Lecture 25: Earth, Venus, Mars

Overview of Mars

• Geological History:
  • Mars initially had a thick atmosphere and abundant liquid water for the first ~1 billion years (Ga).
  • The current Martian atmosphere is thin and hostile.

Key Questions Regarding Mars' Changes

1. Where did all the water go?
2. What happened to Mars’ atmosphere?

Possible Explanations for Atmospheric Loss

• Erosion by solar wind after the geodynamo (which provided protection) shut down.
• A significant impact event (e.g., creation of the Hellas Basin) that stripped away much of the atmosphere.

Evidence of Past Conditions

• Hellas Basin:
  • Remnant magnetism in southern highlands shows that Mars’ dynamo may have only functioned for the first 500 million years (Ma).
  • Lack of magnetism in Hellas Basin and northern lowlands suggests limited protection from solar wind.

Atmospheric Measurements

• The Curiosity rover measured isotope ratios in Mars’ atmosphere:
  • Lighter isotope $^{36}Ar$ escapes more easily than heavier $^{38}Ar$.
  • Current atmosphere shows depletion of $^{36}Ar$ relative to $^{38}Ar$ compared to Earth.

Volcanic Activity and Climate

• As volcanic activity declined, the planet became progressively colder and drier, influencing atmospheric CO2 levels.
• The geologic carbon cycle on Earth:
  • Chemical weathering, subduction, and volcanic emissions regulate CO2.
• Mars’ limited subduction leads to an eventual decline in atmospheric CO2 influx.

Conditions for Liquid Water

• Liquid water requires a minimum atmospheric pressure of approximately $0.006$ bar.
• Mars’ atmospheric pressure averages around $0.0063$ bar, close to the triple point of water.

Evolution of Mars

• Transition from habitable conditions to inhospitable desert due to:
  • Declining volcanism.
  • Lack of plate tectonics.
  • Erosion by solar wind and removal of CO2 leading to cooling and atmospheric thinning.

Remaining Water on Mars

• Some water likely exists as ice, notably in polar caps and subsurface layers.
• Research Goals:
  • Assess quantities and locations of water to understand climate change potential and habitability.

Discoveries by Mars Probes

• The Mars Phoenix lander (2008) found ice close to the surface.
• Mars Odyssey orbiter mapped water ice distribution through gamma ray spectroscopy.
• Equatorial warmth causes ice to evaporate, redistributing water vapor to polar regions.

Potential for Life

• Extremophiles (e.g., certain bacteria) may survive in harsh conditions, including beneath Martian ice.
• Antarctica’s bacteria example raises possibilities for similar organisms on Mars.

Evidence of Possible Life

• 1996: Evidence of bacterial life claimed from Martian meteorite ALH84001 but largely believed to stem from contamination.
• Detection of methane in Martian atmosphere raises questions about biological processes:
  • Methane is transient, suggesting active replenishment mechanisms (likely microbial).

Curiosity Rover Findings

• Sample Analysis at Mars (SAM) has recorded spikes of methane, indicating possible sources.
• Organic Compounds:
  • Discovery of chlorobenzene and long hydrocarbon chains in soil samples, which may have biological significance but are not confirmed as such.

Future Research Needs

• Search for evidence of life requires sample collection and return to Earth for detailed analysis.
• Perseverance rover is currently exploring Jezero Crater, a historic lake bed, to find signs of past microbial life and collect samples.

Geological Context

• Comparison of sedimentary rock layers from Mars (Gale Crater) to lake-bed sediments on Earth, both are potential indicators of past life.