Category Archives: SciComm

A greener earth; Earth’s vegetation is responding to increasing atmospheric CO2


Photosynthesis, a process that had its beginnings about 2.5 billion years ago, has an awesome responsibility; it keeps us breathing. It is a metabolic process in plants that uses the energy from sunlight to drive chemical reactions; reactions that produce amino acids, proteins, sugars and other compounds that create the architecture of plants.  The process takes atmospheric CO2, converts the carbon plus other nutrients to organic compounds, then expels the left-over oxygen. Plants help regulate the composition of the atmosphere – they are our other set of lungs.

It has been shown experimentally that photosynthesis increases in many kinds of plants (some more than others), as the supply of atmospheric CO2 also increases. On a global scale, this is referred to as greening of the earth, where both regional studies, and more recently satellite data show an overall increase in plant growth, and an increase in growing seasons. In Europe and North America, the seasonal leaf-out (or bud-break) for the period 1950 to the 1990s was 2-4 weeks earlier than pre-1950.

Plant growth responds to several environmental factors, such as temperature, soil health, and the availability of water and nutrients. Modelling of plant response to these various controls suggests that globally about 70% of the increased growth (greening) is due to increases in CO2 fertilization. The remaining 30% reflect changes in nitrogen uptake, regional temperature and precipitation, and land-use.

These observations and models are important because they help to define how critical parts of the carbon cycle might respond to increasing CO2 and to changes in climate.  Increased growth means that carbon sequestration in plants, and eventually in soils, is also increasing. A 2015 study has shown that there is significantly greater CO2 uptake in the tropics compared with higher latitudes, although the authors comment that continued deforestation, plus potential changes in climate conditions, might offset this process.

Quantifying the multitude of positive and negative feedback processes in the carbon cycle is a recognized challenge in climate science. Increased uptake of CO2 by plants is a negative feedback that helps reduce the excess gas in the atmosphere.  However, there are important caveats and limitations to this process; some of these are listed below.

  • Plants, in adapting to increased CO2, may reach a point where there is no additional growth. Increased CO2 concentrations result in the closure of leaf stomata. Stomata are the microscopic vents in leaves that allow CO2 to enter, and water to leave (evapotranspiration) the leaf structure. Closure of stomata is used by plants to regulate water loss, but closure will also restrict the amount of absorbed CO2.
  • Nitrogen is a critical plant nutrient. It has been experimentally determined that increased CO2 can lead to a decreased uptake of nitrogen; this negative response persists even when plant growth is enhanced by CO2. The long-term effects of lower N are not well understood. Nitrogen is a key ingredient of plant proteins, so does a potential reduction in its uptake mean lower protein levels in food crops?
  • Other nutrients, such as phosphorous and potassium, are critical for plant growth. Leaf growth cannot continue unabated without sympathetic development of roots, for which both potassium and phosphorous are necessary ingredients. Increased leaf greening may be limited by the availability of these other nutrients.
  • How do soil microorganisms respond to increasing CO2? Microorganisms are responsible for mediating the delivery of most nutrients to plants, but they are highly susceptible to soil erosion, deforestation, and changes in soil chemistry such as pH.

The response by vegetation to increasing atmospheric CO2 is an important feedback in the carbon cycle. The earth is greening, but it is also clear that the current vegetation cover is not capable of sequestering all produced CO2, because atmospheric levels of the gas continue to rise.  Deforestation in tropical regions is certainly not helping in this regard. The importance of understanding such changes cannot be understated; plants, both macroscopic and microscopic, are a critical part of the food chain. Plants can adapt to environmental changes, but that capability is highly variable among the different kinds of vegetation. Plant adaptability also has its limits.


Hold a ‘0’ to the light and look through it – there is nothing, and everything


Several years ago I read Jerry P. King’s The Art of Mathematics (1992). Chapter 3 deals with Numbers, and in it is a statement that has bothered me ever since “Although they are the most fundamental of mathematical objects, the natural numbers are not found in nature.” There are real numbers, but none exist in the natural universe. We may count two people, write the number on a piece of paper, or solve an equation that gives the answer as two, but the number ‘two’ does not exist – we cannot pick it up or put it under a microscope. I have kept an eye out for ‘one’, but even this basic singularity is elusive. Numbers, it seems, are an abstraction.

So where does this leave ‘zero’? Zero means nothing, zilch, emptiness; so is it even a natural number – is it an integer? Several commentators of mathematics and science have suggested that the invention of ‘zero’ was as important as that of the wheel. Our system of numbering depends on it. One becomes 100 by adding a couple of zeros. It is essential to calculus and algebra, and for solving equations. Without zero, there would be no binary code, and no computers (at least as we know them).

In our numbering system, zero is a place holder; it occupies the place where no integer occurs. So in the number 1001, the zeros indicates that there are no 10s or 100s integers; without the 0s we would have the number 11. Our place value system is constructed around a base of 10, usually referred to as base 10. This means that every time we add an integer to a number, the place value column increases by a factor of 10; 1 becomes 10, becomes 100, becomes 1001, and so on. The place value numbering system was probably invented by the Babylonians about 2000 BC. Their numbering system also used a base 60, that was inherited from the Sumerians between 2000 and 3000 BC. However, they had no zero, or any kind of symbol that might represent zero, until later when they introduced spaces between digits to indicate ‘nothing’. In other words, it wasn’t a number.

Zero, as a number, was probably invented in India in the first few centuries AD. Until recently, the oldest known formulation was recorded on a tablet that describes a 9th Century Hindu temple, in the old fortified city of Gwalior. The zeros here are dots, but this notation shows that the base 10 place-value numbering system was probably commonplace by 876 AD.

The dot notation is also found on an ancient birch-bark manuscript – the Bakhshali manuscript, discovered in 1881 (in what is now Pakistan), of which 70 fragments remain. The manuscript is held at the Bodleian Library, Oxford University. There has been significant debate about its age, but what is now generally conceded is that it is a copy of a much older manuscript. Until recently, it was considered to be 8th century, with some scholars placing it as early as 3rd or 4th century. It is a mathematical tome of remarkable sophistication, that contains examples of linear and quadratic equations, formulae for square roots and other functions, and arithmetic and geometric progressions, all of which use zero (dots) as a place holder. Carbon dating of the bark (reported September 14, 2017) now shows that the manuscript is indeed 3rd or 4th century, with the oldest pages between 224-383 AD.

The first definition of zero as an actual number was made in 628 AD by Hindu astronomer and mathematician Brahmagupta.  The work, known as Brahma-sphuta-siddhanta, outlines how zero is derived by subtracting a number from itself. This was a seminal work, not only in astronomy, but in mathematics; its translation into Arabic about 771 AD, was probably an important contribution to the Arabic invention of algebra.

Zero, in the form of a dot, had evolved as place-holder and as an actual number in India by the 7th Century. By some circuitous route, Persian translations of Sanskrit manuscripts provided a basis for Arab developments in mathematics. The first record of zero being represented as an enclosed symbol, a circle (or oval, with nothing inside it) in 976 AD, was in the writings of one Muḥammad ibn Mūsā al-Khwārizmī, a Persian mathematician; it was given the name Sifr (Arabic for ‘empty’).  Not long after, Arabic numerals, that now included ‘0’, reached Europe. Leonardo of Pisa, otherwise known as Fibonacci, was instrumental in advertising the utility of the numeral system in his 1202 composition Liber Abaci (Book of Calculation) – almost 600 years after Brahmagupt’s opus.

According to the Merriam-Webster Dictionary, the word ‘zero’ was first used in English in 1598, but I have yet to find the context for this. The etymology of ‘zero’ mimics the history of its formulation, beginning with the Arabic sifr, that on arriving in Europe, perhaps through Italy (zero) and France (zéro), finally entered the English language as zero.

Hold a ‘0’ to the light, and look through it – there is nothing, and everything. Look through the zero to new worlds, new views of the universe. The history of zero has done just that, from tentative, humble beginnings it now helps us create so many possibilities.  I wrote this piece 2 days after Cassini’s final descent into Saturn’s atmosphere. I wonder if such flights would have been possible if not for the genius of zero?

Check out this YouTube video about the Bakhshali manuscript (Bodleian Library)

Here’s a couple of books on the topic:

The Nothing that is: A natural history of zero, Robert Kaplan, Oxford University Press, 2000

Zero: The biography of a dangerous idea. Charles Seif. Penguin, 2000


Springs and seeps


It rains quite a bit on Mamaku Plateau, the tableland underlain by volcanic debris that was violently deposited 240,000 years ago; an eruption that also gave rise to the Lake Rotorua caldera (central North Island, New Zealand). Some of that rain seeps into the myriad fractures, nooks and crannies, and heads west as groundwater. Fifty to 100 years later, that same water emerges, chilled (a cool 11oC), at Blue Springs (about 40km west of Rotorua). Spring water here flows at 42 cubic metres per minute (9,240 gallons per minute), enough to maintain a decent-sized stream (Waihou Stream).

Fresh water springs have been sources of life, driven conflicts, and kindled the imagination for thousands of years. Greek muses frolicked in the Pierian Spring (somewhere in Macedonia); its waters a fount of knowledge. After spending a lifetime underground in,

         A secret system of caves and conduits; hear the springs

         That spurt out everywhere with a chuckle… (W.H. Auden, In Praise of Limestone, 1948). Gleeful emergence.

Springs may be cold or hot (thermal); they sometimes have dissolved minerals that are deposited near or at the surface where the spring exits.  Dissolved silica or calcium carbonate (tufa) are common in thermal springs. Thermal springs are also fed by groundwater but are heated by deep external sources, like magma chambers. Buoyancy and convection (hot water rises, cold water sinks), play an important role in driving the hot fluids to the surface.

Some springs, like the Devil’s Postpile National Monument in California, have naturally dissolved carbon dioxide, providing a kind of backyard sodastream. Buoyancy forces are also important for driving oil and gas to the surface (hydrocarbons seeps). In this case, the hydrocarbons are derived from oil or gas prone rocks at depth.

Fresh water springs commonly emerge where the watertable intersects the ground surface (water from an unconfined aquifer).  In some regions, particularly those in limestone country, springs discharge from aquifers at depth (confined aquifers) that have elevated pressure, where the water flows through conduits in rock (such as faults and fractures); this is also referred to as artesian flow. Sinkholes are an extreme expression of this kind of groundwater seepage.

The different kinds of fresh water spring all have one thing in common – the direction of flow is dictated by a decrease in pressure (from high to low pressure), or in more technical terms, from high to low hydraulic head.

Gravity is the dominant driving force for groundwater flow in most groundwater systems, including spring flow. The easiest way to grasp this is to look at surface topography; groundwater at high elevations has greater hydraulic potential, or gravitational energy, than that at lower elevations. Thus, if an upland watertable intersects the surface at a lower elevation, for example along a valley, then springs will develop at or below that point of contact – this is illustrated in the block diagram.

In layered rock, groundwater can seep through a permeable aquifer until it reaches an impermeable barrier, such as a mudstone or shale. If the contact between the two layers is exposed, the groundwater can emerge as contact springs. In the example shown here from Grand Canyon, Arizona, springs emerge at the contact between an impermeable limestone below, and the permeable limestone above through which the groundwater has percolated.

Conduits for groundwater flow and spring discharge are provided by fault planes that, in some circumstances, may tap into several aquifers. In the diagram below, groundwater flow in the sand aquifer is diverted by a fault plane. If there is sufficient hydraulic head in the aquifer (think of this as the potential energy available from the force of gravity), the groundwater will emerge where the fault plane intersects the surface.

Fractures and joints are common in hard rock.  Most igneous and metamorphic rocks typically have little or no permeability, except where they have been fractured by tectonic forces, or jointed during cooling (typical of lava flows).  Groundwater can flow through these crevices and emerge as springs. Seepage is common on exposed rock faces; freezing during the winter can produce some spectacular ice shows.

Perhaps one of the more extreme expressions of groundwater seepage and spring discharge occurs in regions underlain by limestone and salt (or salt domes). Both rock types tend to dissolve with groundwater seepage, enlarging fractures, and creating underground caverns and waterways (in limestones, stalactites and stalagmites are part of this process).  As caverns become larger, the roof rock becomes unstable and collapses; the surface manifestation of this is a sinkhole. Florida is a region particularly prone to sinkhole formation, and it is not uncommon for entire houses to be swallowed by the collapsing surface. Sinkholes commonly fill with water, derived from the aquifer below. And while local governments and individuals may find such geological structures a nuisance, it seems that local alligator populations are thrilled with the opportunity for a new beach or swimming hole.


In praise of field work


The class field trip is underway.  Teacher hands out the rap-around, virtual imaging glasses, and you are transported to a green horizon. In the background, there is an annoying kind of buzz, as teacher relates the topic of enquiry, asks questions, provides comments. Fellow students may even be projected into your virtual reality, their essence reduced to pixels. There’s a resounding crash – one student, suffering from vertigo, has fallen off their chair. Another has just thrown up from motion sickness.  All in a day’s field study. Off come the glasses. The green horizon vanishes. All except one of your classmates are still in their chairs, surrounded by the same four classroom walls. What was learned?

Real field schools and class trips, i.e. those outside the classroom, are the bane of educational institution administrators; even teachers find them disruptive. There are health, safety, and liability considerations, budget restrictions and teaching loads, and perhaps worst of all, field trips wreak havoc with those finely tuned curriculum timetables. Field trips are an administrative pain in the neck. And yet they are utterly indispensable to science and the humanities, whether applied to learners or hardened practitioners.

The ‘field’ can be almost anywhere. Geologists traipse the countryside in search of ancient worlds; they could just as easily explore from a drilling rig or the International Space Station. Soil biologists dig dirt. Botanists crawl through the undergrowth. Can you imagine learning chemistry and never experiencing a laboratory, or geophysics and never having the chance to trudge along a gravity transect? Field work is a grand learning experiment that brings together observation and data collection, creative thinking and language, conjecture and hypothesis. In the field, all our senses are assaulted; they engage our brain in three-dimensional visualization. The sights, smells, sounds and feel of the earth, allow us to extend our understanding beyond the immediately visible. Enjoying the vista from a ridge top, while catching one’s breath, contributes to the learning process. My own geological excursions to Ellesmere Island (Canadian Arctic) and Hudson Bay, Hutton’s Unconformity on Arran (Scotland), Atacama volcanoes, or New Zealand wilderness, provided hard data and a myriad tangible and intangible images, the kind of learning that no other experience could provide. In the field, our learning becomes both sensual and cerebral; we can visualize in our mind’s eye the things we cannot sense directly, beneath, within, and beyond the earth. And once we have this learned skill, we can project our understanding to worlds past and future; dimension number four. In the field, we can begin to get a feel for deep time.

Field work provides critical learning in the real world, learning that is conscious and retrievable, and learning that is not always explicable, even subliminal. Technology does not replace this kind of learning, but it can enhance it. We can go into the field with as little as a pencil and notebook. Or we can take with us, instruments that provide measures of earth properties, or real-time satellite imagery. Exploring the deep oceans and our solar system require special equipment. The Mars rovers are a projection of our own field activities (albeit quite expensive) – they observe, take samples, analyse, and transmit data.

Whether our feet are firmly planted on terra firma or leaving footprints on the Moon, our senses, intellect, and language provide us with learning pathways that are necessary to understand the complexity of the universe. Bowing to administrative objections deprives all students of the ultimate learning experience. Field excursions, the kind that go beyond the classroom or office, are utterly indispensable.



Io; Zeus’s fancy and Jupiter’s moon


Zeus, the head-honcho of assorted Greek gods, heroes, nymphs, and mortals, was chiefly the God of the Sky, or Heavens. One of his minor portfolios was the upholding of Honour, but, as mythology relates, he didn’t put much energy into that particular task; he was a philanderer, much to the annoyance of his own wife, Hera (I guess his energies were directed elsewhere).  One such misdirection was Io, a mortal woman, who had the misfortune to be turned into a heifer by Zeus, to hide the infidelity from Hera. Io’s memory now survives as planetary body; one of the Galilean moons of Jupiter is named after her (to be named a moon of the Roman God Jupiter, seems like a historical slap in the face to the Greek deity).

Io is the third largest of Jupiter’s 60 (known) moons, fractionally larger than our own moon, and one of the four moons discovered by Galileo in 1610. It is also one of the most unusual planetary bodies in our solar system, and the only one with active volcanism – over 400 eruptive centres have been mapped. Io’s elliptical orbit brings it to about 420,000 km from the gaseous giant. The resulting gravitational forces produce huge tides such that the surface bulges up to 100m.  These tidal forces generate enough heat to melt rock and sulphur.

Io is quite different to the other Jovian moons in that it’s density is more like that of the rocky planets (Earth, Mars, Venus, Mercury) – in fact it is almost the same density as Mars (3.5g/cm3). In comparison, Jupiter’s average density is little more than that of water, at 1.3g/cm3.  The core of Io probably consists of iron or iron sulphide, surrounded by a mix of solid and molten rock, the composition of which may be similar to that on earth. The outer solid carapace is an intriguing mix of sulphur, which in solid form at the surface, and silicate rock. Sulphur is also the main constituent of Io’s thin atmosphere, but in this case in the form of sulphur dioxide.

Spectacular images taken by the space craft Galileo in 1995, show a moon splashed with colour, a kind of post-impressionist experiment; NASA refers to it in more colloquial terms as a giant pizza. Frequent volcanic eruptions of lava and fine (sulphur?) particles mean that Io’s surface is constantly being renewed; a consequence of this is that impact craters are poorly preserved because they too are recycled.

One of the more exciting discoveries from the Galileo images was active eruption plumes, extending more than 300km above the moon surface (the blue colour is probably due to dispersion of light by plume aerosols).  Several plumes were imaged during different orbits by the Galileo explorer.

Active eruption of lava was also discovered. In the two images below, eruptions about a year apart produced lava flows that contributed to the ridged and fractured surface topography. Also visible are steep-sided depressions, thought to be the remnants of once active calderas, formed by collapse of the surface as lavas were erupted. The calderas are analogous to structures seen on earth.  Older lava flows and deposits from the plumes are buried and eventually incorporated, or recycled into the molten subsurface. On earth, the recycling of rock and water into the crust and mantle is driven by plate tectonics; on Io, sulphur and rock recycling is driven by tidal forces and volcanism.

Space exploration over the last couple of decades has produced a myriad images. There seems to be an endless supply of astronomical vistas, all spectacular, even surreal. Despite this surfeit, I still feel a sense of amazement, a thrill, to witness the details of active Martian sand dunes, the jumbled surface of comet 67P/Churyumov-Gerasimenko, or the stark ruggedness of Pluto’s frozen surface. I am not yet exhausted by this surfeit.


An Italian job; seismic risk-assessment at risk


Life is a risky business. Not a day goes by when some aspect of our lives comes under the gaze of risk assessment, an analysis of potential adversity, the probability that some event will impact our well-being. No black and white determinism here; we have become probabilistic entities. The seemingly simple act of driving your car, is translated into an actuarial assessment that determines the cost of insurance, a government’s health budget, a funeral director’s business plan, or a vehicle manufacturer’s liability. All are predicated on the probability of some event taking place – one chance in x occurrences. No luck, or absence of luck? Luck is when you win the lottery without buying a ticket.

Predicting natural phenomena, like volcanic eruptions, tornadoes, or earthquakes, is the stuff of science. The problem with these kinds of events is that they can have global impacts. How does one prepare for, or mitigate, meteorite impacts or Boxing Day tsunamis? We know there are risks, but quantifying those risks remains, in many cases, elusive. We know a great deal about earthquakes for example, how and where they occur (in terms of the mechanics of the earth’s crust), the most likely impacts, and patterns of aftershocks, but pin-pointing the time and place of the actual events, other than in rather vague, general terms, still eludes us. Seismologists frequently speak of earthquake recurrence intervals for certain faults, based on historical, archeological, and geological records, but there is a world of difference between knowing a fault will probably be active sometime in the next 300 years, and foretelling an earthquake three weeks hence. Seismologists tend to be quite vague, as well they should be, when asked to comment on the likelihood of an event; there are simply no established tools that permit such predictions.  A group of Italian scientists learned this lesson the hard way.

In the early hours of April 6, 2009, a M6.3 earthquake, its epicentre near the medieval city of L’Aquila in the central Italian Apennines, killed 309 people and caused severe damage, rendering more than 65,000 people homeless. The main shock was preceded over several months, by 100s of smaller events (a seismic swarm), mostly less than M3. Understandably, the L’Aquila townsfolk were unsettled by the earthquake swarm. L’Aquila and neighbouring towns are no strangers to earthquakes, but this event had a different outcome in the context of scientific knowledge and its role in risk assessment and management.

Six days prior to the M6.3 event, a group of six seismologists and seismic engineers, plus one government official, met with local L’Aquila officials, presumably to ascertain the level of risk. The government official (head of the Civil Protection Department), announced (as it turned out, unbeknownst to the 6 scientists) that there was no real cause for alarm, that the series of small earthquakes were actually quite helpful because they released some of the pent-up seismic energy, and that the seismic swarm did not elevate the probability of a larger event. It is not clear why the official presented such definitive conclusions, that from a scientific perspective were unjustified – did he not understand the science of seismology or the concept of probability, or was it the response of a bureaucrat trying to please everyone? Six days later, all his pronouncements were shown to be incorrect.

Shock and dismay of a very different kind, rippled through the global scientific community in 2011, after it was announced that the seven experts were to stand trial for manslaughter, based on their inadequate assessment of the risks. While recognising that precise earthquake prediction is not scientifically possible (at the present time), the court did determine that the public had been misled by the Government official’s announcement, and that this had contributed to the death toll. Their sentence – 6 years in prison, pay compensation of 7.8 million Euro to the 29 families who had collectively brought the charges, and a ban from public service.

In 2015, an Appeal Court exonerated the 6 scientists, but not the government official. The court argued that the official had taken it upon himself to make statements that the scientists did not agree with, and that he made the statements before the meeting with the scientists had taken place – the scientists clearly could not be blamed for statements they had no knowledge of, and likely would have disagreed with.

The science community’s collective dismay centred around several issues:

  • the science of earthquake prediction is inexact. Earthquake swarms are not uncommon; some appear to precede major shocks, but many do not. However, the L’Aquila debacle has lent some focus to the study of seismic swarms, in the hope that something predictive may be teased from them,
  • it was an attack on science,
  • that the court did not understand the nature of earthquake science, the nature of probability and risk assessment-management. For example, many seismologists argue that the existence of a seismic swarm does increase the probability of a major event, although the actual probability may still be quite small. In other words, the swarms are possible signals of impending calamity,
  • and, if the convictions had stood, no scientist in future would willingly make public comment about ‘acts of God’. This last point probably extends to disciplines other than scientific, where risks are evaluated.

I expect the six scientists felt a sense of relief, that their judgements about potential risks to L’Aquila were not unreasonable, at least within the context of scientific knowledge at the time. But I also suspect no small degree of frustration, that they were unable to be more definitive in their predictions. At present, earthquake prediction doesn’t extend past statements of recurrence intervals. But I’m optimistic that the science of seismology will eventually find the tools to be more precise.


Life on Mars; what are we searching for?


I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth. Stephen Hawking

I can only imagine H.G. Wells bitter disappointment if he were to learn that Martians were little more than primitive microbes.  All that hype and scare-mongering for nothing. Because that, it seems, is all we are ever likely to find on Mars. They may be intelligent microbes, but microbes nonetheless.

Present conditions on Mars are not conducive to thriving populations of anything living – at least in any life form we are familiar with. Incident UV and other solar radiation, low atmospheric pressure, an atmosphere almost devoid of oxygen, and the presence in soils of oxidizing molecular compounds such as perchlorates and hydrogen peroxide (think bleached hair), all contribute to rather inclement living conditions. It is possible that some life forms have survived these ravages, in sheltered enclaves or buried beneath the scorched earth, but it is more likely that, if life did exist on Mars, we will find the evidence written into ancient sedimentary rocks, or perhaps as chemical signatures.  It is these attributes that current exploration programs, both landed rover expeditions and orbiting satellites, tend to focus on.

Earth-bound life forms, particularly microbes, can thrive in a multitude of environments. We are constantly reminded of this in our own, relatively benign conditions (the right amount of oxygen, clement temperatures and climates), but we also know that microbes and other life forms have evolved in conditions that are extreme; highly acidic geothermal pools, ocean depths devoid of sunlight, the throats of volcanoes, or buried beneath tonnes of ice. The common denominator for all these microbial kitchens is water, water that is used directly, or indirectly via water-soluble compounds such as sulphates, chlorides and carbonates.

Surface water on Mars is scarce; some is locked up as ice at the Martian poles, and perhaps buried in dirt (frozen groundwater – a bit like our Arctic permafrost). However, Mar’s deeper past (measured, like Earth, in billions of years), was quite different.  There is very good evidence (found in sedimentary rocks and minerals), for liquid water that once flowed in rivers, carved ravines and moulded landscapes, rivers that shaped sediment into large deltas, and seas with distant shores.

Scientists can take the watery, Earth-like analogies further in their search for Martian biology, by focussing their attention on muddy rocks. Earthly mud rocks, or mudstones, commonly have evidence for biological activity, either as actual body fossils, or as organic compounds that have survived the vagaries of burial and chemical change; oil and gas deposits represent a kind of end-point of these processes, the culmination of a transformation of organic matter from living creatures or plants.  Certain kinds of molecules, or biomarkers in these deposits, allow us to determine the original kinds of organisms from which the hydrocarbons were derived. We can use the same technology to examine Martian mudstones, to search for biomarker molecules that might point to former organisms.

Specific biomarkers for earth organisms include DNA-RNA, various proteins, amino acids, and steroids, although these molecules tend not to survive intact during geological burial. Compounds like lipids, on the other hand, can be preserved relatively intact – they are found in various hydrocarbons and organic-rich mudstones. A very important distinction needs to be made here, between organic molecules that can be attributed to life forms, and those that form by abiotic processes (not involving life forms). Abiotic organic molecules are no strangers to those who study organic matter on extra-terrestrial flotsam; they have been found on meteorites, identified in the spectra of interstellar gas clouds, and were detected on the European Space Agency’s Rosetta recent mission to comet 67P/Churyumov-Gerasimenko

Mars, as a habitat in its distant, more watery past, may have had some similarity with conditions on earth 3-4 billion years ago, where the earliest life forms (microbes) are hypothesized to have developed in localized prebiotic factories, perhaps associated with hydrothermal and volcanic vents; not unlike Darwin’s “warm little ponds”. Hydrothermal vents, like the black smokers we observe today in deep oceans, could have provided heat and nutrients in the form of chemical compounds. Organisms that obtain their metabolic energy from chemicals like sulphates and iron oxides, instead of sunlight, are called chemotrophs. Mars habitats, it has been suggested, also were localized, but did not have the kind of longevity that earth systems possess. It is also hypothesized that Martian organisms were probably chemotrophic, and, because there was little or no oxygen, anaerobic.

The utility of Earth-Mars analogues, such as organic biomarkers, or comparisons with the life habitats on earth, are useful not because they offer any kind of confirmation of life on Mars, but because they help to focus the search for the kind of evidence that can be interpreted with confidence. This includes a focus on analytical methods, but perhaps more importantly it helps to provide direction on ‘where’ to look, based on knowledge of the geology and Martian landscapes. For example, if the exploration program is to concentrate its search for organic signatures in mudstones, then ancient Martian lakes are good places to start looking. The current mission (Curiosity), and the upcoming ExoMars (European Space Agency, 2020 launch), and NASA’s Mars 2020 missions, will provide initial glimpses into some of these habitats, and perhaps some answers to an age-old question.