Objectives
After viewing the video and participating in one or more of the Hands-On Activities, students
will be able to:
• define "life as we know it" on Earth, and also suggest more fundamental characteristics
• know what "extremophiles" are, and describe places on Earth where they can be found
• discuss places in our solar system, other than Earth, where conditions mean life could
once have, or might still, exist
Program Description
As can be seen in the National Science Education Standard referenced below, the idea that life
can exist anywhere apart from Earth is a comparatively recent one, and still unproven. This
program gives students a chance to listen to two of America's leading "exobiologists"
(researchers who think about life beyond Earth, also sometimes called "astrobiologists"). Chris
Chyba works at the SETI Institute and teaches at Stanford University, and Ken Nealson is Director
of the Center for Life Detection at the NASA/CalTech Jet Propulsion Laboratory.
• In "Life Defined" (5:52), Chyba updates the traditional definition of life as needing
liquid water, biogenic elements (principally carbon), and a source of energy. Ken Nealson adds a
more fundamental definition of life being something that uses energy and creates products, and
thereby leaves evidence of its existence. Chyba describes places where we might expect life to
exist: on the surface of planets or moons, but not in clouds. Did 19th Century astronomers see
evidence of life on Mars in giant canals? Is there, in fact, a "Face on Mars", imaged by the
Viking orbiter? Nealson shows a fossil fish from the Green River formation in Wyoming, which he
calls an unambiguous "bio-signature" (sign of life). He notes that even the reverse side of the
fossil-bearing rock-which seems featureless-shows microscopic clues that life existed on Earth at
that time, in tiny microbes. Does the famous Martian meteorite, ALH84001, show bio-signatures?
Despite what look somewhat like fossil microbes, the program reports there's still no clear
evidence that ALH84001 provides conclusive evidence of past life on Mars.
• In "Extreme Survivors" (3:20) Nealson describes how microbiology has been revolutionized
by the discovery in the past two decades of organisms known as "extremophiles", living in the 300
degree C. heat of water in hydrothermal vents at the bottom of the ocean. As recently as 1998,
microbes have also been found surviving in the extreme cold of the South Pole. Chyba also
suggests that there's a "deep biosphere" under the Earth's surface where there may be more
"bio-mass" (living matter) than in all the trees of all the forests on our planet! Just about
anywhere there's liquid water and some kind of energy source, we now know that life can exist.
• In "Habitable Zones?" (4:00) Chyba and Nealson use these new ideas and discoveries to
analyze where else in our solar system we have reason to look for life. We see how the NASA's
plans to "Follow the Water" has-in spring 2000-shown us what look like "gullies" on Mars, formed
by short-lived gushers of what's presumed to be liquid water. Adds Nealson, we should also think
of moons as candidates, along with planets. Chyba describes Jupiter's large moon, Europa, as a
place with a water ocean that may hold twice as much water as all Earth's seas! Though Saturn's
moon Titan is very cold, there might be sub-surface water. Nealson concludes by saying that in
the search for extraterrestrial life we must keep an open mind: we now know life can exist in
places on Earth we previously thought impossible.
