The Allure
of the Golden Plains
July 2001 - Bo Maxwell
Twenty-five
years ago, on July 20th, 1976, Viking Lander 1 touched down on the surface
of Mars, the first American vehicle to soft-land on another planet.
It landed on the optimistically-called Chryse Planitia, the Plain of
Gold. It was followed on the 3rd September by Viking Lander 2, which
touched down 5,000 kilometres away on the Plains of Utopia.
Viking
was – and still is – one of the most ambitious deep space
missions ever undertaken. Comprising 4 distinct vehicles – two
orbiter craft and two lander craft, the mission was daringly complex.
Coming at a time when NASA was just getting the hang of placing probes
in orbit around other worlds (Mariner 9, Viking’s immediate predecessor,
had been the first American space vehicle to enter orbit around Mars),
Viking proposed putting two craft in orbit about the planet and two
vehicles – complete with robot laboratories down on the surface.
The
Design
Based
on the Mariner 9 vehicle, but of an overall increased size
to allow for carrying a lander vehicle "piggyback",
the Viking orbiter vehicles were designed by the Jet Propulsion
Laboratory as a natural progression of the Mariner programme.
The landers, however, were a different proposition. No one
knew exactly what the landers would encounter on the surface
of Mars, and a range of contingencies had to be planned
for. The vehicles had to be robust enough to survive extremes
of temperature; they had to be as light as possible so as
not to become too heavy to launch, but robust enough to
withstand what amounted to a crash-landing on Mars; and
so on. To meet these criteria, the design and construction
of the landers was handed over to NASA’s Langley research
centre and Martin Marietta.
The
eventual design comprised an ungainly 3-legged craft weighing some 1100kg,
of which 500kg was fuel for the descent engines used to slow the lander
during the last few metres of its descent after the parachutes had been
jettisoned.
With
an overall length of 3 metres and a height of some 2 metres
(to the top of the main high-gain antenna dish), the Viking
lander rivalled several models of small car in its size.
Mated
together, each orbiter / lander vehicle massed 3500kg, of which some
1400kg was fuel. This – combined with an unfavourable 1975 launch
opportunity - lead to the protracted flight time each vehicle combination
would need to reach Mars, 11 months instead of the five months of earlier
Mariner missions.
Concerns
While
confidence in the orbiter vehicles was high – NASA had enjoyed
tremendous success with Mariner 9 and saw no reason why the Viking orbiter
missions with their higher-resolution cameras would be any less successful
– the same could not be said of the lander element of the mission.
For
a start, no one had a clear idea of the surface conditions on the planet.
Vast dune fields had been imaged by Mariner 9, and these were areas
the landing teams clearly wanted to avoid. But what of the rest of the
planet? Could it to be covered in a layer of fine dust? Mars most certainly
suffered huge dust storms – Mariner 9 had arrived in orbit during
one. Would the landers therefore touchdown safely, only to sink into
several metres of dust and sand? Would the surface be too rocky, like
that of the Moon, causing the landers to break their backs on touchdown?
Mission planners weren’t even sure the landing areas they had selected
prior to launch would be of major interest scientifically – the
resolution of the Mariner 9 cameras denied them clear details of the
regions. Viking was also constrained by latitude – if a lander
came down any further poleward than 45 or 50 degrees in either hemisphere,
communications with Earth would be reduced, and the landers would experience
dangerously low temperatures.
To
try and overcome some of these concerns, it was decided to place the
combined vehicles in orbit for a period of time that would allow mission
planners to use the one tool available from Earth that could assist
the landers: radar. Radar scans from Earth could be used to gather information
across a swathe of Mars between latitudes 21oN and 21oS.
While the information gathered was basic, the returned pulses at least
enabled planners on Earth to tell how rough and rocky the proposed landing
sites were by the amount of radar "scatter" in the returned
images. Used in conjunction with images returned from Viking’s
cameras, the radar maps confirmed the region of Chryse originally selected
as Viking lander 1’s touchdown point – the confluence of four
water-like channels – was far too risky. The planned landing date
of 4th July – with all its political significance –
was abandoned while planners searched for a more suitable landing site.
Such
a site was selected, still in Chryse, and in time for the lander to
touchdown on an equally important – and somewhat more fitting –
date: 20th July 1976. Viking lander 2 followed on the 3rd
September, landing in Utopia Planitia – the Plain of Utopia.
Mission
Once
on the surface, both Vikings were programmed to act swiftly – raise
their high-gain antennae, locate Earth, sniff the Martian air around
them, rotate their 360o panoramic stills cameras and…photograph
their own feet.
That
a shot of the landers’ own footpads should be the first images
ever returned from Chryse and Utopia was not a mistake. It was the only
way people back on Earth had of confirming whether the vehicles had
in fact touched down on solid ground rather than shifting dust. If either
lander was going to sink in sand or dust, planners back on Earth wanted
to know about it before the vehicles simply stopped transmitting without
so much as a "help!" or "glug!"
Once
it had been confirmed both vehicles were on solid ground,
the real work could begin. The cameras were calibrated and
rotated to return the first colour images of the surface
of another world. There are few who, on seeing those first
images as they were shown on news broadcasts around the
world and reprinted in a thousand newspapers, could not
have been moved by the vistas returned from Viking. Here
was a world unlike our own, with landscapes vastly different
from those imaged by the Apollo missions. Mars was both
alien and yet hauntingly familiar. The late Carl Sagan summed
up what was perhaps the most common reaction on seeing the
Viking images:
"I
remember being transfixed by the first lander image to show the horizon
of Mars. This was not an alien world, I thought. I knew places like
it in Colorado and Arizona and Nevada. There were rocks and a distant
eminence, as natural and unselfconscious as any landscape on Earth.
Mars was a place. I would, of course, have been surprised to
see a grizzled prospector emerge from behind a dune leading his mule,
but at the same time the idea seemed appropriate."
With
both vehicles down safely, and their cousins imaging the surface of
Mars from orbit, it was time to get down to work.
Each
lander was equipped with a complex laboratory aimed at answering
the age-old question: was their life on Mars? While the
ideas of little green men or implacable intelligences operating
vast machines had been relegated firmly back into the realm
of fiction, few could discount the possibility that Mars
may be the home to tiny microbes. Viking was an attempt
to try and find those microbes.
Two
of the experiments aboard Viking were intended to tell us about the
composition of the Martian soil – or more correctly, regolith.
The remaining three were designed to find evidence of microbial life
within the regolith.
Eight
days after touchdown, Viking Lander 1 extended its sample arm and dug
a trench some 20 centimetres long in the Martian surface, gathering
a sample in its scoop. Returning the sample to the vehicle, the sample
arm deposited it into a hopper that delivered quantities of the sample
to each of the 5 experiments.
The
soil analysis revealed that the Martian regolith had high concentrations
of silicon, magnesium and iron, as well as being rich in iron oxides,
which give the soil its rusty colour. At the Viking Lander 2 site, some
nine days after touchdown, the analysis of samples there revealed exactly
the same components in the regolith. This meant the Martian surface
material was remarkably consistent in two regions some 5,000 kilometres
apart, giving rise to the theory that the surface of Mars is entirely
uniform in its composition – something vastly different to soil
conditions here on Earth.
But
it was the life sciences experiments that drew the greatest amount of
attention. By criteria drawn up prior to the mission launch, two of
the three experiments yielded positive results – there appeared
to be organic reactions caused by the presence of microbes in the samples.
In all, seven samples gathered from two landing sites 5,000 kilometres
apart yielded the same results.
To
some on the programme, the result was clear: Mars had life. But things
are not that simple. The one experiment that should have yielded
positive results had life been present in the samples failed to do so.
What was more, spectrographic analysis of both the samples taken and
of the Martian dust storm of 1971 revealed that montmorillionite clays
were evident in the regolith. When similar clays were added to duplicates
of the Viking labs here on Earth – they produced identical results
to those reported by Viking.
Legacy
Today,
views are still split on the Viking results. There are those,
such as Gill Levin, one of the principal investigators on
Viking, who remain vociferous in their claims that Viking
discovered life on Mars. Others are less than convinced.
Since Viking, items such as the Allen Hills meteorite fragment,
ALH84001, have added to the controversy.
But
the argument for and against Viking having found life is
just one part of the legacy left to us by this remarkable
programme. Before the project finally closed down, it had
returned some 55,500 images of Mars to us. 51,000 of these
had been taken from orbit, revealing the entire surface
of Mars down to a resolution of around 150 metres. The remaining
4,500 had been returned by the Landers themselves. Expected
to last for a little over a year, Viking racked up a remarkable
lifetime. While Orbiter 2 was shut down in 1978, Orbiter
1 continued to function until mid-1980, and was only powered
down when funding became an issue – the vehicle itself
was still operating. Both the landers out-lived their orbital
cousins, with Lander 1 finally ceasing transmission in November
1982, fully five years after its expected expiry date.
Today
Viking stands as a triumph and a challenge. It is a triumph because
it showed us clearly what we could achieve in the name of science and
exploration using modern technology. Even now, it still stands as one
of the most technically complex robotic missions ever undertaken. No
other planetary mission has achieved so much in so many widely varying
areas: photographic reconnaissance, atmospheric analysis, soil analysis
and surface imaging.
Viking
is a challenge. It has revealed much about Mars, and allowed us to build
on that knowledge through the Mars Global Surveyor and Pathfinder missions,
but it has also left us with the mystery of whether or not Martian microbes
exist. Sitting on the surface of Mars, Viking Landers 1 and 2 challenge
us to think boldly. To not only continue in our efforts to explore and
understand Mars through missions such as Europe’s own Beagle 2,
but also to go to Mars ourselves and bend our own intellects and abilities
to the question of life on Mars and open the planet to greater feats
of exploration.
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