Posts Tagged ‘water’
water on Earth – no problemo
So, as described in my last post, H2O in its various forms is plentiful in our solar system as well as beyond it. But, being more or less scientifically illiterate – despite decades of reading stuff on science – I can’t quite work out how liquid water is so abundant on the Earth’s surface. The story has long been told of water-iced asteroids in the time of the heavy bombardment being responsible, with the major proof being that these carbonaceous chondrite asteroids have, or had, the same signature of heavy (deuterium-rich) water as the water we find on Earth. While this seems a strong argument to me, how did the Earth manage to hold on to that water during those super-heated days?
I’ve looked at this in a previous post, sort of, but I’m still not clear on the atmospheric conditions that brought about our soggy planet (much more soggy during the Mesozoic though). In any case, I’ve recently read that bonafide researchers on this topic have also been mystified about the sheer volume of water on Earth.
Enter a new (to me) hypothesis, published in the Journal of Geophysical Research: Planets a little over a year ago. It argues – and other astrophysicists appear to be impressed by the reasoning and the detailed analysis in the paper – that the water came not only from asteroids but also from the solar nebula.
Solar nebula? Never heard of it, but apparently the concept has a long history. The so-called nebular hypothesis for the formation of our solar system was first proposed by Emanuel Swedenborg in the 1730s, and further elaborated by such luminaries as Immanuel Kant and Pierre-Simon Laplace later in the 18th century. Surprisingly for such an early contention, it has stood the test of time and survives today, though the details are still argued, and there are a few competing hypotheses. In any case, without going into too much detail, a nebula of dust and gas began to form around 4.6 billion years ago, and collapsed in on itself due to gravitational forces, spinning around a newly-formed sun. Out of this material, protoplanets gradually formed.
Water in the Earth’s oceans has approximately the same D/H (deuterium to hydrogen) ratio as that of the above-mentioned asteroidal carbonaceous chondrites, so it has always seemed a safe bet that most if not all water came from those asteroids. Yet the sheer volume of water was still a problem. Jun Wu, the lead author of the recent paper, had this to say about the theoretical situation:
The solar nebula has been given the least attention among existing theories, although it was the predominant reservoir of hydrogen in our early solar system.
What has apparently added credence to the new hypothesis is that samples of hydrogen near the core of the Earth have significantly less deuterium and may fit better with the ratio of hydrogen in the solar nebula. Also the isotopic signatures of the noble gases helium and neon found in the Earth’s mantle fit the signatures of these gases from the time of the solar nebula. The explanation of how the lighter hydrogen found itself drawn to the Earth’s centre, in a process called isotropic fractionation, is provided in the paper, apparently. It’s a very interesting story, if true, and it may have implications for liquid water on habitable-zone exoplanets. That’s to say, there’s no reason for it not to be quite common. Here, to finish, are a couple of thought-provoking comments from members of the research team.
… there’s another way to think about sources of water in the solar system’s formative days. Because water is hydrogen plus oxygen, and oxygen is abundant, any source of hydrogen could have served as the origin of Earth’s water.
Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems.
References
How did Earth get its water?
https://www.britannica.com/science/solar-nebula
https://ussromantics.com/2018/09/24/a-little-about-the-chemistry-of-water-and-its-presence-on-earth/
getting mildly excited about water
I’ve generally thought that the extraordinary volume of water on our planet’s surface was a problem, scientifically speaking, but I’m probably wrong. I used to think that the idea that water came to Earth in meteor showers (haha) couldn’t be right, because the days of Earth’s heavy bombardment came early in the planet’s history when everything was molten hot and the water or ice from meteors would’ve just boiled away. But what would I know? And why would meteors, or planetesimals, be so full of water?
As the astronomers are constantly telling me, water in solid, liquid and gaseous form is commonplace in our solar system, our galaxy, our universe. In the habitable zones of our universe it can exist in all three forms close together, and that’s what presumably makes those regions habitable. On Earth we have a hydrological cycle – evaporation and transpiration, condensation, and precipitation – involving the three forms of this precious stuff, more or less. Recently, some fuss was made about water found in the atmosphere of a not-so-distant exoplanet, and the female interviewer was seemingly excited about – hey, water, and maybe life!!! – but the scientist was much more impressed by the detection abilities we’ve developed for working out the chemical signature coming from distant bodies (this one was about 100 light years away – our galaxy is many thousands of light years across). Water in the atmosphere and even on the surface of these bodies is unsurprising, apparently.
When you (I mean I) consider that hydrogen is the simplest and most abundant element in the universe, and oxygen is also a relatively simple and abundant molecule, we shouldn’t be surprised that water is commonplace. As the above-mentioned scientist pointed out, water is found in the interstellar medium between star systems, amongst gas clouds, and within our solar system, especially in the material of the Kuiper Belt and in the ‘ice giants’, Neptune and Uranus. More excitingly for the possibilities of life, liquid, flowing water has been found on Mars – albeit highly salinated and mineral-rich. There’s still a possibility, though, that less ‘contaminated’ water may be found nearer the Martian poles. It’s also seen as a sign that Mars is drying up, water-wise, that it was once a much more watery world, and for a long time. Could it have seeded life on Earth?
Water worlds are being found elsewhere in the solar system too. The Cassini spacecraft has made major discoveries about Enceladus, a tiny, very bright moon of Saturn. Jets of water vapour, ice and surprisingly large quantities of organic chemicals burst out from below the moon’s icy crust at tremendous velocity. Some of the material is added to Saturn’s particulate ring system. The E ring’s particles, where the Enceladus material ends up, have been examined by Cassini, and in short, the examination suggests that there are hydrothermal vents beneath the icy shell of the moon, similar to those underneath the Pacific Ocean. Cassini’s analysis has also strongly indicated an ocean with a depth of around 10 kilometres underneath the thick ice (30-40 kms) at the southern polar region.
There are other promising watery discoveries too, and a relatively new theory about water on Earth, which I’ll leave for another post.
References
NASA discovers a water world in our solar system (mashable video)
https://imagine.gsfc.nasa.gov/features/cosmic/milkyway_info.html
https://solarsystem.nasa.gov/missions/cassini/science/enceladus/
How did Earth get its water?
a little about the chemistry of water and its presence on Earth
So I now know, following my previous post, a little more than I did about how water’s formed from molecular hydrogen and oxygen – you have to break the molecular bonds and create new ones for H2O, and that requires activation energy, I think. But I need to explore all of this further, and I want to do so in the context of a fascinating question, which I’m hoping is related – why is there so much water on Earth’s surface?
When Earth was first formed, from planetesimals energetically colliding together, generating lots of heat (which may have helped with the creation of H2O, but not in liquid form??) there just doesn’t seem to have been a place for water, which would’ve evaporated into space, wouldn’t it? Presumably the still-forming, virtually molten Earth had no atmosphere.
The most common theory put out for Earth’s water is bombardment in the early days by meteors of a certain type, carbonaceous chondrites. These meteors were formed further out from the sun, where water would have frozen. Carbonaceous chondrites are known to contain the same ratio of heavy water to ‘normal’ water as we find on Earth. Heavy water is formed with deuterium, an isotope of hydrogen containing a neutron as well as the usual proton. Obviously there had to have been plenty of these collisions over a long period to create our oceans. Comets have been largely ruled out because, of the comets we’ve examined, the deuterium/hydrogen ratio is about double that of the chondrites, though some have argued that those comets may be atypical. Also there’s some evidence that the D/H ratio of terrestrial water has changed over time.
So there are still plenty of unknowns about the history of Earth’s water. Some argue that volcanism, along with other internal sources, was wholly or partly responsible – water vapour is one of the gases produced in eruptions, which then condensed and fell as rain. Investigation of moon rocks has revealed a D/H ratio similar to that of chondrites, and also that of Earth (yes, there’s H2O on the moon, in various forms). This suggests that, since it has become clear that the Moon and Earth are of a piece, water has been there on both from the earliest times. Water ice detected in the asteroid belt and elsewhere in the solar system provides further evidence of the abundance of this hardy little molecule, which enriches the hypotheses of researchers.
But I’m still mystified by how water is formed from molecular, or diatomic, hydrogen and oxygen. It occurs to me, thanks to Salman Khan, that having a look at the structural formulae of these molecules, as well as investigating ‘activation energy’, might help. I’ve filched the ‘Lewis structure’ of water from Wikipedia.
It shows that hydrogen atoms are joined to oxygen by a single bond, the sharing of a pair of electrons. They’re called polar covalent bonds, as described in my last post on the topic. H2 also binds the two hydrogen atoms with a single covalent bond, while O2 is bound in a double covalent bond. (If you’re looking for a really comprehensive breakdown of the electrochemical structure of water, I recommend this site).
So, to produce water, you need enough activation energy to break the bonds of H2 and O2 and create the bonds that form H2O. Interestingly, I’m currently reading The Emerald Planet, which gives an example of the kind of activation energy required. The Tunguska event, an asteroid visitation in the Siberian tundra in 1908, was energetic enough to rip apart the bonds of molecular nitrogen and oxygen in the surrounding atmosphere, leaving atomic nitrogen and oxygen to bond into nitric oxide. But let’s have a closer look at activation energy.
So, according to Wikipedia:
In chemistry and physics, activation energy is the energy which must be available to a chemical or nuclear system with potential reactants to result in: a chemical reaction, nuclear reaction, or various other physical phenomena.
This stuff gets complicated and mathematical very quickly, but activation energy (Ea) is measured in either joules (or kilojoules) per mole or kilocalories per mole. A mole, as I’ve learned from Khan, is the number of atoms there are in 12g of carbon-12. So what? Well, that’s just a way of translating atomic mass units (amu) to grams (one gram equals one mole of amu).
The point is though that we can measure the activation energy, which, in the case of molecular reactions, is going to be more than the measurable change between the initial and final conditions. Activation energy destabilises the molecules, bringing about a transition state in which usually stable bonds break down, freeing the molecules to create new bonds – something that is happening throughout our bodies at every moment. When molecular oxygen is combined with molecular hydrogen in a confined space, all that’s required is the heat from a lit match to start things off. This absorption of energy is called an endothermic reaction. Molecules near the fire break down into atoms, which recombine into water molecules, a reaction which releases a lot of energy, creating a chain of reactions until all the molecules are similarly recombined. From this you can imagine how water could have been created in abundance during the fiery early period of our solar system’s evolution.
I’ll end with more on the structure of water, for my education.
As a liquid, water has a structure in which the H-O-H angle is about 106°. It’s a polarised molecule, with the negative charge on the oxygen being around 70% of an electron’s negative charge, which is neutralised by a corresponding positive charge shared by the two hydrogen atoms. These values can change according to energy levels and environment. As opposite charges attract, different water molecules attract each other when their H atoms are oriented to other O atoms. The British Chemistry professor Martin Chaplin puts it better than I could:
This attraction is particularly strong when the O-H bond from one water molecule points directly at a nearby oxygen atom in another water molecule, that is, when the three atoms O-H O are in a straight line. This is called ‘hydrogen bonding’ as the hydrogen atoms appear to hold on to both O atoms. This attraction between neighboring water molecules, together with the high-density of molecules due to their small size, produces a great cohesive effect within liquid water that is responsible for water’s liquid nature at ambient temperatures.
We’re all very grateful for that nature.
How do trees transport water such long distances? Part 2: the mechanism remains a mystery (to me)
and I still haven’t found what I’m looking for…
So scientists have learned a lot, though not everything, about water’s travels from soil to leaf in a plant or tree. It’s a fascinating story, and I’m keen to learn more. But the real mystery for me is about energy. As the excellent Nature article, upon which I’m mostly relying, points out, animals have a pump-based circulatory system to distribute nutrients, oxygen and so forth, but plants are another matter, or another form of organised matter.
I actually posed two questions in my last post. How do plants – and I think I should specify trees here, because the massive distance between the soil and their top leaves makes the problem more dramatic – move water such large distances, and how do they know they have to transport that water and how much water to transport?
So let’s look at the Nature Education explanation:
The bulk of water absorbed and transported through plants is moved by negative pressure generated by the evaporation of water from the leaves (i.e., transpiration) — this process is commonly referred to as the Cohesion-Tension (C-T) mechanism. This system is able to function because water is “cohesive” — it sticks to itself through forces generated by hydrogen bonding. These hydrogen bonds allow water columns in the plant to sustain substantial tension (up to 30 MPa when water is contained in the minute capillaries found in plants), and helps explain how water can be transported to tree canopies 100 m above the soil surface.
Notice how we’re again returning to the explanations questioned by Wohlleben – transpiration and capillary action. But we’re introduced to something new – the C-T mechanism. The thesis is that water’s cohesiveness through hydrogen bonding creates a tension (the tension that makes for capillary action) that enables water to be shifted up to 100 metres – all because of the minuteness of capillaries found in plants. And trees? Somehow, I just can’t see it. Perhaps the key is in the phrase ‘helps explain’. There must surely be more to this. The thesis also mentions ‘negative pressure’ generated by transpiration. This is the signalling I wrote about before. Somehow the plant’s chemistry recognises that there’s an imbalance, and of course this happens in all living things, regardless whether they have a complex nervous system. So maybe there’s no need to worry about ‘knowing’. All living organisms respond to their ever-changing environment by altering their internal chemistry, by opening or closing barriers, by selectively adding or subtracting nutrients, and there are unknowns everywhere about precisely how they do that. It’s a kind of organised chemistry that seems like everyday magic from the outside, whether we’re focusing on a beech tree or our own intestines.
The C-T mechanism is only new to me I should add. It can actually be traced back to 1727 and a book by Stephen Hales, in which he pointed out that without what he called perspiration the water in a plant would stagnate, and that it was also required to allow for the capillary movement of water, because ‘the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters’. But this ‘reciprocal proportion’, according to Wohlleben, as quoted in the last post, can only account for a maximum of 3 feet of upward force in ‘even the narrowest of vessels’.
The water transport system, referred to in the last post as the water potential difference or gradient, also has another name, the Soil Plant Atmosphere Continuum (SPAC). I also mentioned something about an ‘apoplastic pathway’. Water enters the tree by the roots, which are divided and subdivided much like branches and twigs above-ground, with the thinnest examples being the fine root hairs. Water enters through the semi-permeable cell walls by osmosis. Cell-to-cell osmosis carries the water deeper into the root system, and thence into an apoplastic pathway. According to this video, this pathway provides an uninterrupted flow of water (no cell wall barriers) which allows a mass flow ‘due to the adhesive and cohesive properties of water’. This is the cohesion-tension theory again. Apparently, due to evaporation, a tension is created in the apoplast’s continuous stream, leading to this ‘mass flow’.
This makes absolutely no sense to me. What I’m so far discovering is that it’s pretty hard to start from scratch as an amateur/dilettante and get my head around all this stuff, and in my reading and video-watching I’ve yet to find a straightforward answer to the how of long distance, fast transport of water in plants/trees – there probably isn’t one.
I’ll try again after a diet of videos – so far I’ve found a large number of videos in Indian English, and their accents defeat me, I’m sad to say. No transcripts available. Meanwhile, I’ve compiled a little glossary (from various sources) to help myself…
apoplast – within plants, the space outside the plasma membrane within which material can diffuse freely. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.
Casparian strip – a band of cell wall material deposited in the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall – the cell wall being made of lignin and without suberin – whereas the Casparian strip is made of suberin and sometimes lignin.
cortical cells – in plants, cells of the cortex, the outer layer of the stem or root of a plant, bounded on either side by the epidermis (outer) and the endodermis (inner).
exudation – An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding.
guttation – water loss, when water or sap collects (at times of low evaporation, dawn & dusk), at tips of grass, herbs (not to be confused with dew, caused by condensation).
hydrostatic pressure – the pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. This increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above.
lignin – a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.
osmosis – the movement of water from an area of high to low concentration through a semi-permeable membrane. ‘Pumps’ in the cell membrane transport the specific ions into the cell which means water moves in by osmosis thus maintaining hydrostatic pressure.
phloem – the living tissue that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation.
plasmodesmata – narrow threads of cytoplasm that pass through the cell walls of adjacent plant cells and allow communication between them.
root pressure – the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves. Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day
sap – a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.
suberin – an inert impermeable waxy substance present in the cell walls of corky tissues. Its main function is as a barrier to movement of water and solutes.
symplast – the network of cytoplasm of all cells interconnected by plasmodesmata. The movement of water occurs from one cell to another through plasmodesmata
tracheid – a type of water-conducting cell in the xylem which lacks perforations in the cell wall.
vascular (plants) – also known as tracheophytes and also higher plants, form a large group of plants (over 300,000 accepted known species) that are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant.
xylem – one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports some nutrients.
On the Trump’s downfall. What a memo. One wonders if the DoJ is running out of patience with the wannabe dictator and his imbecilities, which may bring things to a head sooner rather than later. But those in the know say that Mueller is always thorough and unlikely to be distracted, so I shouldn’t project my own impatience onto him. Dog give me strength to suffer the horrorshow for a while longer.
How do plants transport water? Part 1: xylem, transpiration and a mysterious water potential difference
roots, xylem, upward flow, transpiration – but how does it work? Find out in the next thrilling episode, maybe.
Stolen from Nature Education, with apologies
This post could fit well in the ‘How Stuff Works’ series, always a useful resource, but I doubt if they’ve done a piece on today’s subject. Maybe I’ll check later.
I’ve been reading a book called The hidden life of trees, by Peter Wohlleben, a Chrissy present from a good friend. One of its shortest chapters is titled ‘The mysteries of moving water’. The reason for its brevity is essentially that there’s as yet no solution to the mystery of how water gets from the soil to the leaves of a tree, or any plant for that matter. At least, according to Wohlleben.
This strikes me as amazing, if true. After all, it’s a simple, everyday scenario for any home gardener. You notice on a hot summer day that the leaves of your capsicum plant are wilting. You apply a two-litre dose of H2O to the base, et voilà, within an hour or two (I don’t know, I’ve never timed it), those leaves have become as turgid as much of my writing. And it just may cross your mind that it’s pretty miraculous how plants can do that. But if it’s true that we don’t know how plants manage such an everyday miracle, surely working it out is Nobel Prizeworthy for any ambitious team of botanico-chemists out there, or whatever.
Of course it’s much more likely that botanists have been trying to solve this mystery for decades – isn’t it? But before I look into it, here’s what Wohlleben says in his book:
…water transport is a relatively simple phenomenon to research – simpler at any rate than investigating whether trees feel pain or how they communicate with one another – and because it appears so uninteresting and obvious, university professors have been offering simplistic explanations for decades… Here are the accepted answers: capillary action and transpiration.
Upon reading this I tried to recall what I knew of these terms. With capillary action I drew a blank, though I feel sure I knew about it once. Transpiration, though, was clear enough: it was like perspiration, the evaporation of water from the leaves, rather than the skin (or is perspiration the secretion of water through the pores rather than the evaporation? Later). So transpiration is only about the movement of water from the surface of a leaf to the atmosphere by means of solar energy; it surely has nothing to do with movement through the stem or trunk, though the loss of water from the leaves is presumably a signal to the plant to draw up more water from the earth, but how can we talk of signals when a plant has no brain or command centre to receive them? And how can water be ‘drawn up’ when it has no muscle power or other obvious energy source?
As to capillary action, Wohlleben explains:
Capillary action is what makes the surface of your coffee stand a few fractions of an inch higher than the edge of your cup. Without this force, the surface of the liquid would be completely flat. The narrower the vessel, the higher the liquid can rise against gravity. And the vessels that transport water in deciduous trees are very narrow indeed: they measure barely 0.02 inches across. Conifers restrict the diameter of their vessels even more, to 0.0008 inches. Narrow vessels, however, are not enough to explain how water reaches the crown of trees that are more than 300 feet tall. In even the narrowest of vessels, there is only enough force to account for a rise of 3 feet at most.
Needless to say, plenty of research has been done on the subject of water transport in plants, but I have to agree with Wohlleben that there’s a lot that’s missing. The key to the process is a material called xylem, a structure made from hollow, dead, reinforced cells. Here’s how a BBC science site tries to explain it:
Transpiration explains how water moves up the plant against gravity in tubes made of dead xylem cells without the use of a pump.
Water on the surface of spongy and palisade cells (inside the leaf) evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what’s lost.
As the xylem cells make a continuous tube from the leaf, down the stem to the roots, this acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.
Water doesn’t flow upwards, however. It has to be pumped up, or sucked, as we do when we apply our lips and energy to a straw. The BBC also describes the whole process as transpiration, which just seems wrong to me. Obviously much transpires here, but it isn’t just transpiration. What?
What obviously needs explaining is where the energy comes from to draw the water up against gravity, and how the plant ‘knows’ that water needs replenishing.
A more comprehensive, and richly referenced, attempt at an explanation is provided by Nature, the well-known science magazine, on one of its educational websites. There we’re told that ‘plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth’. This is fascinating, as is the reason for the lack of retention – photosynthesis. Water is lost to the atmosphere from the leaves’ stomata, which are like our pores. These stomata are used to absorb CO2 for the photosynthesis of sugars, but their openness to CO2 increases the transpiration rate, so there’s a tricky balance between the two – water loss versus CO2 and sugar gain.
The xylem mentioned above doesn’t reach down all the way to the base of the root system. First the water must pass through several cell layers that act as a filtration system. But how does it do this? What is the force being applied and where does it come from? The Nature article gives this complex explanation:
The relative ease with which water moves through a part of the plant is expressed quantitatively using the following equation:
Flow = Δψ / R,
which is analogous to electron flow in an electrical circuit described by Ohm’s law equation:
i = V / R,
where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm’s law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).
Got that? I may be wrong, but isn’t this just an analogy? Don’t analogies tend to break down with a little bit of analytic pressure? The idea of hydraulic conductance is clearly drawn from electrical conductance, but electrical conductance relies on a power source, doesn’t it? What is the plant’s power source? Yes, I can see that certain parts of the plant have a greater resistance to the water’s mostly upward movement than others, and that this resistance is measurable by examining the time it takes for water to pass through the different parts with their particular structure and chemistry, but it says nothing about the energy source. In Ohm’s law, V, voltage is the amount of power, which comes from a source of that power, such as a battery. In the above analogy, Δψ is described as the water potential difference that drives flow. I’m possibly being dumb, but how does that happen? What’s meant by ‘water potential difference’?
The Nature article, I must say, is very good at telling us about the materials and obstacles negotiated by water molecules on their journey. First they pass through the root’s epidermis, then the cortex and the endodermis and then on to the xylem. They travel by an apoplastic pathway (more of that next time), or else a cell-to-cell pathway (C-C), and the role of ‘water-specific protein channels embedded in cell membranes (i.e., aquaporins)’ is mentioned, but this role is apparently still much of a mystery. Anyway, the xylem continues into the petiole, to which the leaves are attached, and then into the mid-rib, the main central vein of the leaf. From there the water passes into the smaller branching veins of a dicot leaf, which also contain tracheids – elongated xylem cells for the transport of water and mineral salts. It’s from this network of veins that transpiration takes place.
So I’m learning a lot, but the ‘water potential gradient’ and how it pulls or pushes water upwards, that’s still very much a mystery to me. But there’s more to come.
References
Peter Wohlleben, The hidden life of trees, Collins 2017
Ok, the usual update on Trump’s downfall. Some are saying that the Mueller enquiry is winding up (and I’m not talking about GOP hardheads), but I’m hoping not, because I reckon the financial stuff alone will take years to wade through properly. In the meantime though, I’m hoping that more really dramatic developments occur to light a fire under Trump’s capacious backside, sooner rather than later. The latest news is that the Mueller team are looking at the cover-up re Trump Jr’s meeting with Russian agents. So maybe the cover-ups and the endless obstructing will lead to some justice action soon, while the ‘follow the money’ aspect will continue for some time, and hopefully do the really lasting and permanent damage to the Trump horrorshow.
more on abiogenesis – Greenland and other rocks, water everywhere, and the how question
rock formations that may or may not display signs of life
Jacinta: So I recently watched a Nova video on Youtube, which celebrates, through the geologist and mineralogist Robert Hazen, the relationship between rocks and life, or two worlds we tend to keep divided, the animate and the inanimate, and how they feed off each other. It was fascinating, and I’d like to talk about the effect of photosynthesis on the production of iron in the ocean, but first we should talk about those 3.8 billion-year-old Greenland rocks that we talked about way back when.
Canto: Ah, well, have you heard the latest? It comes from Quebec. Haematite tubes, similar to those produced by microbes around undersea hydrothermal vents, which could be up to 4.28 billion years old…
Jacinta: Yeah, couldabeen, wouldabeen, but I must say the video did argue for a watery planet much earlier than might have been expected, but no clue as yet as to where all that water came from.
Canto: You don’t buy the ‘it came from outer space’ meteor scenario?
Jacinta: I’m no expert but it sounds desperate.
Canto: We’ve found icy oceans on Europa and Enceladus, with probable hydrothermal vents, which we’re keen to explore, so maybe it’s not so weird after all.
Jacinta: Oceans of water?
Canto: Yes, and the Hubble Space Telescope recently observed what’s believed to be plumes of water vapour gushing out from Europa’s surface.
Jacinta: Interesting, but what’s most interesting is the diversity of these early signs of life. They’ve found chemical signatures in ancient microscopic zircon crystals, and ancient microbial mats as far apart as Australia and Greenland, and now, possibly, these very old haematite tubes, all very different from each other, and all very unlikely given what we think we know of the Earth’s early environment.
Canto: And they’re all connected with water, aren’t they? This is one of the mysteries to me, where did all the water come from – on Earth, Enceladus, Europa, Titan…?
Jacinta: Search me. It’s certainly exciting and promising though, NASA scientists say that water, chemistry and energy are the three essential requirements for life, and they reckon those moons have all the requirements. They’re hoping to send back probes to search for that life. But, you mentioned Titan. There’s an environment worth exploring, because, as the NASA boffins tell us, it has rivers, lakes and rain, but it’s not water. So, to steal a phrase, there could be life there, but not as we know it Jim. And if we were able to find a diversity of life in our own solar system, what’s the likelihood of an almost infinitely greater diversity of life amongst the billions of other solar systems we now know to be out there?
Canto: I want to live forever! I want to have infinite time to explore these possibilities! I wanna be a time lord!
Jacinta: Yes but getting back down to Earth. We’re trying to pin down the first appearance of life here but it’s really difficult, and proving to be controversial, unsurprisingly. What isn’t controversial is that there is a window of about 1 billion years between the Earth’s formation and about 3.5 billion years ago when life must have started here.
Canto: Yes and you’re talking about the when, but the where and the how are likely just as controversial and certainly more important. You’ve mentioned Greenland, and I’ve mentioned the remote north of Quebec, and we’re talking about rocky regions that are difficult to get to and explore, and which have undergone great changes over the eons. So there’s plenty of geological argument about them as well. There’s no doubt these regions contain some of the oldest rocks yet discovered, but there’s a fair amount of doubt about their precise age.
Jacinta: Yes they’ve been much deformed over time, but geologists are finding evidence that they formed under the ocean, and that they show distinct signs of hydrothermal vent activity. As you know, hydrothermal vents have come to be associated with the earliest life forms.
Canto: Yes, the evidence appears to be indirect, and based on analogy at this point. Also, some geologists are tentatively putting the date of these rocks as far back as 4.3 billion years, and that’s very early in Earth’s history. I’m talking here about the Quebec material – what’s being said about the Greenland stuff, has it been verified as actual evidence of life?
Jacinta: Well all the reporting on that came out in August-September last year, all based on a paper in Nature, and I’ve not found anything more recent. The claim was that they’d found evidence of stromatolites, that’s the same features we’ve seen in rather a lot of docos recently, growing in shallow waters in Western Australia’s Shark Bay. They’re microbial mats that build up over time to create these mounds. Fossil evidence of stromatolites found in the Pilbara, also in Western Australia, are reliably dated to 3.5 billion years ago, and that’s the current record for earliest life forms, but the contested evidence of stromatolite fossils in Greenland, if validated, would take the record back another 200 million years, at least.
Canto: And these stromatolites evolved in shallow waters, right? Darwin’s warm, energetic little pond. Not like the microbes supposedly found in northern Quebec. Apparently there’s a tension between the fossil evidence, which generally supports the warm pond thesis, and the genetic and biochemical evidence which takes us more towards hydrothermal vents.
Jacinta: Yes, interesting, and anyway water.
Canto: Well we’re not going to be able to solve the water mystery here. Or answer the when question of first life. I’d like to change tack and think on the how question, surely the most interesting one.
Jacinta: Okay so this is where we turn to variations on, or more sophisticated elaborations of, the Miller-Urey-type experiments.
Canto: Yes – finding the recipe, as is emphasised in this documentary on life’s origins. In one part of the documentary, the story’s told about how John Sutherland and colleagues, workers in the field of prebiotic chemistry (a good term for googlers) have created a ribonucleotide, a building block of RNA, through manipulating plausible early-Earth conditions. This was certainly an exciting development, but progress in this field has been frustratingly slow. Sutherland’s work, and critiques of it, are given in more detail here.
Jacinta: Okay so I’ve googled ‘prebiotic chemistry’ as you suggested, and it’s led me to this article in Nature Chemistry which provides a good relatively untechnical intro to the field. Well okay, a bit technical here and there.
Canto: Yeah and it seems quite a small field considering the importance of the question ‘How did life get started?’
Jacinta: Sounds like they’re having trouble with funding. No pay-offs to the research, and it’s not as sexy as fundamental physics or astronomy. No techno-wizardry like LIGO or the LHC.
Canto: Yes, and you’ll only get really incremental advances. A lab-created nucleotide or two seems a bit of a distance from the beating heart of life to most people. And of course it’s impossible to know, when you do manage to create some building-block towards life from simpler chemicals, if that was how it happened here on Earth (if indeed life actually did start here rather than being transported from elsewhere).
Jacinta: A good last point. If all that water came down in a bunch of early meteor showers, that would seem to make life from meteors much more plausible.