1.) Europa
Jupiter's second of
its four large moons,
Europa might at first seem like
it's too far from the Sun to be a good candidate for life.
But Europa has two
special things going for it:
Beneath a surface of
ice, Europa has an enormous ocean of liquid water, and the
heating of its insides due to Jupiter's gravity may create a
situation very analogous to the life-giving hydrothermal vents
on the Earth's ocean floor.
It's not likely to be
life like we see on the surface of Earth, but life that can
survive, reproduce and evolve is life any way you slice it.
Europa,
one
of the solar system's largest moons, orbits Jupiter.
Beneath its frozen, icy surface,
a liquid water of ocean is heated
by tidal forces from Jupiter.
Image credit: NASA, JPL-Caltech, SETI Institute
Cynthia Phillips, Marty Valenti.
2.) Enceladus
Saturn's icy moon is
smaller and has far less water than Europa, but it announces its
liquid ocean (beneath a surface of solid ice) uniquely:
by spewing
300-mile plumes of water into space!
These geysers let us
know for certain that there's liquid water, and in tandem with
the other elements and molecules necessary for life, such as
methane, ammonia and carbon dioxide, there just might be life
beneath the oceans of this world, too.
Europa has more heat,
more water and hence - we think - a greater chance, but
don't count
Enceladus out, since it has a
thinner ice surface and erupts far more spectacularly, meaning
that we could find life with an orbiting mission, rather than
having to drill down beneath the surface!
One of the most intriguing - and least resource-intensive -
ideas
for searching for life in Enceladus' ocean
is to fly a probe through the geyser-like eruption,
collecting samples and analyzing them for organics.
Image credit: NASA / Cassini-Huygens mission
Imaging Science Subsystem.
3.) Mars
The
'red'
planet was once clearly very, very Earth-like.
For perhaps the first
billion years of the Solar System, water flowed freely across
the martian surface, carving rivers, accumulating in lakes and
oceans, and leaving remnant evidence that shows us, today, where
they were once located.
Features associated
with a watery past, like hematite spherules (often associated
with life on Earth), are common. In addition, the Curiosity
rover has found an active, underground and variable source of
methane, a possible signature of life today.
And now that we know
liquid water appears on the martian surface, albeit in a very
salty environment, the door is definitely open.
Mars remains a
tantalizing possibility...
The
flow of a dried-up riverbed
is
an unmistakable signature of
a
water-rich past on Mars.
Image credit: ESA/DLR/FU Berlin (G. Neukum).
4.) Titan
Enceladus might offer
the greatest possibility for Earth-like life in the Saturnian
system, but perhaps life takes on a different form from the
water-based biology here on Earth?
With a thicker
atmosphere than our own planet, the second-largest moon in our
Solar System,
Titan, was found to have liquid
methane on its surface:
oceans, rivers
and even waterfalls!
Could life make use
of methane on another world the same way it makes use of water
on Earth?
If the answer to that
is yes, there just might be living organisms on Titan today.
The
surface of Titan, beneath the clouds,
was
found to contain methane lakes, rivers and waterfalls.
Could it also be home to some type of life?
Image credit: ESA, NASA, JPL, University of Arizona;
panorama by Rene Pascal.
5.) Venus
Venus is hell, literally.
At a constant surface
temperature of some 900 degrees Fahrenheit, no human-made lander
has ever survived more than a couple of hours while touched down
on our nearest neighboring planet.
But the reason Venus
is so hot is because of it's thick, carbon-dioxide rich
atmosphere laden with heat-trapping clouds of sulphuric acid.
This renders the
surface of Venus thoroughly inhospitable, but the surface isn't
the only place to look for life. In fact, speculation is rampant
that perhaps something interesting is happening some 60 miles
up!
Above the cloud-tops
of Venus, the environment is surprisingly Earth-like:
similar
temperatures, pressures, and less corrosive material.
It's conceivable that
with its own unique chemical history, that environment is filled
with carbon-based airborne life, something that a mission to
Venus' upper atmosphere could easily sniff out.
The
surface of Venus,
from the only spacecraft to ever successfully
land and transmit data from that world.
Image credit: Venera landers / USSR.
6.) Triton
You might not have
heard much of Neptune's largest moon
Triton, but it's remarkable and
unique among all the worlds of the solar system.
It has "black smoker"
volcanoes, it rotates and revolves the wrong way, and it
originated from
the Kuiper belt.
Larger and more
massive than both Pluto and Eris, it was once the king of all
Kuiper belt objects, and now, in orbit around our Solar System's
final planet, we recognize that it's covered in many life-giving
materials, including nitrogen, oxygen, frozen water and methane
ices.
Could some form of
primitive life exist at these energy interfaces?
It's certainly worth
a look!
The
Voyager 2 spacecraft
took this color photo of Neptune's moon Triton
on
Aug. 24 1989, at a range of 330,000 miles.
The
image was made from pictures
taken through the green, violet and ultraviolet filters.
Image credit: NASA / JPL.
7.) Ceres
It might sound crazy
to think of the possibility that life might exist on an
asteroid.
Yet when asteroids
fall to Earth, we find not just the 20 amino acids essential to
life, but nearly 100 others:
the building
blocks are all there!
Could the largest
asteroid of them all, the one with those bizarre, salt-deposit
"white spots" on the bottom of its brightest craters, actually
house some form of life?
Although the answer
is "probably not," it's conceivable that it was actually
collisions with asteroids and Kuiper belt objects that brought
either the raw ingredients for life or pre-existing, primitive
life to Earth.
What we consider,
today, to be active biology, might have begun before Earth ever
formed. If so, the signatures might be embedded within a world
like Ceres, which is the best candidate for life in
the asteroid belt.
We just have to look
to find out. And finally…
This global map
shows the surface of Ceres in enhanced color,
encompassing infrared wavelengths
beyond human visual range.
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
8.) Pluto
Who would've expected
that history's outermost world - at a temperature just 100
degrees Fahrenheit above absolute zero - would be a candidate
for life?
And yet,
Pluto has an atmosphere, it has remarkable, changing
surface features, it has the same ices that Triton has, and
objects just like it may be responsible for bringing much of
what looks like Earth's atmosphere and oceans to our planet.
Could it have brought
life as well?
New Horizons will
bring us hints, but to find out for certain, we'll need a
landing mission.
Pluto's atmosphere,
as
imaged by New Horizons
when
it flew into the distant world's eclipse shadow.
Image credit: NASA / JHUAPL / New Horizons / LORRI.
