April 30, 2011
Did Mars long ago develop far enough for life
If so, does anything still live within Mars' dusty plains,
beneath its ice caps, or somewhere underground?
In 1964 the
Mariner IV spacecraft flew by Mars and got a good
look. What it saw looked more like the Moon than the Earth. 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°
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
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
Planet. When the excitement died down, the rovers were set off on
one of the most remarkable journeys in the history of planetary
exploration. Missions like this could one day pave the way for a day
when we'll view images from a real astronaut's camera.
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
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?
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.
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.