On 25 May a new robotic explorer will arrive at Mars.
NASA’s Phoenix lander, formed as the resurrection of two previous missions, will descend from the martian heavens atop a pillar of flame to touch down gently upon the arctic plains of the Red Planet.
The orbiting probes already at Mars have even been manoeuvred into position to observe this newcomer’s arrival.
Phoenix will land in the north polar basin, shown in the map on page 40, and will explore a location further from the martian equator than any probe before.
Although Phoenix isn’t equipped to directly sense life itself, it will wield suite of instruments to assess the habitability of this frigid environment.
The probe has a digger-arm to scoop up soil from beneath the surface and deliver it for analysis in a host of different tests on the body of the probe.
The hope is that Phoenix will make the first actual detection of water on Mars – so far it has only been observed remotely from orbit or inferred from ancient minerals studied by probes on the ground.
Phoenix will also test whether conditions here are compatible with life, assessing the soil in terms of how acidic or salty it is, how chemically oxidising it is, and the presence of any chemical energy sources.
The Holy Grail for this mission will be for Phoenix to detect the building blocks of life on Mars, organic compounds that make up biological molecules such as DNA or proteins essential for life.
It will have to work quickly, though.
The probe will land at the beginning of summer and there will only be about 150 days before the punishing winter arrives and finishes it off.
Over the next four pages we assess how likely it is that Phoenix will discover groundbreaking evidence that life could exist on Mars.
Yes, it will find evidence for life
The Phoenix mission certainly has a lot of unique features.
It will be landing in an arctic region that has never been explored before, digging deeper than any previous probe, and carrying a package that includes several instruments that have not yet been deployed on Mars.
And there are more reasons to be optimistic about a significant discovery.
Ancient Oceans
In 2002, the Mars Odyssey probe in orbit around the Red Planet discovered evidence of vast stores of water ice lying just beneath the surface, around the north and south polar regions.
Early in Mars’s history, huge torrents of water spilled into the northern polar basin – the large area of low-lying ground shaded blue at the top of the map on page 40.
The water came from the southern highlands, carving great flood channels into the martian surface on its journey down.
This water may have pooled in impact craters to form lakes, or soaked into the ground to provide moist surface conditions just long enough for life to develop near the martian surface.
Some researchers believe that an ancient ocean once encircled the north pole.
As evidence, they point to the smoothness of the terrain in this basin, which looks as if it is covered with sea-floor sediment.
And there’s even what appears to be a shoreline running for thousands of kilometres along the northern edge of the highlands.
The Phoenix scientists hope that water ice remains close enough to the surface of the northern polar basin for the probe’s digging arm to uncover it.
They are aiming for nothing less than to be the first to directly sample water on Mars.
Life is tough
Over the past few decades, microbiology research here on Earth has brought about an exciting realisation: that some life forms on our planet are incredibly hardy.
These organisms, too small to be seen by the naked eye, are called extremophiles.
There are examples of extremophile microbes that can tolerate salt levels of up to 100 per cent saturation, and that can live in conditions as acidic as battery acid.
Some are even active at temperatures as low as –20ºC inside tiny channels of liquid water within the hard-frozen ice of a glacier or polar ice shelf.
Although Phoenix won’t be able to detect life itself, its MECA instrument (see diagram above) will be able to characterise the icy martian soil in terms of how salty or acidic it is.
This will tell scientists whether it is an environment where extreme life forms could possibly exist – a subsurface habitable zone.
Many microbes on Earth can also feed off inorganic compounds within rock minerals, and Phoenix will be able to detect such potential energy sources.
Phoenix's Arsenal
Robotic arm
This jointed digging arm has been designed to scrape a ditch up to 50cm deep out of the frozen soil. Soil samples from different depths will be scooped up and delivered to the various analysis instruments on top of the probe.
Microscopy, Electrochemistry and Conductivity Analyzer (MECA)
As well as examining tiny soil grains microscopically, this package will stir warm water into soil samples and measure the concentration of dissolved chemicals to judge the saltiness and acidity of the mud, as well as the presence of destructive chemical oxidants.
Thermal and Evolved Gas Analyzer (TEGA)
TEGA will be used to bake samples of martian soil and measure the amount of water vapour wafting off as well as the minerals present.
It will even sniff for small organic molecules.
Warmer spells
While some terrestrial extremophiles can thrive even at sub-zero temperatures, the martian polar wastelands are currently far too chilly for life as we know it.
However, this deep-freeze does provide excellent conditions for preserving organisms and organic particles, which Phoenix may be able to sniff out.
And just because Mars is chilly now, it doesn’t mean that it always was.
Just as Earth’s climate swings between cold ice ages and warmer interglacial periods, wobbles in Mars’s tilt or orbit around the Sun would provide spells of warmer conditions in the polar regions.
For a short time every few tens of thousands or millions of years, permafrost near the polar surface may thaw, allowing polar life a chance to re-animate after long winters of dormancy.
This theory of transient thawing would be supported if Phoenix discovers increasing levels of salts as it digs down into the surface.
Organics in the soil
The Viking probes that landed on Mars during the 1970s could find no trace of organic molecules in the handfuls of martian grit that they analysed.
To scientists back on Earth, this was baffling: organic compounds were expected to have literally fallen from the heavens aboard meteorites and accumulated in the martian topsoil.
Recent evidence has shown that Viking scientists were right to be baffled. Late last year, researchers announced that work on martian meteorite ALH84001 had revealed that Mars did indeed once host the chemical reactions that create organic molecules, which are the first steps for kick-starting life.
Phoenix has a chemical detector (TEGA) many times more sensitive than similar equipment on board the Viking missions, and it will also be able to dig up soil from deeper underground.
This is where interesting molecules may have survived, so it is hoped this latest explorer will find organics on Mars for the first time.
Although previous missions have taught us that ancient Mars was undeniably more like Earth, the current surface conditions are decidedly inhospitable.
There are a number of good reasons to be cautious about what grand discoveries Phoenix might be able to deliver.
The expert:
William Boynton, professor of cosmochemistry at the University of Arizona, heads the TEGA instrument team on Phoenix.
He’s also in charge of an instrument on board the orbiting Mars Odyssey
What did your instrument on board Mars Odyssey discover?
The Gamma-Ray Spectrometer (GRS) found large amounts of the element hydrogen in the surface around both the north and south poles of Mars.
There’s so much hydrogen there that it can only be in the form of water ice.
What’s more, this ice is buried only a few centimetres below the surface.
We’ve now found that this subsurface layer is about 80 per cent ice by volume.
What does this mean for the habitability of the northern plains?
It means that we’ve found great amounts of water ice in the ground where Phoenix will be landing, and hopefully close enough to the surface so that the probe will be able to scoop it up.
But even more importantly for astrobiology, this ice may experience periods of thawing.
The tilt of Mars’s rotation axis changes over time, and to a much greater extent than the Earth, which is stabilised by a large moon.
When Mars is tilted to a high degree, the polar regions get warmer and so it’s possible that this ice would get warm enough to melt and provide liquid water for life.
What do you hope TEGA (Thermal and Evolved Gas Analyzer) will discover?
I hope TEGA will find ice beneath the surface and that the ice contains significant amounts of organic compounds: the kind of molecules required by life.
If my Mars Odyssey work is correct, the ice will be very close to the surface and we should have no trouble getting to it.
Phoenix has a grinding tool on the robotic arm that will permit us to get a sample of the ice layer that can then be delivered to TEGA.
Hopefully we’ll discover that the ice layer protects the organic compounds from the oxidising layer that’s thought to be on the martian surface.
No, it won’t find any evidence for life
Although previous missions have taught us that ancient Mars was undeniably more like Earth, the current surface conditions are decidedly inhospitable.
There are a number of good reasons to be cautious about what grand discoveries Phoenix might be able to deliver.
Ocean? What ocean?
Although some planetary scientists argue for an ancient martian ocean in the northern hemisphere, many are not convinced.
The problem is that the so-called shoreline does not actually appear to be flat all the way round, as would be expected if there was once a ‘sea level’ in this region.
This mystery may have been solved by research reported last year.
It shows the proposed shoreline is in fact level if the spin axis of the planet had shifted at some point since the ocean dried up.
However, even if a great volume of water did pool in the northern basin, it may not have stuck around very long before seeping down into the crust and freezing, or escaping into the atmosphere.
Although we don’t fully understand how life emerged on our own planet, it is likely to need stable conditions for a relatively long period of time, and the north polar ocean – if it ever did exist – may have dried up too quickly, exposing any fledgling life to much harsher conditions.
By analysing the texture and mineral composition of soil grains at different depths, Phoenix may be able to settle the ocean debate once and for all.
Life isn’t invincible
Although it’s true that various terrestrial life forms can tolerate very acidic or salty environments, they’re not indestructible.
When environments exhibit combinations of these hazards – being acidic and extremely cold for example, it makes the survival of microscopic life very challenging.
As discovered recently, the waters of the ancient martian sea at Meridiani Planum may have imposed impossible conditions on life (see ‘The expert’ on page 41).
In any case, James Cleaves at the Scripps Institution of Oceanography and John Chalmers at the University of California argue that extremophiles are irrelevant to the possibility of life arising in the first place.
“Extremophiles say more about the adaptability of life rather than revealing its origins.
While life can evolve to extremes of acidity or alkalinity, salinity, temperature and so on, pre-biotic chemistry is much more fragile and requires fairly mild conditions,” Dr Chalmers explains.
So depending on what Phoenix learns about the polar subsurface environment, it may be that ultra-hardy organisms might be able to survive there now, but the vast majority of crucial chemical steps that go into producing the molecules of life simply couldn’t have happened.
Oxidising layer
The failure of the Viking probes to detect even the slightest whiff of martian organics was unexpected, but it is now believed to be due to the build-up of destructive chemical oxidants that have formed in the topsoil.
The martian surface isn’t protected from radiation by an ozone layer like the Earth.
The relentless flood of ultraviolet radiation from the Sun that bombards the martian surface creates caustic chemicals such as hydrogen peroxide.
These would tear apart any organic molecules as surely as if the soil were laced with concentrated bleach.
How deep this oxidising layer extends is very difficult to predict and it is a real possibility that Phoenix won’t be able to excavate deep enough to find any surviving organics – one of the prime aims of its mission.
The wet chemistry laboratory in the MECA instrument will assess these oxidising conditions, which will be particularly important if the TEGA chemical nose fails to sense any organics, even from the bottom of the digger arm’s trench.
Time is short
The veteran exploration rovers Spirit and Opportunity have now lasted almost four Earth-years longer than originally planned; indeed, many of their greatest discoveries came after their primary mission was over.
Phoenix, however, will not be able to move once it has touched-down and will only be able to sample within an arm’s reach of its landing site.
The TEGA and wet chemistry experiments also have limited resources and can only be run a handful of times, so scientists will need to be very careful about using them productively.
Unlike its rover cousins, Phoenix will have no chance of a greatly extended lifetime.
Roughly 150 days after landing, the dwindling sunlight of the encroaching polar winter will drain the batteries and kill the mission.
Then, within a few more months, the probe will become buried by gathering carbon dioxide frost – dry ice.
The upside of this is that because of the mission’s short lifespan, it won’t be long before we know once and for all whether Mars is conducive to life.
The expert:
Andrew Knoll is the Fisher Professor of Natural Historyat Harvard University and works with the Mars rovers Spirit and Opportunity
What have you discovered about liquid water on Mars?
The Mars exploration rover Opportunity found both physical and chemical evidence for liquid water in the Meridiani region, at the time its sedimentary rocks formed.
On the other side of the planet, the Spirit rover found evidence for the alteration of volcanic rocks by water in Gusev Crater.
We discovered early on that the water in the Meridiani region was strongly acidic.
That doesn’t mean that astrobiology could not survive there; some terrestrial micro-organisms live at a pH level comparable to, or below, those inferred for Mars.
More recently we’ve been able to work out the saltiness of the water.
It seems that as the waters percolated through Meridiani sediments, they became steadily more saline until most, if not all, terrestrial organisms would have found it a challenging place in which to survive.
What does this mean for the habitability of the planet?
Findings from the two rovers suggest that the martian surface has been very cold, dry, acidic and oxidising for a long time; so it’s pretty inhospitable. But Mars could have been more accommodating during its first 500 million years or so.
What do you think Phoenix will find?
I’m excited about Phoenix.
It carries a suite of experiments and instruments that promise to tell us a great deal about present day Mars.
What, for example, is the nature of the permafrost?
Is there a reservoir of nitrogen in the martian soils?
Do organic molecules accumulate in the dust and broken rock covering Mars’s surface?
Life or no life, carbonaceous meteorites have pummelled Mars throughout its history; does their organic cargo accumulate, or is it destroyed over time?
Given the long-term challenges of the martian surface, I’d be surprised to learn that Phoenix has found evidence of life. But, make no mistake, I’d be delighted if it did.
This article first appeared in the May 2008 issue of Sky at Night magazine
