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| The two Viking landers settled in the regions shown to to the left. The landers among other things, performed tests designed to search for Life As We Know It (LAWKI).
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Using their arms, the Viking Landers obtained soil samples and sampled the atmosphere of Mars.
The landers performed three experiments: (i) Gas-Exchange Experiment: Martain soil was mixed with a broth of organic nutrients brought from Earth and looked for the release of gases from respiration; (ii) Pyrolitic Release Experiment: Martian soil was mixed with carbon monoxide and carbon dioxide brought from Earth to see if the carbon would be incorporated into the soil; (iii) Labeled Release Experiment: Martian soil was mixed organic nutrients to see if products of respiration and metabolism appeared. |
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The labelled release experiment detected an initial burst of oxygen (Plant life?) followed by a slower release (over days) of carbon dioxide (Animals and/or microbes?) when a Martian soil sample was studied. The burst of oxygen quickly subsided (within hours), however. If biology was responsible it was not immediately clear why life would behave in this manner. Later work, however, discovered an interesting thing, Solar ultraviolet radiation striking the Martian soil can produce hydrogen peroxide, H2O2, which then adheres to the soil. If the soil is moistened, the hydrogen peroxide breaks up very quickly releasing a burst of oxygen and forms other compounds. Although, on the face of it this sounds bad as it then suggests that the detected release of oxygen by Viking was due to chemistry, not biology.
A fourth experiment suggested, however, that the Labeled Release result was not due to biology as analysis of the soil found that there were no traces of organic molecules in the Martian soil. The only organics found were chloromethane and dichloromethane compounds, thought to arise as contaminants from the cleaning fluids used to sterilize the spacecraft and instruments. The lack of organic compounds was perplexing.
Phoenix inspired a new look at data from the Mars Viking landers. It suggested that the two landers may have found the building blocks for life after all. The surprise discovery of perchlorates (chemical compounds that contain the perchlorate ion ClO4-) by the Phoenix Mars mission, 32 years after Viking, could mean the way that the Viking experiment was set up destroyed any carbon-based chemical building blocks of life, what the experiment had set about to try and find. "This doesn't say anything about the question of whether or not life has existed on Mars, but it could make a big difference in how we look for evidence to answer that question," said Chris McKay of NASA's Ames Research Center. McKay coauthored a study published online by the Journal of Geophysical Research Planets, reanalyzing results of Viking's tests for organic chemicals in Martian soil.The Viking lander scooped up some soil, put it in a tiny oven and heated the sample. The only organic chemicals identified in the Martian soil from that experiment chloromethane and dichloromethane compounds interpreted at the time as likely contaminants from cleaning fluids used on the spacecraft before it left Earth. But those chemicals are exactly what the new study found when a little perchlorate the surprise finding from Phoenix was added to desert soil from Chile containing organics and analyzed in the manner of the Viking tests. Perchlorate, an ion of chlorine and oxygen, becomes a strong oxidant when heated. It could sit in the Martian soil with organics around it for billions of years and not break them down, but when you heat the soil to check for organics, the perchlorate destroys them rapidly. "Our results suggest that not only organics, but also perchlorate, may have been present in the soil at both Viking landing sites," said the study's lead author, Rafael Navarro-Gonzalez of the National Autonomous University of Mexico, Mexico City.
The Viking experiment results have been controversial over
the years. Some scientists say the experiment actually
found evidence for life, while others say the results
were inconclusive.
McKay said that organics can come from non-biological or
biological sources. Many meteorites raining onto Mars and Earth
for the past 5 billion years contain organics. Even if Mars
never had life, scientists before Viking anticipated
Martian soil would
contain organic materials carried in by meteorites.
"The lack of organics was a big surprise," McKay said.
This interpretation proposed by Navarro-Gonzalez and his for
co-authors challenged the interpretation by Viking scientists that
Martian organic compounds were not present in their samples at the
detection limit of the Viking experiment.
How will we know for sure? The Mars Science Lab mission, with the
car-sized rover called Curiosity
resolved this question in that they found organic molecules in
crater Gale.
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Curiosity was launched on Nov 26, 2011 landing on Mars on Aug 6, 2012 in Gale Crater. Mission to investigate whether Mars once had conditions suitable for life.
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Yellowknife Bay
See layers of sedimentary rock. Initially, the landscape was formed by sedimentation as materials were washed into the region by liquid water and deposited. The materials went through cementation leaving the ground hard. After this, the region underwent multiple episodes of percolation and sedimentation. The region is thought to date from 4 billion years ago.
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Sheepbed
Around the sheepbed area, Curiosity drilled and analyzed samples of mudstone sedimentary rock (rock formed in the presence of water), found important elements for life, sulfur, nitrogen, oxygen phosphorous, carbon and, from the clay, that the water on Mars was not harshly oxidizing, acidic, or salty.
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Windjana is part of a sandstone slab Curiosity analyzed. Curiosity found manganese oxides (MnOs) in the sandstone. MnOs form in the presence of water and strong oxidation. Can come from microbes ("biology") or atmospheric oxygen molecules.
The next mission is Perseverance and Ingenuity in Jezero crater. Perseverance landed last summer and has been steadily working.
Check out a Perseverance page on the Mars 2020 NASA website https://mars.nasa.gov/mars2020/mission/science/landing-site/
Some final comments: comparing the amount of heavy water to normal water suggests that most of Mars's water, but not like on Venus, has been lost to space, methane goes up and down seasonally, streams and lakes existed 3.3-3.8 billion years ago, and there are organic molecules in 3.5 billion year old bedrock.
