Sky and Telescope - November 2017 - 15

a thicker atmosphere, ﬂowing water, and all the ingredients
necessary to sustain life. As a result, beyond the desire to
physically set foot on a new world, the possibility of ﬁnding
hidden alien life awakens the natural human urge to explore.

PAUL H A R RISON / CC BY- SA 3.0

Dusting for Fingerprints
After a barren period following the 1976 touchdowns of the
audacious Viking landers (an interval broken only by two
doomed USSR missions), a concerted effort to reach the Red
Planet began in the 1990s, later morphing into NASA's "follow the water" strategy (see page 16). Using the orbital lineups as opportunities, NASA - joined by other agencies - has
since undertaken a series of missions designed to slice away,
piece by piece, the mysteries of the Red Planet.
Mars Pathﬁnder and its tiny Sojourner rover gave this
quest wheels, and NASA teams later followed up on their success by dropping two Mars Exploration Rovers - Spirit and
Opportunity - onto the planet's surface in 2004 and a much
larger third - Curiosity - in 2012. These latter three rovers
have trundled across the surface uncovering compelling evidence that Mars might once have been able to harbor life.
Yet this life - if it existed at all - was microscopic, perhaps only leaving the tiniest trace of its former existence.
Although the Viking landers did test interesting samples suggestive of ancient life, their results were judged inconclusive
- hardly surprising given that NASA conducted the mission
nearly 20 years before the term "biosignature" had even been
coined to describe evidence pointing to past or present life.
"It's actually really hard to tell what was made by life
and what wasn't . . . unless you're looking at dinosaur
bones," notes planetary scientist Briony Horgan (Purdue
University), co-investigator on the Mastcam-Z camera
system for NASA's Mars 2020 rover - the ﬁ rst NASA mission since Viking with the stated objective of seeking direct
evidence for ancient life. This is why today's search takes
a more holistic view. "You start with what was the type of
environment you are in, what was the chemistry of that
environment, and then - given that - what kind of life
could survive here and what kinds of biosignatures would
that life leave behind."
If found, these biosignatures will be subtle: "We look
for shapes preserved in the rocks that are suggestive of the
past presence or activity of life," says Kenneth Williford (Jet
Propulsion Laboratory), Mars 2020's deputy project scientist.
"And then we look for chemical disequilibria, organic matter,
or biologically important elements distributed in a way that
is correlated with those shapes in the rock."
A great example here on Earth of ancient life providing vestigial clues to its former existence is stromatolites.
Stromatolites form from mats of photosynthetic bacteria that
trap sediments and microbes, building layer upon layer until
eventually they become dome-like rock structures, sometimes reaching basketball proportions. Modern ones are still
developing in the shallow waters of the Bahamas and
(continued on page 18)

p STROMATOLITES These structures form when microbial mats catch
sediment and build it up, layer by layer. Ancient stromatolites' unique
combination of composition and structure provides some of the earliest
records of life on Earth. Scientists would like to find similarly convincing
fossils on Mars.
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

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Sky and Telescope - November 2017

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