Section 8 - Habitability and Extra-Terrestrial life

Question 1

• What are the fundamental requirements for life? What are these informed by?
• What are some common sources of energy for life?
• Why is carbon thought to be needed for life?
• What is likely to be the key limiting factor for extraterrestrial life?

Answer 1

• Energy and carbon sources, and liquid water. Studies of extremophiles such as tardigrades.
• From solar radiation (photosynthesis), or from chemical processes (chemosynthesis).
• Needed for complex organic chemistry, but unlikely to be a neccessary factor as not most abundant element.
• Liquid water. Required by all life on Earth, but is rare in the Solar system.

Question 2

• What are 3 of the most abundant elements?
• Carbon-based life in water likely to be ________.
• Why is carbon unique?
• Water is the most abundant _____ and it is in the liquid state for a ____ range of temperatures and _____. Freezes from the ___ ____, protecting sub-surface ___.

Answer 2

• Hydrogen, oxygen and carbon.
• Ubiquitous (guaranteed)
• Carbon has the unique ability to form complex organic compounds.
• Solvent, wide, pressures, top down, oceans.

Question 3

• What are the functions of hydrothermal vents?

- Where are hydrothermal vents found on Earth?

Answer 3

• Support abundant life, providing fresh nutrients circulated from volcanic rocks. Has been suggested that life may have begun at hydrothermal vents.
• At mid-ocean ridges, where ocean plates are pulling apart and new ocean crust is forming.

Question 4

Water:
- At low pressures (0.006 atm) similar to those on Mars there is no _____ state and ice _____ to ____ phase.
- Water under pressure can remain in ______ state well above 100 deg C, but life probably _____ by fragility of _____ _____ compounds.
Young Sun paradox:
- How long ago was fossil and chemical evidence for life on Earth from? What does this suggest?
- Why is this surprising?

Answer 4

• Liquid, sublimes, gas
• Liquid, limited, complex organic.
• 3.8 Gyr, suggests life began almost as soon as conditions were favourable, which might support the view of finding life in the Solar system.
• Sun was 25% less luminous at the time. Faint Young Sun Paradox. Suggests a stronger greenhouse effect on Earth at the time.

Question 5

• Why has Mars been the focus for the efforts to detect extra-terrestrial life in the Solar system?
• What are the chances of finding liquid water on Mars?
• What is the evidence for past liquid water on Mars?
• What do impact craters tell us about these?

Answer 5

• It is the only other object in the Solar system which can plausably have liquid water at the surface.
• Has an atmosphere and is in the habitable zone of the Sun, where surface temperatures allow liquid water to exist. But atmospheric pressure of Mars very close to the triple point of water, below which liquid water is not possible. But pressure is slightly higher in deepest canyons and craters.
• Evidence for past river valleys and deltas, as well as geological and chemical evidence from sedimentary rock formations and hydrated minerals.
• Show these are ancient, dating from early history of Mars.

Question 6

• What is the habitable zone?
• Where are Earth and Mars located?
• Why is Mars only borderline habitable?
• What about water ice on Mars?

Answer 6

• The distance from a star where an Earth-like planet could have liquid water at the surface.
• In the habitable zone, with Earth at the inner edge.
• Due to low atmospheric pressure due to atmospheric escape. Mars has an enhanced D/H ratio, implying significant loss of water to photolysis and atmospheric escape.
• Water ice and seasonal variations have been detected, suggesting at least the occasional presence of liquid water on Mars.

Question 7

• When has evidence for liquid water on Mars been ambiguous?
• When has evidence for life on Mars been ambiguous?
• Evidence for sub-surface _____ of liquid water below the _____-Polar icecap of Mars. Water likely to be _____-_____ so may not be a suitable habitat for life.

Answer 7

• Recurring Slope Lineae (RSL) are dark streaks of downward slopes that vary with seasons, and were thought to be seasonal water flows, but these are only found on steep slopes, suggesting they are due to slows of dry sand.
• Evidence for fossils found in Martian meteorite, but has been controversial and abiotic explanations proposed.
• Lakes, South, hyper-saline.

Question 8

Venus:

• Where are liquid water droplets available on Venus? What could this provide?
• Suggest a possible lifecycle of microbes on Venus.
• What is a piece of evidence for life on Venus?

Answer 8

• In the form of concentrated sulphuric acid in the middle and lower cloud levels, where the temperature is lower and atmospheric pressure similar to Earth. Provides a potential habitat for microbes to survive.
• Microbial spores carried by updraughts from the hot lower atmosphere to the cloud-forming layer. Spores nucleate water droplets, allowing them to become metabolically active, grow into cells and divide. As water droplets grow and sink, heating triggers spore formation.
• Detected phosphine in atmosphere of Venus. Known product of some bacteria on Earth, and difficult to explain abiotically, as phosphorus should be in oxidised forms in Venus’ atmosphere.

Question 9

Europa:

• The young ____ surface of Europa is very _____, showing ongoing ______ activity. Surface features thought to arise from _____ _____, with the crust fracturing into plates, which move on top of _____ cells, leading to collisions an subduction.
• Why is Europa a likely place for life in the Solar system?
• Why are Ganymede and Callisto less likely to have life?

Answer 9

• Icy, geological, tectonic activity, convection.
• Relatively volatile poor compared to Ganymede and Callisto, but enough water for a global sub-surface ocean. Shallow enough to remain liquid down to rocky interior, with hydrothermal vents likely to cycle nutrients into the ocean.
• Weaker tidal heating and older surfaces, so unclear whether they have liquid water oceans. If they have deep oceans, the ocean floor may be solid ice, due to extreme pressure and lacking hydrothermal vents.

Question 10

Enceladus:

• Surface is young and exhibits ______ _____ similar to Europa.
• Why is enceladus a likely place for life in the Solar system?
• What about Titan?

Answer 10

• Tectonic activity
• Detected water geysers breaking through the icy surface. Found molecular hydrogen in the plumes, indicating hydrothermal vents and providing a potential energy source for life.
• Found lakes of hydrocarbons, and are the only bodies of standing surface liquid in the Solar system. Could replace water as the solvent needed for lifeforms to evolve.