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Life on Mars: Are We Alone? Within our solar system, Mars has always been in the forefront of our search for alien life. As missions shed new light on the Red Planet, we have new hopes for uncovering the fundamental conditions for life. More about Mars and its features is available at Explore Mars Inside and Out.
The new scientific field of astrobiology formed to investigate the origins, evolution, distribution, and future of life on Earth and beyond. Astrobiologists strive to address three questions:
- How does life begin and evolve?
- Is there life elsewhere in the universe?
- What is the future of life on Earth and beyond?
Origin of Life? Scientists believe that life on Earth may have begun as microscopic organisms in extreme underwater hydrothermal environments such as depicted here.
Credit: Lunar and Planetary Institute.
Through our efforts to understand how life began and evolved on Earth, we hope to determine where and how to best look for it elsewhere. The scientific field of astrobiology embraces the search for life both close to home (Earth) and far beyond.
From laboratory and field investigations on Earth, to the exploration of Mars, the outer planets, and planets beyond our solar system, scientists are studying the potential for life to adapt and thrive beyond our home planet.
This research requires partnerships among many fields of science, including molecular biology, ecology, planetary science, astronomy, information science, and space technologies.
How is NASA searching for life? In 1998, in a concerted effort to address the challenges in finding life beyond Earth, the National Aeronautics and Space Administration (NASA) established the NASA Astrobiology Institute (NAI), competitively selected teams across the country that incorporate astrobiology research and training in their programs. For more information about the NAI and its teams, please visit: http://astrobiology.nasa.gov/nai/
What is Life?
Water and Life on Mars
By the end of this section, you will be able to:
- Describe the general composition of the atmosphere on Mars
- Explain what we know about the polar ice caps on Mars and how we know it
- Describe the evidence for the presence of water in the past history of Mars
- Summarize the evidence for and against the possibility of life on Mars
Of all the planets and moons in the solar system, Mars seems to be the most promising place to look for life, both fossil microbes and (we hope) some forms of life deeper underground that still survive today.
But where (and how) should we look for life? We know that the one requirement shared by all life on Earth is liquid water. Therefore, the guiding principle in assessing habitability on Mars and elsewhere has been to “follow the water.
” That is the perspective we take in this section, to follow the water on the red planet and hope it will lead us to life.
Atmosphere and Clouds on Mars
The atmosphere of Mars today has an average surface pressure of only 0.007 bar, less than 1% that of Earth. (This is how thin the air is about 30 kilometers above Earth’s surface.
) Martian air is composed primarily of carbon dioxide (95%), with about 3% nitrogen and 2% argon.
The proportions of different gases are similar to those in the atmosphere of Venus, but a lot less of each gas is found in the thin air on Mars.
Figure 1. Wind Erosion on Mars: These long straight ridges, called yardangs, are aligned with the dominant wind direction. This is a high-resolution image from the Mars Reconnaissance Orbiter and is about 1 kilometer wide. (credit: NASA/JPL-Caltech/University of Arizona)
While winds on Mars can reach high speeds, they exert much less force than wind of the same velocity would on Earth because the atmosphere is so thin.
The wind is able, however, to loft very fine dust particles, which can sometimes develop planet-wide dust storms. It is this fine dust that coats almost all the surface, giving Mars its distinctive red color.
In the absence of surface water, wind erosion plays a major role in sculpting the martian surface (Figure 1).
The issue of how strong the winds on Mars can be plays a big role in the 2015 hit movie The Martian in which the main character is stranded on Mars after being buried in the sand in a windstorm so great that his fellow astronauts have to leave the planet so their ship is not damaged. Astronomers have noted that the martian winds could not possibly be as forceful as depicted in the film. In most ways, however, the depiction of Mars in this movie is remarkably accurate.
Although the atmosphere contains small amounts of water vapor and occasional clouds of water ice, liquid water is not stable under present conditions on Mars. Part of the problem is the low temperatures on the planet.
But even if the temperature on a sunny summer day rises above the freezing point, the low pressure means that liquid water still cannot exist on the surface, except at the lowest elevations. At a pressure of less than 0.
006 bar, the boiling point is as low or lower than the freezing point, and water changes directly from solid to vapor without an intermediate liquid state (as does “dry ice,” carbon dioxide, on Earth).
However, salts dissolved in water lower its freezing point, as we know from the way salt is used to thaw roads after snow and ice forms during winter on Earth. Salty water is therefore sometimes able to exist in liquid form on the martian surface, under the right conditions.
Several types of clouds can form in the martian atmosphere. First there are dust clouds, discussed above. Second are water-ice clouds similar to those on Earth. These often form around mountains, just as happens on our planet.
Finally, the CO2 of the atmosphere can itself condense at high altitudes to form hazes of dry ice crystals.
The CO2 clouds have no counterpart on Earth, since on our planet temperatures never drop low enough (down to about 150 K or about 125 °C) for this gas to condense.
The Polar Caps
Evidence for Microbial Life on Mars: Fossilized Bacteria? | AMNH
In 1996, a team of scientists led by David McKay of NASA’s Johnson Space Flight Center announced that they had discovered evidence for microscopic fossil life in a meteorite from Mars.
Martian meteorite ALH84001, recovered in Antarctica. Some scientists have suggested that physical and chemical features in this meteorite provide evidence for microscopic fossil life on Mars. That interpretation remains controversial. Photo courtesy of JPL/CALTECH/NASA.
From the start, the evidence was both fascinating and controversial, and to this day it remains so.
The meteorite in question had escaped from Mars 16 million years ago when an asteroid or comet collided with the planet and blasted out a crater. The 2-kilogram fragment of Martian rock then moved in an elliptical orbit around the Sun until it was swept up by the Earth about 13,000 years ago.
It landed in glacial Antarctica, where it remained until 1984, when a meteorite-hunting party picked it up it in the Allan Hills. The specimen was designated ALH84001. At first, no one suspected that it came from Mars.
About ten years later, scientists examined ALH84001 more closely and found that it was not an ordinary meteorite, but one of the so-called SNC meteorites, which come from Mars. Meteorites of this class all contain traces of gas having a composition identical to the Martian atmosphere.
Each of the dozen other SNC Martian meteorites then known had crystallized within the last 1.3 billion years, after Mars had become a frozen desert. But ALH84001 was over 4 billion years old, and had presumably existed at a time when liquid water was common on the surface of Mars.
Liquid water is essential to life as we know it. For that reason ALH84001 attracted the attention of McKay and his team, who thought that the rock might preserve microscopic and chemical evidence of ancient life on Mars.
Three Types of Evidence
To avoid the possible terrestrial contaminants picked up by the meteorite in Antarctica, the team obtained their samples from the solid interior volume of the rock.
They found that cracks within the meteorite contain orange-tinted carbonate globules, which resemble limestone cave deposits. This sort of material can form only in the presence of liquid water.
McKay and his colleagues found three kinds of evidence that they interpreted in terms of ancient microbial life on Mars:
- The globules contained traces of complex organic compounds called polycyclic aromatic hydrocarbons (PAHs), which might be the decay products of microbes.
- The globules contained microscopic grains of magnetite (a magnetic iron oxide) and of iron sulfide, two compounds rarely found together in the presence of carbonates, unless produced by bacterial metabolism.
Life Could Exist on Mars Today, Very Close to the Surface
Last week NASA convened a visionary meeting in New Mexico to consider a topic critical to astrobiology—whether life currently exists on Mars, and if so, how to detect it. The site of the conference was near the world-renowned Carlsbad Caverns, which attendees got to visit during a mid-conference workshop.
Caves, along with ice and salt deposits, are likely places to search for possible near-surface life on Mars, and Diana Northup from the University of New Mexico provided an overview of the many diverse shapes and forms microbes can take in caves.
She suggested looking for similar mineral deposits on Mars.
In another talk, Brady O’Connor from McGill University in Montreal reported on cold-adapted and metabolically active microbial communities found within ice in lava tube caves on Earth—again with intriguing implications for similar structures on Mars.
Water ice is believed to be important for locating extant life near the Martian surface, especially at higher latitudes. Carol Stoker from NASA’s Ames Research Center made the case for focusing on this possible microhabitat, and presented information on the proposed Icebreaker Life mission, which would search for biomarkers in Martian ice deposits.
Not all the water on Mars may be frozen, however. Andrew Schuerger’s lab at the University of Florida recently made a startling, albeit preliminary, discovery.
He tested the effect of frost (first discovered on Mars by the Viking mission) on rocks under Martian conditions, and found that liquid water flowed on the rocks for about 15 minutes, before all the water turned into the gas phase.
Would this short time be sufficient to support Martian life if frost occurred on a daily basis at some locations? We don’t know. But it’s an extremely interesting finding, and I’m very interested to hear about his further results.
The search for life in Martian salts was discussed by Shil DasSarma from the University of Maryland and myself.
Both our talks made clear that salts may allow microbes to thrive very close to the surface of Mars—perhaps even just a few millimeters deep, which would allow for photosynthesis. If so, they should be easily retrievable on a future landing mission.
I also pointed out that because Martian life would be exposed to extremely dry conditions for long periods of time, it may have novel biochemical adaptations that Earth microbes do not have.
Penny Boston from the NASA Astrobiology Institute reported on her recovery of viable microorganisms within fluid inclusions in gypsum and quartz. Life may have been preserved in the same type of minerals as the Martian surface became too hostile for life.
A fourth potential habitat discussed at the meeting was the deep Martian subsurface. Although extensive drilling would be needed to explore these regions, future missions could reach depths as low as 100 meters below ground.
One astonishing result, related to both the deep subsurface and salty microhabitats, was presented by Vlada Stamenkovic from the Jet Propulsion Lab.
His computer modeling showed that there is enough oxygen on Mars to support microbes in brines, and perhaps even simple sponges, in some locations.
Near the surface the oxygen would be supplied by the Martian atmosphere, whereas farther below it would come from radioactive decay. No one is claiming that there are sponges on Mars, but this finding challenges many scientists’ assumption that there is no available oxygen beneath the Martian surface.
I could go on listing the many intriguing findings presented at last week’s workshop. The scientists who presented in New Mexico will continue to work on their research, which should help NASA determine where to send a Life Detection Mission (or perhaps a precursor).
And if you ever wondered about the legal status of would-be Martian microbes—whether, for example, they might have certain rights—check out this abstract of a talk by William Kramer from Outer Space Consulting.
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Could There Be Life on Mars Today?
The search for life on Mars shouldn't focus exclusively on the distant past, some researchers say.
Four billion years ago, the Martian surface was apparently quite habitable, featuring rivers, lakes and even a deep ocean. Indeed, some astrobiologists view ancient Mars as an even better cradle for life than Earth was, and they suspect that life on our planet may have come here long ago aboard Mars rocks blasted into space by a powerful impact.
Things changed when Mars lost its global magnetic field. Charged particles streaming from the sun were then free to strip away the once-thick Martian atmosphere, and strip it they did.
This process had transformed Mars into the cold, dry world we know today by about 3.7 billion years ago, observations by NASA's MAVEN orbiter suggest.
(Earth still has its global magnetic field, explaining how our planet remains so livable.)
Related: The Search for Life on Mars (a Photo Timeline)
But this turn of events doesn't necessarily mean that Mars is a dead planet today.
“If Mars had life 4 billion years ago, Mars still has life. Nothing has happened on Mars that would've wiped out life,” said Michael Finney, co-founder of The Genome Partnership, a nonprofit organization that runs the Advances in Genome Biology and Technology conferences.
“So, if there were life on Mars, it may have moved around, it may have gone into hiding a bit, but it's probably still there,” Finney said last month during a panel discussion at the Breakthrough Discuss conference at the University of California, Berkeley.
I’m Convinced We Found Evidence of Life on Mars in the 1970s
We humans can now peer back into the virtual origin of our universe. We have learned much about the laws of nature that control its seemingly infinite celestial bodies, their evolution, motions and possible fate.
Yet, equally remarkable, we have no generally accepted information as to whether other life exists beyond us, or whether we are, as was Samuel Coleridge’s Ancient Mariner, “alone, alone, all, all alone, alone on a wide wide sea!” We have made only one exploration to solve that primal mystery.
I was fortunate to have participated in that historic adventure as experimenter of the Labeled Release (LR) life detection experiment on NASA’s spectacular Viking mission to Mars in 1976.
On July 30, 1976, the LR returned its initial results from Mars. Amazingly, they were positive.
As the experiment progressed, a total of four positive results, supported by five varied controls, streamed down from the twin Viking spacecraft landed some 4,000 miles apart.
The data curves signaled the detection of microbial respiration on the Red Planet. The curves from Mars were similar to those produced by LR tests of soils on Earth. It seemed we had answered that ultimate question.
When the Viking Molecular Analysis Experiment failed to detect organic matter, the essence of life, however, NASA concluded that the LR had found a substance mimicking life, but not life.
Inexplicably, over the 43 years since Viking, none of NASA’s subsequent Mars landers has carried a life detection instrument to follow up on these exciting results.
Instead the agency launched a series of missions to Mars to determine whether there was ever a habitat suitable for life and, if so, eventually to bring samples to Earth for biological examination.
NASA maintains the search for alien life among its highest priorities. On February 13, 2019, NASA Administrator Jim Bridenstine said we might find microbial life on Mars. Our nation has now committed to sending astronauts to Mars. Any life there might threaten them, and us upon their return. Thus, the issue of life on Mars is now front and center.
Life on Mars seemed a long shot. On the other hand, it would take a near miracle for Mars to be sterile. NASA scientist Chris McKay once said that Mars and Earth have been “swapping spit” for billions of years, meaning that, when either planet is hit by comets or large meteorites, some ejecta shoot into space.
A tiny fraction of this material eventually lands on the other planet, perhaps infecting it with microbiological hitch-hikers. That some Earth microbial species could survive the Martian environment has been demonstrated in many laboratories.
There are even reports of the survival of microorganisms exposed to naked space outside the International Space Station (ISS).
NASA’s reservation against a direct search for microorganisms ignores the simplicity of the task accomplished by Louis Pasteur in 1864. He allowed microbes to contaminate a hay-infusion broth, after which bubbles of their expired gas appeared. Prior to containing living microorganisms, no bubbles appeared.
(Pasteur had earlier determinted that heating, or pasteurizing, such a substance would kill the microbes.) This elegantly simple test, updated to substitute modern microbial nutrients with the hay-infusion products in Pasteur’s, is in daily use by health authorities around the world to examine potable water.
Billions of people are thus protected against microbial pathogens.
This standard test, in essence, was the LR test on Mars, modified by the addition of several nutrients thought to broaden the prospects for success with alien organisms, and the tagging of the nutrients with radioactive carbon.
These enhancements made the LR sensitive to the very low microbial populations postulated for Mars, should any be there, and reduced the time for detection of terrestrial microorganisms to about one hour. But on Mars, each LR experiment continued for seven days.
A heat control, similar to Pasteur’s, was added to determine whether any response obtained was biological or chemical.
The Viking LR sought to detect and monitor ongoing metabolism, a very simple and fail-proof indicator of living microorganisms.
Several thousand runs were made, both before and after Viking, with terrestrial soils and microbial cultures, both in the laboratory and in extreme natural environments. No false positive or false negative result was ever obtained.
This strongly supports the reliability of the LR Mars data, even though their interpretation is debated.
In her recent book To Mars with Love, my LR co-experimenter Patricia Ann Straat provides much of the scientific detail of the Viking LR at lay level. Scientific papers published about the LR are available on my Web site.
In addition to the direct evidence for life on Mars obtained by the Viking LR, evidence supportive of, or consistent with, extant microbial life on Mars has been obtained by Viking, subsequent missions to Mars, and discoveries on Earth:
- Surface water sufficient to sustain microorganisms was found on Mars by Viking, Pathfinder, Phoenix and Curiosity;
- Ultraviolet (UV) activation of the Martian surface material did not, as initially proposed, cause the LR reaction: a sample taken from under a UV-shielding rock was as LR-active as surface samples;
- Complex organics, have been reported on Mars by Curiosity’s scientists, possibly including kerogen, which could be of biological origin;
- Phoenix and Curiosity found evidence that the ancient Martian environment may have been habitable.
- The excess of carbon-13 over carbon-12 in the Martian atmosphere is indicative of biological activity, which prefers ingesting the latter;
- The Martian atmosphere is in disequilibrium: its CO2 should long ago have been converted to CO by the sun’s UV light; thus the CO2 is being regenerated, possibly by microorganisms as on Earth;
- Terrestrial microorganisms have survived in outer space outside the ISS;
Life on Mars
We have wondered for centuries whether our neighbouring planet is home to life, just like our own. In the 19th century, the idea of men on Mars took off after astronomer Giovanni Schiaparelli observed straight lines on Mars, which he called canali.
This means channels, but it was wrongly translated into English as canals, and many people interpreted the discovery as evidence of intelligent engineering work.
The lines were later revealed to be illusory, perhaps caused by streaks of dust carried by the wind.
For life to exist on Mars, there must be liquid water. There is plenty of water on Mars, but most of it is frozen in the polar ice caps and buried underground.
Despite the chilly temperatures, there could be liquid water underneath the surface ice in some places, since salt lowers the freezing point of water.
Satellite data suggest there is a permanent lake, 20 kilometres across, hidden beneath the south pole.
Dark streaks called recurrent slope lineae form on Mars’s slopes during warm seasons, and these have been interpreted as flowing briny water. But liquid water on the surface would be hard to explain – and the flows may just be tumbling sand.
In any case, any liquid water on Mars is probably too salty for life to survive.
Geological and chemical evidence suggests that there may have been flowing water on the surface of Mars billions of years ago. This is hard to explain as back then, the sun was less hot, and Mars was even colder than it is now. Greenhouse gases might have trapped some heat, but probably not enough.
If there were once oceans on Mars, they would not have stuck around for long, leaving little time for life to evolve. A barrage of asteroids may have brought the water there, only for it to freeze and disappear within a few hundred million years.
Aside from water, another main clue to look for is the presence of organic compounds, which form the basis for all life on Earth. NASA’s Curiosity rover has found complex organic molecules, which could have been made by ancient life forms.
Methane gas is a possible sign of life: on Earth, most of it is produced by microbes, although it can also be produced by geological sources. We have spotted tantalising glimpses of methane on Mars a few times over the years, but it has been difficult to confirm the detections with other instruments.
In 2019, Curiosity detected the largest amount of methane ever found on Mars. Two satellites were observing the area simultaneously, so it may be possible to confirm the discovery with independent measurements.
Under planetary protection rules, all Mars-bound spacecraft must undergo a rigorous sterilisation procedure to make sure we don’t contaminate Mars with microbes from Earth. These protocols add significantly to the cost of space missions, and some researchers argue that the rules should be relaxed. Sam Wong