North Hawaii News Articles from CFHT

Life in the Universe I: Earth

One of the newest and most exciting branches of astronomy is astrobiology, the study of all living things in our Universe, wherever they might be (here on Earth, on Mars, or around another star) and whatever form they might take (a single cell, a huge elephant or a Star Trek like extravagant creature). One branch of astrobiology is exobiology, the study of life outside of Earth.

I will surprise no one by stating that there is Life on Earth: single cell beings, multicellular ones, plants, animals, intelligent life-forms (that's you)... We take that for granted, and sometimes assume that because Life is thriving here on Earth, it probably is elsewhere in our Universe. Same for intelligent life.

But, what is Life, exactly? How can we define it? What general conditions (temperature, pressure, etc.) are suitable for life? Where in our Solar System could life exist? How can we prove that something is alive, or inert?

What is life? Scientists have not yet found a suitable definition that would precisely include what we know is alive, and exclude what we know is not. A gecko may be a "living system with a definite boundary, continually exchanging material with its surrounding, but without altering its general properties, at least over some period of time", but so is fire, and fire is not alive!

We may adopt a "Darwinian evolution" definition, which includes self-reproduction, genetic variation and natural selection over the course of time, but then, how long do we have to wait for a sign of evolution in a sample we think might be alive? Hours? Months? Years? Centuries?

For every definition that seems reasonable, whether based on metabolism, physiology, biochemistry, genetics or thermodynamics, there are counter-examples of living systems that do not fit that particular definition. Moreover, some chemical reactions sometimes mimic life and might fool us into thinking we have detected a new life-form. What is alive and what is not, can be a tricky question.

We think that life (as we know it) requires in general a source of energy (usually sunlight), a source of carbon (food) and liquid water (remember that our bodies are mostly constituted of water). We also think of reasonable temperature and pressure conditions, and presence of oxygen (just think of the summit of Mauna Kea, where there is only 60% of the oxygen available at sea level, and no animal or plant live - Do you think anything could survive, then, with no oxygen at all?).

But in recent years, scientists have discovered life-forms on Earth where no one could have expected them because one or more of those requirements for Life is missing.

For example, some insects or small reptilians live deep in the dark, in caves that never see the Sun. Those creatures do not have eyes, since they are not useful at all! But they eat plants they find in those caves, reproduce, and die.

Tube worms, spider crabs and even fish survive near sources of very hot water and minerals spewing from the Earth's center, almost crushed by enormous pressure at 3-5 km (2-3 miles) below the ocean's surface. These animals eat microorganisms that use the minerals coming out of those "smokers", as they are called.

Some bacteria happily swim in geothermal sources where the temperature is above the boiling point of water; the upper limit so far is 115 degrees C (239 degrees F)!

It is also possible to find bacteria and multicellular life-forms in rock cores brought from miles down when looking for oil and natural gas, or in tiny cracks of some granite rocks.

Antarctica is known to be a much colder place than Hawaii, with average winter temperatures of -60 degrees C (-76 degrees F) that can go as high as 14.6 C (58.3 F) or as low as -89.2 C (-128.6 F); nevertheless, bacteria, yeast, fungi and algae are present in the snow near the surface of the frozen continent, as well as deep in the ice. These microorganisms survive the extremely cold temperature by hibernating during the winter. Some of those organisms have even been revived after being frozen for over 200,000 years!

Simple life forms can even survive on the Moon! In 1969, after almost 3 years on the Moon, bacteria that had hitchhiked from Earth on the U.S. Surveyor moon lander was retrieved by Apollo astronauts and found to have survived by hibernating. The hundred or so organisms survived launch, the vacuum of space and Moon, years of harsh radiation from the Sun, deep-freeze at an average temperature of only 20 degrees above absolute zero (-253 C, or -425 F), and no nutrient, water or energy source.

Many other examples can be found, such as bacteria that live inside nuclear reactors or even in sulfuric acid pools!

If some life-forms can manage to live in those extreme environments, could then similar beings exist on hot and airless Mercury, pressure-crushing Venus, dry and cold Mars, gaseous Jupiter, the frigid Moon? Where in our Solar System are our best chances of finding Life?

For the moment, scientists concentrate their search for life on Mars, Europa (one of Jupiter's moon) and Titan (Saturn's main satellite), and we will visit those potential life harbors in a coming article...

Nadine Manset
Resident Astronomer
Canada-France-Hawaii Telescope Corporation
Jan. 4, 2001