Its surface became forbiddingly cold and dry even as it was bombarded with radiation. Is anything alive on Mars, perhaps beneath the surface, or in the frozen polar caps?
Two strikes against Mars, Voytek said, are its lack of available water and the absence of plate tectonics — the process on Earth that moves continents over eons and recycles buried nutrients back up to the surface. Strikes in its favor might include detection of methane in the Martian atmosphere.
On Earth, methane, otherwise short-lived in the atmosphere, is replenished by the metabolic action of life forms. Methane also can be produced through reactions of water and rock, but microbial life beneath the surface is another possibility.
Some of these moons could well be habitable worlds; one of them, Titan, has a thick atmosphere, rain, rivers and lakes, though composed of methane and ethane instead of water. We first glide toward Europa, a moon of Jupiter with an icy shell. Beneath the frozen surface, however, space probes have detected evidence of a vast ocean of liquid water.
Two other Jovian moons, Ganymede and Callisto, also are likely to host subsurface oceans, though these might be sandwiched between layers of ice. That makes life less likely, Cable says. A potentially more accessible example can be found among the moons of Saturn, the next planet out.
Enceladus, though tiny, also hides a liquid water ocean beneath an icy shell. But in this case, scientists know the little moon is doing something extraordinary. Cassini flew through the plume, and although its instruments were not designed to analyze ocean-water samples — when it was built, the nature of these distant ocean worlds was unknown — it did pick up important clues. Gases detected in the plume , hydrogen and methane, suggest enough energy is present to provide fuel for life.
So far, no one knows the answer. Though smaller and with lighter gravity than Earth, Titan reminds us of our own world, if perhaps reflected through a fun-house mirror. And Titan is the only other body in the solar system with rain, lakes and rivers — a whole hydrologic cycle in fact. What would human explorers visiting Jupiter's icy moon Europa find when they get there?
It's possible some form of life might already be there waiting for them. Living On Europa Explained: Humans Might Not Be First: Infographic Europa is one of the most viable places in the solar system to hunt for life as we know it, but could humans find a way to settle it? Saturn might not be a place where huamns could live, but its moons Titan and Enceladus might hold more hope for human colonists.
What would it be like for a human explorer to hang out on Titan? How would it feel to bounce around in the low gravity of Titania or Miranda? Find out what it might be like to colonize the moons of Uranus. While Neptune doesn't have much of a solid surface under its layers and layers of gas, its huge moon Triton might be a fun and maybe difficult place for humans to settle in the solar system. Living on Triton: Neptune's Moon Explained Infographic Triton could be an interesting place to live in the solar sytem.
Learn more about what the first human settlers of the world could possibly find. How cold would human settlers on Pluto really be? Living on Pluto: Dwarf Planet Facts Explained Infographic Learn more about what it might be like to live on Pluto, if humans ever make it that far into the solar system. What It Would Be Like to Live On a Comet Living on comets requires great care — the gravity is so weak that you could easily jump off the frozen bodies and into space.
Living on a Comet: 'Dirty Snowball' Facts Explained: Infographic Halley's Comet, a dusty ball of ice and frozen gases, spends most of its time in the chilly outland of the solar system. See what it would be like to live on a comet in this infographic.
There are many unknowns regarding the potentially habitable exoplanet Keplerf, but it may have similar light from its star and gravity as Earth. Living on an Alien Planet: Exoplanet Keplerf: Infographic At last humans are able to make educated guesses about what living on alien worlds might be like. Beyond general sensing of H 2 O 2, genes involved in protein protection, such as groES, dnaK and clp tend to be upregulated thus also serving to protect the cell [ 47 ].
These proteins may be important for stabilizing the enzymes involved in the actual conversion of H 2 O 2 to water and O 2 , including catalases, peroxiredoxins, and peroxidases [ 48 ].
Ultimately, the work performed by Reder et al. Checinska et al. The ability of an organism to survive radiation is paramount if the organism is to survive near the surface of Mars and pose a planetary protection threat.
There are 2 major types of radiation to be concerned with on Mars. The first type of radiation, Galactic Cosmic Rays GCR , originates outside of our solar system and is formed from events such as supernovas.
The second type of radiation, Solar Cosmic Radiation SCR , originates from the sun and consists of both a constant flow of radiation as well as brief bursts [ 39 , 51 ]. In the past, the overall radiation level on Mars has been based solely on calculations and modeling. New studies using data collected from the MSL found that the radiation in flight to Mars is approximately two times higher than the radiation on the surface of Mars 0.
The lower radiation level on the Mars surface is due in part to some atmospheric shielding by the Martian atmosphere, which is not provided to the spacecraft en route, and because radiation from GCR is modulated by SCR [ 51 ]. SCR can consist of both ionizing e. UV radiation. This section will focus mostly on UV radiation since that has been the focus of the majority of previous studies.
It is of note that ionizing radiation can be of more concern since it can penetrate through the Martian soils thus potentially making the first meter of soil inhabitable [ 51 ]. UV-B and UV-C radiation are of the most concern since DNA has high absorption at those wavelengths and can be mutated leading to cellular inactivation [ 39 ]. Radiation of biological cells can cause breaks in molecular bonds including single and double strand breaks in DNA and photolysis of amino acids [ 52 ].
Calculations have suggested that DNA weighted irradiance on the Martian surface would be three orders of magnitude greater than that on Earth meaning that microbes would need to be resistant to much higher levels of UV radiation to sustain life on the surface of Mars [ 53 ]. Studies by Wassman et al. However, when the same studies were performed on the super tolerant Bacillus pumilus SAFR strain, a 7 log reduction in viability was observed [ 55 ].
Comparative proteomic studies showed that superoxide dismutase was present in higher concentrations in the space exposed isolates and exhibited higher UV-C resistance than the ground control counterparts [ 55 ]. Tauscher et al. They concluded that the spores can retain the ability to initiate germination-associated metabolic processes and produce viable signature molecules despite being rendered nonviable.
It has been estimated that spores are 10—50 times more resistant than growing cells to UV radiation at nm. This is due to a difference in the UV photochemistry of the DNA as well as error-free repair of any photoproducts formed by the UV light. Instead of forming thymine dimers as a photoproduct, spores tend to form thymine adducts instead; furthermore, small acid soluble proteins SASPs appear to suppress cyclobutane pyrimidine dimers [ 26 ].
Relative to gamma radiation, spores are significantly more resistant due to the decreased levels of water in the spore coat compared to vegetative cells which may reduce the amount of hydroxyl radicals formed overall [ 58 ]. Many non-spore-forming organisms have also been identified as being UV-resistant. Studies by Montero-Calasanz Mdel et al. A highly radiation resistant isolate from the Moraxella-Acinetobacter group showed increased survival after a repeated exposure to UV light.
Although the ability of non-spore-forming organisms to survive radiation appears to be poorly understood, there are some studies which have given clues to how these organisms survive.
Keller et al. Exposure of the lipids and proteins of Acinetobacter sp. The authors concluded that these changes may account for differences in UV sensitivity. Ultimately, there are many microorganisms, both spore-forming and non-spore-forming, that are able to survive exposure to radiation and could potentially survive on Mars. For example, Deinococcus radiodurans would only be eradicated from the top several meters of Martian soil after a period of a few million years based on the radiation that currently reaches Mars.
However, if the organism were to start growing again, then the clock would start over, and organisms could continue to stay dormant and survive up through today. This has implications for the potential for life to exist on Mars. Unlike Earth, the Martian environment provides very little nutrients to sustain life. Any microbes that may already be on Mars would have to make a living using the limited nutrients that are available. As previously discussed, Mars has a mostly CO 2 atmosphere However, studies by Mumma et al.
Two of the most abundant compounds on Mars are Fe and S and there is evidence that there are large concentrations of sulfur in the Martian regolith [ 65 ]. Perchlorate, a strong oxidizing agent, was shown by the Phoenix Lander to be present in Martian soils in concentrations of 2.
All of these compounds are potential chemical energy sources that can be used by microorganisms to survive. The large methane plumes on Mars are of unknown origin. These plumes seasonally fluctuate but the amount of methane produced is on par with methane plumes on Earth that are known to be of biotic origin. Although the Mars rover Curiosity has found no detectable atmospheric methane, it is possible that the location of the rover prevented the detection of methane in the atmosphere since these methane plumes have been seen at polar regions rather than mid-latitude regions.
Methanogenesis has become a well-known method for microorganisms to conserve energy. Many archaea, such as Methanosarcina , can use various carbon compounds to produce methane [ 63 ]. H 2 can readily be oxidized with the large amounts of CO 2 in the atmosphere to generate energy via methane production [ 64 ].
Once this methane is available, it could be oxidized by methanotrophicarchaea in the presence of sulfate-reducing bacteria to complete a methane cycle which would support at least 3 types of organisms [ 65 ]. An overview of the reaction might look something like this:. The electron donor H 2 could easily be generated by photochemical dissociation of water [ 66 ] and it has already been determined that there are large amounts of sulfate, especially in the form of MgSO 4 and FeSO 4 in the Martian soils [ 17 , 67 ].
More likely energy sources fairly abundant in near surface soils on Mars are inorganics such as iron or sulfur [ 8 ]. Sulfate and iron reduction by organisms on Earth have been very well studied.
These organisms play very important roles in the biogeochemical cycling of carbon, nitrogen, sulfur, and other metals [ 68 ]. Studies by Karr et al. There have also been studies showing that Fe respiration under alkaline conditions is possible. Studies by Williamson et al. These studies show that it is possible for these reactions to occur under cold or alkaline conditions.
Once Fe or S has been reduced it is available for oxidation by other organisms. Perchlorate, detected in soils by the Phoenix Mars Lander, is one of the more interesting potential electron acceptors recently discovered on Mars [ 16 , 71 ]. More than 50 microorganisms on Earth are known to respire perchlorate coupled to the oxidation of H 2 or small organic acids, a metabolism that has been intensely studied over the past decade [ 72 , 73 ].
This group of organisms is quite diverse and many have been found in environments that might seem, on the surface, to be inhospitable such as paper mill waste. Studies by Ju et al. Thus perchlorate reduction would tie in neatly to both the Fe and S cycles. Although Mars seems inhospitable and lacks an abundant supply of nutrients, there are plenty of nutrients available to support anaerobic life on the red planet.
The studies discussed above show that the organisms could work together to supply nutrients for one another within a complex ecosystem. Additionally, many of the organisms discussed above can survive in extreme environments on Earth while still making a living as evidenced by many of these processes still taking place at low temperatures or in alkaline environments.
Despite all that we know, there is still much to be learned with regard to the absolute limits for life. In order to answer these questions, we must have a better understanding of life on Earth. With regard to the potential for indigenous populations on other planets and moons, research has shown repeatedly that life can exist in the harshest of environments.
Although this was not covered in depth in this chapter, life has been found in some of the most dry or frigid environments on Earth such as the Atacama Desert or Antarctica. It is not unreasonable to believe that microorganisms, similar to those found on Earth, could be thriving on locations such as Mars or Europa, especially in the subsurface where radiation would be lower and there would be a better chance for the existence of liquid water.
While searching for life on other planets and moons, we look for the signs of life that are already known such as the presence of carbon and water. It may be possible that if we find life in these distant places that we may discover new limits to life in extremis.
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Edited by Thais Russomano. Edited by Ramesh K. By Robert D. Kothera, Benjamin K.
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