Lecture 25: Venus, Earth, Mars cont.

Mars initially had abundant liquid water on its surface for approximately the first billion years (1 Ga) of its history.
The Martian atmosphere was likely much thicker in its early history, enhancing greenhouse effects conducive to liquid water.

Where did all the water go?
What happened to Mars’ atmosphere?
- Possible atmospheric erosion due to solar wind after the shutdown of the geodynamo (the magnetic field generator).
- A giant impact might have drastically altered the atmosphere, making Mars uninhabitable.

Hellas Basin: This ancient impact site (around 4 billion years old) shows evidence of remnant magnetism only in the southern highlands, suggesting the geodynamo was active for only the first 500 million years (Ma) of Mars' history.
Post-geodynamo, Mars lacked a protective magnetic field, allowing solar winds to gradually erode the atmosphere.
The smaller size of Mars contributed to the difficulty in maintaining a geodynamo or retaining an atmosphere.

Mars' atmosphere lost lighter isotopes of argon more readily than heavier ones.
Curiosity rover measurements show the atmosphere is depleted in the lighter isotope, $^{36}Ar$ , relative to $^{38}Ar$ , indicating significant atmospheric escape over time.

Volcanic activity on Mars declined around the same time global temperatures dropped, resulting in the planet becoming colder and drier.
On Earth, the geologic carbon cycle involves:
1. Chemical weathering of rocks
2. Subduction of carbon into the mantle
3. CO2 release from volcanoes.
Limited subduction and declining volcanic activity on Mars reduced CO2 flux into the atmosphere over time, contributing to climate change.

Liquid water requires a minimum atmospheric pressure of approximately 0.006 bar.
Mars currently has an average pressure of 0.0063 bar, just above this threshold—but very directly at the water's triple point.

The transition from a habitable world to a frozen desert can be attributed to:
- Declining volcanism leading to reduced atmospheric CO2.
- CO2 removal via silicate weathering and solar wind erosion, resulting in a thinner atmosphere.

Water on Mars exists mostly as ice, located in polar caps and potentially underground, possibly buffering the atmosphere.
Major upcoming research aims to locate water reserves to understand climate change on Mars better, and the Phoenix lander found ice just below the Martian surface in 2008.

Mars Odyssey orbiter used gamma ray spectroscopy to map ice distribution in the top meter of Martian soil.
Ice cannot survive near the equator due to higher temperatures; hence, water vapor moves to higher latitudes where it freezes.

Life can exist in extreme conditions; scientists found bacteria thriving beneath ice in Antarctica, suggesting a potential for similar life beneath Mars' surface.

Certain extremophile bacteria tolerate low temperatures and could survive on Mars despite harsh conditions, possessing natural antifreezes in their cells.

In 1996, NASA scientists thought they found ancient microbial life in a Martian meteorite (ALH84001) based on purported structures and organic compounds; however, many now believe this to be contamination.

In the early 2000s, methane plumes were detected on Mars, which can originate from biological processes on Earth.
The Curiosity rover's Sample Analysis at Mars (SAM) instrument detected sporadic methane spikes, with potential sources still under investigation.

SAM discovered chlorobenzene, indicating the presence of organic compounds in Martian sediment, though their origin remains uncertain; they could stem from biological or non-biological processes.

Collecting and returning Martian samples to Earth for analysis is essential for resolving questions about Mars' past life.
Perseverance rover is specifically collecting samples from an ancient lakebed, Jezero Crater, aiming to search for signs of past life.

Mars features thinly-bedded, laminated rock layers indicating past lake deposits, similar to those on Earth that contain evidence of ancient life.
Ongoing studies will further elucidate the environmental history of Mars and potential for past life forms.