure water is life?
@1amnerd@AstroKatie@BBCOS that's why we always care so deeply about the subsurface oceans of moons like Europa/Enceladus as well 🙂
Such otherworldly influences present yet more possibilities; all you need is another interstellar smuggler of life to crash into a conducive laboratory. Consider the saturnine moon Titan. While hydrocarbons – the principal constituents of terran life – on Earth are thought to have gassed up and out from the mantle since its formative years, Titan already boasts entire lakes of methane (CH4), a simple hydrocarbon. A 2004 paper by Steven Benner et al discusses the implications of this in detail, arguing that liquid methane could actually be a better medium than water for certain simple chemical reactions that are the precursors of life to occur in.
Another Solar System candidate that shows signs of habitability is Titan’s peer Enceladus. In April this year, teams of scientists studying data from the Cassini space probe said there was evidence that Enceladus hosts a giant reservoir of liquid water 10 km deep under an extensive ice shell some 30-40 km thick. Moreover, Cassini flybys since 2005 had shown that the moon had an atmosphere of 91% water vapor, 3-4% each of nitrogen and carbon dioxide, and the rest of methane.
These examples in our Solar System reveal how the conditions necessary for life are possible not just in the Goldilocks zone because life can occur in a variety of environments as long some simpler conditions are met. The abstract of the paper by Benner et al sums this up nicely:
A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen– helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding.
As humans, we enjoy the benefits of some or many of these conditions – although we know what we do only on the basis of what we’ve observed in nature, not because some theory or formula tells us what’s possible or not. Such is the amount of diversity of life on Earth, and that should tell us something about how far from clued-in we are to understanding what other forms of life could be out there. In the meantime, as the search for extra-terrestrial life and intelligence goes on, let’s not fixate on the pessimism of Fermi’s words and instead remember the hope in Sagan’s (and keep an eye on McKay’s checklist).
