Robotic rovers are currently exploring the surface of Mars. Part of the rover’s mission is to survey the planet for signs of life. Maybe there is nothing to find – but what if there is, and the rovers can not “see” it?
New research published today i Nature Communication suggesting that the rover’s current equipment might not be up to the task of finding evidence of life.
As an extreme environment microbiologist, I know the challenges of searching for life when it seems almost impossible.
In astrobiology, we study the diversity of life in locations on Earth with environmental or physical features similar to regions already described on Mars. We call these terrestrial environments “Mars analog” sites.
The new research, led by Armando Azua-Bustos at the Astrobiology Center in Madrid, tested the sophisticated instruments currently in use by NASA’s Curiosity and Endurance rovers – as well as some newer laboratory equipment planned for future analysis – in the Mars analogue of the Atacama Desert.
Azua-Bustos and his colleagues found that the testbed equipment of the instruments for analyzing samples in the field had a limited ability to detect traces of life that we would expect to find on the red planet. They were able to detect the mineral components of the samples, but they were not always able to detect organic molecules.
For my team, our analog sites on Mars are cold and hyperic deserts in the Dry Valleys and Windmill Islands in Antarctica.
In both these settings, life exists despite great pressures. Finding evidence of life is challenging, due to the harsh conditions and scarcity of microbial life there.
First, we need to define the biological and physical limits of life that exist (and are being detected) in analog “extreme” environments. Then we need to develop tools to identify the “biosignatures” for life. These include organic molecules such as lipids, nucleic acids and proteins. Finally, we determine how sensitive instruments need to be to detect these biomarkers, both on Earth and on Mars. This tells us the limits of our perception.
The search for a dark microbiome
In my field of gross microbiology, “microbial dark matter” is when the majority of microscopic organisms in a sample have not been isolated and/or characterized. To identify them, we need to define next-generation sequencing. Azua-Bustos’ team goes one step further, proposing a “dark microbiome” containing Earth’s relict and extinct species.
Azua-Bustos’ team found that sophisticated laboratory techniques could detect dark microbiomes in hyper-arid Martian soil samples like the Atacama Desert. However, the rovers’ current equipment would not be able to detect it on Mars.
In samples with such sparse biomass, we use highly sensitive laboratory methods to detect microbial life, including gene sequencing and visualization of cells using microscopic analysis. Prototypes are being developed for genome sequencing in the field, but they don’t have the sensitivity needed for low biomass samples – yet.
Different planet, different rules
The search for life on other planets also depends on our understanding of what life would need to exist, and the simplest list is energy, carbon and liquid water.
On Earth, most organisms use photosynthesis to extract energy from sunlight. This process requires water, which is almost entirely unavailable in dry desert environments such as Antarctica and the Atacama Desert – and, presumably, Mars. We think that a process we call “atmospheric chemosynthesis” could fill this gap.
My team discovered atmospheric chemistry for the first time in the cold soils of the Antarctic desert. In this unexpected metabolic process, the bacteria literally “live on thin air” by consuming trace levels of hydrogen gas and carbon monoxide from the atmosphere.
We think that dry desert microbiomes may depend on this process for energy as well as water, which is a by-product of the process. Ecosystems like the ones we’ve found in Antarctica hold one of the most promising ecological models in the search for Martian life.
We now believe that there is potential for life in the ice-cemented subsurface of Mars. My team – together with collaborators at NASA and the University of Pretoria – plan to investigate this in Antarctica’s University Valley, by defining the environmental limits for energy, metabolic water and carbon production through the use of trace gases.
We will not find what we cannot define
Our new knowledge of target biomarkers and the level of sensitivity required to detect them will be critical when designing or optimizing future instrumentation for use on missions aimed at finding life.
The goal of future missions to Mars, including the Icebreaker Life mission planned for 2026, is to search for evidence of life. The Icebreaker Life will sample ice-cemented land, such as dry permafrost in the Antarctic, and if it finds signs of life, a Mars Sample Return mission would be a high priority.
Bringing samples back to Earth for laboratory analysis is dangerous. As we found with our Antarctic soil samples, challenges can include contamination, preservation at cold temperatures during transport, and the need for specialized quarantine laboratories, to analyze samples without spoiling them.
But as Asua-Bustos suggests, bringing samples to Earth for detailed laboratory analysis may be the only sure way to detect or confirm the presence (or past presence) of life. excluded.
Armando Azua-Bustos, dark microbiomes and very low organic matter in the Atacama fossil delta reveal the limits of the detection of life on Mars, Nature Communication (2023). DOI: 10.1038/s41467-023-36172-1. www.nature.com/articles/s41467-023-36172-1
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