Cosmic Journeys – Mars: Earth that Never Was
There is a region on Mars, roughly the size
of Australia, that rises high above the surface of the planet. Three of the largest volcanoes
in the solar system line up to guard its western flank. To the East, a vast canyon, six to seven times
deeper than the Grand Canyon, cuts into the barren Martian plain. This strange region,
once so baffling to scientists, recalls the planet's violent past, a time long ago when
the planet's core erupted, pushing molten rock to the surface. It's part of a larger story, a planetary tragedy,
in which Mars began its descent into the cold, dry, and lifeless state that we see today.
We are now scouring its surprisingly complex surface for clues to the events that long
ago doomed the Red Planet, Mars.
Since the early 1960s, we've tried 46 times
to send spacecraft to Mars across the 55 million kilometers of its closest approach to Earth. Over half failed at launch or upon arrival.
The rest flew around the planet, snapping pictures, recording data. Or they landed to
test its soil and rocks, and crawl around its canyons and craters. These probes may one day pave the way for
human explorers, who will dig deeper still, in search of answers to our most pressing
question: Did Mars, at some point long ago develop far
enough for life to arise? If so, does anything still live within Mars' dusty plains, beneath
its ice caps, or somewhere underground? Mars does not give up its secrets easily.
Over the years, that has led observers on this planet to jump to all sorts of conclusions. In the year 1877, the Italian astronomer Giovanni
Schiaparelli noted markings on Mars' surface, a latticework of lines. He called them "canali"
in Italian, meaning "channels" in English. A careful and thorough observer, Schiaparelli
began to sketch them and name them, connecting them in a vast global network. Over a 15-year period, beginning in 1894,
the American astronomer, Percival Lowell, closely examined these features.
He saw a
remarkable drama unfolding on our neighboring planet. In his view, Schiaparelli's channels were
artificial canals, designed perhaps to carry melting snow from the poles to the dry interior.
After all, on Earth, the Suez Canal had been open since 1869. Construction on the Panama
Canal had just gotten underway. The Martian canals, Lowell surmised, had been
built by a sophisticated society confronting an environmental catastrophe on the grandest
of scales. Its inhabitants faced an urgent choice: move water across vast arid regions,
or perish on an increasingly dry planet. In a series of three best-selling books, Lowell
took his case to the public. The public responded with some ideas of their own. With the means to remake an entire planet,
perhaps these Martians were more advanced than humans. Some of us began offering schemes
for making contact.
Giant mirrors to flash greetings. Light beams. Mental telepathy. Lowell vision fell by the wayside in 1964.
The Mariner Four spacecraft flew by Mars and got a good look. What it saw looked more like
the Moon than the Earth. Three more Mariners followed, culminating in the arrival of Mariner
Nine in 1971, the first spacecraft to go into orbit around Mars. These missions documented a heavily cratered
landscape, pocked with huge dormant volcanoes… and cut with the deepest and longest canyon
in the solar system. They saw no traces of life, present or past.
Then, in the mid-1970's, two lander-orbiter
robot teams, named Viking, went in for an even closer look. The landers tested the soil
for the chemical residues of life. All the evidence from Viking told us: Mars is dead.
And extremely harsh. The mission recorded Martian surface temperatures
from -17 degrees Celsius down to -107. We now know it can get even colder than that
at the poles. The atmosphere is 95% carbon dioxide, with only traces of oxygen. And it's
extremely thin, with less than one percent the surface pressure of Earth's atmosphere. And it's bone dry. In fact, the Sahara Desert
is a rainforest compared to Mars, where water vapor is a trace gas in the atmosphere. On
Earth, impact craters erode over time from wind and water… and even volcanic activity.
On Mars, they can linger for billions of years. Earth's surface is shaped and reshaped by
the horizontal movement of plates that make up its crust driven by heat welling up from
the planet's hot interior. At half the width and only 11% the mass of Earth, Mars doesn't
generate enough heat to support wide-scale plate tectonics.
Nor does it have the gravity to hold a thick
atmosphere needed to store enough heat at the surface to allow liquid water to flow.
Nonetheless, some areas that looked to Viking-era scientists like craters and volcanic areas,
were later shown to be riverbeds, lake bottoms, and ocean shorelines. If water once flowed on Mars' surface, where
did it all go? This was the scene at NASA's Jet Propulsion
Lab in 2004. The twin rovers Spirit and Opportunity had just bounced down on the Red Planet. When
the excitement died down, the rovers were set off on one of the most remarkable journeys
in the history of planetary exploration. Opportunity had come to rest in a small crater
near the equator, at a spot called Meridiani Planum. Here, in plain view, on a nearby crater
wall, its camera revealed exposed bedrock, the first ever seen on Mars. Not far away,
the rover found layered rocks on the face of a cliff. On Earth, they typically form
as sedimentary layers at the bottom of oceans. And at every turn, Opportunity rolled across
tiny, smooth, round pellets.
They became known as "blueberries" because they appeared purplish-brown
against Mars' rust-colored surface. Initially thought to be volcanic in origin, they turned
out to be iron-rich spherules of the type that form within cavities in the mud at the
bottom of an ocean. Drilling into rocks, the rover inserted a
spectrometer to read the mineral content. The readings showed significant amounts of
sulfate salt, a tracer for standing water. That wasn't all. Spirit's broken wheel, dragging
behind it, exposed soils saturated in salt. Clearly there once was water on Mars' surface,
but how long ago? And, if there is anything left, where would you find it? One possible
answer: the North Pole. From orbit, this region seemed to be covered in frozen CO2 – what
we call dry ice. But was there water ice below the surface? Enter Phoenix, a lander that touched down
near the North Pole in early 2008. Radar readings from orbit, taken by the Mars Express mission,
hinted at the presence of ice just below the surface.
The Phoenix lander's descent thrusters blew
away the top layer of soil, allowing its camera to snap pictures of what looked like ice.
Scientists instructed the robot to conduct a simple experiment: reach out and dig a trench,
then watch what happens. As expected, clumps of white stuff appeared.
A couple of days later, it was gone. Vaporized. That means it can't be salt or frozen CO2,
which is stable in the cold dry temperatures of the Martian pole.
So it had to be water,
the first ever directly seen on Mars. There are indications that the North Pole
was actually warm enough in the recent past for water ice to become liquid. The Mars Reconaissance
Orbiter, or MRO, used radar pulses to peer beneath the surface of the ice cap. These
data reveal that the ice, just over a mile thick, formed in a succession of layers as
the climate alternated between warm and cold. Our planet avoids mood swings like this in
part because its spin is stabilized by a massive moon. Mars' spin is not, so it can really
wobble, with the pole tilting toward the sun for long periods.
New observations by the
MRO spacecraft show that these wobbles can lead to dramatic releases of CO2, and warming
periods due to an increase in the greenhouse effect. The ice now detected below Mar's surface is
a remnant of a much earlier time. The thinking is that not long after its birth, the planet's
molten interior would have spewed out enough gas to form an atmosphere. Carbon dioxide and water vapor began to trap
heat from sunlight. Temperatures rose high enough to allow liquid
water to flow on the surface, creating myriad rivers, ponds, lakes and oceans. Evidence of this thicker atmosphere landed,
literally, in Opportunity's backyard. The rover spied a strange bluish rock, a nickel
iron meteorite named Block Island. Streaking through a thin atmosphere, this massive chunk
of metal should have been obliterated on contact. Instead, its fall was likely slowed, and its
impact softened, by a much thicker atmosphere. What then caused the atmosphere, and the water,
to disappear, and the planet to grow cold and dry? The answer comes from data recorded
by the Mars Global Surveyor just after it went into orbit in 1997.
Its instruments detected the presence of a
weak magnetic field emanating from the planet, a reading that scientists eagerly compared
to that of Earth. Our planet's magnetic field is generated by molten rock deep in its core
that rises and falls into a vast region below the outer crust, called the mantle. It has turned Earth into an electric dynamo.
The rising and sinking motion within, combined with the spinning motion of the planet, generates
a strong magnetic field.
You can trace this field back to Earth's early years, when large
amounts of heavy elements such as iron sank into its core. Radioactive decay began to
generate heat and the planet's mass is large enough to hold it in. Earth's magnetic field extends far enough
out into space to deflect the wind of high-energy solar particles.
Without a similar electromagnetic
"deflector shield" on Mars, solar radiation lashed the planet, gradually stripping it
of its atmosphere. What water Mars had would have vaporized into space or frozen underground. However, there is evidence that at one time
Mars did have a robust magnetic field. Rocks in some of the older craters bear a strong
imprint of this field, while newer craters indicate a much weaker field. What happened to it? The answer lies deep
in Mars' past, in events so powerful they are still written on the landscape.
This is a simple elevation map of Mars' surface, from data gathered by the Mars Odyssey spacecraft.
The South Pole, colored in red and orange, is piled high with ice. Moving off these southern highlands, we make
our way north. The landscape is pocked with craters. The largest and oldest ones have
faded, their edges softened by windblown dust. Moving up along the equator, we pass into
a region called Tharsis, based on a biblical name for the western edge of the known world. On the edge of this vast high altitude plateau
is a series of enormous volcanoes: Ascraeus Mons: 18 kilometers high.
Pavonis Mons: 14
kilometers. Arsia Mons: 16 kilometers. Just beyond, is the largest volcano in our solar
system, Olympus Mons, 25 kilometers in elevation. The thinking is that the Tharsis region bulged
out when a giant dome of magma pushed up from the planet's core. The volcanoes grew large
because Mars lacks the constant shifting of crustal plates that, on Earth, leads to chains
of smaller volcanoes like the Hawaiian Islands. Just to the East is the great Valles Marineris
– named for the Mariner Nine mission that found this vast gash in the Martian landscape.
It's about 4000 kilometers long and up to 200 kilometers wide. On Earth, Valles Marineris
would stretch from Los Angeles all the way to the Atlantic coast. If you went to Valles Marineris, you'd see
dust devils sweeping along the plains above it and dust blowing up the canyon walls. Here's
a realistic rendering of data captured by spacecraft. Giant landslides have caused the
walls to slump off and pile onto the valley floor. Feeding into the valley: a maze of side channels.
Scientists think these and other tributary features were formed when underground water
flowed into the main basin, and the land above collapsed.
Wider parts of the canyon are regarded
as possible landing sites for a manned mission. They offer flat surfaces and possible access
to liquid water that may remain below the surface. The theory is that Valles Marineris formed
when the planet began to cool. Its sides were pulled apart as the Tharsis plateau, just
to the west, began to rise up. That chain of events is now being linked to a much larger
planetary event, what one scientist called "the" defining moment in Mars' history. Travel north, down the slopes of Mars's great
volcanoes. The elevation drops as we move across what appears to be an immense ocean,
colored here in blue. With this so-called Borealis Basin in the north, and the high
elevations of the South, Mars is a lopsided planet.
In fact, there is a difference of
about 30 kilometers in the thickness of the crust in these two regions. Here's the reason: Early on, when the Solar System was young,
Mars was hit by at least 15 large asteroids. Scientists have linked these events to a time
around 3.9 billion years ago, known as the late heavy bombardment, when rock samples
from the Apollo landings show that our own moon was seriously pummeled. One theory holds that these impacts heated
the outer subsurface layer of the planet, Mars' "mantle." That prevented molten rock
in its core from rising up… and caused the crust to thicken. This had the effect of shutting
off Mars' magnetic field, exposing the planet to damaging solar winds, and over time, turning
it into a wasteland. One model says that as the newly formed giant
gas planets, Jupiter and Saturn, moved in their orbits, they hurled a rain of asteroids
and comets at the inner solar system. There are other theories that explain the disappearance
of Mars magnetic field and its atmosphere.
What's certain is that at some point early
in its history, the Red Planet grew increasingly desolate. The Martian landscape we see today is replete
with coded signals from those early times, ancient riverbeds and lake bottoms, flood
plains, and volcanic cones, as well as the battering it received from impacts. But is Mars a dead world? Maybe. Maybe not.
An infrared telescope on Hawaii's Mauna Kea volcano was trained on Mars over several years.
Astronomers used a spectrometer to split the light into its individual wavelengths… to
identify chemical fingerprints in the atmosphere. They found the signature of methane gas, in
amounts that change from place to place. Because methane should disappear quickly in Mars'
atmosphere, there must be some source that constantly replenishes it. That source could amount to nothing more than
the chemical reactions in Mars' crust. Or it could be biological in nature, perhaps
microbes alive and well in heated pools underground. So far, neither the satellites flying over
Mars, nor the robots on the ground, have turned up anything close to clear proof of life.
It may take the searching eyes, flexible minds, and nimble fingers of human explorers to find
that buried treasure, if it exists.
In the meantime, we are finding that even
if Mars is dead, it's certainly not dull. Mars has nowhere near the dynamism of Earth,
with its oceans, atmosphere, volcanism, and shifting continents. But it does do some fascinating
things: If you take an atmosphere, however tenuous,
add heat from the sun. You get the renowned dust devils of Mars. With no trees to hold
the soil down, landslides are common. The Mars Reconnaissance Orbiter has returned
images showing that Mars is actively remaking its surface, not in canals built by the Martian
engineers of Percival Lowell's imagination, but in sand dunes shaped by the wind, and
in landscapes molded by a gradually changing climate.
Some scientists have even turned
up hints of low-level plate tectonics. Whether or not we ever find life-forms on
Mars, we can still marvel at the beauty of our neighboring planet, its surface subtly
sculpted over eons of time, on a world that never was. 7.