Life on other Planets 

Does life exist on other planets? Two possibilities exist: either we are alone in the universe, or we are not. Both would be amazing.

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Astrobiology is an interdisciplinary scientific field concerned with the origins, evolution and distribution of life in the universe. It asks the questions: does extraterrestrial life exists, and if so, how can humans detect it. So far, astrobiologists have not found any biology beyond the Earth.

Of the other planets within our solar system, Mars is considered the most likely to have ever harboured life. Mars is in the liquid water zone of our sun and has evidence of dried-up rivers and floodplains on its surface. But it seems to have lost the atmosphere necessary to maintain liquid water, and perhaps any life along with it. However, it still has water in ice form buried underground, particularly at the poles. In 2016, Boron was discovered in the rocks. This can only occur between temperatures of 0-60°C, and further supports the existence of liquid water in the past. Boron might also be necessary for life to form since it stabilizes some of the molecules involved in forming DNA. There are currently almost 20 spacecraft on Mars, although only 2 are still operational. The next nuclear powered robotic rover will be launched in July 2020, when Mars is at its closest to Earth, and will land in February 2021. This super fancy remote control car will look for habitable conditions and signs of life. The name of the rover will be chosen next month from ideas proposed by school students [Perseverance].

The Drake equation

But what about life beyond our solar system? While no life has yet been found beyond our home planet, Astrobiologists came up with an equation for how many planets within our galaxy are likely to have evolved intelligent life capable of communicating with us. It is known as the Drake Equation. It goes something like this:

Multiply

1. Number of stars in the galaxy (basically known)

2. Average number of planets orbiting each star (we are getting there)

3. Fraction of those planets with conditions suitable for life (we are starting to have some idea)

4. Fraction of those suitable planets where life will actually begin (we have no idea, since Earth is the only time we know life started)

5. Fraction of times life will evolve to the level necessary to communicate with us, such as by sending radio-wave signals (we have less than no idea)

6. Fraction of time the civilization survives long enough to overlap with ours (we don’t even know how long we will survive, but it is unlikely to reach into the millions of years)

While there is an astronomically small chance that life will spontaneously begin on any one exoplanet, there is an astronomical number of planets in the universe. A very small probability multiplied by a very large number of opportunities could still make it happen many times.

Possibility of life

There may be no place like home, but perhaps there are other habitable places where life could exist. So how many chances are there for life to begin? How many habitable planets might exist in our galaxy or universe? 

Ingredients of life

Our best guess is that life requires at least the following ingredients:

1. A source of energy to power chemical reactions

This will usually be radiation emitted from the nuclear fusion that occurs in stars, but could include the heat from the molten core of a planet or even gravitational energy.

2. A variety of elements that allow complex chemical bonds to form.

Carbon can bond into a greater variety of molecules than any other element, but life might arise from other atoms too.

3. A liquid to facilitate the atoms interacting.

H2O water is the most likely candidate, since it is abundant in the universe and can remain in liquid form over a large range of temperatures.

Ammonia and methyl alcohol might also support life, although they are less abundant in the universe and remain in liquid form over a shorter range of temperatures.

4. A sufficient time of habitable conditions for life to arise and evolve.

Even in a liquid, it takes time for energy to bond atoms into what we might consider life, and even longer for complex life to evolve. This process can be interrupted by the star blowing up or burning out, or by asteroid collisions or other such cosmic events.

5. Last but not least, the process of atoms forming complex life amid a chaotic universe requires a healthy dose of luck.

Chance, probability or uncertainty; whatever you want to call it, it will be incredibly difficult to predict where life will begin.

Habitable distance

The planet must orbit the star at a distance where liquid could exist: not too far, where liquids will freeze, and not too close, where liquids will boil. The water is not too hot, not too cold, but just right for life. This was known as the Goldilocks Zone, but is now usually referred to as the Habitable Zone. Since not all planets containing water will be habitable, it would be more accurate to describe it as the Liquid Water Zone. Due to the magnetic attraction between polar molecules, water can remain a liquid over a greater range of temperatures than any other substance found in abundance in the universe. However, the range is still relatively small and fragile compared to the temperatures of stars. Stars larger than our own burn hotter and faster. The liquid water zone would be further out, but their shorter life span might make them unsuitable to host life and evolution. Smaller red dwarf stars burn slower and cooler than our sun, and are much more abundant than every other type of star combined. However, the liquid water zone would be much closer, making planets much more vulnerable to solar flares and X-Ray radiation. Red dwarf stars also emit little light at the frequency required for photosynthesis by plants as we know them. Furthermore, planets orbiting close to their parent star are likely to become tidally locked, like our moon is to Earth. With one side in perpetual sunlight and the other in perpetual darkness, life would have a difficult time taking root. For example, the tidally-locked exoplanet called WASP-76b gets so hot on one side that it might vaporise and rain iron. Life would not be possible here. However, on a tidally locked planet with a liquid ocean and thick atmosphere to spread the heat, life may still be possible in a belt near the boundary between day and night. The closest star to our sun is the red dwarf Proxima Centauri. In 2016 an exoplanet was discovered orbiting it, which is tidally locked to its parent star. It orbits within the theoretical liquid water zone, but faces extreme solar radiation on ½ of its surface, and does not seem to harbour significant amounts of water any more, let alone life. Trappist-1 is also a red dwarf star, but since at least one of these planets contains an ocean of liquid water, there is still a chance that life could be sheltered below the surface. 

Habitable size

But just because a planet orbits in the so-called habitable zone where liquid water may exist does not mean that it is actually habitable. Many dynamically interacting factors contribute to the habitability of a planet, but the most significant one is perhaps the mass of the planet. It contributes greatly to the atmosphere and magnetic field of the planet.

Atmosphere capture

The mass of a planet influences its atmosphere due to gravity. If a planet is more massive than Earth, its gravity will be enough to suck in any passing gases and form a thick atmosphere. Jupiter, with over 300 times the mass of Earth, has about 90% of its mass in the form of an incredibly thick and dense atmosphere. At the solid core, the air pressure would be a thousand times stronger than at the deepest part of the ocean, which is a thousand times stronger than at the beach. The temperature would also be hotter than the surface of the sun. Not exactly ideal conditions for life to thrive in. Even planets only several times the mass of Earth tend to be gas giants with little solid ground, bone-crushing air pressure and scorching heat. It is difficult enough for life to begin at the best of times, and such conditions are not the best of times. Smaller, Earth-sized planets have a better chance of being habitable. But if a planet is much smaller than Earth, however, it might not have enough gravity to trap gases like hydrogen, which would just drift off into space. An example of this would be Mars, which has only 10% the mass of Earth. While Mars is in the habitable zone of our sun and has evidence of flowing water on its surface in the past, it probably lost its atmosphere then liquid water due to its low mass. It now has only 1% the air pressure of Earth. It was recently calculated that planets with as little as 2.7% the mass of Earth could theoretically trap enough atmosphere to support liquid water, but only if they were much closer to their star. This lower limit is a planet about half the mass of Mercury or twice that of the moon. Of course, it is not always that simple. Saturn’s largest moon, Titan, is only ¼ the mass of Mars, yet has an atmospheric pressure greater than that of Earth. Jupiter’s even more massive moon Ganymede, however, has almost no atmospheric pressure. And Venus, considered Earth’s twin in size and mass, has almost 100 times more air pressure — about the same pressure as being a kilometre below the ocean. So it is complicated. But for a planet to be habitable, it is probably fair to say that it requires some atmosphere, but not too much. Conditions need to be just right.

Air pressure

Atmospheric pressure also affects the range of temperatures under which a liquid will remain a liquid. Under lower air pressure, water will boil at a lower temperature and freeze at a higher temperature. This is what occurs at the top of a mountain, where the air pressure is less. If you keep decreasing air pressure until it is less than 1% of that on Earth, you will reach a point where H2O can exist as ice, water and stream at the same temperature. This is called the Triple Point of water. If the pressure drops any lower, such as on planets with less atmosphere than Mars, solid ice will skip liquid form and go straight to gas. At such low pressures, there is no liquid water zone to speak of, and life seems unlikely to flourish. Under higher pressure, however, water will remain in liquid form over a greater range of temperatures. While the freezing point will not change much, the boiling point of water will continue to increase. At normal air pressure, water will boil at 100°C. At double pressure, water will boil at 120°C; at 10 times pressure at 180°C and at a hundred times pressure at 310°C. So larger planets with dense atmospheres are likely to have an expanded liquid water zone, although if the pressure is too high, they are unlikely to be hospitable to life at all.

Magnetic field

Another important factor for habitability is the presence of a strong magnetic field. Even if a planet is orbiting a sun-like star at a relatively safe Earth-like distance in the liquid water zone, there will still be waves of harmful radiation that could wipe out any life that may have emerged. Magnetic fields deflect some of this radiation and may be necessary for life as we know it. Our magnetic field is produced by the electrically charged core of metal spinning around within our planet like an electrical generator. This is dependent on the speed of its spin and the planet being big enough to keep its inside melted and flowing long enough for life to emerge and evolve. Gas giants without a solid rocky core may also have magnetic fields produced by the circular flow of hydrogen compressed to such pressure and temperature that it becomes like an electrically charged metallic liquid. Jupiter has the strongest magnetic field in our solar system, due to its abundance of metallic hydrogen liquid and fast spin. Saturn, Uranus and Neptune also have stronger magnetic fields than Earth, but ours is the strongest of the terrestrial planets that we know of. Strong magnetic fields have already been detected in some Hot Jupiter exoplanets, many of which are stronger than Jupiter itself. However, detecting magnetic fields of Earth-sized exoplanets is still in its early days. This is another factor to consider for the habitability of a planet.

Habitable planets 

In the heyday of the Kepler mission, estimates of how many habitable exoplanets may exist were optimistic. In 2011, NASA predicted that 1.4% - 2.7% of all sun-like stars would host Earth-sized planets within the habitable zone. Since then, estimates have ranged from under 1% to over 100%, and from 1% to 20% in the last year alone. Since they all used the same Kepler data, this says more about their assumptions on habitability than anything else. How wide is the liquid water zone for a given star? How big can a planet be before habitability seems unlikely? Thus far, we do not know. Even assuming 1% of sun-like stars are orbited by Earth-sized planets in the liquid water zone, there could be billions of such planets in our galaxy alone, and trillions upon trillions in the observable universe. But the actual discovery of such habitable planets has not been so fruitful. To date, almost 200 exoplanets are known to spend most of their orbits in the liquid water zone. But most of these are gas giants like Jupiter that are unlikely to support life. Only about 20 are expected to be Earth-sized terrestrial planets. If we accept that planets with a mass up to 10 times that of Earth might still harbour life, then the number goes to about 50. But more accurate distance measurements by the GAIA space telescope suggest that many of these planets are not really in the liquid water zone. The actual number of Earth-sized planets discovered orbiting in the liquid water zone may be less than 10, to say nothing of their atmospheric pressure and magnetic field strength. So while our discovery of habitable planets is lagging behind our discovery of exoplanets, progress is set to accelerate over the coming decades. A recent study suggests that 24 known exoplanets may be more habitable than Earth.

Habitabitating beyond the habitable zone

We have been talking about the possibilities for life to emerge. Before we move onto the probability that life will actually begin on habitable planets, I want to briefly mention other opportunities where life could arise. Not all planets orbiting in the habitable zone are habitable, and not all places outside are inhabitable. Solar radiation is not the only potential energy source for life.

Geothermal energy

Firstly, even rogue planets not orbiting a star at all may produce liquid water by the heat energy within their core. For example, if Earth flew off into interstellar space, the ocean would freeze over, but liquid water would remain around geothermal vents and underwater volcanoes. This is a particularly interesting scenario since life on Earth most likely began around deep-sea vents. So even planets orbiting outside the range where liquid water may exist on the surface may still have liquid water and even life deep below. But this is only for rocky planets with molten cores. 

Gravitational energy

Secondly, ice may be transformed into liquid water by gravitational energy and friction. Take for example the moons orbiting Jupiter. Io is slightly larger than our moon and makes an oval orbit very close to the biggest planet in our solar system. The strong gravitational force changes at different parts of the orbit, causing the moon to bulge and shift, creating friction and heat in what is called Tidal Flexing. This results in Io being the most volcanically active body in our solar system, with volcanoes that explode hundreds of kilometres into the air. As a result, Io has the least water of any body in our solar system, and would perhaps not make a great home. Further out, Europa is also subjected to intense tidal flexing on its oval orbit, but not quite so intense. Europa is slightly smaller than our moon and, like Earth, is mostly made of rock, has an iron core and is covered with large amounts of H2O. It even has oxygen in its atmosphere. It appears that the entire surface is covered in frozen water. But evidence of floating plates of ice and geysers of water spraying high into the air suggests that tidal flexing has melted much of the water below the surface. Further out still, Ganymede is the largest moon in our solar system, even bigger than the planet Mercury. It has an iron core, a rocky crust, an atmosphere with some oxygen and perhaps more H2O than planet Earth. It is also the only moon with a confirmed magnetic field, probably caused by the flowing or spinning of the iron core. Under more than a hundred kilometres of solid ice, Ganymede is thought to contain an ocean of salty water sandwiched between layers of ice. The Juno spacecraft has been orbiting Jupiter since 2016. A Gravity Assist slingshot back around Earth increased its speed by 4 kilometres per second, but the total travel time was still 5 years to arrive at Jupiter. Juno is scheduled to burn up in Jupiter’s atmosphere in 2021. The new and improved JUpiter ICy moons Explorer (JUICE) is set for launch in 2022, and the Europa Clipper will be launched by 2025. These explorers will provide us with much more information about these moons and their potential to harbour life.

In 1997, the Cassini space explorer was launched to Saturn, and arrived in 2004. It watched the planet and its moons for over a decade, and made many fascinating discoveries. Several of the moons have solid cores and large amounts of H2O, mostly in the form of solid ice. But in 2005 Cassini took photos of huge geysers spraying water and other elements out of the moon Enceladus. This suggests that tidal flexing from Saturn and its other moons has melted an ocean of liquid water below the solid ice cap. Later evidence seems to support this theory, indicating a subsurface ocean 10 kilometres thick only around the south pole. Over 90% of the atmosphere is also water vapour. Enceladus is particularly interesting in the search for extraterrestrial life because of its abundance and variety of organic compounds. While it would be difficult to drill down to the liquid water, the geysers bring it up for us. Over a hundred geysers have been identified, and in 2008 Cassini swooped down to within 50 kilometres of the moon to fly through one such geyser. This was an incredibly difficult and risky maneuver! The samples indicate that the water contains salt crystals, simple organic compounds, some larger organics molecules and even a few very large and complex building blocks for life. Evidence from 2018 further supports these findings. So the underwater vents that power these geysers create complex organic compounds, and may perhaps power the creation of life. Incidentally, these geysers also create one of Saturn’s rings, since they are so powerful that they shoot out into space. Most of the water, however, falls back down as snow.

Saturn’s largest moon, Titan, may have even more potential to harbour life as we know it. Catching a ride with the Cassini spacecraft, the Huygens probe detached from its mothership and parachuted down to the surface of Titan in 2005, taking photos the entire way. This was the furthest landing from Earth, and the only landing on a moon other than our own. Titan is bigger than Mercury and has more in common with planets than moons. It has an atmosphere rich in nitrogen with clouds, rain, weather and even seasons. It has plate tectonics and mountain ranges over 3 kilometres high, many named after mountains from Lord of the Rings. It also has rivers, canyons, lakes, deltas, sand dunes and even small seas. That’s right, it has liquid on its surface! But there is one important difference: the liquid is made of methane and ethane, not water. The surface of Titan receives only 1% as much sunlight as Earth, and its temperature is far below the freezing point of water. But the average temperature just so happens to be in the small range that methane is in liquid form. This range is increased slightly due to the higher air pressure on Titan compared to Earth. So on Titan, the surface is made of ‘rocks’ of deep frozen ice and rivers of liquid methane. Ice volcanoes, called Cryovolcanoes, spit out melted ice, not melted rock. The biggest river is almost as long as the Nile, and the biggest sea (called Kraken Mare) is bigger than the biggest lake on Earth (called the Caspian Sea). That is almost twice as big as the Great Lakes of North America combined. Like many of these other moons, Titan is expected to have an ocean of liquid water below its thick ice cap, since continental drift is occurring at such a fast pace. Furthermore, Titan contains organic molecules and even some oxygen. Just over 2 years ago, after nearly 20 years in operation, the Cassini spacecraft executed its Grand Finale Kamikaze mission. While sending back valuable data as long as possible, it weaved between the rings of Saturn 22 times before entering the atmosphere and burning up like a meteor. The next mission is set for launch in 2026 and arrival at Saturn in 2034. The Dragonfly is a large drone with 8 rotors that will explore Titan for prebiotic chemistry and other signs of life. Its electric motors will be powered by the nuclear fission of elements created in supernovas, and will recharge its battery during the night for longer flights. The drone will be largely autonomous, since it will take over 2 hours for the roundtrip journey of a signal travelling at the speed of light.

Despite being far from the liquid water zone of the sun’s energy, these moons still contain liquid water due to gravitational and geothermal energy. These moons are a good candidate for life, and are much closer to explore than even the closest exoplanet, over 4 light-years away. Within our own solar system, there are about 200 moons. No exomoons have yet been discovered, but they almost certainly exist in abundance. This multiplies the opportunities for life to emerge dramatically. While there is still an astronomically small chance that any one exoplanet or exomoon will have the conditions necessary to support life, there is an astronomical number of them in the universe. The very small percentage of them that are habitable, multiplied by the very large number of them, could still result in many habitable planets and moons in our galaxy alone, let alone the entire universe.

Probability of life

Even if we have a planet or moon with all the conditions necessary to support life, how likely is it that life will actually emerge out of the chemical soup? What is the probability of life spontaneously arising? While it is bordering on absurdity to calculate the probability that something will happen that may have only happened once, Astrobiologists have an equation for it. The probability of an Origin-of-Life event occurring on a given planet or moon considers the following:

1. Number of available building blocks for life

2. Number of building blocks needed to create a living system or organism

3. Availability of those building blocks during a given time

4. Probability that these building blocks will actually assemble into a life form

While we have almost no idea about the probability that these building blocks will form into life forms, except the remarkable quickness that it occurred here on Earth, we have some idea about the building blocks that go into making them.

Building blocks

The matter within every known living thing on Earth, and therefore the universe, consists of mainly 4 elements: hydrogen, carbon, oxygen and nitrogen. All the other elements combined account for less than 1% of the atoms. These are also the top 4 most abundant yet reactive elements in the universe. Hydrogen is by far the most abundant, accounting for ¾ of all matter in the universe and ⅔ of the atoms in the human body. Helium accounts for all but 1% of the remainder, but is unreactive and therefore unsuited to forming life. Carbon is formed in the carbon core of stars, and would be the next most abundant element, except when it is fused with a hydrogen or helium atom it becomes oxygen or nitrogen respectively. After this transformation, oxygen becomes the 3rd most abundant element in the universe, and makes up ¼ of the atoms in the human body. By weight rather than number of atoms, the human body is 65% oxygen, 18% carbon and 10% hydrogen. Over half of this weight is water, which is the two most abundant yet reactive atoms in the universe bonding together. Methane, ethane and all other potentially liquid substances are also made from these same elements. DNA is made from these 4 most abundant building blocks, plus phosphorus, which is the next most abundant atom in living cells. So the building blocks for life appear to be quite abundant in the universe. They are also abundant here on Earth. These same 4 elements make up 99% of the present atmosphere, almost half of the ground beneath our feet and all of the water on our planet.

Panspermia

Furthermore, once life emerges, it can spread to enliven more of these building blocks. Perhaps it can even spread between planets or moons. If so, maybe this is how life arrived on Earth. Life need not begin independently every time. This theory is known as Panspermia. Rocks are definitely exchanged between neighbouring planets and moons. Indeed, most of the small meteors arriving to Earth are rocks dislodged from the moon by larger meteor impacts. Through the same process, we exchange rocks with Mars and Venus. But I’m not sure if this would be any more likely than life beginning from scratch. How likely is it that even the hardiest microbes could survive the impact that blasted it out into space and the re-entry through another planet’s atmosphere hitching a ride on a meteor? Furthermore, the chance of exchanging rocks with a neighbouring planet decreases exponentially the further away it is. For example, in the closely packed planets orbiting Trappist-1, it would be thousands of times more likely than between Earth and Mars. The probability that a microbe hitched a ride across the void of interstellar space, and happened to land in a habitable place to tell the tale, seems so unlikely that it might be easier for life to just start from scratch. Nevertheless, if we ever find signs of life, we should definitely look at nearby planets and moons to see if life was able to jump ship.

Fermi paradox

If life is made from such abundant building blocks, and there are so many possibilities for it to do so in the universe, then where are all the aliens? This has come to be known as the Fermi Paradox. Nobel prize winning Dr Fermi created the first element beyond Uranium, worked on the first nuclear bomb and the first nuclear reactor. While on lunch break, Fermi was discussing recent UFO reports and the possibility of faster-than-light travel with some of his colleagues. After considering the number of planets and the immense age of the universe, he suddenly exclaimed: Then where is everybody? There are basically 3 possible explanations:

1. They do exist, but it has been covered up

2. They do exist, but they have not contacted us; or

3. They do not exist

While alien cover-ups are popular in science fiction, in practice, it would be difficult to maintain for long. One leak would change everything permanently. Furthermore, if the government of the USA already knew about aliens, why would it spend billions of dollars to scratch around in the rocks on Mars? Having said this, I think it is important to keep an open mind. 

In my opinion, it is more likely that extraterrestrial life exists but has yet to contact us. The Great Filter is the suggestion that there is some kind of extremely difficult step or steps in the evolution of life that prevent it from developing interstellar communication or travel. There are a few main candidates. Firstly, the evolution of tiny cells merging into larger ones with a nucleus of DNA took 1-2 billion years, whereas the emergence of life out of the primordial ocean took only a few hundred million years. Maybe simple life emerges often, but rarely makes it past this threshold. Second, it took about 2 billion years for these large complex cells to evolve into multicellular organisms. Maybe complex life emerges often, but rarely becomes multicellular. Third, life above the surface of the water only became possible under the protection of an ozone layer and magnetic shield. Maybe multicellular life emerges often, but rarely colonizes the land. Fourth, life on Earth was almost wiped out by at least 5 mass extinctions caused by changes in habitable conditions. Many of these were triggered by asteroid impacts. Places need to be continuously habitable for advanced life to evolve. Maybe complex ecosystems emerge often, but rarely survive long enough for intelligent life to arise. Fifth, humans invented agriculture and industrial machines, providing us with the surplus necessary to develop interstellar communication and travel. Maybe intelligent life emerges often, but rarely achieves such a surplus of resources. 

Finally, perhaps the greatest filter is the duration that an advanced civilization will survive and the distances between such civilizations. How long before human civilization will be annihilated by celestial bombardment, environmental change or our own folly? We have developed nuclear weapons capable of extinguishing all life on Earth long before the capability for interstellar travel. Even if we survive for many million more years, it is still only a tiny fraction compared to the age of the universe. What chance is there for another origin-of-life event to surpass all of these filters, overlap with our short existence and be within a distance that we can interact? Due to exponentially expanding space, only 3% of galaxies in the observable universe are reachable at the speed of light. Even if some super-advanced civilization developed warp drive capabilities, how would they find us? We are out at the edge of the Laniakea supercluster, amongst a hundred thousand galaxies, near the end of an offshoot from one of the spiral arms of our galaxy, on the 3rd planet out from a yellow dwarf star. Since only a few decades ago, radio waves have been leaking out into space and could theoretically be detectable. But it would take an incredible instrument to detect such weak leaking signals amongst the cacophony of radiation waves emitted in our galaxy alone. In 1974, humans broadcasted the first intentional interstellar radio message to our potential neighbours. They focused the beam on a nearby globular star cluster, and hoped that it would still be there when the message arrives 25 thousand years later. The so-called Arecibo message, partially written by the creator of the Drake equation, contained information about life on Earth. This information included the structure and composition of DNA and the location of our planet in our solar system. Even if the message is received, it will take 50 thousand years to hear a reply. Let’s hope humanity survives that long and we gain an interstellar pen pal. And that they survive long enough to receive our next message, so we can catch up on old times.

Copernicus principle

The 3rd possible solution to the Fermi paradox is that we are indeed alone in the universe. But I consider this highly unlikely based on one simple fact: we are not the centre of the universe. We are not even the centre of our solar system. This is known as the Copernicus principle. Contrary to popular opinion, Copernicus put forward a theory in 1543 that the Earth orbited around the sun, not the other way around. This was not proven until 50 years later and was not widely accepted for over a century. But now it is self-evident. Even if we never find proof of extraterrestrial life, I cannot accept that we are the most complex beings in existence. We may be unique in the universe, but we are not special. While some people find this disheartening, I find it an utter relief.