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The Possibility of Extraterrestrial Life in
Our Solar System and on Exoplanets
Stephen R. Cantor
Teaching Lecturer
Plymouth State University
Plymouth, NH
Sunday Morning University
Congregation Bnai Israel
St. Petersburg, FL 33710
February 22, 2015
3 Adar 5775
Life History of a Star Similar to Our Sun
• Life history of a star whose mass is near that of the sun. The
sun is now halfway through its estimated lifetime of 10 billion
years.
• For the sun, we can expect a billion more years of warming as
the sun’s temperature gradually increases. Life on earth will
ultimately become impossible, not as one thought which was
because the sun would cool off, but rather because it will
grow too hot.
• In 5 or 6 billion years, the sun will expand to reach its
maximum size as a giant larger across than the earth’s current
orbit.
• Later its fuel gone. The sun will collapse into a white dwarf as
its outer layers stream off as a planetary nebula.
Mars
• Over billions of years, solar winds stripped away the Martian upper
atmosphere.
– This atmospheric erosion transformed Mars from warm and wet, and
presumably more hospitable to life, to cold and dry.
– The Opportunity Robotic Rover discovered evidence for the most
ancient Martian environment where life could have existed, roughly 4
billion years ago.
• Dry lake beds and river channels carved into the Martian landscape
suggest that the planet once had a thick encasing atmosphere.
• Mars should have been cold and dry in the past, as it is now, and it
has been a mystery how warm and wet periods occurred in its
history.
– One possibility is that water vapor and carbon dioxide injected into the
Martian atmosphere by volcanoes were responsible.
Mars (Continued)
– Another possibility follows from the discovery that
the Martian river valleys and craters made by
giant asteroid impacts are about the same age.
– The kinetic energy of an asteroid of rock and ice
100 to 200 kilometers across (62 to 124 miles
across) that smashed into Mars would have
heated the planet’s surface, and the asteroid’s ice
content would have become a scalding rain that
fell for many years, caused flash floods, and fed
the streams that carved the valleys seen today.
Mars (Concluded)
– Frozen ground water would also have melted and
come to the surface.
– After some thousands of years, Mars would have
cooled down and the water turned to ice,
although underground water could have remained
liquid for much longer.
– There seem to have been 25 such impacts 3.5
billion years ago, spread 10 to 20 million years
apart.
Is There Life On Mars?
• Mars rotates on its axis in a little over 24 hours.
• Its revolution about the sun requires nearly 2 Earth
years, and its axis is inclined to the plane of its orbit at
nearly the same angle as the Earth’s.
• Thus the Martian day and night have about the same
lengths as ours. The Martian seasons are 6 months
long, and at least as pronounced as ours.
• Although the conditions on Mars were once as
hospitable to life as those on Earth, if life does exist
today on Mars, it would have adapted to an
environment that would soon destroy most Earthly
organisms.
Is There Life On Mars? (Continued 1)
• Martian climates are severe by our standards.
– Over 50% further from the sun than the Earth, Mars receives much
less solar energy per square foot than we do.
– The Martian atmosphere, which is largely carbon dioxide (only 0.13
percent is oxygen) is extremely thin, which is equivalent to the Earth’s
atmosphere at an altitude of nearly 40 km (24.8 miles), so little of the
heat from the sun remains after nightfall.
– Daytime temperatures in the Martian summer at its equator rise to
over 0°C (32°F), but at night drop to a chilly -23°C (-9.4°F).
– The average surface temperature of the entire planet is about -55°C (67°F).
• The scanty Martian atmosphere is unable to screen out harmful
solar ultraviolet radiation, a function carried out in the Earth’s
atmosphere by the ozone present at high altitudes.
• At times, giant dust storms blow for months across the entire
planet.
Is There Life On Mars? (Continued 2)
• NASA’s Curiosity Rover recently recorded (reported in
December 2014) a burst of methane gas that lasted at
least 2 months.
– One possibility of the explanation for the methane is that
it is the waste product of certain living microbes.
– The presence of methane is significant because that gas
can not last for long.
– Calculations indicate that sunlight and chemical reactions
in the Martian atmosphere would break up the molecules
within a few hundred years, so any methane there now
must have been created recently.
– On Earth, most of the methane in the air is produced by
plants and bacteria.
Is There Life On Mars? (Continued 3)
• Scientists have also confirmed the presence of
carbon-based organic molecules in a rock sample.
– The so-called organics are not direct signs of life, but
they lend weight to the possibility that Mars had the
ingredients required for life, and may still have them.
• Extreme Life
– The fact that most terrestrial life requires a regular
supply of liquid water and oxygen plus protection
from solar ultraviolet radiation does not mean that life
of some kind could not exist without them.
Is There Life On Mars? (Continued 4)
– The life processes of certain bacteria on Earth do not
require oxygen, so an oxygen-containing atmosphere is not
indispensible, at least for primitive forms of life.
– Sulfur is abundant on Mars, and organisms that obtain the
energy they need from chemical reactions based on sulfur
are, in principal, possible.
– Conceivably there could have evolved on Mars life forms
that can thrive on traces of water gleaned from minerals in
surface rocks. Also, shells of some sort might protect
Martian life from ultraviolet radiation.
– Or, life on Mars could exist underground where perhaps
water ice or liquid water is present, the energy source
being heat from the interior rather than sunlight, as in
some places on Earth.
Is There Life On Mars? (Concluded)
– In fact, extra heat may not be needed: bacteria have been found
deep in the ice sheets of Greenland and Antarctica that seem to
have survived for hundreds or thousands of years at
temperatures as low as -40°C (-40°F). One species remained
alive at -196°C (-321°F) when immersed in liquid nitrogen.
• Since conditions long ago on Mars may have been
comparable for a long period to those on the Earth, life of
some kind could have come into being on Mars.
– The loss of most of the carbon dioxide of its atmosphere, vital
for the greenhouse effect, and the disappearance of its surface
water, some ending up underground, were gradual, and it is not
at all absurd to speculate that living things could have adapted
to the progressively harsher environment and survived in some
form to the present.
Europa
• The Jovian satellite Europa is nearly the size of
our moon, but unlike our moon, it is covered with
a layer of water ice several kilometers thick.
• The ice is scarred by a network of curved,
intersecting fracture lines, like a cracked eggshell.
– A surface of this kind is expected if the ice is floating
on on a vast ocean of liquid water.
– Currents in the ocean regularly break up the ice, and
water wells up into the cracks and freezes there to
give the patterns we see.
Europa (Continued)
• Where does the heat come from that keeps
Europa’s subsurface water liquid and powers
its currents?
– Some may be the result of radioactivity in
Europa’s rocky core, but most arises from the
continued squeezing and stretching due to tidal
forces caused by the gravitational pulls of Jupiter
and its other large satellites, as in the case of Io.
• Given liquid water and heat, could life have
developed on Europa?
Europa (Concluded)
– After all, organisms have been discovered on Earth that thrive in
perpetual darkness with no need for sunlight, and living things
have been discovered beneath Antarctica ice.
– Comet impacts on Europa may well have dumped on its surface
billions of tons of such elements needed for life as carbon,
nitrogen, and sulfur.
• Many scientists think that Europa, not Mars, is the most
likely candidate fror extraterrestrial life in our solar system
today.
• NASA hopes to launch a spacecraft to visit Europa, Callisto,
and Ganymede.
– If liquid water is indeed under Europa’s ice coat, a lander may
follow that will inspect the cracks, and burrow through the ice
to look for life.
Life Thrives Under Antarctica
• An unseen ecosystem flourishes in the darkness,
entombed beneath 800 meters (1/2 mile) of ice
in Antarctica.
– In January 2013, a U.S. team reported finding live cells
in water from Lake Whillans, 800 meters below the
West Antarctica ice sheet.
– In August 2014, that team announced that it had
found not only life but a thriving ecosystem there.
– The team identified genetic traces of 3,931 microbial
species or groups of species in water from the lake.
Life Thrives Under Antarctica
(Concluded)
• The finding bolsters the idea that life could
exist elsewhere in the solar system, such as
under Mars’s polar ice caps or in a subsurface
ocean on Jupiter’ frozen moon Europa.
Titan
• Titan, Saturn’s largest satellite, is believed to have a
rocky core surrounded by a thick layer of water ice.
• It is the only satellite in the solar system with a dense
atmosphere, which seems to consist largely of nitrogen
with some methane and small amounts of other
organic compounds.
• Titan’s atmosphere is more like ours than of any other
body in the solar system.
• Reddish clouds of organic compounds float in the
atmosphere and send showers of methane rain and
perhaps snow from time to time to feed the liquid
methane rivers, lakes, and seas on Titan’s surface.
Titan (Continued)
• Methane apparently plays the same geological role on
Titan that water plays on Earth.
• Titan and the Earth are the only bodies in the solar
system on which rain or snow of some kind falls to the
ground.
• Could life have developed on Titan?
– The answer seems to be that it might have if Titan had
been warmer.
– Titan’s atmosphere is similar to the Earth’s before life
emerged.
– However, chemical processes are slower at lower
temperatures.
Titan (Concluded)
– If life took a half billon years to come into being on
the Earth, which is a reasonable estimate, then
the solar system is not old enough for for this
process to have occurred on the frigid (-179°C [290°F]) surface of Titan.
– However, conceivably heat from Titan’s interior or
from comet impacts produced pools of liquid
water that could persist under the crusts of ice on
its surface for long periods, which makes a search
for life more reasonable.
Enceladus
• There appears to be a saltwater ocean, and a
potentially habitable environment, beneath the
icy crust of Saturn’s moon Enceladus.
– Salty ice geysers erupt through cracks in Enceladus’
south polar ice sheet.
– Heat wells up through the fissures, presumably from a
deep, warm reservoir.
– In April, 2014, data from the spacecraft Cassini was
used to map an ocean hiding underneath Enceladus’
south pole. The sea holds about as much water as
Lake Superior.
Enceladus (Continued)
– In July 2014, evidence was found connecting the
fountains to an underground water supply.
– Using images from the Cassini spacecraft and
maps, it was found that warm spots at the base of
each geyser are too small to power the jets, so
that spray could not originate at the surface.
– Most likely, Saturn’s gravity repeatedly opens and
closes the fissures allowing water and heat to
escape from the interior and vent into space.
Enceladus (Concluded)
• The findings suggest that water jetting out of Enceladus
probably comes from a warm, briny subsurface sea atop a
rocky core.
• The vapor plumes emanating from Enceladus show traces
of organic compounds.
• Even though distant from the sun, Enceladus is warm
enough for subsurface liquid water to be present, perhaps
heated by the same tidal kneading that heats Jupiter’s
moon Europa.
• Given warmth, water, and organic compounds, did life of
some kind appear?
– Since many of the ingredients of life seem in place, right now
there is nothing to rule it out.
Charon
• Pluto, regarded as a dwarf planet, has 5
satellites, the largest of which is Charon.
• Charon’s diameter is about 1200 km, a little
over half that of Pluto’s.
• Charon probably consists largely of ice with
some rock as well.
• There are signs that Charon, like Jupiter’s
moon Europa, has an ocean of liquid water
under a covering of ice.
Charon (Concluded)
• It is possible that Charon, like Europa, is
heated by tidal forces that kneed its interior.
• Could there be underground life on Charon
that draws its energy from this heat rather
than from the feeble sunlight?
– What little we know about Charon does not seem
to rule it out.
Exoplanets (Planets Orbiting Other Stars)
• Prior to 1992, no planets outside our solar system had
been discovered
• To-date, over 1700 confirmed exoplanets have been
discovered
– In 2014, 715 new worlds were discovered by the Kepler Space
Telescope
– In April 2014, the Kepler team reported the smallest possible
habitable exoplanet, about 490 light-years away. It is just 10
percent wider than the Earth, and orbits its dim, red star at a
distance where liquid water could exist. However, astronomers
do not yet know if that planet can support life.
– As of January 6, 2015, the number of confirmed exoplanet
worlds that may be in their stars’ habitable zone with the
potential conditions for life is now 13.
Exoplanets (Planets Orbiting Other Stars)
(Concluded)
• As of January 6, 2015, the total number of
exoplanet candidates discovered by the Kepler
Space Telescope is 4,145, including 800 with
diameters roughly the size of Earth’s.
– Among those far-off locales are a few worlds that
might not be too different from Earth.
Extraterrestrial Life
• The recipe for creating life begins with a warm
rocky planet, whose gravity is able to hold an
atmosphere and whose distance from its parent
star is such that it receives enough but not too
much radiant energy.
• Then, provide an atmosphere of certain quite
common gases, allow the planet to cool down a
bit so that water condenses out, and wait
perhaps a billion years.
• This recipe was followed successfully on Earth.
Goldilocks Zone For Planets
• From the point of view of life in the universe, just how
many billions of planets there are is not really important.
• What is important is that a certain proportion of these
planets apparently meet the requirements of a suitable
distance from its star, near-circular orbit, proper mass, a
favorable atmosphere, and reasonable rotational period, all
of which are necessary for life as we know it on Earth.
• Some of the rocky exoplanets that have been discovered
are in the “Goldilocks Zone”, that is not so near their parent
star to be too hot, nor too far away to be too cold for liquid
water to exist on them.
Goldilocks Zone For Planets
(Concluded)
• But, not any location in our galaxy will not do.
– Elements heavier than hydrogen and helium, which
are needed for life and are created in supernova
explosions are more abundant toward the center of
the galaxy.
– However, supernova explosions, which would destroy
life on a planet anywhere near it, are also more
common toward the center of the galaxy.
– A region between 22,000 and 30,000 light-years from
the center of the galaxy, about a tenth of the galactic
disk, seems about right. Our sun is in the middle of
this zone.
Would Extraterrestrial Life Resemble
Life on Earth?
• Given a suitable parent star, size, composition,
and environment in space, life could appear.
• Would that life necessarily resemble life on
earth?
• Terrestrial life has many interesting examples of
what is called convergent evolution.
– Organisms that came from different genetic lines are
found to have developed similar forms and functions
as they responded to similar environmental pressures.
– Thus eyes like ours independently evolved in six
different types of animals.
Would Extra-Terrestrial Life Resemble
Life on Earth? (Concluded)
– Bats and dolphins independently developed echolocation.
– Birds, bats, and pterodactyls independently developed
flapping wings.
– Regarding intelligence, despite very different evolutionary
histories and brain structures, crows and people both
exhibit such aspects of intelligence as tool use, deception,
and intricate social behavior.
• Extra-terrestrial life is not likely to mirror life on Earth
exactly, but if a planet elsewhere is much like Earth,
then the organisms that arise there may well be
recognizable variations of organisms that now or in the
past flourished here on Earth.
How Much Advanced Life?
• The Earth came into being about 4.6 billion years ago.
• Less than a billion years later, life of some primitive
sort appeared.
– On a cosmic scale, that is pretty fast.
– This strongly suggests that life could come into being on
planets around other stars.
• But, what kind of life?
– The next step from one-celled micro-organisms to
multicellular organisms took over 3 billion more years on
Earth.
– And not until now, 4.6 billion years since the Earth’s origin,
have living things on Earth progressed to the point where
interstellar communications can be taken seriously.
How Much Advanced Life?
(Concluded)
• The development of an advanced civilization, as
distinct from merely complex life forms, may
require too many chance events to be inevitable
by now, wherever life occurs.
– The dinosaurs were successful enough to be the
dominant animals on earth for 140 million years, but
they never developed an advanced civilization.
– If it were not for the asteroid that drove them into
extinction, we the human species may not be here
today.
– Although life may probably be common in the
universe, advanced life may not be.