Today I get
to talk about a subject near and dear to my heart-life on other planets and
moons. I have no doubt that life existed on Mars and still exists underground
today. I feel that advanced and intelligent life once existed on Mars. Why
don't we see ruins of cities from long ago? It is because this advanced life
lived in caves and underground structures.
I also believe that a group of meteors from
Mars launched life on our planet. Whatever life we find on Mars will have a lot
of similarities to life on earth.
Intelligent life could have taken
infinite possibilities when we go further out in our solar system and to
planets in other galaxies. How do we recognize it and communicate with it?
Some astrophysicists have come up with
some "think outside the box" ideas published in The Economist
magazine this week. I will share these with you now:
How to
improve the search for aliens
So far,
people have sought Earthlike biology. That will change
May 25th
2022 (Updated May 26th 2022)
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For decades,
astronomers searching for life beyond humanity’s home planet have had a simple
strategy: follow the water. Water is the sine qua non of terrestrial life and
as thousands of new planets have been discovered orbiting faraway stars, the
greatest levels of excitement have usually been reserved for those in the
“habitable zones” of their systems—in other words orbiting at a distance where
liquid water could exist on the planet’s surface.
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The next
step has been to look for biosignatures—molecules which might betray the
existence of biological processes. These could include oxygen or methane in a
planet’s atmosphere. On Earth, those molecules persist only because living
things constantly regenerate them.
The problem
with both these approaches is obvious—they are restricted to finding life as
currently known. But, as Natalie Grefenstette, an astrobiologist at the Santa
Fe Institute in New Mexico, points out, “we don’t know if other forms of life
would necessarily have the same signatures, if they would have the same
metabolisms, if they would be based on the same genetic molecules or any of the
same molecules at all.” Life on Earth could have evolved in the way that it has
because the specific chemistry of the planet at crucial times gave rise to
selective pressures which might not be present on other worlds. “And so we’ve
been thinking—if life were different, how do you even look for that?”
Exotic
beasts and how to find them
From May
16th-20th, at AbSciCon, a biennial astrobiology meeting organised by the
American Geophysical Union and held this time around in Atlanta, Georgia,
astrobiologists including Dr Grefenstette considered that question and
discussed ways to expand their searches in the coming decades, so that they
might stand a better chance of recognising more exotic forms of life than are
currently being sought. To do this, they will need several strategies.
The first
begins by imagining the various different chemistries which exotic forms of
life might employ, and using those to devise a wider set of potential
biosignatures. On Earth, the most important molecules of life almost all
involve carbon atoms. These are particularly versatile because they can form
chemical bonds with up to four other atoms, including other carbons, to create
complex molecular structures. Carbon is the fourth most abundant element in the
universe and the molecules it forms can survive for long periods in the sorts
of temperatures and pressures found on Earth’s surface.
An exotic
lifeform might plausibly, however, rely on silicon instead of carbon. Silicon
sits just underneath carbon in the periodic table and thus shares with it the
ability to bond with up to four other atoms. Familiar examples of the results
are most of the huge diversity of minerals which make up rocks, for silicon is
the second most common element in Earth’s crust. It is also the seventh most
abundant in the universe, which means there is plenty of it available for
potential silicon-based lifeforms to use.
Alien life
might, though, have its roots in something yet more exotic. In the laboratory,
metal oxides known as polyoxymetalates have shown some remarkably lifelike
abilities, such as being able to form membranes (dubbed “inorganic chemical
cells” by Lee Cronin, a chemist at the University of Glasgow) and being able to
assemble, with some chemical help, into complex structures reminiscent of dna.
Whatever its
building blocks, though, life will need a solvent in which to function. On
Earth, that solvent is water.
Water is a
good solvent because it is a “polar” molecule, meaning its electrical charge is
unevenly distributed. In a molecule of H2O the oxygen has a slightly negative
charge and the two hydrogen atoms are, by way of counterbalance, slightly
positive. This polarity causes water molecules to stick to similarly polar
molecules, making them good at dissolving other chemicals—which, in turn, once
thus in solution, can interact with each other. That enables water to support
the myriad functions of life, and no other abundant chemical on Earth matches
this versatility.
Other
chemicals can, however, fulfil some of the roles water plays. Life elsewhere
might, perhaps, have found a way to employ ammonia. This, like water, is polar,
and therefore good at dissolving things. It is not quite as good at doing so as
water, though, and it also stays liquid (at terrestrial atmospheric pressures,
at least) only between -78°C and -33°C. But that would make it available in
liquid form in frigid places such as Europa, a moon of Jupiter, and Titan and
Enceladus, moons of Saturn, where water itself would be frozen.
Possible
solutions
Titan in
particular is believed to host vast ammonia-rich underground lakes which might
act as cradles for chemically exotic life. But other possibilities exist there,
too. Dr Grefenstette says astrobiologists are also intrigued by the lakes of
liquid methane that cover Titan’s surface (the average temperature of which is
-179°C). Methane exists on the surface of Titan in much the same way that water
does on Earth—in liquid, gaseous and solid forms.
Methane is
not a perfect solvent for life. It is not polar and therefore not as versatile
in that regard as water. And it remains liquid (again, at terrestrial
atmospheric pressures) only between -182°C and -161°C. Since chemical reactions
proceed more rapidly at higher temperatures, on Titan’s surface they would be
pretty slow. But astrobiologists hypothesise that life composed of different
materials to those on Earth—smaller hydrocarbons and nitrogen, for
example—could feasibly eke out an existence there.
Perhaps the
most promising general-purpose alternative to water is formamide, a colourless
organic liquid composed of carbon, hydrogen, oxygen and nitrogen (all elements
common in the universe) that can dissolve many of the same chemicals as
water—including proteins and dna. It can also stay liquid at up to 210°C,
making possible a large range of chemical reactions on planets with more
extreme surface temperatures than Earth’s. Formamide is such an intriguing
alternative to water that some astrobiologists even argue that it might have
been the main solvent used by the earliest forms of terrestrial life. This
chemical has been located in vast clouds at the edge of the solar system and
also in more distant nebulae where stars are forming, according to Claudio
Codella, an astronomer at the Arcetri Astrophysical Observatory in Florence,
Italy. Finding it definitively on another world would surely pique interest
among those searching for exotic forms of life.
The units of
life on Earth—cells—are contained within lipid membranes. These keep the
chemical reactions which sustain life concentrated inside a cell, and the
exterior world outside it. Such membranes would not be stable in a medium such
as liquid methane. But exotic lifeforms on Titan might instead build membranes
from structures called azotozomes. These are molecules, currently hypothetical,
made from nitrogen-rich organic compounds, according to Paulette Clancy, a
chemist at Cornell University who came up with the idea. They would, she
thinks, be capable of operating in the ultra-low temperatures of a place like
Titan.
Or perhaps
there could be life without any membranes at all. Lifelike chemical reactions
have been shown to occur on the surfaces of certain minerals, including pyrites
and various clays. These often contain networks of pores and cavities that
could serve the compartmentalising function of lipid-based cells. Or biological
reactions might be contained within drops of liquid floating in planetary
atmospheres.
Finally,
life needs to store information about itself and pass that information on to
its offspring. Terrestrial organisms do this using molecules called nucleic
acids. These employ four different molecular units known as nucleotides to
carry a code of instructions that can build 20 different amino acids, which
then link up in various combinations to form proteins. But laboratory
experiments and samples from meteorites show that many more nucleotides and
amino acids than these exist. Though they have not been incorporated into life
on Earth, they could form the basis of alternative systems of genetic
information.
Identifying
exotic life forms made from different materials is thus a matter of widening
the search from Earthly biosignatures—oxygen, methane and so on—to include
chemicals that might be made by various imagined biochemical systems. One tool
for this search is the mass spectrometer, a device that ionises samples and
then filters those ions by mass.
Mass action
Mass
spectrometers have been the eyes and ears of decades of space exploration, said
Luoth Chou, an astrobiologist at Georgetown University. Successive generations
of these devices, flown into space, have permitted researchers to characterise
chemicals everywhere from the surface of Mars, via the atmospheres of Venus and
Titan, to the plumes of water ejected from geysers on Enceladus.
The next
generation of mass spectrometers, though, will be smaller and yet more
powerful. And they will be carried aboard a range of missions far and wide into
the solar system. Dragonfly will hop around the surface of Titan in the
mid-2030s and take a close-up look at the molecules there. davinci will orbit
Venus in 2031. The Jupiter Icy Moons Explorer will explore the Jovian satellite
system, starting in the early 2030s. And Europa Clipper’s mass spectrometer
will provide a high-resolution characterisation of that body, beginning at the
end of this decade.
If exotic
life does exist, however, it could use chemistry that goes way beyond anything
astrobiologists can currently imagine. To get around that means thinking of
biosignatures which depend not on chemistry but rather on the patterns of
behaviour associated with life.
There is no
universal definition of life. But astrobiologists often default to nasa’s
operational definition of “a self-sustaining chemical system capable of
Darwinian evolution”. Living things self-replicate and make large amounts of
specific complex molecules (for example, proteins or dna). They also draw
energy and consume resources from their environments to fuel their metabolisms.
Based on these ideas, so-called agnostic biosignatures could include looking
for excesses in certain elements or isotopes in an environment, or for specific
patterns within groups of chemicals that cannot be explained by abiotic
processes alone. Peter Girguis, an evolutionary biologist at Harvard
University, told the AbSciCon meeting that this new class of biosignatures
would be “indirect proxies for a living organism”.
One example
would be to search for gradients in an environment—zones of sharp change in,
for example, heat or electrical voltage or chemicals. According to Dr Girguis,
“all living organisms that we know of establish gradients of one kind or
another to maintain themselves at a kind of disequilibrium from the
environment.”
Some of
these gradients occur at cellular and microscopic scales, and can be incredibly
sharp and therefore distinguishable from non-biological processes. Others are
larger-scale. In marine sediments on Earth, for example, microbes work together
to oxidise methane, a process tied to the chemical reduction of sulphate ions.
“We see gradients in methane and sulphate concentration over centimetres, and
they’re really pronounced,” says Dr Girguis. “This is a biological
manifestation of their activity and yet this is detectable by simply making
abiotic measurements.”
Another
tactic would be to study the complexity of the molecules at a particular
location. Biological molecules are selected and shaped by evolution to do
specific jobs within an organism, such as assembling or disassembling other
molecules, or signalling between cells. That often requires unusually energetic
chemical processes, which in turn need the help of catalysts. On Earth, these
catalysts are protein molecules called enzymes which are, themselves, the
product of evolution. Finding complex molecules of any sort might thus be
considered a potential biosignature.
A related
concept is what Chris McKay, a planetary scientist at nasa Ames Research
Centre, calls the “Lego principle”. The idea here is that life is recognisable
by its use and reuse of a selected set of molecules. Abiotic samples scooped up
from an alien world would be expected to contain a wide array of organic molecules,
some of them in fairly small amounts. A biological sample, by contrast, would
contain large numbers of just a few distinctive molecules. Molecules that are
chemically similar (left-handed and right-handed versions of an amino acid, for
example) might have markedly different concentrations if they came from a
biological sample, whereas they would probably be present in near-equal numbers
in a non-biological one. Spotting patterns like these would be independent of
the specific biochemistry involved.
The past as
a clue to the present
Such methods
would widen the astrobiological search wherever it was possible to obtain a
sample—in other words any world in the solar system to which researchers can
send a probe—and apply to it tools such as miniaturised, space-hardened mass
spectrometers. For planets going around other stars, though, things are
obviously trickier. Few people think human beings or their machines will visit
any of the rapidly expanding population of these exoplanets anytime soon.
Astrobiologists are instead considering other ways to search for new agnostic
biosignatures. Michael Wong, an astrobiologist at the Carnegie Institution for
Science, in Washington, dc, presented a technique that applies what is known as
network science to data about exoplanets’ atmospheres. These data can be
gathered using telescopes on, or orbiting, Earth.
Any chemical
system, the chemicals within an atmosphere included, can be represented by a
so-called network diagram, in which molecules that react with each other in
some way are connected by lines. Dr Wong showed that, when compared with those
of other planets in the solar system, Earth’s atmospheric network stands out
like a sore thumb. In fact Earth’s network more closely resembles those of
biological systems, such as marine food webs. This technique is a work in
progress and Dr Wong said it would need a lot more development before
astrobiologists could include it in their life-detection toolkit. But it is an
intriguing approach.
Dr Girguis
told the meeting that future searches for exotic life in the universe would do
well to learn from mistakes made by explorers searching for life in Earth’s
oceans in the 19th century. In one expedition, for example, Edward Forbes, a
prominent naturalist from the Isle of Man, was dredging in the Aegean Sea. He
noticed that the farther plants and animals were from the water’s surface, the
less well they thrived. In 1843 he extrapolated his incomplete data to propose
his azoic hypothesis, which stated that life would not exist at all below 550
metres.
It took
several decades to prove him wrong, an effort that involved some of the first
scientific endeavours designed to explore the deep ocean—such as the Challenger
expedition that sailed from 1872 to 1876. These, said Dr Girguis, were some of
humanity’s earliest life-detection missions. “Let’s not be too quick to
extrapolate,” he warned his fellow astrobiologists. “And let’s never
underestimate the capacity of living organisms.” ■
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This article
appeared in the Science & technology section of the print edition under the
headline "Life, but not as we know it"
Science
& technology
May 27th
2022
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