Lesson 63: Life in the Solar System
In this life, we will explore the possibility of life on other worlds, starting
with a look at Mars and then other worlds in our solar system. We will also
learn about the requirements for life on the surface and the concept of a
habitable zone around a star. Finally, we will discuss scenarios in which life
might develop on worlds very different from Earth.
A habitable world is a world that has all the necessities of life (nutrients,
energy and liquid water).
Scientists have considered Mars to be habitable due to early evidence of liquid
water. The most direct way to search for life has been to send rovers and
landers. Unfortunately, results from experiments including ones conducted by
the two Viking landers that reached Mars in 1976 have been inconclusive.
Martian soil is very different from
Earth soil. While it contains organic molecules, it also contains perchlorate,
a substance known to kill organic molecules.
Apart from rovers, scientists have also studied Mars using meteorites that
appear to have come from Mars, and by analyzing methane gas that is believed to
be abundant on Mars. If methane is truly being produced on Mars, it would
indicate at the very least geological activity, and maybe even the possibility
of underground liquid water where life could flourish.
Can life exist on other planets in the solar system?
This question has intrigued scientists for a long time. One planet that
possibly could possibly harbor life is Jupiter. Jupiter has three moons -
Europa, Ganymede and Callisto, all of which appear to have subsurface oceans.
Europa is considered the best candidate for potential life because the ice and
rock that formed Europa also includes the chemical ingredients necessary for
life. Europa's internal heating is similar to the seafloor vents in Earth's
oceans.
Given the amount of ice on Europa, it would be difficult to penetrate to the
ocean below to investigate possible life forms. Another potential candidate is
Saturn with its two moons, Titan and Enceladus.
What do you need to live on the surface?
There are four essential conditions that
have made Earth habitable for long term life. They are:
- the proper distance from
the Sun to allow water vapor to
condense so that rain occurs and oceans form but not too far so that all water
freezes
- volcanism to release trapped gases such as water vapor and carbon dioxide to
make an atmosphere and oceans
- plate tectonics to support a climate-regulating carbon-dioxide cycle
- A planetary magnetic field to protect the atmosphere from the solar wind
Habitable zone is the region around the star in
which it is possible for a planet to
have just the right conditions for surface liquid water. This zone is dependent
on many factors such as the planet or star's mass as that affects temperature
and total energy output as well as the distance from other planets and stars.
A habitable surface is the surface area that supports life located on a planet
in a habitable zone that is large enough to support volcanism and plate
tectonics. To live on a planet's surface, both internal heat and volcanism are
necessary.
Earth also has a global magnetic field that
protects conditions of life. To maintain this field, a planet has to have
enough internal heat for core convection and rotate fast enough to twist and
distort the convection pattern. Finally, a planet may also need to be big
enough - at least Earth's size - to support volcanism and plate tectonics.
The variety in
extrasolar planets has led scientists to
consider scenarios which may look quite different from what we have on Earth
but still provide conditions suitable for life.
Other types of worlds that may prove to be habitable are:
- Moons with habitable surfaces
- Super Earths and Water worlds
- Worlds with Subsurface Habitability
- And Orphan planets
We would need much more powerful telescopes to test out these ideas fully,
using information from
images and infrared spectra to look for the presence of atmospheric gases
needed to support organic life forms.