Astrobiology and the Search for Life on Mars

Overview

Is there life on Mars? This question has driven planetary exploration since the Viking landers in 1976. In this lesson, students approach this question as scientists: examining evidence, understanding the chemistry of life, studying organisms that thrive in Mars-like conditions on Earth, and designing experiments that could definitively detect life on Mars. Along the way, they develop critical thinking skills for evaluating extraordinary claims.

Background for Teachers

The Astrobiology Framework

Astrobiology asks three fundamental questions:

1. How does life begin and evolve?
2. Is there life beyond Earth?
3. What is the future of life in the universe?

For Mars specifically, the search focuses on biosignatures — measurable indicators of past or present life:

Morphological biosignatures: Physical structures that suggest biological origin (microfossils, stromatolites, biofilms)

Chemical biosignatures: Molecular evidence of biological processes:

• Organic molecules with biological complexity patterns
• Isotopic fractionation (living systems preferentially use lighter isotopes)
• Chirality (life on Earth uses only left-handed amino acids and right-handed sugars)
• Lipid biomarkers preserved in rocks

Atmospheric biosignatures: Gases that are difficult to explain without biology:

• Methane (detected on Mars — source unknown)
• Oxygen in disequilibrium with reducing gases

Key Evidence from Mars

Supporting habitability:

• Ancient lake and river systems (Gale Crater, Jezero Crater)
• Clay minerals and sulfate salts indicating sustained liquid water
• Organic molecules detected by Curiosity (thiophenes, aromatic and aliphatic compounds)
• Seasonal methane variations detected by Curiosity
• Subsurface water ice confirmed by multiple missions
• Boron detected in Gale Crater — an element important for RNA stability

The ALH84001 Controversy: In 1996, NASA scientists announced that Mars meteorite ALH84001 (found in Antarctica) contained possible evidence of ancient Martian life: carbonate globules, mineral assemblages, and structures resembling bacterial fossils. The claim sparked enormous debate. Most scientists now believe the features can be explained by non-biological processes, but the case remains instructive as an example of how science evaluates extraordinary claims.

Extremophiles Relevant to Mars

Lesson Procedure

Day 1: Evidence and Extremophiles (45 minutes)

Opening: The Viking Experiment (10 minutes)

In 1976, NASA’s Viking landers conducted the first life-detection experiments on Mars:

• Labeled Release: Added nutrients with radioactive carbon to Mars soil. Detected radioactive gas release — possibly metabolism?
• Gas Exchange: Exposed soil to water and nutrients, monitored gases. Detected oxygen release.
• Pyrolytic Release: Tested if organisms could incorporate CO2 in the presence of light. Ambiguous results.
• Gas Chromatograph-Mass Spectrometer: Found no organic molecules in the soil.

“The biology experiments gave positive results. The chemistry experiment gave a negative result. Scientists were divided. What would you conclude?”

Discussion: This ambiguity drove decades of further research. The GCMS result (no organics) was later partially explained by the discovery of perchlorates in Mars soil, which would have destroyed organic molecules during heating.

Evidence Evaluation Activity (15 minutes)

Provide student groups with a “Mars Life Evidence Packet” containing 8-10 pieces of real evidence, each on a separate card with the source mission/instrument cited.

Students categorize each piece of evidence into a matrix:

Each categorization must include a brief justification.

Debrief: Compare group categorizations. Where do groups disagree? What additional data would resolve the disagreement? This mirrors real scientific debates.

Extremophile Investigation (20 minutes)

Option A (with lab materials): Students examine prepared slides of extremophile organisms under microscopes, sketching morphology and noting adaptations.

Option B (without lab materials): Students analyze detailed case studies of 3-4 extremophiles, completing a structured analysis for each:

• What extreme condition does this organism tolerate?
• What biochemical adaptation allows survival?
• Where on Mars might similar conditions exist?
• Could this type of organism survive on Mars? What additional challenges would it face?

Key discussion: “The existence of extremophiles does not prove life exists on Mars. It proves that life as we know it COULD potentially survive in some Mars environments. There is a crucial difference between ‘could’ and ‘does.’”

Day 2: Experimental Design and Critical Thinking (45 minutes)

The ALH84001 Case Study (15 minutes)

Present the chronology:

1. 1984: Meteorite found in Allan Hills, Antarctica
2. 1993: Identified as Martian origin (gas composition matches Mars atmosphere measured by Viking)
3. 1996: McKay et al. publish in Science — four lines of evidence suggesting ancient biological activity
4. 1996-2010: Extensive debate; each line of evidence challenged with non-biological explanations
5. Present: Scientific consensus is that the evidence is not conclusive, but the meteorite remains under study

Student analysis: For each of the four original lines of evidence, students examine:

• The original claim
• The alternative non-biological explanation
• What additional evidence would be needed to distinguish between the two

Lesson: “Extraordinary claims require extraordinary evidence” (Carl Sagan). The burden of proof for claiming the discovery of extraterrestrial life is extremely high — and appropriately so.

Life-Detection Experiment Design (25 minutes)

Students work in teams to design a life-detection experiment for a future Mars mission.

Design constraints:

• Must detect at least one type of biosignature (morphological, chemical, or atmospheric)
• Must include a control that distinguishes biological from non-biological processes
• Must function in Mars surface conditions (low pressure, cold, radiation)
• Mass limit: instrument must weigh less than 10 kg
• Power limit: must operate on less than 50 watts

Design template:

1. Hypothesis: What specific type of life are you looking for? (Past or present? Microbial? In what habitat?)
2. Biosignature target: What measurable indicator will you detect?
3. Instrument design: What does your instrument do? How does it work?
4. Sample type: What Mars material will you analyze? (soil, rock, ice, atmosphere)
5. Control experiment: How will you distinguish a biological signal from a non-biological one?
6. Positive result: What result would indicate life? What confidence level would you assign?
7. Negative result: What result would indicate no life? Can you ever prove life does NOT exist?

Teams present their designs for peer review. Class evaluates each design for scientific rigor and feasibility.

Wrap-Up: The Significance of the Question (5 minutes)

“Finding life on Mars — even fossilized microbes — would be one of the most profound discoveries in human history. It would mean life arose independently on two planets in the same solar system, suggesting that life may be common throughout the universe. When humans reach Mars, they will continue this search with the most powerful tools of all — human eyes, hands, and minds, working directly on the Martian surface.”

Assessment

• Evidence matrix: Accurate categorization with scientifically sound justifications
• ALH84001 analysis: Demonstrates understanding of the distinction between correlation and causation in scientific claims
• Experiment design: Complete, scientifically rigorous design with appropriate controls and clear criteria for positive/negative results
• Critical thinking: Written reflection on what constitutes sufficient evidence for claiming the discovery of extraterrestrial life

NGSS Alignment

• HS-LS1-2: Develop and use a model to illustrate the hierarchical organization of interacting systems
• HS-LS2-7: Design, evaluate, and refine a solution for reducing the impacts of human activities on the environment and biodiversity
• HS-ESS1-6: Apply scientific reasoning and evidence from ancient Earth materials, meteorites, and other planetary surfaces to construct an account of Earth’s formation and early history
• HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems

Extensions

• Research the Mars Sample Return mission — what life-detection analyses are planned for the returned samples?
• Debate: Should planetary protection protocols prevent us from sending humans to Mars (who carry trillions of microbes) before we have searched for indigenous life?
• Investigate the concept of panspermia — could life have traveled between Earth and Mars via meteorite exchange?
• Analyze real Curiosity SAM (Sample Analysis at Mars) data for organic molecule detections
• Research the Europa Clipper and Dragonfly missions — how do life-detection approaches differ for ocean worlds vs. Mars?