- Those on-the-prowl Mars robots -- Spirit and Opportunity
-- are sending back extraordinary images and science data about the red
planet and its history of climate and water.
-
- Both rovers have found evidence of water at their respective
landing sites. But the question remains open as to whether Mars was, or
is today, a planet capable of supporting life.
-
- The tell-tale clues of water left behind hint that some
spots on Mars did have a persistent wet look that might have been sociable
to extraterrestrial creatures. While Mars scientists have their eyes focused
on finding tiny microbes, the question remains: just how far along could
martian biology, if any, have evolved?
-
- Yet answering this question is a difficult task to answer
robotically and it might take rock-splitting fossil hunters, hammer in
hand, to chronicle the true life on Mars saga.
-
- Eye of the beholder
-
- Peter Schultz, a planetary geologist at Brown University
in Providence, Rhode Island, said you don't have to look out to Mars to
see how hard it is to spot fossils.
-
- "We even have trouble identifying fossils on Earth
that are older than 3 billion years old. There continues to be debate today,"
Schultz said. Many primitive life forms leave only subtle traces and often
require sophisticated techniques to prove that they indeed were produced
by something living, he said.
-
- Those studying the incoming Mars rover images clearly
get an eye-full. For many a casual observer, the zones in which the robots
tread appear to be chocked full of objects, from weird rocks to leftover
remains of long-gone life.
-
- "There are slow geochemical processes that can create
spherical shapes and filamentous forms. The 'eye of the beholder' may guide
us where to lookbut that is not proof," Schultz advised.
-
- Schultz explained that astronomer Percival Lowell, a
keen observer who, in the late 1880s into the early 1900s, advocated that
Mars was a populated world, once ironically said: "So easily are we
the dupes of our own prejudice."
-
- "This certainly is true for what we think we may
see in these unprecedented close-up views of Mars [provided by the rovers].
Science requires testing and proving, not simply suggesting," Schultz
concluded.
-
- Recognizable patterns
-
- The identification of fossils is often difficult, explained
Ron Greeley, Mars Exploration Rover team scientist from Arizona State University
in Tempe, even by scientists observing them with the full spectrum of lab
instruments.
-
- "Remember that fossils are defined as the traces
-- such as leaf imprints in rocks, or the remains, such as shells or bones
-- of formerly living organisms. Typically, recognizable patterns are sought,
such as bilateral symmetry," Greeley told SPACE.com . "Unfortunately,
similar patterns often occur in rocks that result from non-biological processes,
which make the identification more difficult."
-
- On Mars, Greeley said, there is no reason to expect the
same patterns as fossils that are seen on Earth. Nonetheless, patterns
of some sort are being formulated by the astrobiological community -- so-called
biomarkers -- in the on-going search for life elsewhere. Furthermore, while
the Athena science gear onboard the two Mars rover are great assets, "analyzing
patterns and other features remotely is not so easy," he added.
-
- For Greeley, Mars fossil hunting has a bottom line: Unless
something really obvious pops out in the images and/or other information,
"it's going to take a while to sift through the data and derive some
clear answer."
-
- There is a lot of interest in trying to see fossils in
the Mars rover images, Greeley said. "The team is looking at the data
fairly rigorously, but nothing has emerged along these lines."
-
- Preserved in stone
-
- Prior to 21st century astronauts putting foot and exploration
flag down on Mars, there is much that can be done remotely. Future on-the-surface
rovers are already being designed, profoundly more capable than the golf
cart-sized Spirit and Opportunity.
-
- "Ultimately, it will take sample return of any putative
'fossils' to convince the scientists of the world. But we can tell a lot
in situ (on-the-spot) before such a time," said James Garvin NASA
(news - web sites) Lead Scientist for Mars and Lunar Exploration in the
Office of Space Science at the space agency's headquarters in Washington,
D.C.
-
- Garvin, a long-time fossil hunter himself, cautioned
about the definition of the word. Fossils come in many varieties, Garvin
advised, from the micro- and nano-fossils of single-celled primitive microbes,
to preserved-in-stone bones of organisms as big as automobiles.
-
- "Searching for fossils also comes in many flavors,
from microscopic siftings through tiny grains, to overland reconnaissance
for suitable bedding settings to uncover bones," Garvin said.
-
- Microscopic to macroscopic
-
- Assuming that Mars rover data confirms the existence
of sedimentary systems of rocks on Mars, searching for fossils can then
take many forms, Garvin related. "One would involve the recognition
that depositional environments were interrupted and that they are preserved
at scales ranging from microscopic to macroscopic."
-
- On Earth, such "preservational" environments
are almost always sedimentary, but can include volcanic and impact 'sedimentary'
environments.
-
- "So, if the kind of putative sedimentary deposits
we have identified on Mars from orbit are validated, then we can gainfully
start our first searches for at least indirect, morphological (form and
structure of plants and animals) signs of life in such localities,"
Garvin suggested.
-
- However, expecting fossil formation to operate on Mars
the same way as it does on Earth is not a sure bet, Garvin pointed out.
-
- Nooks and crannies
-
- The recent finding by the Opportunity Mars rover of a
very high concentration of sulfur in the rock outcrop at Meridiani Planum
bodes well for biology and fossil hunting. The chemical form of this sulfur
appears to be in magnesium, iron or other sulfate salts
-
- Moreover, Mars is rich in landscape where vestiges of
life may be found.
-
- "My overall fossil-hunting bias is heavily weighted
to natural caves and fissures and overhangs," said Penelope Boston,
Director, Cave and Karst Studies Program at New Mexico Institute of Mining
and Technology in Socorro, New Mexico. "I know from personal experience
how these environments serve as both original home for organisms and as
great places to preserve the evidence," she said.
-
- "I'm particularly excited about the sulfate resultsbecause
so much of the material we are looking at is in sulfur rich caves where
gypsum and other sulfate minerals play a huge role in the biology and the
subsequent preservation of traces of that biology," Boston observed.
-
- Along with caves, Boston said some top-notch nooks and
crannies on Earth to look for fossils are in things with vertical relief
that slices down through layers, like canyon walls, fault escarpments,
and river/stream outflow channels. For Mars and other crater-rich environments,
obviously crater walls where things are excavated down to some deeper strata
are first-rate sites.
-
- "The best thing about these places is that large
-- as in macroscopic -- buried fossilized material is usually weathering
out of the outcrops. One can see them laying about and follow the fragments
to the source," Boston said. "For microscopic fossils, of course,
no clear obvious clues are usually present. The exception is large scale
microbial mats like those that made stromatolites and algal travertiles
and tufa."
-
- Happy hunting ground
-
- Thanks to picture-taking Mars orbiters, snapshots of
Newton Basin on Mars reveal it to be a site were ponded standing water
appears to have been present, Boston said. That landscape feature is a
great place to look for significant sedimentary rocks, "not to mention
Martian critter remains," she added.
-
- "That kind of a no-doubt-about-it pond with minimal
shallow excavation could be a goldmine of microfossil material! I'd want
to sample its beautiful rims...and do a small core down through the middle
somewhere," Boston explained.
-
- Another promising happy hunting ground for biologists
on Mars may be in the cracks within so-called patterned ground or polygonal
terrain.
-
- "On Earth, the bottoms of these cracks can house
organisms in a much more pleasant environment than the surface at large.
Obviously, if such communities thrive or thrived on Mars, some of the evidence
may well be at the bottom of these cracks," Boston said. "We
have much imaging evidence of these terrains on Mars for comparison to
the similar terrains here on Earth, not just at the poles but many other
high latitude and high altitude places."
-
- Place your bets
-
- So far there have been no shouts of "eureka!"
from Mars rover scientists spotting a signature of past or present life.
-
- "I flat out see no evidence for any fossils in our
data," said Jim Rice, a Mars Exploration Rover team scientist and
a planetary geologist from Arizona State University in Tempe. "If
a trilobite, for example, evolved on Mars and we came across a rock with
it, we could resolve it."
-
- Rice noted that the twin Mars rover's each carry a Microscopic
Imager. It has a resolution of 30 microns per pixel. That device, however,
has the ability to only see objects that are about 100 microns across and
above, he added.
-
- "Fossil finding will be a very difficult job on
Mars. I don't think robotic missions will do it," Rice said. "It
will take astronauts and even then it will be a tough job," he said.
-
- And that job ahead most likely means drilling and cracking
a lot of rocks open. On Earth, Rice added, even when you go to a known
fossil field location it still requires work to find them.
-
- "We are a lot farther along in the game now that
we have identified a rock outcrop on Mars that involved liquid water. We
still have much to learn about where to go for any future fossil hunting
on Mars. It will be sheer luck if robots discover conclusive evidence for
fossils on Mars," Rice said, making the point: "I am putting
my bets on astronauts."
-
- Copyright © 2004 SPACE.com
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