NO PLANET in the Solar System can match Mars for highs and lows. When the
first space probe to map Mars, Mariner 9, arrived there in 1971, all it could
see was a vast cloud of dust enveloping the planet. Suddenly, above the dust
cloud, it spotted four small humps. These turned out to be the mighty peaks of
the Tharsis volcanoes, monstrous slabs of rock that thrust their broad, flat
summits up through the thin Martian atmosphere and out into space. Gradually, as
the storm subsided, the Mariner cameras saw the depths of the great impact
craters, the floors of the canyons and the low-lying northern plains. The lowest
points were 32 kilometres lower than Mars鈥檚 volcanic heights, whereas Everest
rises a mere 20 kilometres above the deepest trenches of the Earth鈥檚 oceans. It
was a dramatic landscape, and the image of Mars as rugged and mountainous has
been with us ever since.
Now, however, our picture of Mars is being changed. A new mapping mission has
discovered that although Mars is a pretty uneven proposition on a global scale,
up to a third of the planet鈥檚 surface is astonishingly flat. Large parts of the
plains of the north鈥攖he Vastitas Borealis that circles the pole, Arcadia
Planitia, Acidalia Planitia and Utopia Planitia at mid-latitudes, and Amazonis
Planitia reaching down to the equator鈥攁re flatter than any pancake you
could ever hope to see. There is the occasional crater and apart from that,
nothing; cover the place with felt and you鈥檇 have the Solar System鈥檚 biggest
pool table. If you travelled100 kilometres, the terrain might rise by only a
metre or two, says Jim Garvin of NASA鈥檚 Goddard Space Flight Center.
No relief
Garvin is a part of a team studying the Martian surface using a laser
altimeter on board the Mars Global Surveyor spacecraft. Since it reached its
orbit last September, the spacecraft has been carefully measuring its own height
above the Martian surface. By sending out pulses of laser light and timing them
as they bounce back from the surface they illuminate, measuring the elevation of
the Martian surface becomes 鈥渁t least two orders of magnitude better, maybe
three鈥� than any made before, according to Garvin. Estimates that were sometimes
off by kilometres are being replaced by measurements with an absolute accuracy
of about ten metres and relative precisions of less than a metre.
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And it is that precision that has also revealed that big chunks of Mars lack
almost any relief. From the prairies of Kansas to the grassy plains of northern
Europe, from the great Asian deltas to the smoothest parts of the Sahara, the
landscapes humans live in on Earth look hilly by comparison. 鈥淲e flew spare
parts of the altimeter on the space shuttle over some of the flattest places on
Earth,鈥� says Garvin, 鈥渁nd these places on Mars are flatter.鈥� The only things
that come close are lake-bed basins such as the Bonneville salt flats in Utah,
where the world land-speed record used to be contested, and the salars of South
America鈥攑laces so smooth that you can drive across them blindfolded. But
these desert flats are, on a planetary scale, tiny. The great Martian flats
stretch for thousands of kilometres.
However, Earth does have its own huge, flat plains鈥攊t鈥檚 just that
they鈥檙e hidden by kilometres of sea water. Between the mid-ocean ridges and the
continental shelves, the sediments that lie over the oceans鈥� abyssal plains can
be very flat indeed. Material scoured out by rivers from the surrounding land
falls slowly to the ocean floor in a fine rain. Far below the reach of waves or
weather, the smooth, flat sediments build up in layers that are kilometres
thick, forming abyssal plains as smooth as the northern wastes of Mars. And
scientists studying Mars have long thought that, if the planet ever had any
oceans, they would have been in the north.
So is it possible that the Mars
Global Surveyor is measuring the first seabed beyond the Earth? Tim Parker of
NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, which operates the Mars
Global Surveyor, is convinced it is. He has believed in the idea of Martian
oceans for a long time and reckons that the new measurements mean the theory is
鈥渄arn near confirmed鈥�.
The fact that there was once water on Mars was Mariner 9鈥檚 great
discovery鈥攎ore stunning even than the Tharsis volcanoes. The spacecraft
found scars on the planet鈥檚 surface that looked like evidence of free-flowing
water. Some of these scars were channels through which massive floods appear to
have coursed. And the grandest of these grand channels open onto the low plains
of the north. Hence the idea of a northern ocean.
Or oceans. Victor Baker, of the University of Arizona, Tucson, thinks that
unlike terrestrial oceans, the northern ocean on Mars may have come and gone
over time. The upper layers of Mars鈥檚 crust are quite porous, and could easily
contain more than an ocean鈥檚 worth of liquid water. Baker and most of the other
ocean-hunters imagine this water trapped below a layer of ice near the surface.
Baker thinks that massive volcanic episodes early in Mars鈥檚 history may have
melted vast tracts of this ice, releasing huge amounts of water to flow down the
channels and out to create a northern ocean. 鈥淎t peak discharges, you could
build the body of water in the northern plains over weeks,鈥� he says.
Ancient shores
The ocean would stay in place for thousands of years, warmed by an atmosphere
thickened with water vapour and with carbon dioxide from the volcanoes. Then it
would fade away, some of the water freezing, some escaping into the atmosphere,
some returning to the crust to wait until it was next summoned by volcanic fire.
Such a series of oceans could do a good job of leaving sediment around most of a
hemisphere. They would have plenty of sediment to offer鈥攅stimates of the
amount of crust scoured out of the channels put it at around 4 million cubic
kilometres.
But sporadic oceans are not the only possibility. Michael Carr of the US
Geological Survey in Menlo Park, California prefers a subtler story, with
underground reservoirs welling up in separate regional floods鈥攔eleased by
a single volcanic eruption, say, or an asteroid impact鈥攖o produce
something much more like big lakes than an ocean. Each time this happened, the
water would flow to the lowest available point and then freeze. So each new lake
would form around the existing ones, filling in any hollows and cracks. The ice
would then be coated with a layer of dust. In other words, the flatness measured
by the altimeter would be the surface of a mosaic of frozen lakes rather than
the floor of a defunct water ocean.
This idea is being tested using the laser altimeter data. Craters made in
thin layers of dirt solid ice will look different from those made in crust,
because material thrown out during the collision would vanish as the ice
sublimed.
Meanwhile, the altimetry data seem to be giving more support to the idea of a
fully-fledged ocean. In the late 1980s, Parker found features that he
interpreted as shorelines at different heights when he studied images taken by
the Viking orbiters. These shorelines reflected either the highest levels
reached by separate oceanic episodes or different levels of one long-lived
ocean.
The altimeter has measured the height of the most prominent of these
features, which Parker calls the Deuteronilus shore and the Arabia shore, and
which those not so committed to the idea of an ocean call contact A and contact
B. If they marked the positions of an ancient ocean, you would expect these
shorelines to lie at the same height around the basin. According to James Head
of Brown University in Rhode Island, another member of the altimetry team, the
proposed shorelines are not level.
However, in the three billion or more years that have passed since most
experts believe Mars finally lost its water oceans, the planet鈥檚 crust must have
been deformed in many ways鈥攐f which the Tharsis volcanoes are the most
spectacular example. Any relic of a shoreline would have been affected by all
this, and with that in mind, says Head, the ups and downs of the two contacts
make sense. They go up where you would expect the surface to have risen and down
where you would expect it to have fallen. Parker鈥檚 analysis of the data, carried
out with his colleague Bruce Banerdt, also of JPL, shows much the same
thing.
At May鈥檚 American Geophysical Union meeting in Boston, Head pointed out two
other intriguing bits of news for the pro-ocean camp. Statistical measurements
of the roughness of the surface show that it is rougher above the higher of
Parker鈥檚 鈥渟horelines鈥� than it is between the two of them鈥攁nd that it is
smoothest of all below the second contact, where it would have spent more time
under water. What鈥檚 more, the precise altimetry has allowed a much better
estimate of the volume of water that an ocean with those shores would have
contained. This sits comfortably with estimates of the amount of water Mars had
for filling oceans鈥攅nough to cover the entire planet to a depth of 400
metres, according to Carr.
When Mars Global Surveyor manoeuvres itself into a circular mapping orbit
next year, it will produce much more altimetry data and imagery for the various
camps to go on. And a future mission will carry a camera well suited to spotting
shorelines鈥攖he camera on Mars Global Surveyor is designed to take detailed
pictures of small areas rather than moderately good pictures of regions.
If the final consensus is that these are indeed ocean beds, they will become
prime targets for future exploration鈥攏ot least because on Earth, the
oceans beds are rich with microscopic fossils. The plains of Mars may be flat,
but it looks as if they鈥檙e going to be anything but dull.
MARS is a planet of two halves. The south is high and full of craters, the
north is low and flat. Earth, too, has its highs and lows, which reveal
something important about the way the planet works. Earth has two different
types of crust: one is thin and dense and is pulled inwards more strongly by the
planet鈥檚 gravity, the other is thicker and more buoyant. The planet鈥檚 water
naturally comes to sit over the thin, dense stuff, which is why it has come to
be called the oceanic crust. The processes that create and destroy oceanic crust
are the engines that drive plate tectonics, pushing the thick buoyant
continental crust around.
If the northern plains of Mars once held an ocean, it was because they are
lower than the rest of the planet. But is that because they, like the Earth鈥檚
ocean basins, are underlaid by a different sort of crust? And does that mean the
engines of plate tectonics once started up on Mars, cooling its core and
rearranging its surface? Norm Sleep of Stanford University in Palo Alto thinks
so. He believes there are telltale signs of tectonic activity around the great
northern basin which show that it was created through the formation of new
crust, like an earthly ocean basin.
Others are not so sure. George McGill of the University of Massachusetts in
Amherst points out that some bedrock structures in the north clearly predate the
features that Sleep thinks mark the start of the tectonic process. McGill
favours some novel mechanism whereby the ancient surface in the north sank to
its current depth as a single unit. This would explain why some features around
the edge of the basin seem younger than some structures at its floor: the edge
features formed as the surface dropped, when the floor had already been
formed.
The third possibility is catastrophe. Mars, like all planets, suffers from
asteroids and comets colliding with the surface. In the south, there are two
vast circular impact craters, Hellas Planitia and Argyre Planitia, which may
once have been seas themselves. Some researchers believe that the northern
lowlands are the same sort of thing writ even larger. One camp suggests that
they were formed in a series of big overlapping craters. Others, who think that
the odds against all the biggest impacts in Martian history hitting a single
part of the planet are large, prefer to think of them as the result of a single
event鈥攁n impact crater 7000 kilometres across, created by an object bigger
than any of the asteroids now seen in the Solar System.