A different perspective on the world

No comments

A grand vision of global cooperation promises to boost the opportunities for chemical analysis from space. Andrew Scott looks at the findings from existing satellites

Results from space-based chemical analysis are confirming and challenging predictions in existing environmental pollution models. Work is under way to capitalise on this information.

At the third Earth Observation Summit in Brussels in February, 60 nations and more than 40 key international organisations endorsed a 10-year plan to create a single, comprehensive system to monitor the health of the planet. The planned global earth observation system of systems (GEOSS) includes work to integrate a network of existing and future satellites to analyse the environment from space. A significant part of its work will involve chemical analysis of the planet using instruments in orbit.

It is a grand vision of global cooperation, although much of the funding to make it a reality is not yet secure.

The potential of space-based chemical analysis is being demonstrated by the results now flowing in from the European Space Agency’s (ESA) Envisat satellite - the world’s largest and most sophisticated satellite for environmental monitoring.

John Burrows of the University of Bremen’s institute of environmental physics in Germany emphasises there are two key advantages of doing environmental chemistry from space: it gives a dynamic picture of change over time, but also a global picture. ’I think it is vital for us to get a better understanding of this complex interacting chemical system that is our home,’ he enthuses, ’and you can argue that it is both cheaper and better to do it from space.’

One of Envisat’s key instruments was devised by Burrows - a spectrometer called Sciamachy - a Greek word meaning ’chasing shadows’, which also works as an acronym for the scanning imaging absorption spectrometer for atmospheric chartography.

A high resolution map of global atmospheric nitrogen dioxide pollution, recently released by ESA, is an impressive example of what Sciamachy can do. This map is the result of 18 months of accumulated observations, and vividly illustrates how human activities affect air quality. Nitrogen dioxide is released from power stations, heavy industry and road transport, so it is not surprising to see the swathes of high NO₂ concentration across the industrialised belts in North America, Europe and south-east Asia. Some large population centres such as Mexico City also stand out as vivid hotspots of pollution amid surrounding areas of relative purity.

It is also interesting to see the vast smudge of high NO₂ levels blanketing much of Africa, for example, as a result of the combustion of biomass for cooking and heating. NO₂ from the smoke stacks of ships also makes some major shipping routes visible, such as in the Indian Ocean between the southern tip of India and Indonesia, and in the Red Sea.

’We have got some very interesting measurements of the greenhouse gases,’ says Burrows. Although it is too early to publish details, the Bremen researchers think they may be gathering important new information about the subtle interactions of forests and greenhouse gases.

In results released in March, Sciamachy has also provided the first space-based measurements of the global distribution of near-surface methane, one of the most important greenhouse gases. In agreement with existing simulations, the measurements reveal enhanced methane concentrations over India and China caused by emissions from rice paddies and domestic ruminants such as cattle.

But in large parts of the tropics there is a considerable difference between the methane distribution patterns found by Sciamachy and those predicted by current models. Further research will be needed to make sense of the new insights offered by this view of methane chemistry from space.

Sciamachy records the spectrum of sunlight, transmitted, reflected and scattered by the Earth’s atmosphere or surface. It works in the ultraviolet, visible and near infrared wavelengths, allowing gases, water vapour, clouds and dust particles to be quantified throughout the atmosphere. With a 960km wide swathe, it covers the entire planet every six days.

When the data arrive on the ground, operators use differential optical absorption spectroscopy (DOAS), to retrieve the very weak trace gas absorption patterns within the overall spectrum of backscattered light. DOAS removes the predominant spectral noise from air particles, Rayleigh scattering of light and the absorption patterns from the oxygen, nitrogen and water that comprise most of the atmosphere. The remaining spectral absorption patterns of the trace gases are identified by comparison with known samples.

This technique is sufficiently sensitive to detect NO₂, for example, at just a few parts per billion. Above highly polluted industrialised regions the NO₂ values can be as high as a hundred parts per billion.

Although Sciamachy was devised and launched at a time when interest in the ozone layer was the primary driving force, it can analyse a wide variety of atmospheric gases. These include gases implicated in ozone destruction, such as bromine monoxide, which is a stratospheric ’ozone-killer’ generated as part of natural atmospheric cycles beginning in the troposphere.

Space-based analysis of bromine monoxide is already providing a better understanding of the so-called ’bromine explosions’ - bursts of bromine production that can occur in the troposphere. This is a helping to improve understanding of the processes involved in ozone destruction and regeneration.

Other key chemical targets for Sciamachy include formaldehyde produced by biomass burning and biogenic emission, the various sulphur and nitrogen oxides, and methane, carbon dioxide and water.

Sciamachy has also demonstrated that it is possible to analyse the chemistry and biology of the Earth’s waters from space. The instrument has achieved the first space-based measurements of chlorophyll-a in the oceans. This provides data about the biological productivity and health of the water, allowing algal blooms and related biological processes to be monitored.

Envisat carries 10 scientific instruments, and two of the others are also devoted to analysing atmospheric chemistry. MIPAS (michelson interferometer for passive atmospheric sounding) is a fourier transform infra-red spectrometer which is able to analyse the concentration of about 20 trace gases, including the complete NOx family and several CFCs. The use of MIPAS was suspended in March 2004 because of an instrument anomaly.

GOMOS (global ozone monitoring by occultation of stars) is a medium resolution spectrometer. It is designed to measure the concentrations of ozone and other atmospheric trace gases with very high accuracy and over long time periods, so that trends become visible. In January, GOMOS also had an instrument anomaly, which is now being investigated.

Rather than basking in the glow of hard-fought achievement, the scientists who put chemical analysis at the heart of Envisat’s mission are growing increasingly worried that Europe’s current lead in the field will be short-lived. ’We are not putting sufficient investment into the follow up,’ says Burrows, adding, ’I am very concerned that we have missed opportunities to push on in [space-based] atmospheric chemistry.’

Joerg Langen, of ESA’s atmosphere unit, comments that Burrows’ concern is actually one that applies all over the world. He points out that ’after NASA’s new Aura craft, there is no follow up, so everyone is worried about the future’. This puts the bold plans for GEOSS, announced at the Third Earth observation summit, into some perspective.

Europe’s specific contribution to the development of GEOSS will be a joint initiative between ESA and the European Commission known as global monitoring for environment and security (GMES). ’The next phase of GMES includes five new space missions with missions four and five relating to atmospheric chemistry satellites,’ says Langen. But he emphasises these are still at the planning stage and will only be implemented if the funding eventually materialises.

The Aura satellite Langen refers to was launched by NASA in July last year. NASA describes it as a 24-hour global pollution monitoring service that will allow daily chemical forecasts from space.

Aura should be serviceable for at least five years. It carries four instruments (see p43), some of which have been developed with European input.

’Aura’s early results are nothing short of astounding,’ says NASA’s Mark Schoeberl. The most significant results will take about a year to accumulate. This is because some of the most revealing analysis requires combining data from many passes over a region under study, in the same way as the Sciamachy NO₂ map was created from observations taken over 18 months.

Nevertheless, NASA claims Aura is ’already providing the first daily, direct global measurements of low altitude or tropospheric ozone and many other pollutants that affect our air quality. with unprecedented clarity over a region’.

NASA representatives also comment that politics will inevitably become involved alongside the chemistry. They point out that space-based chemical analysis can prove conclusively which countries are the biggest polluters and which may or may not be improving their pollution record in line with political claims.

Such analysis could, in future, include monitoring global environmental treaties and providing atmospheric data on a commercial basis. One large company in the sunscreen business is already paying to get data to make UV forecasts.

So, there could be many applications beyond the major ones of ozone monitoring, pollution analysis, and weather forecasting.

As the GEOSS initiative develops, the science will inevitably be just one part of a cocktail including politics, international diplomacy, competition and cooperation. Regardless of exactly how the 10-year plan unfolds, doing chemistry from space will be a part of it.

Speaking at the Brussels summit in February, Timo Makela, director in the EC directorate-general for the environment, commented: ’Our environmental policies and legislation are based first and foremost on our knowledge of the state of the environment.What we know is that we do not know enough at the moment, and satellites and space can bring something additional to our existing monitoring.’

Getting chemistry performed from space included in that level of political acceptance has been a long road for scientists, such as Burrows, who have been in the field from the start.

’When I first proposed that we should be able to do these sorts of chemical measurements from space in the 1980s it was ignored,’ says Burrows, ’ but the discovery of the ozone hole created a huge wave of interest. And we pointed out that we could get data on a lot more things besides just ozone’. That data is now beginning to be delivered, albeit at considerable expense.

Andrew Scott is a writer and lecturer based in Perth, UK

If you want to know what happened in the past, it’s usually best to ask someone who was there. Astronomers are using a similar technique to try to piece together how the solar system came into being. By analysing some of the most ancient bodies in the solar system, which are thought to have been around since its formation, they hope to reveal details about the processes and material involved.

The easiest solar bodies for astronomers to study are meteorites, because they arrive of their own accord. The oldest type of meteorite is the carbonaceous chondrite, which scientists think formed from dust and rocks within the pre-solar system protoplanetary disc. These meteorites consist mostly of tiny igneous rocks, but also contain calcium- and aluminium-rich inclusions (CAIs) and large amounts of carbon.

Only around 100 have been found, with the most famous being the Murchison meteorite, which was discovered in Australia in 1969. Scientists have identified over 70 amino acids in this meteorite, raising the possibility that carbonaceous chondrites might have helped deliver the organic material needed for life to start on the early Earth.

However, scientists analysing another carbonaceous chondrite in 2001 - the Taglish Lake meteorite - found hardly any amino acids. This implies that it either formed in a different way to the Murchison meteorite or that any amino acids that it did possess have been destroyed by excessive heat.

Recent analysis of the CAIs in carbonaceous chondrites has also given scientists a window on the formation of the solar system. They have uncovered evidence that CAIs once played host to short-lived radionuclides of iron and chlorine, which were most likely produced in supernova explosions. This implies that our solar system formed in a fairly active region of space, close to large stars that were going supernova, rather than forming in relative isolation, as has always been believed.

Another way to analyse meteorites is by sampling interplanetary dust particles (IDPs), which can be collected by planes flying at high altitude. More interesting, however, is that IDPs also consist of particles from comets. These conglomerations of ice and dust formed at the same time as the solar system and are thought to preserve much of the material from the protoplanetary disc.

But scientists can only discover so much about comets from IDPs, which are often chemically altered by their passage through the Earth’s atmosphere. To know more, scientists need to study comets close up.

Analysis of Halley’s comet by three spacecraft in 1986, as well as later ground-based analyses of the comets Hale-Bopp and Linear, has already revealed that, aside from ice, comets consist mostly of silicate particles and great quantities of complex organic molecules. In January 2004, Nasa’s Stardust spacecraft flew close to the nucleus of the comet Wild 2 and also detected a large amount of organic matter and some sulfur compounds, but no traces of amino acids.

More detailed information will come in the next few years. In July 2005, as part of the Deep Impact mission, Nasa will fire a 372kg copper probe into comet Tempel 1 and analyse the debris, while, in 2014, ESA’s Rosetta mission will land a probe onto comet Churyumov-Gerasimenko.

Jon Evans

Aura instruments

HIRLDS, the high resolution dynamics limb sounder, is an Anglo-American infrared radiometer analysing the upper troposphere, stratosphere and mesosphere to determine the concentrations of O3, H2O, CH4, N2O, NO2, HNO3, N2O5, CFC11, CFC12, ClONO2 and aerosols.

MLS, a microwave limb sounder instrument, measures lower stratospheric temperature and concentrations of H2O, O3, ClO, BrO, HCl, OH, HO2, HNO3, HCN, and N2O. These measurements are focused on diagnosing and analysing ozone depletion, transformations of greenhouse gases and their impact on climate change.

TES, the tropospheric emission spectrometer, is a high-resolution infrared-imaging fourier transform spectrometer that captures both the natural thermal emission of the surface and atmosphere and reflected sunlight, allowing day and night coverage. TES molecules including NOx, CO, O3, H2O, OH and SO2. It is designed to monitor long-term variations in the quantity, distribution and mixing of minor gases in the troposphere, including their sources and sinks, their exchange between the troposphere and stratosphere and the resulting effects on climate and the biosphere in general. TES will provide global maps of tropospheric ozone and its photochemical precursors and monitor the main contributors to acid rain.

OMI, the ozone monitoring instrument, is another European contribution to this NASA craft, having been built by the Netherlands’ Agency for Aerospace Programs in collaboration with the Finnish Meteorological Institute. It observes solar backscatter radiation in the visible and ultraviolet range. In addition to ozone monitoring, it can also measure key air quality components such NO2, SO2, BrO, and aerosol characteristics. It can distinguish between different types of aerosol, such as smoke, dust, and sulfates. OMI can also examine the key characteristics of clouds and map global distribution and trends in UV-B radiation.

Titan’s chemistry

When the Huygens space probe fell through the atmosphere of Saturn’s moon Titan, on 25 December 2004, it carried two instruments with a specific role of performing chemical analysis on an alien world. The aerosol collector and pyrolyser gathered aerosols and subjected them to evaporation and pyrolysis, before their chemical composition was analysed by the linked gas chromatograph and mass spectrometer. Together, these instruments allowed the chemical components of Titan’s atmosphere to be identified and quantified.

Methane and nitrogen

The atmosphere was sampled from an altitude of 160km all the way down to the surface. Initial results from the descent and after landing have confirmed the atmosphere is largely a mixture of methane and nitrogen, with the methane concentration increasing steadily to the surface. Clouds of methane speed across the skies about 20km up, and a fog of methane and ethane drifts near to the surface. Combining the results of all Huygens’ analyses suggests Titan’s surface resembles wet sand or clay with a thin solid crust. It is mainly a mixture of dirty water ice and hydrocarbon ice, with flowing liquid that is principally methane. The discovery of argon 40 in the atmosphere has been used to infer that Titan has sufficient volcanic activity to generate a lava of water ice and ammonia. The results also suggest ammonia is abundant beneath the crust and serves as the source of most of the nitrogen that dominates the atmosphere. According to Jonathan Lunine, an interdisciplinary scientist at the European Space Agency (ESA), Titan appears to have ’the biggest hydrocarbon reservoir of any of the solid bodies in the solar system’. This is of great relevance to any chance that Titan may have something to tell us about prebiotic chemistry and the origin of life.

Interpreting data

One of the more sophisticated chemical questions the Huygens’ team are addressing is whether or not Titan carries more complex organic chemicals relevant to the origins of life - both on Earth and on Titan now, in the past or in the far future. They are also trying to interpret their data to reveal what chemical reactions are currently occurring. However, we will have to wait a while for clear and detailed answers. ’The teams are working hard to understand all their complex data about the chemistry before releasing any further information,’ says Jean-Pierre Lebreton, ESA’s Huygens project scientist and mission manager.