Sunday, January 12, 2014

The Dyson Sphere and the Search for Alien Life

For decades now, the people at SETI (Search for Extraterrestrial Intelligence) have been scouring the skies looking for evidence that suggests the universe is occupied by beings other than humans. Besides a few promising mysterious signals (most famously the “Wow! signal” that was detected in 1977) with unknown origins, the search has largely come up empty handed.
If intelligent beings exist, where are they? Why aren’t we picking up their radio signals? Are they picking up ours? Perhaps they’re out there, but we’re just not looking in the right place? Perhaps we don’t even know what to look for, and we could be looking directly at it without realizing what we’ve found?

A New Proposal?

One proponent of such a theory is Geoff Marcy, the new chairman for SETI at the University of California at Berkeley. Marcy is a seasoned American astronomer who is famous for having discovered over 100 exoplanets (that’s 100 more than me). He thinks we should focus our search for ET around Dyson spheres. In case you need brushing up on your sci-fi, a “Dyson sphere” is a hypothetical massive set of solar panels that would surround a star, collecting a vast amount of the stars energy output. Thus, any intelligent civilization (see: super-intelligent civilization) would be provided with all of the energy it could ever need.

Quite the undertaking...
Quite the undertaking… (Image source)
A highly technological civilization would be heavily dependent on energy, just as we are. So a Dyson sphere, or something of a similar construct, seems like the most logical endgame in terms of supplying fuel to a growing population and environmental preservation. Which brings us to another question entirely: if a Dyson sphere encompasses a star to soak up all of its energy output (meaning little to no light escapes), how would we go about finding one? Well, stars give off all sorts of things besides visible light. Heat being one of them. The Hubble Telescope(and the Spitzer Space Telescope, which is excellent at detecting infrared), is equipped with several tools that can capture ultraviolet light, infrared, and x-rays…along with visible light (obviously). If we spotted out a section of the cosmos where a Dyson sphere was in use, we could see a blackbody object that’s radiating in the far infrared around 10 microns in wavelength. The star would be radiating light in a blackbody spectrum that’s captured by the Dyson sphere, and released back out at a distance of 1 AU (the average distance from the Sun to the Earth). The blackbody temperature for the region would be about 100 to 600 Kelvin for a full or partial Dyson sphere.

 What Does It All Mean?

Two eyes? Two nostrils? A mouth?
…Unlikely. (Image Source)
Now, detecting an area of our galaxy that has the signature mentioned above isn’t a sure fire sign of a Dyson sphere being used by an alien race, but it would have some interesting implications. The major point being that we need to open our minds a little bit further and take an nontraditional approach in our search for extra terrestrial intelligence. So far, the methods we’ve been usingaren’t providing us with anything. Perhaps we are in dire need of changing the way we envision what an alien intelligence might be?  If we ever find them, it isn’t likely that they’ll meet the preconceived notions we have of them. That, may ultimately be our downfall.

Friday, December 27, 2013

The Benefits of Colonizing Space: Space Habitats and The O’Neill Cylinder

Many argue that the world is in a state of crisis and that the human race is the cause. As a species, we are approaching an important turning point in our history, and if we make the wrong decisions we might be facing a future of deprivation, over population, hunger, and instability. Ultimately, many believe that we will eventually be forced to colonize space. Last year, the 100 Starship Symposiumset on course a project to design and build an economical and practical spacecraft for interstellar travel.

But with the very immediate worries about over population, it might not be a good idea to wait for interstellar travel and the colonization of other worlds. Fortunately, there are also many suggestions in place for large space structures designed as places for people to live in their millions, much like a city is on Earth. Of course, building a space habitat comes with thousands of challenges, including: construction in space, recreating a livable atmosphere, recycling waste, producing artificial gravity, transporting food and materials to the habitat, and convincing people such a venture is worth it.

Image credit: Mars One graphics
There’s no strict definition for a ‘space habitat’, but it’s generally agreed to be a permanent human living facility on a celestial body such as ‘Mars One’ (extra-terrestrial planets, moons, or in a spaceship orbiting the Earth). We may have no choice but to build one of these in the future, be it initiated as a matter of survival or an undeniable demand because of our desire to explore and gain new knowledge by expanding in space. Ultimately, there are a number of incentives to building such a habitat.

For governmental bodies and world leaders faced with a huge and unsustainable population, the concept of a space habitat would be attractive. Using the materials available in the Solar System, there is the potential to build enough surface area within space habitats to possibly house billions and even trillions of people. Populations would have the space to expand sustainably without destroying any current ecosystems, as well as relieving the pressure off Earth to provide resources. The planetary population could be stabilized and supported with the extra space to inhabit and develop agricultural plantations for food.

The expansion into space also offers up a wealth of privatized opportunities, such as access to energy and other interplanetary resources. On Earth, utilizing the Sun’s energy via solar cells is a disappointingly inefficient process with unavoidable problems associated with the atmosphere and night. In space, solar panels would have access to nearly continuous light from the Sun, and in Earth’s orbit this would give us 1400 watts of power per square meter (with 100% efficiency). This abundance of energy would mean that we could travel throughout much of the Solar System without a terribly significant drop in power.

Image credit: Ricky
Material resources would also be in abundance throughout the entire Solar System (especially if you include mining opportunities on Mars, Luna, and other moons). Asteroids contain almost all of the stable elements in the periodic table, and without gravity, extracting and transporting them for our uses could be done with ease. NASA is working on a project where one could manufacture fuel, building materials, water, and oxygen just from resources found the Moon. The shift from Earth based manufacturing and plantation to industries in space may not just become feasible, but incredibly economically beneficial.

So now that we’ve laid down some reasons as to why organisations may want to unite and build a space habitat, I want to introduce you to the O’Neill cylinder. My personal favourite suggestion is the O’Neill cylinder, a space settlement design proposed by Gerard K. O’Neill nearly 40 years ago, in 1976, when he published his book ‘The High Frontier: Human Colonies in Space’Gerard K. O’Neill was a lecturer at Princeton University, as well as a physicist and space activist. He designed and built the first mass driver prototype, and he developed new concepts to explore particle physics at higher energies than what had ever been possible (he was quite an awesome guy). But his lasting legacy was based in his work on space colonization. He founded the Space Studies Institute, an organisation devoted to research into this field.

Image credit: Rick Guidice
The design for his cylinder was spawned from a task that he set a group of physics and architectural students. The goal was to invent large structures that could be used for long term human habitation, and the results inspired the idea of the cylinder. The title is a little misleading, because it is actually two cylinders that rotate on bearings in opposing directions (to cancel gyroscopic effects). Each one would be 20 miles long and 8km in diameter, with 6 stripes along its length (3 windows and 3 habitable surfaces). Industrial processes and recreational facilities were envisaged to be on the central axis where it would effectively be a zero-gravity zone.

One difference between a planetary/moon-based space habitat and a man-made structure is the need for artificial gravity, and the O’Neill cylinder does this in a beautiful manipulation of basic Newtonian physics. As the two colossal cylinders rotate on their axis it utilizes the centripetal force on any object on the inner surface to create the appearance of gravity! Using the dimensions of the cylinder, the equation a=v²/r and the acceleration due to gravity on Earth (9.81m/s²), we can deduce that the cylinder will only need to rotate around 28 times every hour in order to simulate an equal force (though about 40 times is what the plans suggest).

Image credit: Donald Davis
The next box on the check list for a planetary habitat is maintaining an atmosphere with a composition and pressure that is similar to that ofEarth’s. The cylinder is designed to have a carefully controlled ratio of gases much the same as Earth, but the pressure will be half of that at sea level. This will create a minor difference to how we breathe, but the advantages are the need for less gas and less of a requirement for thick walls. It also thought that the habitat will be able to generate its own micro-climate and weather systems that we could control using mirrors and by changing the ratios of gases in the cylinder.

Habitats also have to deal with a variety of problems that come as a consequence of living in space. With the colony situated in a vacuum the cylinder essentially turns into a giant thermos flask! O’Neill’s design to overcome this issue uses a series of mirrors hinged to each of the 3 windows. They are able to direct sunlight into the cylinder to simulate day time and warm the air, and turn away at ‘night’ so that the windows look out onto the blackness of space. This period of ‘night’ would allow heat in infrastructure, and that produced biologically, to radiate out just as theEarth’s atmosphere does (at night time too).

Another serious issue is that of small meteoroids or even man-made space debris. Radar systems based all around the outer skin of the cylinders will continuously map the region around the habitat to locate possible dangers. It was predicted that small scale collisions are inevitable; so to counteract the effect the windows would be built up of small panes built around a strong steel frame. The loss of gas would be so insignificant compared to the volume of the cylinder that repair jobs would not be an emergency. Though much larger pieces of rock would be a threat to the habitat, and methods of deflection or vaporization would be required.

Stephen Hawking said that he has predicted the extinction of the human race within the next thousand years, unless we build habitats in space or on other planets/moons in the next two hundred. That’s quite a statement, and with the current economic problems facing many developed countries around the world, it is highly unlikely that any big projects such as an O’Neill cylinder will be started soon. But with pioneers such as SpaceX and Mars One, what do you think the human race will do in the next 100 years?

Sunday, December 22, 2013

TW Hydrae: The Planet That Shouldn’t Exist

Image via NASA
Image via NASA (click for a larger view)
In a physical world that contains phenomena such as pulsars, neutron stars, and black holes, one would think that we have a pretty good grasp on the basics–how our universe evolves and functions; however, you might be surprised to know that some of the basics are actually difficult to discern. One of those problematic areas is planetary formation. That’s right.. .we really have a somewhat-amateur understanding of how  planets form, despite actually living (and evolving) on one.

Our current models say that our solar system, and other planetary systems circling other stars, form from a protoplanetary accretion disk encircling a young star. This elliptical disk of material is typically comprised of gas, ice, rock, and grain. These build up slowly before ultimately coalescing into a planet, shaped by gravity. This process is thought to take tens of millions of years to occur from start to finish, and of course the amount of material concentrated around the protoplanetary disk dictates the number of planets a star has (and their size), which means that the planets should all form during a similar time-frame and there should also come a period when planetary formation is no longer possible.

Recently, Hubble took a closer look at a youngish orange-dwarf star (similar in size and mass to our parent star) that is located a mere 176 light-years from Earth in the constellation of TW Hydrae. Hubble spotted a large gap in the protoplanetary disk encircling the star. The gap, which is attributed to a large, yet-to-be-detected planet forming (similar to a snow plow), stretches more than 1.9 billion miles wide (the disk itself is about 41 billion miles wide, located about 7.5 billion miles (66 billion kilometers) away from the star), which is perplexing given the young age of the star. (Less than 10 million year sold).

The star, known simply as TW Hydrae, has been a popular star for astronomers a while now, particularly since the star its positioned in such a way that its poles align directly toward Earth, giving us an opportunity to study the star’s protoplanetary disk (Said disk is known to host a substantial amount of water too), which should not be in the process of forming planets just yet (though I must say that this is not the first claim of a detected exoplanet. The last of which, occurred back in 2007 and was ultimately discredited after it was shown to be a starspot.).

As I mentioned earlier, planetary bodies are thought to form over the course of tens of millions of years, allowing for the needed time for enough material to collect bit by bit from the protoplanetary disk. This is obviously problematic, as TW Hyrade is only an estimated 8 to 10 million years old. Furthermore, the distance is also disconcerting. The planet (estimated to be between 6 and 28 times more massive than Earth) is orbiting its parent star from a great distance of 7.5 billion miles (on average, Pluto is a mere 3,667,000,000 miles from the sun).

According to John Debes of (from The Space Telescope Science Institute): “A planet 7.5 billion miles from its star should take more than 200 times longer to form than Jupiter did at its distance from the sun because of its much slower orbital speed and the deficiency of material in the disk. Jupiter is 500 million miles from the sun and it formed in about 10 million years.” Therefore.. this planet should not exist.. at least it shouldn’t based on our current models.

Also hindering the stars’ ability to produce a planet so quickly is the noticeable lack of dust grains located in the outer region of the protoplanetary disk. (Starting out at about 5.5 billion miles from the star) No grains of material, no planet. So assuming this discovery pans out, it could be yet another strike against conventional models.

Thursday, December 19, 2013

Crows Could be the Key to Helping Humanity Understand Alien Intelligence


Yes, you read that right, understanding how a crow’s brain works could lay the foundation to helping us understand how an alien mind might work. In case you don’t know, crows are some of the most intelligent animals on Earth. They use toolsand pass this knowledge onto their children, they will unite to fight a common enemy or intruder, and their ability to remember is almost unmatched. Recently, a study was conducted to see just how reasoning inside the bird’s brain works, because a avialan brain is very different from a mammalian brain. Here, the rabbit hole goes deeper than we could have possibly imagined.

Mammalian brains are all pretty much the same. We all share a “recent” common ancestor that roamed the earth some 200-million years ago. The mammalian brain has been studied with great effort and we have an OK understanding of how it works (at least, we have a much better understanding of how a mammalian brain works as compared to reptilian or avialan brains). On the side, you can clearly see how various mammalian brains have the same basic structure. When you look at the big picture of the evolution of intelligence, birds begin to make an appearance.

The Last Common Ancestor and the Evolution of Intelligence

Mammals and avialae shared a common ancestor about 300-million years ago during the Permian period, an era that predates the time of dinosaurs. This ancestor was probably about the size of a raccoon, looked like an unholy marriage of a rodent and a reptile, and was probably governed by instinct instead of “higher” brain functions that we normally associate with cognition.

This great-great-great-……….-great grandparent of ours had a region of the brain called the pallium. The pallium is the bit of the brain that eventually evolved to control the “sentience” (for lack of better words) of animals. For mammals, the pallium evolved into the prefrontal cortex (PFC) and is cram packed full of nerves that allow us the luxury of abstract reasoning. In birds, the PFC doesn’t exist. Instead, the pallium moved up and became the nidopallium caudolaterale (NCL).


In both brains, the PFC and the NCL are responsible for “working memory, reversal learning, and reward prediction” and also “shares important properties such as dense innervation by dopaminergic fibres and connectivity patterns with multiple sensory input, limbic and motor output regions” according to the paper published by Lena Veit and Andreas Nieder, who have taken the study of the avialan brain to heart. In other words, the PFC and NCL are receptive to neurotransmitter stimuli, govern emotion, memory, and movement, deals with abstract reasoning, and is generally the command center of the brain.

The Reasoning of Crows

We know that the NCL is equivilant to the PFC, so now scientists want to watch a crow reason and solve a puzzle in real time to see what the brain does. You know the “one of these things isn’t like the other” style puzzle? Veit and Nieder trained some crows to solve this type of puzzle and either identify two pictures as a match or as a mismatch.

Basically, the crows would be shown a single picture. They would be given a visual or auditory stimulus that told the crow to select a matching or a not matching picture. Then, the crow was presented with two pictures at the same time (one picture is the same as the one before, the other is different). The crow was then suppose to select either the picture that matched or the picture that did not match the one they were shown at the beginning of the test.

The results were nothing short of remarkable. While working the puzzle, the crow’s brains would behave exactly the same way as the mammalian brain on the neuron level. Even though mammals and birds have “intelligence” in very different portions of the brain, it behaves in a remarkably similar way.

This display of intelligence is a perfect example of a phenomena called “parallel evolution” where two different species evolve to posses the same trait(s) completely independent of one another. As a matter of fact, the brain-to-body size ratio for birds and primates is pretty much the same. It is unsurprising that we might find mammalian type intelligence in these animals because they have all of the same equipment as we do, it’s just wired a little differently. You could almost consider crows, and birds in general, as a type of “feathered ape” from a cognitive perspective.

OK, What about the Aliens?

parrot-bird-wearing-glasses-funnyI promised that understanding a crow’s brain could help us understand aliens, and I wasn’t lying. By cross-referencing what we learn about avialan and mammalian intelligence, we could start to learn what makes intelligence tick. What attributes does each type of animal share? If we encounter an alien species evolved enough to have a brain-like structure (as we understand it), it’s possible we could cross what we know about intelligence with the alien species to see if there are any similarities. In more “real time,” understanding how intelligence works could help us to reverse engineer it in our efforts to build a smarter artificial intelligence.

In the end, crows give us an awesome opportunity. Here, we have two very different brains that are able to reason in similar ways. The crow’s brain is the first “alien” brain, as compared to mammals, that humans have been able to study. This can serve as a beacon of hope for us since it teaches us that intelligence isn’t just found in one type of brain – it’s not just an evolutionary fluke that affected mammals. There are other types of brains that can house intelligence. In other words, we are not alone.

Side note: In this particular study, “intelligence” is being defined as the ability to reason abstractly. Intelligence can be defined in different ways by different people and different fields of science, but this is the way the term was defined for these tests.

You can see an awesome video of a crow solving a multiple step problem here:

Wednesday, December 18, 2013

Are Earth’s Radio Signals Being Intercepted by Aliens?

This image was created by Abstruse Goose. Click on it to see a larger version.
This image was created by Abstruse Goose. Click on it to see a larger version.

The SETI (Search for Extraterrestrial Intelligence) Institute has been searching for intelligent life in our universe for the last 29 years. People have been searching for signals for longer than that – I presume since the dawn of radio astronomy. Of course, they haven’t turned up any signs. But let’s dive into a hypothetical real quick. Suppose tomorrow, we detected the first radio broadcasts of an alien civilization that lives on a star 1,000 light-years from Earth – on the other side of the Milky Way.

We listen to their initial broadcasts, and eventually start seeing their earliest television broadcasts. The greatest minds on Earth eventually decipher the alien’s language. and we are able to understand what we see and hear. We watch their politics, culture, science, and religions evolve over time. We potentially learn some lessons along the way as the aliens crack problems we struggle with. We listen to our new friends for a thousand years. Maybe, something happens that destroys the civilization – through their own cataclysmic wars, a naturally occurring apocalyptic event they just didn’t see coming, or their mishandling of the environment. Maybe they switch to a different way of broadcasting that we simply can’t hear. Maybe, they vanish–radio silence–and we don’t know why. Maybe, an alien race 1,000 light-years away eavesdrops on our own radio and television broadcasts and experiences the same thing as our own civilizations rise and fall over time. Could such a scenario occur?

Not really.
Image Credit: Shutterstock
Image Credit: Shutterstock
As you undoubtedly know, Earth is surrounded by a sphere of electromagnetic radiation that has been generated by humans since we started broadcasting electromagnetic waves wirelessly. This concept has been exploited across our entertainment scene, such as the movie “Contact” where ET sends us back the first television broadcast humans made powerful enough to punch into interstellar space – Hitler opening the 1936 Berlin Olympic Games. In reality, such an event will probably not happen, and it’s all thanks to the inverse square law.

The inverse square law is a law in physics that states that a “specified physical quantity or intensity is inversely proportional to the square of the distance from the distance of that physical quantity.” In short, if you double the distance, you quarter the power of force. To understand this, I’ll use Voyager 1 as an example.

Currently, Voyager 1 is the furthest man-made object from Earth. It’s about 123.6 AU away (well beyond the orbit of Pluto), and it is preparing to enter interstellar space sometime (hopefully) before it ultimately loses power. Because of the inverse square law, when voyager communicates with us, the signal we receive is about a 20-billionth the power of a watch battery. We are able to receive and understand Voyager’s communiqu├ęs mostly because we know exactly where to look, when to look, and the signals are aimed directly at us in a very precise fashion. Unfortunately, when we send Star Trek: The Original Series out into the cosmos, our intention wasn’t to regale aliens with our TV programs.

As far as these signal go – either from the perspective of humans hearing ET or ET hearing us – the most we can hope for is that the signal itself stands out from the cosmic background noise. Even then, the intelligent species doing the searching (either the aliens or us) will have to gather a ton of data on any given system in order to discern with reasonable certainty whether or not an anomaly that would potentially be an intelligent signal exists. Using that information, we could conduct more extensive searches to confirm the signal as sufficiently peculiar, but we won’t be watching ET’s late night television.

Image Credit: Alamy
Image Credit: Alamy
If that wasn’t sufficiently dampening enough, it gets worse. From the human side of things, we have progressively transitioned to digital entertainment (instead of broadcasting powerful analogue signals out into the cosmos). The digital counterpart is far less powerful (about four times so) and will be much harder to detect. As Cecil Adams says so eloquently, “the age of pumping high-power terrestrial noise into the ether is likely to be a mere blip lasting less than a century.” Whereas our galactic radio presence won’t vanish completely, Earth will certainly become considerably less noisy.

Then again, maybe ET never used radio in the same way we did. Maybe they kept all of their communications cable-based. It could also be possible that this progression of technology is natural, and there is only a centuryish window to detect noticeable signals from an intelligence. Considering there are hundreds of billions of stars in the Milky Way, and we’ve only surveyed 750 or so of them for ET, such a situation would prove problematic for detecting ET in such a very short window.

At least, we can walk away from this thankful that the first human ET sees won’t be Hitler… Jersey Shore will likewise be lost for all time, and they will never suffer through songs from Justin Beiber.

Monday, December 16, 2013

The Oort Cloud: Home of the Icy Giants

Image Credit: R Mewaldt & P. Liewer, JPL/NASA/FQTQ

Humans generally like stability. We are used to our small, predictable world: Every 24 hours, we rotate on our axis; every 365 days, we revolve around the Sun. We have followed this pattern for millennia, and we will continue to follow this pattern for ages henceforth. For the most part, when we are thinking in cosmic terms, the Earth is steady and unchanging…but the same cannot be said for the rest of the solar system.

Credit: NASA/JPL (Source)
Chaos reigns a mere 1.9 light-years from Earth. At this icy distance, in the cold recesses of our solar system, we find the Oort cloud. At just over 11 trillion miles from the solar surface, the dense cloud is at the outer region of the Sun’s influence. This portion of our neighborhood is nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun (which gives you some idea of just how far the Sun’s influence extends). Some scientists believe that you could travel half way to Proxima Centauri before you truly leave the Sun’s sphere of influence (meaning that you would have to essentially travel to the next star in order to leave our solar system).

This dark region of space is home to several trillion individual objects larger than 0.62 miles (1 km) and several billion with a diameter over 12 miles (20 km). This mass of material is believed to be the leftover remnants of the early solar system. Ultimately, this chaotic amalgamation of primordial material consists of whatever didn’t coalesce into planets, fall into the sun, or become a part of the asteroid belt that orbits between Mars and Jupiter.

Credit: NASA/JPL (Source)
Unlike the planets, which orbit (more or less) on the same plane of the solar system, the Oort Cloud is a dense smattering of material that envelopes the Sun. As you can see in the image, there is no rhyme or reason to the orbit of these icy bodies… There is no single, definitive orbit. Rather, tidal interactions between the materials cause the pattern to constantly alter and shift, throwing some objects into the inner planetary region and slinging other objects out of the solar system entirely.

The objects in this cloud are grouped into two basic groups: short period comets and long period comets. Short period comets have an average orbital period of some 200 years. Conversely, long period comets have orbits that are, well, long. For example, Comet Hyakutake was last seen flying by the Sun in 1996, a mere 9.2 million miles from Earth (14.8 million km). Astronomers were able to capture many magnificent pictures of this icy interloper as it careened into the inner solar system. Unfortunately, no one alive today will ever see it again. Hyakutake’s current orbital period is more than 70,000 years long (so unless you live to the year 71996, you probably won’t see it). Hale Bopp is another long period comet, though its orbit is a mere 2,500 years (hardly the blink of an eye when compared to Hyakutake).

This image just doesn't do it justice
This is an image by NASA taken of Hale-bopp in 1997. Go on and take a minute to process that amazing distance (Source)

When these distant travelers make their way into our neck of the woods they put on some fantastic shows. When Hale Bopp last passed by, astronomers were graced with a fantastic view of the flaring material evaporating off its icy body. Ultimately, they estimated that its sodium tail was about 373,000 miles wide and about 31 million miles long (600,000m and 49.8 million km, respectively)

Once these foreign beasts are sent in close to the Sun, they may careen through the solar system for thousands of years until they are ejected into interstellar space…or until they collide with another body such as a planet — the luckiest return to a semi-stable orbit in the far reaches of the Oort Cloud, though such an event is unlikely (a close pass to the Sun is generally a death sentence). I know that we are all fascinated by black holes, supernovae, and hyper giant stars; however, we should remember that tiny objects can be just as interesting…especially where there are billions upon trillions of them flying about so close to home.

Wednesday, December 11, 2013

Life Could Have Hitched a Ride to the Moons of Jupiter and Saturn

Montage showing Jupiter and its Great Red Spot and, from top to bottom, the moons Europa, Ganymede and Callisto. Image credit: NASA/JPL

Life on Earth or Mars could have been brought to the moons of Jupiter or Saturn on rocks blasted off those planets, researchers say. These findings suggest if scientists ever detect life on those moons, they might have to contemplate the possibility that it came from elsewhere rather than originating there on its own. The idea that life can spread through space is known as panspermia. One class of panspermia is lithopanspermia — the notion that life might travel on rocks knocked off a world's surface. If these meteoroids encase hardy enough organisms, they could seed life on another planet or moon. Although lithopanspermia might seem farfetched, a number of meteorite discoveries suggest it might at least be possible. For instance, more than 100 meteorites originating from Mars have been discovered on Earth, blasted off the red planet by meteor strikes and eventually crashing here.

Some researchers have even suggested that life on Earth may have originally been seeded by meteors from Mars. A great deal of research has explored whether the red planet once harbored lifeand whether life might still exist there today, based on findings that Mars might once have been significantly more hospitable to life than it is now, and that refuges for life could remain hidden under its surface. One Martian meteorite, Allan Hills 84001 (ALH84001), was even initially claimed to contain evidence of life. However, research since has revealed that every item on this meteorite that was potentially suggestive of life could be generated inorganically. 

Past computer simulations also have suggested that matter blasted off Earth by cosmic impacts could have escaped the pull of Earth's gravity and landed on the Moon. Billions of years of Earth dust may have accumulated on the lunar surface — as much as 22 tons (20 metric tons) of Earth material is spread over every 38 square miles (100 sq. km) of the Moon. If true, the Moon could hold fossils of some of the earliest microbial life on Earth. 

Martian meteorite ALH84001. The meteorite is sliced to show its interior. Found in the Allan Hills ice field in Antarctica in 1984, the four-billion-year-old rock is one of the oldest in the world. The meteorite likely originated just below the surface of Mars. About 16 million years ago, another meteorite struck the area, blasting it off into space before it landed on Earth about 13,000 years ago. Credit: NASA
Martian meteorite ALH84001. The meteorite is sliced to show its interior. Found in the Allan Hills ice field in Antarctica in 1984, the four-billion-year-old rock is one of the oldest in the world. The meteorite likely originated just below the surface of Mars. About 16 million years ago, another meteorite struck the area, blasting it off into space before it landed on Earth about 13,000 years ago. Credit: NASA

The discovery of organisms on Earth that can survive in environments once thought too harsh for life has piqued interest over whether the moons in the outer reaches of the solar system, such as Jupiter's moon Europa orSaturn's moon Titan, could host life. 

"There have been previous simulations looking at transfer between Earth and Mars, but we wanted to scale the simulations up in the hopes of seeing transfer to Jupiter and Saturn," said study lead author Rachel Worth, an astrophysicist at Pennsylvania State University. 

Worth and her colleagues analyzed where batches of several thousand rocks traveled once ejected off both Earth and Mars. "We ended up simulating over 100,000 individual fragments," Worth said. 

Most of these meteoroids slammed back into their home planet. A great many rocks also were either swallowed by the Sun or left the solar system entirely. In addition, large numbers hit planets more inward in the solar system from their home planet — for Earth, that means Venus and Mercury, and for Mars, that means Earth, Venus and Mercury. However, a small fraction of meteoroids did hit planets outward from their origin. 

The researchers calculated that over the course of 3.5 billion years — roughly the amount of time Earth is known to have possessed life — about 200 million meteoroids large enough to potentially shield life from the rigors of space were blasted off Earth. They also estimated roughly 800 million such rocks were ejected off Mars during the same period. More rocks escape from Mars because Martian gravity is a little more than a third that of Earth's. 

Past research suggested moderately-sized rocks ejected from impacts could protect organisms from the dangers of outer space for up to 10 million years. The scientist calculated about 83,000 meteoroids from Earth and 320,000 from Mars could have struck Jupiter after traveling 10 million years or less. Also, roughly 14,000 from Earth should have hit Saturn in that time, and no more than 20,000 from Mars.

Since the moons of those giant worlds are relatively close to their planets, many of them might get peppered by these meteoroids as well. The researchers calculated that Saturn's moons Titan and Enceladus and Jupiter's moons Io, Europa, Ganymede and Callisto should each have received between one and 10 impacts both from Earth and from Mars. 

These findings suggest the possibility of transfer of life from the inner solar system to the outer moons, although very rare, currently cannot be ruled out. "When planning missions to search for life on Europa or other moons, scientists will have to think about whether they can distinguish between life that is or is not related to that on Earth," Worth said. 

The researchers caution they are not saying "that life has made it to any of these moons, just that it could," Worth said. "To know for certain that this kind of transfer has happened, we would need to actually identify an Earth or Mars rock on one of the moons in question. We tried to make our estimates as realistic as we could, but they are still estimates, and we can never know for sure what will be discovered in the future that might change our assumptions." 

For instance, "we don't really know the probability that an ejected rock fragment would have microbes in it, or that they would be the type of microbes that might survive all the trauma of ejection and space travel," Worth said. "There's also the question of just how habitable they might find the moons if they did make it there." 

Still, the researchers note the icy moons of Jupiter and Saturn were all once warmer and likely had little to no icy shell to prevent meteorites from reaching their liquid interiors as they do now. In addition, Europa currently has the thinnest ice crust of the six moons the researchers examined, and roughly 40 percent of its crust appears to be covered with "chaos regions," uneven terrain hinting that it often breaks into large chunks separated by liquid water that later refreezes. Any meteorites on top of such regions therefore might have a chance of falling down into the underground oceans that moon is suspected to have. 

"I think the possibility of any life in Europa's oceans is exciting, whether it is descended from Earth life — showing us a novel evolutionary path in a very interesting environment — or life that comes from an independent origin, which would point towards life being fairly common in the universe." 

Worth noted one factor not included in their simulations that could be significant was the Yarkovsky effect, where rotating objects about 4 inches (10 centimeters) to 6 miles (10 kilometers) large will radiate heat that can help propel them through space. 

"We expect that this effect would basically spread the ejected rocks out faster, as some would be propelled outward and others inward, so we might see slightly faster transfer times," Worth said. 

The scientists added that rocks crashing back onto their home planet could help reseed life on that world after the cosmic impact that created them partially or completely sterilized the planet in question, serving as refuges for life in space while the world's surface cooled enough to permit survival. This could help explain how life on Earth survived the era known as the Late Heavy Bombardment about 4.1 billion to 3.8 billion years ago, when untold numbers of asteroids and comets pummeled Earth, the Moon and the inner planets. 

Worth and her colleagues Steinn Sigurdsson and Christopher House detailed their findings online Dec. 6 in the journal Astrobiology.