Our Solar System Part 1: Earth

An Exploration of How Earth Was Formed and How Life First Took Hold

A Violent Beginning

Looking at this photo of Earth taken during the Apollo 17 lunar mission, you might get the impression that Earth has always been this beautiful blue life-filled planet.

Far from it. Our planet was born of fire and violence, and life, once it took hold, hung on tenaciously in one form or another over several mass extinctions. Many events came together in just the right order to allow us to exist here today.

4.6 billion years ago, Earth and the entire solar system itself, was a cloud of dust and gas composed of hydrogen and helium as well as heavier elements spewed away from supernovas, near the edge of a galaxy called the Milky Way. A nearby supernova may have recently exploded, creating a shock wave that created localized pockets of denser matter, dense enough to trigger this cloud to collapse. It began to rotate, thanks to the angular momentum of its particle atoms and gravity, and inertia slowly flattened it into a disk, called a protoplanetary disk. It might have looked something like this artist's conception.

Material began to concentrate in the center and heat up. Its rotational speed increased much like how a skater rotates faster as she draws her arms in. The force of gravity and angular momentum conspired to increase the overall energy of this central mass and, with no way for the energy to escape into the vacuum of space surrounding it, eventually its temperature and pressure became so great that hydrogen atoms began to fuse, igniting a nuclear fusion reaction. At this point the central mass called a protostar, ignited into a star, our Sun. Internal energy, directed outward, soon balanced the inward force of gravity to create a stable state called hydrostatic equilibrium. Radiation from the blast blew much of the dust and gas outward but not all. Bits of small dust-like grains composed of silicon, magnesium, aluminum and oxygen as well as other trace elements collided with each other and accreted. These chunks are preserved in chondrite meteorites. These bits formed larger and larger chunks of matter as they collided with and captured each other within their orbital neighbourhoods, eventually forming planets, moons and asteroids. The thick dust around the newly formed Sun would have made it appear red and opaque as chunks of planetesimals were colliding with each other to form Earth. The accretion process is illustrated here in this artist's rendering of Earth forming from a disk of accreting chunks of rock.

When Earth was forming from an accreting mass of orbiting matter, it regularly melted from the energetic impacts and this process caused the matter inside Earth to differentiate according to mass. 

A core evolved from the densest elements, mostly iron (89%) but other heavy elements as well, including radioactive elements. A mantle composed of a mixture of oxygen (45%), silicon (22%) and magnesium (23%) as well as many other elements, formed around the core. Finally, a crust composed of lighter elements and compounds, most of which is silicon dioxide (61%) and aluminum oxide (16%), would eventually form as the planet cooled.  This process is called planetary differentiation.

The Moon is Born From Catastrophe

Earth had just formed. It was about 100 million years old and it had just differentiated into core and mantle but no crust existed yet because it was still completely molten, when it collided with a Mars-size planet called Theia, shown here in this artist's depiction. Theia is the name of the mythical Greek titan who gave birth to the moon goddess, Selene.

Computer simulations suggest that Theia's iron core sank into Earth's core on impact while Theia's mantle and a great deal of Earth's mantle were ejected into orbit around Earth. It probably took less than a thousand years for this material to coalesce into the Moon. Earth gained significant mass and angular momentum from this collision. After impact it was rotating so fast a day was only 5 hours long. And the new Moon loomed very large on the horizon. It was about ten times closer, about 30,000 km away. This is what Earth might have looked like then.

The Moon's orbit is expanding at a rate of 3.8 centimetres per year and as it does so, Earth's rate of rotation slows, gaining 2 milliseconds every one hundred years. The reason for this is that the Moon's gravitational attraction literally squishes Earth into an egg shape (actually the distortion is only on the scale of a few meters) and there is a lag time for Earth to reform a sphere shape so the Earth's and Moon's bumps don't perfectly face each other. This creates a torque on the spin of the Earth and a similar back reaction on the Moon, which slows down Earth's rotation and injects energy into the Moon's orbit, pushing it away. All the heat lost from the rhythmic deformation of Earth also translates into lost rotational energy. The moon used to rotate faster but tidal forces slowed its rotation until it became tidally locked with Earth, happening just 50 million years after its formation. That is why we see only one face of it. The moon will continue to move away from the Earth until it is about 1.6 times the distance it is now but its rate of orbit expansion has slowed so much it will take 15 billion years to do so. By then, the Earth and the moon will have long been engulfed by the Sun when it expands into a red giant and the Sun itself will be nothing but a brown dwarf dying star remnant. 

Add Ingredient For Life #1: A Magnetosphere

A part of the differentiation process, which was well under way when Theia collided with Earth, is what is called the iron catastrophe, in which the spinning molten metal outer core created Earth's magnetosphere, shown here.

Earth's magnetosphere formed when the Earth was about 500 million years old, not long after the Theia impact. Contrary to the name "catastrophe," the creation of the magnetosphere was essential for life to take hold on Earth. Without it, solar radiation would have stripped away the solar radiation-deflecting atmosphere and any surface water would have dispersed into space before life could evolve.  

Add Ingredient for Life #2: An Atmosphere

Planets as large as Earth had enough gravitational pull to hang onto gases that came from outgassing within the mantle and from comet impacts. These captured gases created Earth's first atmosphere, which replaced a thin haze of hydrogen and helium, leftover from Earth's formation, with carbon dioxide, nitrogen and water vapour. Water vapour increased in abundance with each comet impact (and there is new evidence that asteroids may have contributed much of Earth's water in addition to the contributions from icy comets. Earth was heavily bombarded with comets and asteroids during this violent and chaotic phase of the solar system's evolution.

Hydrogen and helium gases, being very light, dissipated into space as they formed and were replaced over time with the heavier gases from mantle outgassing. As the mantle began to settle, volcanic outgassing contributed to the atmosphere tremendous amounts of carbon dioxide (CO₂), as well as hydrogen sulfide, sulfur dioxide, methane and some ammonia, and additional water vapour released from mineral hydrates within the mantle.

As the Earth began to cool, the surface solidified into a crust and vast amounts of accumulated water vapour in the primordial atmosphere condensed and rained down to form the first ocean. Surface temperatures then were about 230°C. Liquid water existed only because the CO₂-heavy atmosphere was extremely dense, creating enough pressure to prevent water from vapourizing.  Frequent super-cyclonic storms raged across the surface, fueled by the Earth's rapid rotation and energetic atmosphere. The Moon, much closer then, may have created tides of superheated water up to a thousand times higher than those today, perhaps 100 km high, crashing over land every few hours! Frequent cosmic collisions continued up to about 3.8 billion years ago. Their impacts regularly re-vapourized part or even the entire ocean, creating high altitude clouds that completely enveloped the planet. As the bombardment slowed, clouds dissipated as water vapour rained out of the atmosphere into the ocean.

Earth As a Giant Organic Chemistry Lab

Volcanic gases such as carbon dioxide, sulfur dioxide and hydrogen chloride readily dissolved in the ocean into acids that would have been neutralized by various minerals. Eventually the ocean became a reducing soup of organic compounds in an environment energized by intense UV radiation. Even though the young Sun was fainter then, most UV radiation reached the surface because the ozone layer had not built up yet. Volcanic activity and intense radioactivity within the young core also contributed energy to the environment. Intense lightning from volcanic eruptions may have played an important part in producing HCN (hydrogen cyanide) from CH₃ (methyl groups) and NH₃ (ammonia) or N₂ (nitrogen gas). HCN is essential for the synthesis of amino acids and nucleobases, parts of the building blocks of proteins and DNA, respectively. Small chemically reactive intermediate molecules such as formaldehydes, ethylene, cyanoacetate and acetylene, which can recombine into more complex intermediates that can in turn form stable biochemicals, would have formed under these conditions, but we don't know how concentrated they might have been in the primordial ocean because it is very difficult to pinpoint what the temperature and pH of that environment was. However, we do know that amino acids did in fact form because life made its first remarkable appearance about 3.8 billion years ago. Experiments such as the Miller-Urey experiment, which involve simulating the conditions of this primordial environment, have successfully created over 20 different amino acids.

Oxygen and Nitrogen

The earliest atmosphere contained very little oxygen gas. What little oxygen was present came from the dissociation of water vapour in the upper atmosphere by ultraviolet radiation. Much of this oxygen would have been photochemically converted into what was then a very thin ozone layer, further reducing the abundance of it in the atmosphere.

Researchers have puzzled over the abundance of nitrogen in our atmosphere. It was a predominant material during Earth's formation, probably in solid form, much of which may have been released as gas from the mantle through volcanic eruptions. Nitrogen is an inert gas that is quite stable under solar radiation so it may have built up slowly and remained in the atmosphere so that now it comprises about 78% of the atmospheric content today.

So far I have described the Hadean (a name aptly derived from Hades or Hell) era Earth, a period lasting from Earth's formation to about 3.8 billion years ago when life was about to appear for the first time.

A few rocks surviving from this very early period have been discovered in Greenland, Canada and Australia. The oldest dated rocks, zircons, have been dated at 4.4 billion years old. Life evolved when there was no oxygen and no protection from UV radiation, when volcanoes ravaged the world and the ocean was a roiling soup. The sky was an ominous reddish colour and the ocean was dead grey. It is possible that life arose more than once under these conditions as the primordial ocean repeatedly vapourized and condensed during repeated asteroid impacts. Life may have evolved in shallow clay-rich waters and/or near hydrothermal vents that spew out concentrated ammonia and methane, two molecules that could serve as building blocks for more complex organic compounds. Some theorists also propose that life may have gotten its start when primitive RNA-like sugar-rich molecules hitched rides on the backs of meteorites. It is difficult to model how these sugar molecules, which form the backbone of RNA and are called ribose, could have been created from Earth's primordial ingredients.

A Molecule That Can Copy Itself

In the chaotic and energetic environment, a molecule somehow gained the ability to replicate itself. An early form of abiotic (meaning nonliving or pre-life) evolution has been postulated in which different potential methods of replication were attempted and either improved upon or eliminated based on the natural tendency of any system to move toward the lowest possible energy state. It is possible that within the ocean's organic soup, the energy from lightning and UV (ultraviolet) radiation drove reactions creating more and more complex molecules from simple compounds such as methane and ammonia. Among these molecules were amino acids and nucleobases, the building blocks of life. Reactions occurred randomly, and by chance a replicator molecule was formed, perhaps something much simpler than but superseded by DNA, perhaps with RNA as an intermediate. DNA is now life's universal replicator molecule, except for some viruses and prions.

A Cell Membrane

Some kind of protective envelope to house the replicator molecule would have been needed and this would likely have come from a primitive phospholipid bilayer sphere, which can form spontaneously when phospholipid molecules are placed in water. Eventually a self-contained organism, a single cell prokaryote, evolved, which used DNA as its genetic code, RNA for information transfer and protein synthesis and enzymes to catalyze reactions. There may have been many kinds of protocells and this one line out-survived the others and evolved. Many of these terms may be new to you. Exploringorigins.org provides an excellent and easy to understand tutorial on this entire process, explaining the importance of RNA, DNA and proteins and how they may have come together to create the first living cell under the extreme conditions of early Earth. It comes with many short animations to illustrate the processes involved.

An Energy Source For Life

The first cells to evolve likely used surrounding organic molecules for energy, much like many extremophiles do today. An example of an extremphile, are primitive bacteria-like unicellular organisms called endoliths that survive by feeding on traces of iron, potassium or sulphur. They can survive long ice ages by simply slowing down their cellular processes.

A Much Better Energy Source For Life: Sunlight

At some point, about 3 billion years ago, cells evolved a new strategy for capturing and using the energy in sunlight, and this evolution drastically changed the atmosphere and climate of Earth. Highly successful cyanobacteria (bacteria that obtain their energy from sunlight) utilized a new cellular process called photosynthesis, using CO₂ and water as raw materials to create energy-rich sugars, with oxygen as a byproduct. These bacteria evolved in shallow water, trapping sedimentary grains in their bacterial biofilms to create large structures called stromatolites. Stromatolites were the first life to colonize Earth. A few very rare colonies still exist in hypersaline lakes where animals can't graze on them. The oxygen gas released from enormous colonies of stromatolites was captured by organic matter and by dissolved iron in the ocean, but as these minerals became saturated with oxygen, it began to accumulate in the atmosphere. This process is sometimes called the Oxygen Catastrophe because it probably caused the greatest extinction event of all time. Oxygen would have been toxic to most other kinds of bacteria because it destroys organic compounds. As free oxygen combined with atmospheric methane, a very potent greenhouse gas, and as atmospheric carbon dioxide diminished through photosynthesis, eventually a glaciation event was triggered that was so severe that Earth was entirely enveloped in ice, the first of several "Snowball Earth" episodes to come. This ice age, called the Huronian glaciation, began about 2.3 billion years ago and lasted between 300 and 400 million years. The Oxygen Catastrophe also marked the beginning to a new opportunity for life to evolve because until now, life was energetically limited to fermentation reactions. Now cells could take advantage of the much more effective metabolic process of respiration. The geology of Earth changed dramatically as minerals became oxidized. Mitochondria, organelles acting like little cellular power plants, evolved soon afterward, which could turn sunlight into highly concentrated energy storage molecules called ATP. A new kind of cell called a eukaryote, housing all of these enhancements, made its first appearance about 2 billion years ago. Because it had more energy at its disposal, it could grow larger and even more complex. Meanwhile, some of the oxygen was converted into ozone through ultraviolet radiation, creating a UV-protective layer in the upper atmosphere which allowed cells to colonize the surface of the ocean, and later, on land as well. Before this, the DNA in cells would have been vulnerable to high rates of lethal mutations caused by the radiation. At this time, volcanic islands began to coalesce into one large supercontinent called Nuna or Columbia, which existed around 2 billion years ago and began to fragment 1.6 billion years ago, starting a pattern of repeated supercontinent assembly and fragmentation driven by plate tectonics that continues to occur today, ending with the latest supercontinent, Pangaea. This brief but interesting National Geographic video  illustrates Earth's early formation.

Life Hangs In There Through Extremes

The three types of unicellular organisms, archaea (a group that contains, for example, extremophiles and methanogens which live in our gut), bacteria, and eukaryotes (at that time confined to living in the water) continued to diversify, possibly during the extreme cold of the ice age as well as after conditions warmed. By about 1 billion years ago, plant, animal and fungi lines had all split and multicellular life was beginning to evolve as cells began to accumulate in colonies and a division of labour began to take place. The first simple multicellular organisms such as green algae and sponges began to evolve.

Multicellular Life Evolves Twice!

Another supercontinent called Rodinia formed about 1.1 billion years ago, and like Nuna, it was entirely barren because no terrestrial life yet existed. This supercontinent was centered on the equator and some theorists believe that because of its location, the rate of chemical weathering of its rock increased, sequestering CO₂ and removing its greenhouse gas function from the atmosphere. Two more Snowball Earth episodes followed, around 710 and 640 million years ago. Permafrost decreased chemical weathering and as atmospheric CO₂ levels gradually built back up through volcanic activity, each ice age ended. This ushered in a period of intense evolution of multicellular life forms called Ediacara biota. Plants and animals with tissues performing specific functions evolved. Fossil evidence suggests that these early organisms were completely replaced by those of the later Cambrian explosion, because most current body plans of animals appear only in the Cambrian fossil record and not in the older Ediacaran period. You might be wondering how the fossils of these soft-bodied animals could exist today at all. Life on Earth is made up of organic compounds. These molecules are based on carbon to which atoms of oxygen, hydrogen, nitrogen and other elements are attached. When some of these animals died, their soft bodies were quickly buried in sand, preserving them very well. Hydrogen, nitrogen and oxygen are volatile elements and they were driven off of the fossils over time. What remained is carbon and so the overall shape of each organism was preserved in ancient rocks as a carbon layer, an imprint. Canada has an excellent collection of these fossils. I urge you to click on this link and browse the online exhibit.  This website gives you an idea of how these animals might have lived and what they might have looked like. At around 540 million years ago these once quite plentiful older fossils, representing impossible to classify Ediacaran organisms with discs, tubes, mud-filled bags and quilted mattress-like bodies, vanished. No one is sure why but perhaps these organisms represent a failed first attempt at multicellularity. By about 510 million years ago, the Cambrian explosion was underway. Most of the major phyla of organisms made their appearances as the rate of evolution accelerated and the diversity of life exploded and organisms eventually exploited every available ecological niche. This 32-minute online lecture explores Cambrian life in detail. It is still unclear what triggered the Ediacaran die-out and the Cambrian explosion, but perhaps fluctuating atmospheric oxygen levels contributed to them.

Land Plants Appear

About 510 million years ago, at the start of Cambrian explosion, land plants made their first appearance, evolving from branched filamentous algae that lived in shallow waters. This 5-minute video

gives you an idea of just how essential plants are to all life on Earth. There is some evidence that simple photosynthetic algae lived in fresh water depressions and lakes perhaps as early as 1 billion years ago but they did not colonize in high enough numbers to impact atmospheric gases. Several more mass extinctions, triggered by climate changes or catastrophic events or a combination of these, would threaten life but not obliterate it in the eons to come.

Earth Teaches Us How To Look for Life On Other Planets

The atmospheric effects of life on Earth have helped astronomers to define their search for possible markers of life on extrasolar planets. The presence of liquid water and gases such as methane, carbon dioxide and especially oxygen mark the possibility that life which uses biochemical pathways similar to those used on Earth could be supported. An increasing understanding of how life evolved on Earth may help astronomers refine their search toward eventually finding even more specific biomarkers of life.

Next up: Part 2: Mars

Our Solar System Part 2: Mars

Mars, the fourth planet from the Sun, is named after the Roman god of war and agricultural guardian. This is a mosaic image of the planet taken by Viking 1 in 1980. It was first seen close up by Mariner 4 as it flew by the planet in 1965. Until then, many researchers believed the light and dark patches and long channels visible in this image were signs of liquid water in the form of seas and rivers, possibly even irrigation channels, suggesting that intelligent life lives there. We now know that Mars is far less hospitable than we once thought and yet, after decades of intensive study, Mars still intrigues us with its many mysteries. 

Water

Water does, in fact, exist on Mars. The lines and depressions you see in the image above are optical illusions. But, there is a large quantity of water ice at the poles, as revealed by radar data from two current missions, Mars Express and Mars Reconnaissance Orbiter. In fact, there is so much water ice in the southern polar cap that, if melted, it would cover the whole planet in water 11 meters deep. However, liquid water can not exist for long on the surface of Mars because any liquid water would quickly freeze and sublimate into the atmosphere. As well, the surface temperature is, on average, too cold for water in its liquid state and the atmospheric pressure is far too low (it's about equal to the air pressure on Earth at 35 km above sea level, that’s about 3 times higher up than a typical jet's cruise altitude) to support liquid water. At this extremely low pressure it just sublimates and disperses into space, although a very tiny amount can and does exist as water vapour in Mars' thin atmosphere.

These are two colour images taken by NASA's Phoenix Mars Lander in 2008. The white patches reveal water ice just beneath the planet’s surface regolith.

The right image was taken 4 days after the left one. If you look closely you can see that some ice has sublimated away during this time.

In the following NASA image you can see the northern polar cap of Mars very clearly.

This ice is not to be confused with the water ice I just mentioned. This is dry ice, or frozen carbon dioxide, CO₂. 

On Mars, during a pole's winter, the pole exists in continuous darkness. The surface becomes very cold, averaging around -87°C (compare this to the absolute coldest Earth temperature on record at -89°C) at Volstok Station in Antarctica in 1983). At this temperature, about a third of Mars' atmosphere condenses out (freezes) into solid dry ice. When Martian spring arrives, the dry ice will sublimate back into the atmosphere as it is exposed to sunlight and warms up. This seasonal cycle drives powerful winds, as fast as 400 km/h, sweeping off the poles and gives rise to high cirrus clouds, shown below as bluish wisps in this beautiful photograph of the Martian sky just before sunrise, taken by the Imager for Mars Pathfinder in 1996.

The thin atmosphere on Mars consists of almost all carbon dioxide, 95%, with 3% nitrogen and traces of argon, oxygen, water vapour and other gases. The atmosphere extends almost twice as high into the Martian sky as it does on Earth. This is because Mars has much lower gravity to keep the atmosphere contained, about 38% that of Earth.

Methane Mystery

Interestingly, Mars also has a tiny bit of methane in its atmosphere. Although methane is a bit chemically unstable, each methane molecule on Mars should last for hundreds of years, maintaining a fairly stable atmospheric level. However, observations show that its levels fluctuate yearly, and they coincide with the seasonally changing presence of atmospheric water vapour. It is estimated that Mars produces about 270 tons of methane per year and there is currently some heated speculation about where it is coming from. Asteroid impacts should contribute less than 0.8% of this amount per year. There is also the nagging question of what is destroying the methane because it also seems to have a strangely high turnover rate in the atmosphere. Methane can come from volcanic activity, comet impacts, microbial activity or mineral processes such as serpentinization. In this case, a mineral called olivine, which is abundant on Mars, reacts with water and carbon dioxide to create serpentine, magnetite (these are two minerals) and methane gas. There are other possible geological sources of methane on Mars as well.

None of these possibilities has yet been ruled out. Until recently, it was widely believed that Mars has not been volcanically active for billions of years. Yet new atmospheric data suggests that Mars might have experienced at least one volcanic eruption, with resultant water flowing on its surface, within the last 100 million years. For some researchers, in particular the European Space Agency, the coincidence of methane levels and water vapour hints at a biological methane source, especially since these two gases seem to be concentrated in three equatorial regions. The significance of this will be explained in a moment. But first things first: how can life on Mars exist? The surface of Mars is very inhospitable to life. It is constantly blasted with deadly UV radiation, micrometeorites and solar radiation because Mars has no protective magnetosphere to shield it (this is also why Mars' atmosphere is so thin; we will get into this in more detail in a moment). However, recent images from the imaging system onboard NASA's Mars Odyssey orbiter reveal seven possible cave entrances on the flanks of the Arsia Mons volcano. These caves, known as the seven sisters, might provide a haven for possible methanogenic microbes. Researchers at the European Space Agency believe that perhaps these caves are deep enough and warm enough to sustain some liquid water, and being caves they could shield microbes from radiation and micrometeors. To test this hypothesis, NASA will launch the Mars Science Laboratory in 2011 (landing on Mars about 9 months later). It will measure the isotopic proportions of carbon-12 and carbon-14 in Martian methane. The idea behind this measurement is that living cells absorb carbon-14 at the same rate as they absorb carbon-12 into their tissues. Their living cell ratio of carbon-12 to carbon-14 is the same as that of their surroundings. However, when the cells die, the ratio of carbon-14 to carbon-12 decreases as the unstable carbon-14 decays. Meanwhile the ratio in nonliving things, such as the atmosphere and rocks, stays the same. The two ratios can be compared and the once-living fossil can be dated. This dating method, though, is not perfect and it has upper and lower reliable time limits. As well, the atmosphere on Mars is not equivalent to Earth’s atmosphere, making this kind of test potentially unreliable. So, the ratio of ethane to methane will also be tested. A ratio less than 0.001 suggests a biological methane source, whereas nonbiological chemical reactions produce nearly equivalent amounts of ethane and methane. Perhaps when these results come back, we can put to rest one way or another the question of life on Mars. Personally, I hope there is life, even microbial life. Such a discovery would suggest that life on other planets in our and other solar systems might be more likely than we ever imagined.

Martian Meteorites

Meanwhile, NASA has amassed a catalogue of 34 Martian meteorites. So far, these are the only physical samples of Mars we have because no probes sent to Mars have yet included return missions. That makes these meteorites extremely valuable to researchers. Scientists are fairly certain these meteorites are of Martian origin because they have the same elemental and isotopic compositions as rocks and gases as those analyzed on Mars. Most of these meteorites are quite young and this, along with the new atmospheric data mentioned above, suggests that Mars might have been volcanically active more recently than once believed. Rocks could have been spewed from volcanoes and launched into space, released from Mars' low gravity. For example, 7 nakhlite meteorites have been found so far. These unassuming-looking meteorites are named after El-Nakhla in Egypt where a large (10kg) meteorite was discovered in 1911. About a dozen of them fell as a meteor shower in the area. One is shown here.

These are all igneous rocks formed from basaltic magma about 1.3 billion years ago and ejected from Mars by an asteroid impact about 11 million years ago. The interesting thing about these meteorites is that they are all about the same age in terms of their formation age (1.3 billion years old) and their cosmic ray exposure age (11 million years). This points to a single origin of these rocks: a single location on Mars and a single impact. Perhaps most significantly for us is that the rocks formed 1.3 billion years ago, meaning that a volcano must have existed as recently as then. More recent data from the European Space Agency’s Mars Express orbiter suggests that some lava flows on Mars are as recent as 2 million years. All of this means that while Mars dos not appear to have ever had plate tectonic activity, it has, up to quite recently, been geologically active.

The most intriguing, and controversial, Martian meteorites contain what appear to be fossilized Martian microbes. The most convincing meteorite fell about 13,000 years ago in Antarctica. This is what it looks like.

It was discovered in 1984 and it made quite an international stir in 1996 when scientists announced it might contain fossilized Martian microbes. The rocks itself is very old; it is thought to have formed from molten rock about 4 billion years ago and later blasted off the Martian surface about 15 million years ago in an asteroid impact. It then floated in space until it landed in Antarctica. A rock this old may have come from a young wet Mars. The structures are very small, 20-100 nanometres (nm) in diameter, smaller than any known Earth bacteria, which are on average about a micrometer (1000 nm) in diameter. However, some very recently discovered round bacteria are as small as 400 nm. This bacteria, called Nanoarchaeum equitans, lives in boiling hot hydrothermal vents and has a simple archaic-appearing genome never seen before. Here is what the Martian candidates look like under a scanning electron microscope:

If they are tiny microbes, they are the first concrete evidence of extraterrestrial life. Researchers have done many tests on the rock to look for organic compounds, which would point to the presence of life processes. They have found some possible organic signatures of life such as amino acids and polycyclic aromatic hydrocarbons, but these compounds could be nonbiological in origin, or they could be the result of contamination by organic compounds within the Antarctic ice. As well, researchers have come up with both biological and nonbiological mechanisms by which these bacteria-like shapes could be formed. I will continue this debate in the following section.

Mars in The Beginning

Let’s now focus on early Mars – how was it different from today and could early Mars have supported possible life?

So far, I have hinted at an early Mars with liquid water (and an implied significant atmosphere) and active volcanic activity. Mars Express orbiter and Mars Reconnaissance orbiter have both found clay minerals that are signatures of a wet environment on Mars, at least in its southern highlands, where surface rocks are about 4 billion years old. Ancient dried up valley networks, and chaotic flood plains are also evident on Mars’ surface. This evidence of water along with the possible microbial fossils of a similar age in the meteorite mentioned above suggest that simple microbial life may have gotten a very early foothold on the young planet, possibly much earlier than life on Earth, which is thought to have begun about 3.5 billion years ago . However, we must keep in mind that a nonbiological origin of the “fossils” can also be argued. And we must add to this extensive evidence on the Martian surface of a meteorite bombardment about 3.9 billion years ago, at the time that our young Moon was bombarded by impacts. This makes a case for early tenuous Martian life to have been obliterated during this period of intense meteorite bombardment before it had any chance to evolve. Keeping in mind these scientific pros and cons, a large question remains: How did Mars ever hold onto liquid water (and an atmosphere) in the first place? What conditions existed then and not now?

 If you have read some of the other planetary articles in this series you may by thinking in the direction of magnetosphere. Mars doesn’t have one, and without a magnetosphere, any atmosphere formed on Mars would be rapidly ionized by solar UV radiation and then picked up and swept away by magnetized solar wind (solar wind is magnetized by the Sun’s very powerful magnetic field). In 1989, the Soviet Phobos probe directly measured Mars’ atmospheric erosion. When the data is extrapolated backwards 4 billion years (and changes in solar wind taken into consideration) it fully accounts for the planet’s lost atmosphere. You might argue that, for liquid water to exist on Mars, it must have also once had an atmosphere and it, therefore, must have once had some kind of magnetosphere to protect that atmosphere from solar erosion. You would be right: In 1998, magnetometers on NASA’s Mars Global Surveyor discovered the remnants of one. A series of magnetic loops are arrayed across the southern hemisphere. In these areas the surface magnetic field is about as strong as that on Earth. However, elsewhere the magnetic field is 100 to 1000 times weaker. The southern magnetic fields harbour localized pockets of gases ionized by solar UV radiation. Earths’ magnetosphere is created by an active dynamo, the result of electrical currents circulating in its liquid metal core. A similar dynamo once churned inside Mars, and we now even have evidence for when it stopped. A giant impact basin, about 4 billion years old, is demagnetized. This means that the crust that reformed after the impact was not under the influence of any magnetic field. This, in turn, means that the dynamo must have stopped before then. The question we are left with is why? Mars’ crust points to a possible answer that explains both the formation and the demagnetization of the impact basin. Its northern hemispheric crust is much thinner and lower in elevation than the crust in the southern hemisphere. This could be the site of what would be largest impact crater in the solar system, roughly the area of Europe, Asia and Australia combined, on a planet about one tenth the mass of Earth. Mars could have been struck by a meteor one tenth to two thirds the size of the Moon, depending on the velocity of the impact. The impact would have to have been violent enough to blow off a significant amount of crust off the northern hemisphere and not enough to melt the whole planet, as was the case with early Earth in which a planetoid is believed to have impacted early Earth and formed the Moon. It would also have to have been violent enough to disrupt Mars’ core dynamo. This is currently a hypothesis based on computer modeling, and as promising as it is, the scientists themselves admit that it needs more verification. Scientists are also currently looking at other possible mechanisms by which Mars lost its early magnetosphere.

We are left with a somewhat haunting image of a young newly formed planet, possibly endowed with all the ingredients necessary for life to take hold and evolve, violently jarred from its future path onto a different path that leads to dry dead planet, whose past is now a puzzle for us, the inhabitants, the evolutionary products of a much luckier planet, to solve. I leave you with this Cosmic Journeys 25 minute video called "Mars World That Never Was." It offers great imagery and a good recap of all that was discussed here.



Next up: Part 3: Venus

Our Solar System Part 3: Venus

Venus, named after the Roman goddess of love and beauty, seems less aptly named as we learn more about the planet. It is the brightest object in the night sky, after the moon, and it is often called the morning star or evening star, reaching its brightest point just before sunrise and just after sunset, bright enough to cast shadows here on Earth. Venus glows in our night sky along with Mars (extreme upper center/left) and Pleiades, a distant cluster of young stars (just above Venus), in this image.

Venus is slightly closer to the Sun than Earth, averaging about 0.718 AU with Earth being one AU (astronomical unit) away from the Sun. It appears so bright to us because its thick cloud layer reflects as much as 72% of the sunlight that strikes it. Here is a real-colour image of the planet.

(image processing by R.Nunes at http://astrosurf.com/nunes/)

Venus at first glance seems very similar to Earth. It's mass (0.81 that of Earth), density (5.204 g/cm³ compared to Earth’s 5.515 g/cm³) and surface gravity (0.9 g) are all very similar to Earth. Compare it with Earth in this NASA image of the solar system, with the planets in correct order of distance from the Sun at the bottom of the image and each planet enlarged to show detail at the top of the image. From left to right, for example,  you can see Mercury, Venus, Earth and then Mars.

Venus was formed about the same time as Earth was in about the same region around our young Sun, drawing from the same pool of elements within the solar dust left over from the Sun’s formation. Yet with all these similarities, Venus is nothing like Earth. Its surface is more akin to Hell than anything. There is no surface water and almost no oxygen. The average surface temperature is over 460°C, hotter than the brutally baked surface of Mercury, and its atmosphere is so dense that the pressure on its surface is the same as it is nearly a kilometer beneath our ocean. A human on the surface would be instantly charred, crushed and suffocated. But what if you could survive in a special spacesuit? Winds of up to 300 km/h buffet the upper atmosphere, driven by convection, whereas surface winds travel only a few km/h. But don’t mistake them for gentle. With an atmosphere so dense, this breeze would blow you over and you could not walk. During your stay each single day would pass agonizingly slowly, once every 243 Earth days, about as long as one year lasts on Venus, with the Sun rising in the west rather than the East. Venus is the only planet in the solar system that spins clockwise and this may be because it suffered some kind of catastrophic event such as a collision. We will revisit this idea shortly. Each long day would pass much like the next as Venus experiences no seasonal changes thanks to its extremely dense and universal cloud deck. You would never see the Sun or the starry night sky, each dim day gradually eroding into darker night and back again, punctuated by frequent lightning. Venus, named after the goddess of love, will never be the future interplanetary romantic getaway some of us might like to imagine.


What happened to Venus? Why isn’t it more like Earth?


Venus has an extremely dry and dense carbon dioxide rich atmosphere. Two recent flybys by the European Space Agency’s Venus Express Orbiter have revealed some clues. In 2007, the probe discovered hydrogen and oxygen streaming off the night side of Venus in a ratio of 2:1, suggesting that what little water there is in the atmosphere is being ionized by UV radiation and blown away into space by solar wind. The rate of water loss suggests that Venus has lost enormous amounts of water over the eons. However, this doesn’t necessarily mean that Venus once has liquid water oceans, as Mars appears to have once had. Researcher Eric Chassefiére of France developed a computer model that suggests the water was mostly atmospheric and existed only when the planet was very young and its surface was still molten. The ionization and escape of water into space would have removed energy from the atmosphere and triggered the solidification of the surface. This doesn’t rule out the possibility that Venus was later bombarded with water-laden comets that contributed a new source of water and maybe even oceans, at least temporarily, to the planet. This modeling, based on Venus Express data, needs to be followed up with more data such as how water and volcanic activity together may have shaped a young Venus.

So much water would not be stripped away from Venus if it had a magnetosphere as robust as Earth’s. Venus’ lack of a strong magnetic field poses yet another mysterious difference between it and Earth. One would guess with all the similarities, Venus would have a similar magnetic field, yet it does not have a dynamo as Earth does. This could mean that either Venus doesn’t have a solid inner core or a molten outer core, or that its outer core isn’t cooling and creating any convective currents. It might mean that its entire core has already solidified. Or, perhaps its lack of any dynamo may have something to do with its very slow rotation. Venus, unlike Earth, also has no plate tectonic movement. It is possible that some kind of catastrophic event, alluded to above, may have shut down whatever plate tectonic movement that may have once been active and this could have reduced heat flux through the crust and caused the mantle to heat up, reducing the heat flux from the core. In this case, heat from the core is reheating the planet’s crust rather than driving a dynamo that in turn could generate a magnetic field. Venus does have a very weak induced magnetic field that derives from the rotation of ionized gases high in its atmosphere, which interact with the solar wind. However, it is not strong enough to prevent the escape of hydrogen and oxygen and possibly many other light gases from Venus’ atmosphere. If you have read the articles on Mars, Earth and Mercury, you may have noticed a trend among the rocky planets: Strong magnetosphere > significant atmosphere; weak or no magnetosphere > either no atmosphere or a thin exosphere. So why does Venus, with a very weak magnetosphere, have the densest atmosphere of all the rocky planets in the solar system? This is a significant scientific puzzle. We need to figure out through which mechanism Venus holds onto its dense CO₂-rich atmosphere, and perhaps we need to re-explore the connection between magnetosphere and atmosphere.


A partial explanation comes from recent data coming mostly from the European Space Agency’s Venus Express Probe launched in 2005, and it has to do with the weak induced magnetic field around Venus. The Sun’s powerful magnetic field carried by the solar wind creates field lines that wrap around Venus. These field lines induce a magnetic field around Venus that is created by the charged atomic ions in its ionosphere (the ions are themselves the result of the Sun’s UV radiation striking gaseous atoms and molecules high up in Venus’ atmosphere). The induced magnetic field around Venus has a bow shock and a magnetopause. The bow shock slows down supersonic particles streaming from the Sun to subsonic speeds, while a space created between the magnetopause, located about 300 km from the surface, and the  ionosphere, located about 250 km from the surface, creates a special magnetic barrier that prevents solar wind particles from penetrating deeper into the Venusian atmosphere. This diagram helps to explain this concept.

(copyright RuslikO, http://commons.wikimedia.org/wiki/File:Venusian_magnetosphere.jpg)

This barrier is not a perfect atmospheric seal. In fact, atmospheric gases continuously leak from even powerful magnetospheres like Earth’s. In Venus’ case, the magnetic barrier is much weaker, and light gases such as oxygen, helium and hydrogen continuously and significantly leak trough the magnetotail. The water loss, as mentioned, suggests that Venus once had significant water, if not in an ocean than in a moist atmosphere.


Venus as Run-Away Greenhouse Effect


A nagging and important question remians:  If Earth and Venus were created by the same raw materials at the same time and existed in similar environments (similar distances from the Sun for example), then how can Venus have such a dramatically different atmosphere and no internal dynamo system? Should the planets not both have a similar crust, mantle and core makeup?

Some clues to this mystery unfold as we explore Venus’ atmosphere in more detail. The atmosphere on Venus is extremely hot and dense. It is enveloped in opaque sulfuric acid clouds. The atmosphere is almost all carbon dioxide (96.5 %) with 3.5 % nitrogen and trace amounts of sulfur dioxide, water, helium and other gases. These pie charts reveal Venus' atmospheric makeup.

These gases rapidly spin around the planet within the upper atmosphere at a rate of about 350 km/h. The atmosphere is so dense on Venus that the carbon dioxide at the surface is technically a supercritical fluid rather than a gas, giving it unusual properties such as the ability to effuse through solids like a gas and dissolve minerals like a liquid. Carbon dioxide is an effective greenhouse gas, trapping energy from sunlight, and it is this property that caused the surface of Venus to heat up to about 460°C, hot enough to melt lead, tin and zinc. With such thick cloud cover, this temperature doesn’t change much. During the night, which lasts about 58 days, it remains just as hot. Venus rains sulfuric acid high within its thick sulfur dioxide clouds but it never reaches the ground before it evaporates in the extreme heat. Unlike Earth, sulfur, which is released from volcanoes on both planets, does not become sequestered into solid compounds but rather circulates in the atmosphere in various compounds such as sulfur trioxide, sulfur dioxide and sulfuric acid.


Carbon also becomes sequestered and released in what is called the carbon cycle on Earth, where much of it is stored in limestone (CaCO₃). It is released from limestone into the ocean and atmosphere (as well as into living things) whenever limestone is subducted or weathered. Atmospheric/oceanic carbon is then sequestered back into limestone when shell-bearing marine animals such as corals and shellfish die and leave their carbon-rich shells behind. The carbon cycle on Earth is made possible through both plate tectonics and surface liquid water, and it actually creates an elegant negative feedback loop that regulates Earth’s surface temperature. There is no such carbon cycle on Venus. For one thing Venus has no plate tectonics, in which giant plates of crust move, bump into one another and slide or subduct under one another. Subduction effectively traps massive amounts of carbon underground on Earth but Venus’ carbon, released from volcanoes as well as baked out of surface rocks, has nowhere to go but into the atmosphere. Venus is a planet of volcanoes. It has produced more volcanoes than any other planet in the solar system. Radar mapping of the surface reveals many volcanoes and lava plains. However, measurements of the density of impact craters on the surface reveal that volcanism may have quieted down about 500 million years ago, when a major resurfacing event may have occurred. This, along the mystery of Venus’ solidified crust, no (dynamo-creating) core convective activity, and its slow and backward rotation, suggests that something might have happened to Venus to take it on a path much different from that of Earth. What exactly happened to Venus, if anything, is still a matter of heated debate, as is the possible connection between such an event and the build-up of carbon dioxide in its atmosphere. New evidence from Europe’s Venus Express orbiter suggests that large plateaus on Venus may consist of granite, a rock that, on Earth, needs water and plate tectonics to form. Could these granite plateaus be the ancient remnants of oceans and could Venus have once had active plate tectonics? As previously mentioned, some researchers do not think that Venus ever had liquid oceans. Granite was detected by measuring small differences in surface infrared (heat) radiation off different rock formations. This was, of necessity, done from orbit, through the dense Venusian clouds and atmosphere, where infrared radiation could be scattered.


As you can see, the story of Venus is far from complete. Although Venus Express has garnered a great deal of invaluable data from the planet and other probes, such as two recent data-gathering flybys by NASA’s Messenger Mercury probe and Japan’s Akatsuki probe launched in 2010, may add to that data, more detailed research is clearly needed not only to further understand the geological and atmospheric processes on Venus but to further our understanding of these processes here on Earth as well.


NASA has proposed the Venus In-Situ Explorer, a mission that would land on the surface of Venus and study rock core samples, measuring their composition and mineralogy. This might help answer questions about early Venus and possible ancient plate tectonics and surface water. This mission obviously has many technical challenges in terms of constructing a probe able to withstand extreme surface conditions. A launch has been proposed for 2013 but current fiscal restraints will likely push this date significantly back. This is a NASA image of what it might look like as it approaches the surface of Venus.

As well, Europe is planning to send the Venus Entry Probe, to be launched at the same time, which would consist of a special balloon-like probe that will study the Venus’ atmospheric layers in detail.


In spite of Venus' hostility, some researchers, such as Geoffrey Landis of NASA's Glenn Research Center, believe that a case can be made for future colonization on Venus, not on its hellish surface but, instead, about 50 km up in its atmosphere. Here, air pressure drops to one atmosphere and the temperature ranges from 0°C to 50°C. The notion has several advantages: no bone loss due to low gravity living and Venus, being the closest planet to Earth, makes back and forth travel feasible. However, the problems are severe: almost no water and oxygen, the clouds are extremely corrosive, and the surface is practically unlivable. Human life there would have to consist of some kind of floating colony. Cases for colonization on Mars have also been made and in fact, NASA has created an interesting page full of external links exploring colonization, exploration and terraforming of Mars here. Both Mars and Venus come with their own difficulties in terms of human habitation and exploration.  If you are curious about the habitability of other planets including those in other solar systems, I recommend keeping an eye out for a future article exploring what is called the Goldilocks Zone. This is a theoretical distance from a star in which a planet can maintain liquid water and sustain Earth-like life. In the meantime, I hope that these articles on the rocky planets thus far have given you an idea of just how much more we need to learn about planet dynamics, how rough indeed our outline is, of what constitutes a “habitable” planetary zone, and perhaps even a “habitable” planet.


Before you head off exploring, I recommend these Venus videos. I hope you enjoy them.


The Planet Venus (10 minutes, excellent info on all the exploratory missions to the planet)




All About Venus (6 minutes, a good introduction for beginners)



Venus: Death of a Planet (22 minutes, comprehensive)



Next we will explore the Goldilocks Zone.

Our Solar System Part 4: The Goldilocks Zone

Now that we have explored Earth, Venus and Mars in some detail, it is very tempting to conclude that Venus is too close to the Sun so it is too hot for life as we know it. Mars is too far away from the Sun so it is too cold, and, as the Goldilocks fairytale goes, Earth is just the right distance from our Sun, neither too hot nor too cold to support life. This is the essence of the concept of the Goldilocks Zone, a specific distance from a star in which a planet receives just enough energy to support liquid water on its surface, and thus, life.

However, if we think about these three planets for a moment, we might notice some complications to this idea. First of all, of these planets, only Earth has a significant magnetic field that protects its atmosphere from solar wind. Why? It is this atmosphere that moderates Earth’s temperatures and allows liquid water to exist, between 0°C and 100°C. On Mars, with no magnetic field, any atmosphere that may have once existed has been stripped away over the eons by solar winds. Its surface is now ravaged by ultraviolet and other solar radiation. Earth is also the only planet that exhibits plate tectonics and this means that nutrients and gases can cycle into different forms, keeping their atmospheric concentrations buffered within certain extremes. For example, carbon dioxide is moderated by the carbon cycle on Earth and this prevents a runaway greenhouse effect, which seems to have occurred on Venus, sending its surface temperatures skyrocketing up to about 460°C.

Does distance from the Sun have anything to do with internally created core dynamos or plate tectonic movement? Does how a planet form impact its later habitability? Do catastrophic events early on in a planet’s life affect its later habitability as much or more than its distance from its star? For example, did Earth’s surface water come from within Earth or did it come from comet impacts during the Heavy Late Bombardment? Let’s see what the Goldilocks Zone theory has to offer in terms of answering these questions.

What the Goldilocks Zone Is

The Goldilocks Zone or habitable zone is, technically, the intersection of two regions in space that must both be favourable to life. One region is confined to the planetary system of interest and the other region is where this system exists within its galaxy. The zone around a star, also called the circumstellar habitable zone or ecosphere, is where the star’s energy output allows for water to exist as a liquid rather than freezing or boiling away. The galactic habitable zone is a hypothesis that is met with a bit more skepticism than the ecosphere zone. The idea here is that the center of the universe acts in much the same way as a star does in the ecosphere zone. The favourable zone must exist close enough to gather enough heavy elements to form a habitable planet but far enough away to be protected from radiation from the galactic centre. Planets or moons could exist within this intersected region and possibly support carbon-based life. However, it is important to remember that planets within the Goldilocks Zone may not all be habitable. For example, gas giants in this zone are unlikely to support life.

The Goldilocks Planets Of Gliese 581

The red dwarf star, Gliese 581, is of particular interest to scientists studying Goldilocks planets because it has a planetary system and one planet in particular, that lie within the Goldilocks Zone. This image produced and copyright by the European Southern Observatory (Gliese 581’s planetary system is described in detail on this Wikipedia page) compares Earth’s and Gliese’s Goldilocks planets.

(copyright attributed to European Southern Observatory)

You can see that both Earth and Mars are within the Goldilocks Zone with Venus just on the inner edge of it. Gliese 581 g exists well within the habitable zone around its star, with Gliese 581 c and Gliese 581 d straddling the zone. Scientists currently hold all three exoplanets as possibly life-supporting, with reservations. Gliese 581 g appears to be a perfect candidate for alien life. However, recent evidence indicates that it may be tidally locked, meaning that one side always faces its star. And I should also note that both planets g and f are listed as unconfirmed because they have not been detected by new spectrograph analysis. In both solar systems, planets such as Gliese 581 f and Jupiter do not receive enough solar radiation to make up for radiative losses, and surface water freezes. Planets such as Gliese 581 e and Mercury, on the other hand, absorb too much solar radiation and any surface water simply boils away. Keep in mind that the Goldilocks Zone must be calculated for each star system based on the energy output of that star. The Goldilocks Zone for Gliese 581 is much closer to the star than our Sun’s habitable zone. Gliese 581 has only 0.2% of the visual luminosity of our Sun (but while our Sun radiates mostly in the visible spectrum, Gliese 581 radiates mostly in the near infrared, meaning that although it is much fainter than the Sun it gives off a much higher percentage of its radiation as heat than the Sun does). This NASA image gives you an idea of how the Goldilocks Zone (shown as a green belt) is affected by the host star’s energy output.

Kepler Space Mission Results

The Kepler Space Observatory launched by NASA in 2009 is designed to discover Earth-like planets orbiting within the habitable zones of other stars in the Milky Way galaxy. This is Kepler’s targeted star field, only 1/400^th of the night sky, courtesy of NASA.

We do not yet have the capacity to directly observe these planets but we can detect them indirectly by monitoring fluctuations in brightness of other main sequence stars. Fluctuations in brightness indicate one or more planets and/or moons crossing the star’s surface. This NASA image shows how this works.

In February 2011, the Kepler Space Observatory Mission Team released a list of 1235 possible exoplanets, with 54 of them being both Earthlike in size and existing within the Goldilocks Zone. This 10 - minute NASA video introduces us to the Kepler Mission.

This is a very exciting time for astronomy. These results have allowed scientists to estimate for the first time that about 6% of all stars host Earth-size planets and 19% of all stars host multiple planets. Astronomer Seth Shostak believes that, based on the Kepler findings, there are at least 30,000 habitable worlds within a thousand light years of Earth!

New Missions To Study Goldilocks Planets?

We need to confirm the existence of the Kepler Mission exoplanets and to study them in more detail and for that we will need a new generation of observational missions tailored to study exoplanets. Several such missions have been proposed and await funding. The Darwin, a European Space Agency cornerstone mission, was proposed but scrapped in 2007, because of technical and funding problems. It would have detected exoplanets and then carried out more detailed analysis of their atmospheres, looking specifically for the presence of oxygen. Finding just atomic oxygen does not necessarily mean life, however. Europa, one of Jupiter’s moons, has a tenuous oxygen atmosphere that is produced by the radiolysis of water molecules (this means that solar radiation breaks water molecules into ionized atoms) high up in its atmosphere. To find oxygen produced biologically through some kind of photosynthesis-like process, astronomers must look for the simultaneous presence of ozone, water and carbon dioxide. Oxygen produced at high altitude as it is on Europa immediately attacks atmospheric ozone and prevents its accumulation. If oxygen is produced low in the atmosphere, say through photosynthesis, and little water gets high into the atmosphere, then there are no ions that can attack ozone. Therefore, scientists now believe that ozone, water and carbon dioxide (required for photosynthesis) along with oxygen comprise a reliable biosignature in an alien atmosphere. This mission would have looked for this signature, with the Gliese planets being good first candidates for study.

Other missions that have been proposed and scrapped or put on hold include the New Worlds Mission, PLATO, the Space Interferometry Mission, the Terrestrial Planet Finder and the Transiting Exoplanet Survey Satellite, all of which were planned at least in part by NASA except for PLATO, which was another planned European Space Agency mission. Right now, the Kepler Mission, still active, is our best exoplanet explorer and it has been very successful so far. As our technology improves, new missions to study these worlds will undoubtedly be planned and we will move forward in this exciting new field of research.

Goldilocks Zone – A Concept In Infancy

In the meantime, it is important that we refine our search for habitable planets. The Goldilocks Zone concept is in its infancy and it will be refined and expanded upon as we learn more about what makes planets habitable. We are doing that right now as we study the mysteries of Mars, Venus and Earth right here in our own stellar neighbourhood. We also need to question and refine our definition of life itself. As we discover new forms of life never before believed possible in extreme environments right here on Earth, where there is no oxygen or temperatures are well below freezing are well above the boiling point, we must open ourselves to new possible extreme habitats in which life finds a foothold. And we must explore the possibilities of biochemistries that may not be based on carbon or water at all. Why is liquid water so important for life to exist? Carbon compounds dissolve in water to form the basis of all life as we know it, from enzyme functions to the building of cells, tissues and more complex structures. But are other biological solvents possible? A potential biosolvent should exist as a liquid over the range of temperatures an alien organism might encounter, and pressure must be accounted for. For example, while hydrogen cyanide has a very narrow temperature range as a liquid at 1 atmosphere (the surface pressure on Earth), it can exist as a liquid over a wide temperature range on Venus where the surface pressure is almost 100 times greater. Observations from NASA’s Spitzer telescope hint that planets around cool stars such as M-dwarfs and brown dwarfs, which are widespread around the Milky Way, might offer a prebiotic chemical soup that is different from that of our young Earth. The disc chemistry around these cool stars is different from that around our Sun, containing significant amounts of hydrogen cyanide, for example. Hydrogen cyanide is an active molecule that can combine to form adenosine, an essential building block of DNA. Perhaps it could function as a building block in the biochemistry of some kind of alien life, perhaps on a young planet such as the one in this NASA artist’s conception.

Methane, hydrogen fluoride and perhaps even molten salts could also theoretically be used as biosolvents. As well, rather than carbon, life could use silicon atoms. Silicon is chemically similar to carbon and it is far more abundant on rocky planets than carbon is. However it cannot create as many diverse functional groups as carbon can and it can’t readily form the double and triple bonds important to carbon-based biochemistry. But it may be useful under temperatures and pressures different than those of Earth or it could be used in roles less analogous to carbon. These kinds of studies will affect what we refer to as the Goldilocks Zone and perhaps even do away with the concept all together.

Here are some intriguing articles to explore:

Life As We Didn’t Know It – An ecosystem that thrives in complete darkness

A New Form Of Life – A new extremophile is discovered in California’s exotic Mono Lake

‘Goldilocks Zone' Bigger Than Once Thought - To find worlds within the "Goldilocks" zone, where conditions to support life are just right, look no further than our own solar system

Habitable or Goldilocks Zone  - an article from an excellent space science blog called Weirdwarp

Cruising the Goldilocks Zone - The Search for "Earth's Twins" - this article outlines a comprehensive shopping list for the ingredients to make a habitable exoplanet

Next up, we explore Mercury.