alien life ‘microfossils’
Earth’s oldest fossils may be billions of years old – although such claims are highly controversial.
These kinds of discoveries usually start with finding what appear to be preserved outlines of microscopic lifeforms that look similar to microbe fossils found in much younger rocks. The rewards for exploring truly ancient life are high, because as well as providing a glimpse at some of the earliest life on Earth, these microfossils can also act as a model for what evidence we might find for life on other planets such as Mars.
But how do we know these really are fossils and not some other non-organic geological feature? In 1996, Nasa scientists claimed micro-structures in a meteorite could be the remains of extraterrestrial life, but further investigation suggested the structures didn’t have biological origins after all.
A recent study, published in Royal Society B, has further examined this question by demonstrating how it is possible for some microfossil-like objects to form without any actual microbes. This study may lead to doubts about specific fossils.
But ultimately it will help scientists focus their attention on the strongest and most likely case studies for the earliest evidence of life – on this planet or others. Microbes don’t have easily fossilized hard parts like the bones or teeth of animals. Sometimes a geologist may be lucky and stumble across glassy silica deposits complete with entombed and immaculately preserved multi-billion-year-old microorganisms.[/text]
[text text_format=”]More commonly, we find some feature that at first glance could be a microfossil of a billion-year-old microscopic lifeform. But on closer inspection, we realize it is perhaps just a “pseudofossil”, like a speck of microbe-shaped dirt or a lookalike mineral that grew in a crack in the rock. Just because it looks like a microbe doesn’t actually mean it’s a microbe.
To distinguish pseudofossils from real microfossils, geologists and astrobiologists look at the shape, structure, and chemical composition of specimens.
To know what we are looking for, we can compare these features to those from modern fossilized microbes in environments such as alkaline lakes, or from uncontroversial well-preserved examples from the older rock record. We can also conduct experiments to try to make microfossil-like objects without microbes and compare the results with purported ancient microfossil examples.
Sometimes microfossils are preserved as hair-like tubular filaments of organic carbon. Sometimes the organic carbon has long since disappeared and a microorganism-shaped hole in the rock is all that remains. And sometimes filaments of iron oxide have taken the shape of the organism.
This last category includes some recent high-profile finds, including those from 4-billion-year-old rocks in Canada. ‘Chemical gardening’ can produce mineral features similar to those claimed to be microfossils. Sean McMahon/Royal Society, CC BY The new study shows that not all hollow tubular filaments made of iron oxide are biogenic microfossils.
In an experimental process known as “chemical gardening”, researcher Sean McMahon created a pocket of acidic fluid enclosed within a membrane, sat in a highly alkaline carbonate or silicate solution. As the outer solution flowed into the pocket, it would grow and eventually rupture, ejecting a jet of the internal fluid that is then rapidly enclosed by a new, tube-like membrane.
This process ultimately created complex arrangements of hollow tubular filaments that can superficially resemble shapes of microbial lifeforms. Looking like a microbe isn’t enough This study is another reminder that extraordinary claims, such as finding Earth’s oldest fossils or life on Mars, must be supported by extraordinary evidence.
Finding a microfossil-like shape is no longer enough without supporting evidence that might include three-dimensional high-resolution microscopy, chemical analysis and a thorough consideration of the context and alternatives.
In his paper, McMahon indeed contends that the reported 4-billion-year-old iron oxide filaments are more likely to be the result of fluids chemically reacting with the ancient seafloor than Earth’s oldest fossil lifeforms.
But this research also doesn’t write off the possibility of true ancient iron oxide microfossils or their use as a model for evidence for life on other planets. It is notable that while the chemical experiments reproduced some microfossil-like shapes, they didn’t reproduce other shapes we could use to help identify microscopic life, such as septate (partitioned) filaments with cell walls.
Ultimately, finding the answers to these fundamental questions of when and where life existed requires a long journey, and we still don’t know if we are nearly there yet. [/text]
Mushrooms on Mars? Five unproven claims that alien life exists
A recent study claims to have found evidence for mushroom-like life forms on the surface of Mars. As it happens, these particular features are well known and were discovered by cameras aboard Nasa’s Mars Exploration Rover Opportunity, shortly after it landed in 2004.
They are not, in fact, living organisms at all, but “hematite concretions” – small sphere-shaped pieces of the mineral hematite, and their exact origin is still debated by scientists. Haematite is a compound of iron and oxygen and is commercially important on Earth. The spherical rocks on Mars may have been created by the gradual accumulation of the material in slowly evaporating liquid water environments. They could also have been produced by volcanic activity.
Either way, mushrooms they are not. The area around Opportunity’s landing site is littered with them – they can be seen all over the surface and were also found buried beneath the soil and even embedded within rocks.
Fossilized worms
These space “mushrooms” were not the first claim of alien life. On August 7, 1996, the then US president Bill Clinton stood on the White House lawn and announced the possibility that scientists had discovered the ancient, fossilized remains of micro-organisms in a meteorite that had been recovered from Antarctica in 1984.
The meteorite, ALH 84001, is one of a handful of rocks we have from Mars. These were blasted off the surface of the planet by volcanic eruptions or meteorite impacts, drifted through space probably for millions of years, before ending up on Earth.
The tiny structures discovered within, using powerful microscopes, resemble microscopic worm-like organisms and are likely to be billions of years old. The debate over the true origins of these structures continues today – many scientists have pointed out that well-known inorganic processes are quite capable of producing structures that resemble living organisms. In other words, simply because something might look a bit like life (mushrooms or otherwise), that does not mean it is.
Mystery gases
In the 1970s Nasa’s Viking robotic landers carried a series of experiments designed to test the Martian soil for the presence of microorganisms.
The experiments chemically treated small samples of Martian soil in reaction chambers on board the landers. In one of them, nutrients containing radioactive carbon-14 were added to the soil samples. In theory, this should be absorbed by any growing and multiplying microbes. The carbon-14 would then increasingly be “breathed out” over time, showing a steady increase in concentration within the reaction chamber.
After the chemical analyses, each soil sample was steadily heated to hundreds of degrees to destroy any microbes, with the intention of seeing whether any such reactions in the soil ceased. Intriguingly, this particular experiment did show a steady increase in carbon-14 over time which was indeed terminated after heating to above the boiling point of water. Several inorganic chemical reactions have been proposed as an explanation. These results, therefore, remain inconclusive and are still debated today.
More recently, minute quantities of methane have been found in the Martian atmosphere. This is also intriguing as living organisms on Earth are known to release methane. Once again, however, it must be stressed that this not conclusive proof of life. Methane can also be produced by several inorganic processes, including by heated rocks.
Wow!
In 1977, the Big Ear radio telescope in the US detected an unusual radio signal while scanning the sky. The signal lasted for just a couple of minutes, was very high-powered, and was detected over a narrow range of frequencies. These factors make it quite difficult to envisage a natural cause, as most natural radio sources can be detected across a wide range of frequencies.
The exact signal has not been detected again since, despite frequent radio surveys of the same part of the sky. The signal was so remarkable at the time that the astronomer on duty, Jerry Ehman, circled the printout of the signal with a red pen and wrote “Wow!” next to it.
Various explanations have been proposed over the years including, recently, that the signal was generated by a passing comet or transmissions from an Earth-orbiting satellite. The exact origin of the Wow! the signal is still not fully agreed upon today and remains an intriguing mystery.
Tabby’s Star
A key tool of planet-hunting is the dimming method – observing light from a star to see if it periodically dips in a regular fashion as an orbiting planet passes in front of it. In 2015, professional astronomers working with citizen scientists from the Planet Hunters project announced the discovery of a nearby star displaying unusually strong and consistent dimming over time.
Tabby’s Star is named after astronomer Tabitha Boyajian who was the lead author on the paper announcing the discovery. Data from the Kepler Space Telescope showed not just a regular dimming, as one might expect from a planetary orbit, but highly irregular dips in the light and, interestingly, a consistent decrease in light output over several years.
This highly unusual behavior prompted numerous theories to explain the observations, including cometary dust or debris from a massive impact gradually spreading out to cover the face of the star. Some also speculated that these were signatures of an advanced alien species building a structure around the star. But further observations have found no corroborating evidence to support this possibility. For example, radio telescopes have failed to detect any unusual radio emissions from the star. Today, the scientists behind the discovery believe that the unusual dips in light are caused by clouds of cosmic dust passing across the face of the star.
As exciting as they are, it is important to treat claims of alien life with a healthy dose of skepticism, and this is indeed what scientists do. No conclusive evidence that extraterrestrial life exists has been found … yet.
Originally Published at the conversation
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