Lecture 23: Earth, Venus, Mars cont.

Differences from Earth:
- Surface Temperature: Extremely hot due to a runaway greenhouse effect.
- Lack of Plate Tectonics: Possibly caused by dry mantle conditions resulting from the greenhouse effect.
- Absence of Magnetic Field: Problems related to the runaway greenhouse effect may also be linked.
Runaway Greenhouse Effect:
- Defined as a state where a planet’s atmosphere thickens with greenhouse gases, trapping heat and preventing water from existing in liquid form.
- Without liquid water, plate tectonics cannot occur because the mantle lacks hydration, leading to a stiff state.
Current Understanding of Venus:
- Scientists are uncertain if Venus always had a runaway greenhouse state or was once habitable. There is a theory that it had oceans initially before evolving into a runaway state due to solar luminosity increase.
Surface Composition:
- The presence of granite (high-SiO2) versus basalt (lower-SiO2) could provide clues to Venus's past oceans and tectonic activity. Ancient granite indicates a history of liquid water.
Comparison with Earth:
- Venus may have been more Earth-like initially, with oceans that were lost over time, leading to its current state.
- Future changes in Earth, influenced by solar evolution, may mirror Venus's fate in 1-2 billion years.

Why Stable Temperature?:
- Earth’s temperature has remained steady despite an increase in solar energy over 4.5 billion years due to the long-term carbon cycle.
Carbon Cycle Interactions:
- Atmospheric CO2 levels are regulated by volcanic activity and chemical weathering.
- If temperatures rise, weathering increases, reducing atmospheric CO2 and cooling the planet, creating a negative feedback loop.

Himalayas and Climate:
- The Indian subcontinent's collision with Asia led to the Himalayas’ formation, which increased silicate weathering and reduced atmospheric CO2, contributing to planetary cooling.
Feedback Mechanisms:
- Negative feedback (like chemical weathering) stabilizes climate, whereas positive feedback (like ice-albedo) can lead to dramatic shifts.
Snowball Earth Events:
- Geological evidence suggests instances where carbon cycling led to global glaciation, reinforcing the idea of climatic tipping points.

Basic Overview:
- Mars has about 11% of Earth's mass and orbits at 1.5 AU from the Sun, receiving around 50% solar energy. There is extensive evidence for past volcanism and signs of liquid water.
Mission History:
- Mariner 4 was the first mission to photograph Mars, revealing a heavily cratered surface. Since then, over 50 missions have improved our understanding of Mars despite various failures.
Geological Characteristics:
- Mars hosts large volcanoes in the Tharsis Region, which likely formed from a long-lived mantle plume. Olympus Mons is significantly larger than terrestrial volcanoes due to Mars’s lack of tectonic activity allowing for continued growth over time.
Volcanism and Habitability:
- The evolution of Mars’s surface and its volcanic history are interconnected, suggesting that the planet may have been warmer and wetter in its early history before transforming into its current arid state.
Current Volcanic Activity:
- Mars might still be volcanically active, as evidenced by Martian meteorites dating back 170 to 180 million years.

The geological history of Venus and Mars highlights the importance of liquid water, greenhouse gases, and plate tectonics in shaping planetary environments and their capacity to support life. Earth’s dynamic systems maintain a stable climate compared to the extremes observed on Mars and Venus, suggesting future challenges as the Sun continues to age and climate systems change.