Lecture 24: Earth, Venus, Mars cont.

Mars is a terrestrial planet with a complex geological history involving a variety of features, including valleys, volcanoes, and evidence of water.

Volcanic Features:
- Mars has fewer volcanic features than Venus or Earth, suggesting significant differences in geological activity.
- Most volcanism took place during the "pre-Amazonian" period (older than ~3 billion years).
Plate Tectonics:
- Presence of features like Valles Marineris raises questions about the existence of plate tectonics on Mars.
- Evidence of lateral offset indicates possible transform motion, akin to faults on Earth (e.g., San Andreas fault).

Valles Marineris:
- Located east of the Tharsis bulge, it is 4000 km long and 200 km wide, dwarfing the Grand Canyon.
- May have formed in response to geological processes related to Tharsis volcanoes.
Martian Dichotomy:
- The stark contrast between heavily cratered southern highlands and smooth northern lowlands may indicate a history of dual geological forces, including possible giant impacts or tectonic actions linked to mantle convection.

Historical Evidence of Water:
- Noachian period (>3.7 billion years ago): Strong evidence suggests the presence of rivers, lakes, and potentially shallow seas in the northern hemisphere.
- Features like dendritic channel networks indicate historical drainage from precipitation.
- Cracked surfaces suggest the existence of dried lake beds and flash flood events during the Hesperian period.
Current Water Presence:
- Today, water mostly exists as ice at polar caps or below the surface as permafrost.
- Mars has ice caps composed mainly of water ice and capped with CO2 ice during winter, demonstrating significant reserves of water.

Ancient Warmth:
- Conditions were likely warmer and wetter early in Mars' history, suitable for potential life.
- Over time, volcanic activity waned, and with it, the greenhouse effect diminished, resulting in a colder, dryer environment.
Atmospheric Changes:
- Possible loss of atmosphere due to solar wind erosion, particularly after the geodynamo ceased, leaving the planet unprotected.
- Theories suggest giant impacts may have contributed to atmospheric depletion, transitioning Mars from habitable to inhospitable.

Geologic Carbon Cycle:
- On Earth, carbon cycles through volcanic activity, weathering, and subduction.
- On Mars, limited subduction and volcanic processes contributed to decreased CO2 levels, affecting temperature and atmospheric pressure.
Atmospheric Pressure:
- Liquid water existence requires a minimum pressure of ~0.006 bar; Mars’ atmospheric pressure is approximately 0.0063 bar, right at the threshold for liquid water stability.

Mars evolved from a planet with conditions potentially suitable for life to a barren desert due to:
- Declining volcanic activity.
- Absence of plate tectonics leading to reduced CO2 influx into the atmosphere.
- Escalating removal of CO2 by silicate weathering.

Continued investigation into Mars’ history and geological features can provide critical insights into its past conditions, climate evolution, and potential habitability.