Download Habitable Zone - Wando High School

2.2.1 - Requirements for Life
https://lcogt.net/spacebook/what-are-requirements-life-arise-and-survive/
Background: We are alive. So obviously life exists on Earth. It is also possible that it exists elsewhere. But what are some
major requirements for life? There are many different requirements for life depending on which source you refer to, but it
appears that most sources come to a point with these 5 requirements: Planet being within the Habitable Zone, Liquid Water,
Complex Chemicals, Protection from UV Rays, and Plate Tectonics. The following will explain why.
Habitable Zone
And at some point 4.1 bya, these conditions were met for the first time. But
what was necessary for this to happen on Earth? There seem to be a few
requirements of that a planet must meet in order to be habitable for life.
First, the planet must fall inside the Habitable Zone. As you can see in the
image to the right, how far away the habitable zone is depends on the
size/strength of the sun. The bigger the sun, the further away the habitable
zone is. Stars that are much larger than the Sun have much short lifetimes,
which it is unlikely that there would be enough time for any kind of life,
particularly complex life, to develop. If the star is too small, the planets in the
habitable zone are likely “tidal locked”, which means that like our moon, the same hemisphere of the planet faces the star all the
time, causing extreme temperatures on both sides of the planet that likely don’t allow for liquid water, which is the key to all of
this.
Distance from Sun
With our sun, Earth is the only planet inside that zone. If the planet was closer to the sun, global temperatures would increase
due to greater solar heating. If Earth were 0.88 AU, and not 1 AU from the sun, it would be exposed to enough extra solar
heating that it would overheat the atmosphere and trigger a runaway greenhouse effect much like Venus is today. Conversely,
if the planet were further from the sun, solar heating would reduce. At a distance of 1.4 AU (compared to our current 1 AU),
water would freeze out and the planet would “Snowball Earth.”
Size of Planet
Astronomically, there are 2 other factors that seem massively important. The first is that the planet can’t be too big or too
small. If the Earth is too small, the interior cools too rapidly. This could cause plate tectonics to stop and its magnetic field
would be lost, causing it’s atmosphere to be lost to solar wind. Essentially, it becomes too small to retain a warm atmosphere.
Mars is a great example of this. It is nearly inside the Sun’s habitable zone, but it is a frozen desert world with a solid interior,
no magnetic field, and virtually no atmosphere because it is 1/3 the size of Earth.
On the other hand, if the planet is too large, it becomes capable of retaining too much H and He in its atmosphere, which
causes problems with liquid water and the basic chemistry interactions with the planet. This implies that not only do you have
to be in the Habitable Zone, but you have to be the right size. A rough estimate is that the planet must be between 0.2 – 10
Earth masses. The second major astronomical factor is that having a giant planet near you seems important, as it’s massive
gravity pulls in asteroids towards it that might have impacted our hopeful planet if that giant had not been there.
Liquid Water
One other major reason this Habitable Zone is so important is that within this zone, liquid water is possible. Earth is suitable for
life because of its unique orbital position that allows for water to exist in all three phases on the surface. Water makes Earth
unique. The hydrosphere, Earth’s mass of liquid water that is constantly on the move, is vital to life within it and also to life on the
land. Liquid water is essential because biochemical reactions take place in water. Water has many unique physical and
chemical properties that make it well suited to support the complex chemistry required for life. Life forms are usually made
primarily of water. In fact, our human bodies are more than 60% water. Water is also an excellent solvent that easily dissolves
many substances and it also has a high heat capacity, which means it takes a lot of energy to cause water to change
temperature. This property of water gives Earth its moderate climate.
Complex Chemicals
The planet must contain essential complex chemicals. Life as we know it
contains specific combinations of the 6 elements required for all life: carbon,
hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS) that are
required for all life. They are used in different combinations to make proteins
(used for structure, hormones, enzymes) carbohydrates (served as fuel for
energy), lipids (stored energy and to make cell membranes) and RNA/DNA
(stores genetic information used to make new cells). All the basic elements
are formed in stars and distributed throughout space as a result of giant explosions called supernovas. Since these essential
chemicals are quite common in other places in the Universe we can expect that the development of life somewhere else is also
possible.
Protection from Harmful UV Radiation
Life also needs protection from harmful ultraviolet (UV) radiation given off by the sun.
UV light can damage or break complex molecules, causing mutations (changes to the
genetic code in RNA/DNA) that may inhibit the ability for organisms to reproduce.
Enough exposure to UV will kill organisms, and it is commonly used as a disinfectant
(for instance, it is how we clean the lab goggles). As you saw yesterday, life first
developed in the oceans, because water is a wonderful shield from UV radiation. Life
couldn’t thrive on land until it had some protection from that radiation, which means it
was nearly 3 billion years after photosynthesis developed that there was finally enough
wasted oxygen given off that the atmospheric oxygen levels were great enough for the
ozone layer (O3) to form. This ozone layer finally offered protection from UV radiation
outside of the ocean, and within 70 million years of that life began to thrive on dry land.
Currently, most life depends on oxygen in the atmosphere as well (there are cases of
bacteria called extremophiles that can survive on deep oceanic hydrothermal vents or deep inside rocks, but they are far and
away exceptions), but that oxygen was a byproduct of photosynthesis and not required for initial life to begin. This is a giant
example of how life can drastically change the conditions of it’s surrounding environment (photosynthesis develops and the
atmosphere begins to be filled by oxygen), as well as how changes in the environment can affect which organisms are
successful (bacteria that were dependent upon high levels of CO 2 and CH4 in the air became extinct as atmospheric oxygen
levels increased significantly because of this photosynthesis).
Plate Tectonics
Unlike on Venus and Mars, the crust of the Earth is constantly being recycled as the crust moves via plate tectonics. This keeps
the carbon dioxide levels in the atmosphere from getting too high or too low. If the levels become too high, (as they did on
Venus) they act as a greenhouse gas and the planet becomes too hot. Liquid water evaporates and the surface of the planet
dries up. If the levels become too low, the planet cools and an ice age begins. This has happened several times in Earth’s
history. However, Earth has been pulled out of multiple ice ages because of the motion
of the plates and the continued recycling of the carbon in rocks. Plates interacting
causes volcanism, which releases CO2 into the atmosphere. When this occurs over
millions of years of an ice age, atmospheric levels of carbon dioxide rise to a high
enough point that it begins to warm the planet again. Without this carbon cycle, planets
don’t seem to be able to maintain a climate balance to sustain life.
Recap
All of the aforementioned factors that were necessary for life to exist on a planet were abiotic factors. These are factors that are
non-living factors that chemical and physical factors that affect the atmosphere (air around a planet), the hydrosphere (all the
water on a planet’s surface and in the atmosphere), the geosphere (a planet’s rock and mineral content from surface to the
core), and the biosphere (all the life on a planet). These conditions necessary for life to start didn’t take into account anything
biotic (living components of an ecosystem), which certainly also affect an organisms ability to find food, avoid becoming food,
and find a mate to reproduce with. In short, the abiotic factors necessary for life to form are:
 Planet being within the Habitable Zone  so that liquid water forms and stays on the planet
 Liquid Water  biochemical reactions take place in water and water is great at dissolving and moving minerals
 Complex Chemicals  CHNOPS need to be present to combine and form Proteins, Carbohydrates, Lipids, RNA/DNA
 Protection from UV  Oceans, protective atmosphere, or deep crevasses to prevent UV radiation from killing organisms
 Plate Tectonics  to facilitate the carbon cycle and balance CO2 in the atmosphere, maintaining livable climate
Review Questions
1.
2.
3.
4.
What are the 5 requirements for life to appear on a planet?
What is the problem with very large stars, in terms of life span, when it comes to life forming?
What occurs if a planet Earth’s size is close enough to a star that it is no longer in the Habitable Zone?
What happens if the planet is too small? Based on what you read later about plate tectonics, why does that
seem important?
5. Why is liquid water such an important factor? Give at least 2 reasons?
6. What are the 6 elements necessary for all life?
7. What are the 4 basic biomolecules those elements can make and what is the purpose of each one?
8. Why do we need protection from UV radiation? What protected the dinosaurs (and us humans) from UV?
9. What is the difference between biotic and abiotic factors?
10. List and define the 4 “spheres” on Earth.