Ordinary Meeting, 2003 November 29
Mars, the Inside Story
Ms Couper expressed her delight at having been invited to address the Association, having previously held the Presidency 1984-6. As to her subject, there were many reasons for taking special interest in Mars at present, both for amateurs and professionals. The former group had, of course, been treated to an exceptional view of the red planet in recent months as it passed closer to the Earth than any previous approach of the two planets for many thousands of years. Furthermore, four space probes destined for Mars were due to arrive within a few months; firstly, the Japanese Nazomi probe, though this one seemed rather unlikely to perform well, as a series of crippling malfunctions in-flight, and solar flare damage from 2002 April, had forced substantial revision of the flight plan. Indeed, Nazomi had been intended to reach Mars in 1999, but a thrusting fault early in the mission had forced a new interplanetary trajectory to be devised, bringing it into Martian orbit in 2003. America had launched two twin landing rovers, Spirit and Opportunity, scheduled to arrive 2004 January, and Europe's Mars Express, scheduled to arrive at a similar time, was carrying the British-led Beagle 2 landing probe. All these spacecraft appeared to have survived unscathed the radiation hazards posed by the intense solar storm activity of late 2003 October.
The speakers commenced their historical survey of observation of Mars in 1894, with Percival Lowell's industrious efforts from Arizona. Travelling south from Massachuchets to find clearer skies, Lowell had founded his own observatory shortly before the opposition of that year, using funds from his family fortune. Subsequently, he had concentrated his work almost exclusively upon Mars for a period no less than 15 years. He had famously claimed to find canals, and constructed elaborate theories to explain how an intelligent race had built these extensive systems of waterways to irrigate withering vegetation in the equatorial regions using water from the melting polar caps. He pictured Mars as a dying planet, and even after only his first night of observation, he already used the word 'desert' to describe what he saw. The speakers suggested that cultural influences such as the inauguration of the Suez Canal in 1869 may have biased Lowell's imagination at this time, as well as the desert surroundings of Arizona. Some inspiration was sure to have come from Schiaparelli's 1877 discovery of what he termed 'canali' – translating either as 'canals' or 'rivers' – though Schiaparelli did not himself believe them to be anything other than natural phenomena. The speakers speculated that Lowell's use of aperture stops to reduce the brightness in the eyepiece may have introduced an anomalous sharpening of filamentary features, leading Lowell to over-interpret them.
Little more was learnt about the red planet until 1965, when NASA's Mariner 4 flew past, becoming the first probe to obtain close-up images of any of the planets. Results returned by its instruments suggested the Martian atmosphere to be only 1% the thickness of that on Earth, and to be composed primarily of CO2. Furthermore, Mariner found minimal magnetic field to protect the surface from the ionising particles of the solar wind. Bathed in such radiation, it would be rendered a particularly harsh environment for life.
This coming at the height of the Cold War era, the Soviets also worked strenuously on their own programme of spacecraft destined for Mars, though it was dogged by so many failures that the phenomenon became known as the 'Mars Curse'. The closest the Russian missions came to success were the Mars 2-7 projects, launched in the early 1970s. These followed a series of at least eleven entirely fruitless attempts, most of which failed on launch. Mars 2 and 3 were combined orbiters and landing modules, of which one lander failed 20 seconds after commencing data return; the other was destroyed at touchdown. The two orbiting modules returned data for no more than a few weeks. In 1973, the Russians launched four further missions, two orbiters, Mars 4/5, and two landers, Mars 6/7. The first failed to enter orbit, and only managed to return a few distant images of the planet. Mars 5 failed 10 days after reaching its destination, and returned minimal data. Mars 6, a landing probe, returned data during decent, but failed on landing. Meanwhile, Mars 7 entered solar orbit after missing its target completely.
The USSR would not attempt any further Mars missions until the 1988 launch of Phobos 1 & 2, both with considerable assistance from other European nations and communication support from NASA's Deep Space Network. These ambitious projects intended to examine Mars' moon Phobos, thought by some to be a captured asteroid. They would perform chemical tests on it by directing an intense laser beam onto the surface below, vapourising tiny samples. The spectrum of the resultant glow would be analysed in the hope of detecting emission features allowing identification of the rock composition. However, trajectory errors would lead both to miss their target.
The speakers reported that western missions to Mars had fared little better. For example, NASA's 1993 Mars Observer probe was lost three days prior to insertion into Martian orbit. In the absence of any information beyond a loss of radio contact, the failure remains unaccountable to this day, but a report into the incident found the most probable explanation to be a breach of the probe's propulsion system during an orbital insertion burn.
The final blow to Russia's programme was the loss of Mars '96 in 1996, a massive spacecraft, weighing 6180kg at launch, and with both orbiting and landing probes. The latter sought to perform extensive tests on the soil, including penetrating below the top layers which are bathed in solar UV radiation. Sadly, however, the fourth stage of the launch vehicle failed to provide a crucial second burn to move the craft from Earth orbit onto an interplanetary trajectory, and a few orbits later, the payload crashed back to Earth.
In late 1999, the American programme suffered the twin failure of Mars Climate Orbiter (MCO) and Mars Polar Lander (MPL). MCO's loss was believed to be the result of confusion between imperial and metric units, which led to an incorrect orbital insertion burn. MPL failed on landing, and enquiries found the most likely cause to be a software bug in a control system programmed to cut-off the landing retrorockets upon feeling the jolt of touchdown. In the event, this was prematurely triggered by the jolt of the landing gear unfolding at an altitude of 1.5km.
The speakers moved on to discuss the notable success stories, starting with the memorable Mars Pathfinder mission of July-September 1997, which included the Sojourner rover. This had performed detailed surveys of rock chemistry around its landing site, finding results similar to those of the Viking mission of 1976-82. Mars Global Surveyor (MGS) arrived soon after, in November 1997, and continues to return data. The findings of these two missions have brought many past paradigms into question, in particular by returning evidence of geological processes at work in the recent past, suggesting that their present absence is a period of dormance, not extinction. Perhaps the most unexpected discovery of MGS was the identification of gullies and flow-like features, reminiscent of water-erosion seen on Earth. These appeared to imply the presence of water in the recent past.
Further evidence for recent activity had emerged from studies of lava flows in the vicinity of Olympus Mons. These appeared to be newly formed within a timescale of 10 million years; incompatible with previous ideas that Martian volcanism had ceased 100 million years ago. Though an issue of debate, many now argued Olympus Mons to be dormant, not extinct, and likely to erupt again in the future, this view being shared by Henbest and Couper.
The speakers concluded their talk by reviewing the evidence for life on Mars. The most important results in this regard came from the two Viking missions which landed in 1976. One experiment, termed Labelled Release (LR), sought to measure whether radioactively-tagged nutrients, when applied to soil samples, were subjected to respiratory reactions and released as gas, as would happen for samples containing terrestrial microbes. The results favoured the presence of respiring microbes. However, the results of another experiment, the Gas Chromatograph – Mass Spectrometer (GCMS), which measured the soil carbon abundance, had seemingly excluded the possibility of life. It reported a complete absence of organic molecules, and was influential in leading NASA to conclude that the evidence favoured a lifeless planet. A few researchers continued to advocate the presence of life, including Dr Gilbert Levin, who led the Viking LR experiment, and Henbest and Couper shared their conviction. The speakers explained that the inherent insensitivity and possible miscallibration of GCMS seemed a likely explanation for its negative results.
Further excitement had been raised in 1996 with the discovery of structures resembling fossilised bacteria in carbonate deposits within meteorite ALH84001, the Allan Hills meteorite, which was believed to be of Martian origin. The speakers eagerly awaited the results from Beagle 2, due to land in December, which, if successful, would perform extensive chemical tests seeking to resolve many of these questions.
To close, the speakers speculated about possibilities for future human colonisation. Following the applause, the speakers invited questions, and a lively debate ensued on many of the more controversial issues raised. With reference to the Allan Hills meteorite, Mr Nick James pointed out that many had discredited claims that the observed structures were fossils, because, at 100nm, they were around ten times smaller than anything comparable on Earth. However, the speakers felt that this scale-difference did not entirely refute a 'nanobacterial' interpretation. Mr James further wished to clarify the circumstances of the failure of MPL and MCO. He pointed out, with reference to the Young Report, that the failure of MPL was considerably less simple than had been implied earlier, and in particular, it had plunged only the final 40m to the surface with suicidal speed. With regard to the loss of MCO, the confusion of units was very subtle as compared to how it had been portrayed by the speakers. The error had occurred when ground-teams had modelled the small effect of the rotating angular momentum dump wheels on their thrust calculations. The effect was very small indeed, and some blame for the loss might be directed at the navigation teams who failed to identify the cause of a string of small trajectory errors earlier in the mission. The speakers disputed the details of Mr James' account.
With regard to the Viking experiments, Mr James argued that the speakers' account was rather selective. There had been a total of four soil experiments, three of which were incompatible with the presence of life. Even in the unlikely event that GCMS had failed, the results of two further experiments had to be explained. The combined evidence of these three experiments had led the majority of the community to believe Mars lifeless, especially since the LR results were themselves rather ambiguous and doubtful. The speakers disagreed, arguing that the LR results could not be ignored.
Moving on, the speakers were asked, in view of the low pressure of the Martian atmosphere, whether the formation of water-erosion features on Mars at the present epoch was plausible, as had been implied earlier, as water would surely boil too rapidly. Mr Henbest maintained that water might exist for long enough to cause erosion before vapourising.
Following a vote of thanks proposed by the President, the meeting broke for tea. After the break, Mr Martin Mobberley was welcomed back to present his latest Sky Notes.