"There are often evolutionary parallels on the different worlds because creation tends to be economical." - This quote from Julian May just might capture the essence of how humankind proceeds to go about it in search of extraterrestrial life. 

The presence of life beyond Earth is something that humans have dreamed about since the beginning of time with a seemingly intuitive sense that there just had to be other planets out there as well as the fact that some of them might support life. That's where the study of astrobiology comes in. 

The funny thing about biology is that it can never be certain that a biological process exists forcibly in an extraterrestrial domain, but what we can be certain about is what it can rule out, what is speculative. We'll take you on a journey through the magical realm of astrobiology with this series of articles, but for that, first, we need to look at a classification of organisms called "Extremophiles."

So what exactly is an extremophile?

It's an organism that can thrive under extremely harsh conditions that are quite challenging to other carbon-based life forms. Some of the examples could be Environments that reach extreme conditions of heat, acidity, pressure, cold, and even space!. And it's a natural thought to look at these organisms for mapping out life on other planets. If we consider that the current planetary surface conditions on Earth (such as mean temperature, redox state, and oxygenic atmosphere) have only occurred for a short period compared to the existence of Life, we might conclude that the extremophilic way of Life has dominated the evolutionary history of life on our planet. One of the extremophiles that were sent to space for further research was Tardigrades. 

Above is the picture of a tardigrade so that you don’t get bored by all these words. Although my Co - Author thinks they are cute, I tend to disagree with her opinion. And now, due to her fascination *cough* Obsession *cough*, she is going to take you through all that you need to know about these creatures.

Tardigrades, water bears, moss piglets, slow steppers, there are multiple adorable names for this creature who looks like a mini transparent Demogorgon. 

They are not more than 1mm long, that too microscopic. They have eight tiny legs with ferocious claws resembling those of the great bear, thus the name water bears. 

 Their mouth is a serious weapon with dagger-like teeth that can easily spear prey. 

 More than 900 known species of Tardigrades feed by sucking moss, lichens, and algae juices. In contrast, others are carnivores, sometimes even preying on other tardigrades. Did you know that they are like ancient- ancient? They've been surviving and thriving and evolving since the Cambrian Era.  

They can even live without water for decades! But how do we know? 

There were many instances through which it had been proven that even dried tardigrades could be brought back to life by simply rehydrating them. In 1948, an Italian Zoologist – Tina Franceschi, claimed that tardigrades found in dried moss from museum samples over 120 years old could be reanimated. After rehydrating a tardigrade, she observed one of its front legs moving. ]In 1995, dried tardigrades were brought back to life after eight years! 

But the question remains – how do they do it? How do they keep coming back to life? We all know that when a typical cell dries out, its membranes rupture and leak, its proteins unfold and aggregate together, making them useless. DNA will also start to fragment the longer it is dry. But since water bears can survive drying, they must have tricks for preventing or fixing the damage that cells like ours would die from. 

The trick is,  

When a tardigrade dries out, it retracts its head and eight legs. It then enters a deep state of suspended animation that closely resembles death. Shedding almost all the water in its body, the tardigrade curls up into a dry husk. Baumann called this a "Tönnchenform," but it is now commonly known as a "tun." Its metabolism slows to 0.01% of the standard rate. It can stay in this state for decades, only reanimating when it comes into contact with water!!! (cryptobiosis)   But tardigrades are not alone who do this. Some yeast, nematode worms, and bacteria can survive even desiccation. They produce a tremendous amount of a sugar called Trehalose, which forms a glass-like state inside their cells that stabilizes key components like proteins and membranes, which would have been destroyed. Trehalose wraps itself around water, stopping them from rapidly expanding if the temperature rises. Rapidly growing water molecules are dangerous because they can rupture cells, which can be fatal.

Though most of them do, not all tardigrades can make this sugar. Even if they do, it is in undetectable amounts.

THEN HOW DOES THIS TRICK WORK?  

We know that they make a unique protectant when they dry out, allowing them to survive from entirely drying out. But these protectants are still a mystery. They also seem to make a lot of antioxidants. Chemicals like vitamins C and E soak up dangerously reactive chemicals. This may mop up harmful chemicals in the tardigrade's cells. 

"Reactive oxygen species" pose a particular hazard to tardigrades. These compounds are formed as a by-product of regular cell function. Yet, they have the potential to degrade a cell's fundamental components, including its DNA. Many of them float about in animals exposed to environmental stress. 

The antioxidants may explain one of the tardigrades' neatest abilities – If a tardigrade stays in its dry tun state for a long time, its DNA gets damaged. But after it reawakens, it can fix it quickly.

Temperature doesn't bother them.

Hot? 

In its tun state, it could survive being heated to temperatures of 125 °C for several minutes. Gilbert (Definitely not the name of a scientist) tardigrades came back to life after heating them to 151 °C for 15 minutes. 

But at scorching temperatures like 150 °C, proteins and cell membranes should unravel, and the chemical reactions that sustain life cease to happen. 

Many creatures have evolved to survive in hot spots (such as hot springs and snorkeling deserts) and generate so-called thermal shock proteins. They serve as protein chaperones in cells and enable them to maintain their form and repair proteins damaged from damage. 

Cold? 

Another Scientist immersed them in liquid air at -200 °C for 21 months, in liquid nitrogen at -253 °C for 26 hours, and in liquid helium at -272 °C for 8 hours. They sprang back to life as soon as they came into contact with water. 

Guess the cold doesn't bother them anyway, either. 

The biggest hazard tardigrades face in the cold is ice. If ice crystals form inside their cells, they can tear apart crucial molecules like DNA. 

(animals like fishes make antifreeze proteins that lower the FPT of their cells, but tardies don't even have that)

BUT GUESS WHAT? 

These sneaky creatures can still survive despite their cells being all icy. Either they can protect it or repair it, but how? We don't know that either. Though, they may produce chemicals called ice nucleating agents. These encourage ice crystals to form outside their cells, protecting the vital molecules. Trehalose sugar may also protect those that make it, as it prevents the formation of large ice crystals that would perforate the cell membranes. 

But there is no conclusive evidence that tardigrades produce these chemicals. *shrugs* 

RADIATION? 

Exposed to X-rays, UV, alpha, gamma – they coped – weren't even in the tun state. 

It was the most significant threat they faced while being sent to space, and yet the mortality 

wasn't 100% 

PRESSURE? 

They can cope with extreme pressure that would squash most animals flat. Scientists found that tardigrades in the tun state could survive a pressure of 600 MPa.

Proteins and DNA break down under these crushing pressures. The fat cell membranes turn solid in a refrigerator like butter. Most microorganisms can halt metabolism at 30 MPA, and not much more than 300 MPa can survive. 

But not all tardigrades are this resistant (notable survivors).

Their resiliency is partly due to a unique protein in their bodies called Dsup—short for "damage suppressor"—that protects their DNA from being harmed by things like ionizing radiation, which is present in the soil, water, and vegetation. It involves binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals.

At these crushing pressures, proteins and DNA are ripped apart. Extreme heat and cold, radiation, and high pressures all have one thing in common: they harm the tardigrade's DNA and other cell components. Both heat and cold induce proteins to unfurl, clump together, and cease to function. Radiation rips DNA and other vital components apart. High pressures cause the lipid membranes that surround cells to harden.

So if all the stressors cause similar problems, maybe the tardigrades only

need a handful of tricks to survive them

In 2007, these tiny tardigrades were attached to the satellites and were blasted into outer space. 

Surprisingly, many of them survived, even reproduced, laid eggs and the babies were healthy too! Also, there are probably tardigrades on the Moon(Lunar tardies, yay). 

It was a failed mission. When the equipment returned, it contained a colony of tardigrades, which scientists believe are highly likely to have survived the impact. 

Yet despite their rather tedious lifestyle, they have evolved to cope with environments so extreme they don't even exist on Earth.

Woah, that was an interesting read about tardigrades. Now, I’ll be taking the mic to continue our discussion.  

Another notion that the researchers had was that sunlight is fundamental to the nature of Life, that no life exists without it being provided energy by the sun in some way, shape, or form. However, this changed when scientists discovered colonies of giant tube worms, clams, mussels, crustaceans around the Hydrothermal vents deep inside the oceans ( Hydrothermal vents are essentially underwater geysers created by tectonic plates on convergent or divergent boundaries). They were surprised to find this self-sustaining ecosystem where Inorganic chemical compounds fuel primary productivity as energy sources instead of light (Also known as chemolithoautotrophy).

Chemolithoautotrophic bacteria are able to use the energy of chemicals within the hydrothermal vent water to synthesize the carbon compounds they require to grow and reproduce. Although a fraction of the bacteria is dependent on the oxygen provided by other photosynthetic organisms, the majority get their energy from the chemicals buried deep inside the Earth as they are anaerobic in nature. These bacteria use sulfur compounds, particularly H2S and sometimes NH3, to produce organic material (This info might be relevant in the upcoming articles). Another interesting organism to look at is the green sulfur bacteria capturing geothermal light for anoxygenic photosynthesis.

Now, to generalize a bit, the organisms hidden in the deep underwater vault of the Earth live very, very slowly, rarely dividing, their energy consumption at times six orders of magnitude lower than that of cells living in surface habitats. They divide once per maybe thousands of years, and we don't even know the lifespan of these organisms (And we just found out about them decades ago!). It would be like measuring a tree's age if our lifespans were of a year (Many challenges are associated with analyzing them). 

We have found organisms like these as deep as 5km into the ground, and there are literally billions of them per cubic centimeter. Many researchers argue that "Life might have gotten its start on the surface of the Earth, where it found creative ways to survive and spread, including to deeper environments. But it's also possible that Life began underground, at some fortuitous juncture of rock and water—eventually also making its way to the surface and figuring out how to harness the sun's energy." as it's still an open question. If we hope to tell tales of life beyond the stars, then it's very natural to look at the world beneath our feet, seemingly alien to us. 

Okay, now we have sort of established where does humanity get the motivation to tackle the challenge of how should we even proceed? What knowledge might aid us. Now we’ll be looking at what are astrobiologists trying to achieve? What are the objectives of this research? 

Why is Earth habitable? How, when, and why did it become habitable? Are, or were, any other bodies in our Solar System habitable? Might planets orbiting other stars be habitable? What sorts of stars are most likely to have habitable planets? These are just a few of the questions that astrobiologists are trying to answer today. 

So now, What's the strategy for the future? How is humanity going about the research? We'll be talking about the plan of NASA in this Article. 

In preparing this strategy, Scientists all across the globe have tried to define the objectives for astrobiology research moving forward. The community identified six important topics of research in the field today:

• Identifying abiotic sources of organic compounds

• Synthesis and function of macromolecules in the origin of Life

• Early Life and increasing complexity

• Co-evolution of Life and the physical environment

• Identifying, exploring, and characterizing environments for habitability and biosignatures

• Constructing habitable worlds

1) Identifying Abiotic Sources of Organic Compounds

How did the building blocks of life come into being? Where did they come from? A major goal of research on this topic in astrobiology is to understand how the abiotic (non-biological) production of small molecules led to the production of large and more complex molecules, prebiotic chemistry, and the origin of life on Earth.(feels too repetitive for some reason idk) This also aims to understand what roles primitive icy bodies (asteroids and comets) play in the origin of Life? And whether Life or prebiotic chemistry could or did evolve on icy worlds such as Enceladus, Europa, and Titan. 

( Enceladus and Titan being the moons of Saturn and Europa being the moon of Jupiter. We have to thank Cassini Spacecraft for discovering that these moons could be potentially habitable.

First, to talk about Enceladus. Many researchers believe that Life as we know it requires three primary ingredients:

• Liquid water.

• A source of energy for metabolism (This need not be sunlight, as we discussed earlier).

• The right chemical ingredients, primarily carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur.

We'll hopefully go into why we suspect that and why these biosignatures are promising but bear with me here. With this finding, Cassini has shown that Enceladus, a small icy moon a billion miles farther from the sun than Earth, has nearly all of these ingredients for habitability (A lot of exomoons seem pretty promising). 

3) Early Life and Increasing Complexity 

Understanding the history of Life on Earth is key to a full understanding of what Life is and how it works (Which we might go into further detail later).

Over four billion years, Life on Earth has gone through a lot of changes to become what it is today. Astrobiologists face the challenge of deciphering overarching rules for evolutionary processes, drawing on theory and observation to make a general model of Life.

4) Co-Evolution of Life and the Physical Environment Life affects its environment.

At the same time, the environment affects Life. Astrobiologists are focused on understanding the relationship between Life and environment well enough to inform the search for potentially habitable environments beyond Earth. Examples of major transitions in biological evolution that affected our planet include the origins of photosynthesis, multicellularity, and intelligent Life.

Major changes in the physical state of the planet that have affected biology include the emergence of plate tectonics and continents, as well as climatic transitions such as "Snowball Earth" episodes. 

Snowball Earth - The Snowball Earth hypothesis proposes that, during one or more of Earth's icehouse climates, the planet's surface became entirely or nearly entirely frozen. Throughout Earth's climate history, its climate has fluctuated between two primary states: greenhouse and icehouse Earth. Icehouse climate basically refers to the period where both poles have ice sheets. (We are currently living in an Icehouse climate period!). So, in the snowball earth phase, life's surface was basically a frozen shell. 

I am not covering the further points here because it’ll go on too long. If you have made it this far into the Article (btw, thanks for reading it :) ), you can check out the detailed plan by NASA in the resources section.

So, that's all folks!

We'll be covering some exciting stuff in the next Article like biosignatures, how we detect it, some mathematics related to it, and a whole bunch of other cool stuff!

More about Europa - https://solarsystem.nasa.gov/moons/jupiter-moons/europa/in-depth/

References -

• The Habitability of Titan and its Ocean | News | Astrobiology (nasa.gov)

• Enceladus: A Habitable Environment? | NASA Solar System Exploration

• Enceladus plume White paper (nasa.gov)

• NASA Astrobiology Strategy 2015

• snowball1.eps (caltech.edu)

• The Exoplanet Era – Many Worlds

• Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context (nih.gov)

• High School Ocean Lesson Plans for Reference: The Remarkable Ocean World: The Library: Black Smokers and Giant Worms (courseworld.com)

• Inside Deep Undersea Rocks, Life Thrives Without the Sun | Quanta Magazine

• https://www.nasa.gov/centers/ames/news/2013/bacteria-sent-into-space.html 

• https://elifesciences.org/articles/47682#s1

• https://microbiologysociety.org/publication/past-issues/what-is-life/article/astrobiology-what-is-life.html

• https://askabiologist.asu.edu/tardigrade-anatomy