Mars, Mars, Mars:
About Planets
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The word planet comes to us from ancient Greek, in which language it meant "wanderer". To the ancients, who did not have telescopes, and whose astronomy was thus limited to what the naked eye could perceive, the night sky contained only three things: the Moon, the "Milky Way", and stars. Now the stars, being--as we have come to know--at huge distances from us, all appear to be fixed in the sky (though the sky itself seems to revolve around the North Star, an illusion caused by the rotation of the Earth): it is just as though they were dots painted on the inside of some gigantic sphere with the Earth at its center (and, as we will see later in more detail, some early views of the universe pictured them so). But there were (and are) five seeming "stars" that do not hold their fixed place among the others but seem to, well, wander about the sky--and so the Greeks called them "wanderers".
To better understand the how and why of that, let's look at what planets are and how they come to be. (This account is not only risibly brief, it also--as a necessary consequence--much oversimplifies complex matters; nonetheless, it is a reasonable beginning point for those not well acquainted with astronomy and the associated sciences.)
Overview
The universe was born roughly fourteen billion years ago as an unimaginably small, unimaginably hot bubble of space-time full of energy. As it expanded, it cooled, and eventually some of that energy condensed into subatomic particles that combined to form a universe-wide cloud of the simplest element, hydrogen gas. In time, that gas--under the influence of gravity--segregated out into large spinning individual cloudsthe precursors of stars. As each proto-star further condensed under gravity's pull, it became hotter and denser at its core, till that core ignited in nuclear fusion. In time, the stars themselves began to clump up under the influence of gravity, and came together into vast, slowly spinning assemblies of millions or even billions of stars, what we recognize as galaxies. Meanwhile, throughout the universe, the outer gas layers associated with each original proto-star flattened out into a disk spinning around its parent star, and cooled enough to become bits of dust; and--at many, perhaps most, stars--over time, those dust bits drifted together, again under gravity's force, becoming individual masses--planets. Some planets are relatively small and rocky--our Earth and Mars are examples--while others are buried in huge masses of cold gases or liquids (and are called "gas giants"). The conditions needed for life as we think of it are such that in our Solar System only a few planets--and perhaps some large moons--might be, or have been, home to life.
Contrary to popular belief, the "Big Bang" theory of cosmology does not actually posit that universe popped into existence out of nothingness as a literally infinitely tiny, infinitely hot point of energy. Present-day scientific theory has a fairly satisfactory history of the universe back to a moment that, to a lay person, seems as close to zero as never mind, but that is quite significant to a cosmologist: roughly 10-43 seconds. In numerals (to impress), that is:
0.0000000000000000000000000000000000000000001 seconds
In that period--called the Planck epoch--between a hypothetical "time zero" and that first mark, which all occurred roughly 13.7 billion years ago, conditions were such that our current knowledge does not cover them (we need to unite quantum-mechanical theory and gravitational theory, for one thing).
At the end of that unthinkably brief Planck epoch is where our understanding of the history of the universe starts. At that instant, the early universe was filled with energy; there was, however, no matter at that stage, because the average density of energy was so high that nowhere was it cool enough for matter to form. But that initial almost infinitesimally tiny bubble of space-time was expanding at an amazing pace: by a mere millionth of a second after its birth, the universe had expanded enough that its average energy density had dropped to the point that some of that energy began to "condense" into matter.
When energy changes to matter, it forms what we now call subatomic particles, the best-known being the positively charged proton, the electrically neutral neutron, and the much lighter, negatively charged electron. Although opposite electrical charges attract, the earliest matter was still so hot that the subatomic particles could not form lasting electrical bonds; but by the time a full second had passed, the still-expanding early universe had further cooled enough that pairs of protons and neutrons could draw together to form atoms of hydrogen, the very simplest element possible, its normal atom being just one electron and one proton (isotopes of hydrogen containing one or more neutrons also exist, but are relatively rare).
Helium, the next-simplest element--two protons, two electrons, two neutrons--was also formed, at about one-quarter the amount of hydrogen, at this time (the great initial matter-formation phase ended perhaps three minutes into the life of the universe); there was also a trifle of Lithium, the next-lightest element after helium, formed as well, though the lithium formed then was much outweighed (literally and figuratively) by the hydrogen and helium.
Because that newly made hydrogen gas was not exactly "smooth" in filling the universe, gravity slowly worked to pull atoms from the less-dense areas toward the more-dense areas, and much of the hydrogen gas eventually separated out into clouds or clumps. At somewhere between a half million and a million years into the universe's history, those huge gas clouds started turning into the universe's first stars. The transition from a loose cloud of gas to a burning star is a simple one (in general, though the details can be complex). We must remember that those clouds of gas, the proto-stars, were immense. As such a cloud slowly condenses under the pull of gravity, it evolves a definite shape: it will have a central core that, owing to gravitational force, is essentially spherical, but at its "waist" (its equator) it will spin out an attached disk of material (artist's conception of protostar at left).
In that spherical core zone, the weight of the outer layers exerts pressure on the inner regions; that is no different than, for example, the ever-increasing water pressure one encounters as one goes deeper undersea. It is a simple law of physics (Boyle's Law) that as one compresses a gas, its temperature rises. (A gas, or any substance, contains a certain amount of heat energy, but, as we shrink its size, the average amount of heat energy in any part of the remaining space goes up; that accords with everyday experience--twenty people in a large room will not warm up much, but in an elevator they will.) Eventually, the pressure of the outer layers of a proto-star on its inner zone is so vast, and the gas in that inner zone so hot, that nuclear fusion is ignited at the core. You can see a typical proto-star amidst its parent gas cloud in the Hubble photograph at the right; below to the left is a false-color image showing a straight-on view of a young star surrounded by its disk.
We saw earlier that the hydrogen atom is the simplest possible (one electron associated with one proton) and that the next-simplest is the atom of helium (two electrons associated with an atomic nucleus of two protons and two neutrons). To oversimplify even more than we have been doing, the pressure--or, really, the consequent temperature rise--in the core of a proto-star forces pairs of hydrogen atoms together to bond into single helium atoms. The physics of nuclear fusion are such that the reaction gives off energy; in a star being born, that energy emission from the newly ignited core "pushes back" on the gases being pulled down by gravity, and in short order, a stable state is reached: the immense energy being released by nuclear fusion in the star's core just balances the immense weight of the gas around that core (which gas is also, however, heated by that fusion-released energy).
When a proto-star ignites and becomes a true star, its radiation tends to blow away most of the gas then remaining in the disk around the core. But, as the proto-star core has been forming, things have also been happening in that surrounding disk. Temperatures there were much lower than near the core, such that bits of matter could condense out of the cloud. Now we have said that the cloud is just hydrogen gas, which does not condense to solid form--so where did the other elements involved come from? We'll see in a moment, but for now, let's just note that besides hydrogen, other light elements near the bottom of the periodic table are present in the cloud in small amounts. We say "small amounts", but even a small fraction of an astronomically large quantity is itself pretty large; those other elements are present in amounts large enough to form solid dust particles.
While the core is collapsing into a proto-star, the dust particles in orbit around that core--within the gas disk--are accreting from the effects of gravitational tugs one on another. As a small ball forms in stellar orbit, it sweeps up, by its gravitational pull, more dust particles; as it becomes larger, its gravity becomes yet stronger, and soon (in a relative sense!) most of the solid matter in the stellar disk has accreted into one or a few such balls of dust each in orbit around the stellar center. To the right is an artist's conception, derived from NASA data, of the current gas-and-dust cloud around the star Beta Pictoris; we can see the "tracks" or "grooves" where growing balls of solid matter are starting to "sweep lanes" in the disk cloud.
These dust balls, like galaxies and stars, have great pressures at their cores from the weight of the material above; but unlike proto-stars, these dirt balls do not have enough total mass--that is, their weight does not generate great enough pressures--to start any nuclear reactions in their cores. Instead, they melt their core material to a liquid; and, farther out from the core zone, they crush and solidify what were once individual dust particles. These balls of cold matter are early planets, and--owing to their mass--they will remain in orbit even when the proto-star ignites and blows away most of the free gas in the disk.
Before looking at planets in more detail, let's clean up the fascinating question of where those heavier elements in the stellar dust came from. It turns out to be a literally vital question, because without those heavier elements, the entire universe would contain nothing but hydrogen and helium gas--no planets, and thus (saving bizarre improbabilities) no life.
As we have seen, helium is, in a sense, the "ashes" of the hydrogen "fires" that burn in new stars. But, as common sense suggests, you can't burn anything forever: soon or late, you run out of fuel. And that happens in both fireplaces and stars.
When a star has burned up most of its hydrogen "fuel", the energy it can generate by fusing hydrogen into helium is no longer sufficient to offset the terrific gravitational forces its mass generates--and it starts to collapse again. What then happens is that the immense energy resulting from the collapse of the correspondingly immense mass of the star starts up a new form of fusion reaction, in which the "ash" element, helium, is itself fused to make heavier elements (beryllium, oxygen, carbon). This is a yet more energy-generating process than helium fusion, and the energy it generates blows the star out to a huge new size: it becomes a "red giant" star. But now its days are numbered, for the helium-fusion process cannot be sustained for long (in a relative sense--it can last for a few million years). After this stage comes an even briefer period in which oxygen and carbon are fused into yet heavier elements: neon, sodium, magnesium, sulfur, silicon; then, in yet later (and yet briefer) periods, reactions fuse some amounts of those elements into calcium, iron, nickel, chromium, copper, and a few others. It is in those relatively brief eras in their history that dying stars bring the heavier elements into the universe.
How do the heavy elements generated in a dying star end up in the gas disks of newly forming stars? How a dying star finally ends its stellar life depends on several factors, particularly the total mass of the star--but for many a star, when all the progressively shorter forms of fusion "burning" have run out of fuel, the star explodes violently, becoming a so-called supernova. What happens is that when the final possible fusion reaction has no more "fuel" left to burn--so that the star's radiation drops off--gravity resumes its tremendous compression of the star's mass. If the mass is over a certain threshold size, the gravitational implosion is so rapid and extreme that the hyper-compressed core material "bounces back" in a violent explosion. We need some scale on "rapidly" and "violent": stars, whose lifetimes are measured in hundreds of millions or even billions of years, undergo their final collapse and explosion in a matter of hours! And the energy they release if they go supernova is so vast--often billions of times what it was during their normal time as a star--that some supernovas were seen from Earth in the daytime sky.
A famous example was seen 1604, and was recorded by many astronomers of the time; today, four centuries later, the effects of that stupendous blast continue, as the breath-taking photo to the left shows. The "ring of pearls" is clouds of diffuse gas being heated to incandescence by the expanding shockwave of material blown off from the star by its explosion (the ring, about a light-year across, already existed when the star exploded--astronomers believe the star shed the ring about 20,000 years before going supernova). The elongated, expanding cloud in the middle of the ring is debris from the supernova blast; it glows because it is being heated by radioactive elements (principally titanium 44) created in the supernova. The debris will continue to glow for many decades. The energies at work here are almost incomprehensible to the human mind.
The cloud of material ejected from a supernova (or its smaller cousin, a nova) continues to expand till some of it is captured by the gravitational field of some astronomical body--typically another star or proto-star. The earliest universe was empty of any element but hydrogen. When the first stars formed, helium was generated in quantity as the stars "burned" their hydrogen. But it was not until some of that first generation of stars died and exploded that the universe became saturated with small quantities of the heavier elements, generated by the death throes of the earliest stars. As the millennia rolled on, heavier elements became widespread--though always, even today, quite scarce compared to hydrogen and helium on the universal scale. The gas clouds that became newer proto-stars now contained nontrivial amounts of those heavier elements, and so--when those proto-stars ignited--the disks surrounding them were, as we saw earlier, able to partially condense into solid balls of matter: planets.
But before we get to looking at planets in general and Mars in particular, we should--for completeness--stop a moment and look at the fascinating other end of the size scale in the universe: galaxies.
The early universe, at, say, a million years out from its birth, still contained (and to this day still does contain) great amounts of hydrogen and helium gas dispersed throughout it. But by then it also contained the first generation of stars. As time went on, those early stars, under the influence of gravity, began to start associating into larger structures. And just as a star is begun by individual atoms (or molecules) forming a ball that has a wide disk extending about its equator, so the forming clumps of stars acquired rotation, and started to flatten out while forming a rounder core. But the stars in a grouping are much less tightly bound together than the atoms in a collapsing ball of gas, so the structures they formed had, in proportion, much smaller cores and much larger and wider disks. (At right: Dwarf galaxy I Zwicky 18, the youngest known galaxy--just starting to round and flatten.)
Those gigantic spinning clouds of stars that emerged early in the history of the universe were each the beginning of a galaxy. (The word "galaxy" also derives from Greek, galaxias referring to what we now call the "Milky Way", in turn from gala, milk; the Milky Way in the sky is the disk of our home galaxy as seen by us from within it.) And just as the galaxies themselves formed out of the many free-floating stars, so also did the young galaxies bunch up into larger structures--what we today call galaxy groups and clusters (there is no sharp dividing line between the types--"groups" typically comprise up to fifty galaxies, "clusters" typically comprise 50 to 1,000 galaxies); and those, in turn associated into yet larger structures call galaxy clusters (which can comprise thousands of galaxies); and those in turn associated into what we call super-clusters. Yet even those are not the very largest structures known to exist in our always-amazing universe.
To the left is a view of a far galaxy, about the farthest yet found, as viewed through the amazing Hubble orbiting space telescope. Because light travels at a finite speed, and this galaxy is mind-bogglingly distant from us, we are seeing it as it looked roughly 13 billion years ago, when the light that we are only just now seeing actually left it; that means that this galaxy--designated HUDF-JD2--formed within the first 5 percent or so of the present lifetime of the universe, and so will be one of the very earliest galaxies.
Galaxies, as they spin, not only flatten out--their disks tend to separate into "wisps" or "streamers", commonly called the arms of the galaxy; galaxies of that common form are
called "spiral galaxies", for reasons obvious when one sees a picture of one. That can be seen in the image of HUDF-JD2, but it is not very clear; a more emphatic example--galaxy M74--is
shown below to the right.
Not all galaxies form such emphatic "whirlpool" patterns: some become what are called "barred" galaxies, a name deriving from their seeming to have a central "bar" of stars. You can see an example--galaxy NGC 1300--below on the left. (Recent studies suggest strongly that, over time, galaxies slowly switch back and forth between the spiral and barred forms, as a function of complex interactions with the clouds of gas that still remain associated with them.)
It is almost beyond human understanding that each of these galaxies is composed of staggeringly many individual stars (a single typical galaxy comprises from ten million to a trillion stars), of which our own sun is just one more--and that there are probably, well, let's put it in numerals, roughly 1,000,000,000,000 galaxies in the part of the universe that we can actually see from Earth. That's a lot of stars in the universe.
The only planets we know much about are those associated with our own home star, the Sun. Indeed, until very recently, no one knew whether planets elsewhere were extremely rare or quite common--or even existed at all--and opinions divided sharply. But in just the last few years, careful observation, made possible by such important new tools as the Hubble orbiting telescope, have made it plain that planets of some sort seem the rule, not the exception, in stellar systems (the best current guess is that about half of all stars have what it takes to make planets).
From examining our own solar system, we find that planets seem to divide into two or perhaps three rather distinct classes: relatively small, relatively cool rocky masses (of which our own Earth is one and Mars another), so-called "gas giants", and possibly--it's a matter of opinion if this is a distinct class--"Kuiper Belt" captures (we'll explain that in a moment). Within each of the classes there is a fair amount of difference from one planet to another; but the differences between the classes themselves are huge and dramatic.
In our solar system, the four so-called "inner planets" (Mercury, Venus, our own Earth, and Mars) are all solid, rocky masses; beyond them lies the asteroid belt, a wide ring of small rocky lumps mostly ranging in size from a few miles wide down to pebbles and dust (a very few are slightly larger); beyond that lie the four "outer planets" (Jupiter, Saturn, Uranus, and Neptune), all gas giants; while beyond Neptune lie Pluto and perhaps some others like it. The illustration at the right shows the planets to scale, in their order going out from the Sun--it is easy to see why "gas giants" are so called.
"Giant" is a relative word: as you see above, the gas giants are indeed gigantic compared to the four inner planets--yet Jupiter, the largest, represents only one-tenth of one percent of the mass in the solar system (the Sun, our parent star, dominates everything, being 99.86 percent of all the mass in the system). The gas giants are called that, and are so huge, because most of the mass of each is not solid material but rather very dense gases (in fact, most likely they are mainly liquids). It is hard to say just where a gas giant's true atmosphere leaves off and its surface begins. Nonetheless, it is almost certain that a gas-giant planet has, underneath that gigantic shell of gases or liquids that constitutes most of its size and bulk, a small rocky or metallic core--perhaps much like one of the inner planets.
"Gas" is also something of a misnomer, in that although the mass of this type of planet is mostly hydrogen and helium, those substances--normally gases--are under so much pressure, owing to the weight of the planetary mass, that they are mainly in liquid (or possibly, deep down in the planet, solid) states. Some astronomers suggest further subdividing the "gas giant" class of planets into two parts: true "gas giants" like Jupiter and Saturn, which are both mostly hydrogen and helium; and "ice giants" like Uranus and Neptune, whose compositions are mixtures (or perhaps layers) of rock, water, methane, and ammonia (all mostly or entirely frozen).
It was long thought that gas giants could only form at some distance out from a sun, where little solar radiation--heat, that is--can reach a planetary surface, else that solar radiation would have boiled off the gases that make up most of these planets. But recent discoveries of planets orbiting other stars show some that seem, from their size and especially their mass, to be "gas giants"--many much bigger than our own Jupiter--yet with orbits lying quite close to their parent suns. Various theories attempt to explain such planets: perhaps they are not true gas giants but some other form, or perhaps they are quite young yet and their sun has not yet had time to "boil them down". Improvements in our ability to detect and analyze such remote bodies will eventually tell us more.
(That the only extrasolar planets so far detected are all very large and quite different from our Earth means little, in that our methods of detection are still raw enough that the only planets likely to be found are very large ones; it is almost universally believed that as our techniques improve, we will also find many samples much like the inner planets of our own solar system.)
Though conventional thought holds that gas giants are extremely unlikely to harbor life of any sort, owing mainly to their very low temperatures and their lack of the chemical bases of "life as we know it", some imaginative biologists have conceived of exotic life forms that might exist in the higher layers of such planets' atmospheres. But even modestly conservative opinion on the subject rules out gas giants as possible homes to life. (But it is not impossible that some of the moons of gas giants might harbor life--we will discuss moons, and that aspect of them, farther below.)
Besides true planets, and their associated moons, the solar system comprises several orbiting rings of "junk": the Asteroid Belt, lying almost entirely between Mars and Jupiter; the Kuiper Belt, ranging from within Neptune's orbit out to the so-called "Kuiper Cliff" bound; and--it is commonly believed--the presumed Oort Cloud, a huge distance beyond any of the planets. All three contain cold, rocky material, though each differs somewhat from the others.
In the Asteroid Belt, the millions of objects are generally very small, some being just pebbles or even dust. There are only about 220 asteroids larger across than 60 miles, and the very largest (Ceres) is only about 600 miles across. All the asteroids added together (and Ceres alone is about one-third of their mass) would sum to less than the mass of tiny Pluto--which, in turn, is smaller than Earth's Moon, as the illustration to the left shows.
The existence of the Kuiper Belt is a relatively recent discovery, and it is by no means fully explored (by telescope) yet. Computer simulations of its origin (which involve gravitational effects from massive Jupiter on the bodies of the early solar system) suggest that it might harbor things as large as true planets, say Mars- or even Earth-sized. So far, fewer than a thousand bodies have been identified in the Belt, but many astronomers include on that list the "planet" Pluto and its moons (Charon and the two smaller ones just found), as well as the recently discovered Quaoar--fully half the size of Pluto--plus Varuna and Orcus. (The newly found planetoid Sedna is usually considered part of the Oort Cloud). The traditional count of planets in the solar system has been, since Pluto was spotted in 1925, nine; but it is now widely felt that Pluto should not be counted a true planet, but "merely" a "Kuiper-Belt Object", though the reduction in count to eight planets is still a matter of modest controversy. (What would happen if a Mars-sized, or even Earth-sized, body were ever found in the Kuiper Belt is anyone's guess.)
The Oort Cloud is a postulated--but not yet definitely observed--mass of objects, primarily comets, surrounding the solar system more like a sphere then an orbiting belt. Its existence was deduced, and seems necessary, as a source for comets, since typical comets--very fragile structures--are destroyed by just a few passes through the inner solar system, so there must be a supply of material for new ones to spring from. The planetoid Sedna is thought to be the first definite Oort Cloud object to be discovered, though it is not universally accepted yet as such an object.
Here is a set of illustrations that place the three planetoid belts in proportion to one another and the solar system as a whole; start at the upper left and follow around clockwise--each box enlarges from the one preceding it. As you can see, the Oort Cloud is, compared to the known planets, at a huge distance out from the Sun--but still within its field of gravity, and so still indeed a part of "the solar system". Incidentally, a year (one revolution around the Sun) on Sedna takes roughly ten thousand Earth years!
The numerous planetoids in the solar system--and there may be more things yet to be found (such as planet-sized bodies in the Kuiper Belt)--are interesting but minor players in the planetary scheme of things.
(Recently a planet-sized object, officially designated 2003_UB313 but nicknamed "Xena", has been discovered, accompanied by a moon nicknamed "Gabrielle"; it seems to be about 20% larger than Pluto, and has an eccentric orbit that sometimes brings it closer to the Sun than Pluto, though it is usually much farther out. We can expect more and more such bodies to be discovered in time.)
While we have the term "gas giant" for the outer planets, there is no corresponding universal term for the type of planet represented by Mars or our own Earth; we can call them "rocky planets" or "solid planets" or any of several other similar terms. But, by whatever name, they are the ones that for many hold the most interest, especially as they are the ones most likely to hold life.
The four inner solid planets of our own Solar System demonstrate fairly well the narrowness of possibility for life, at least life as we currently think it possible. Science-fiction books and movies may suggest that life can exist anywhere in any form, but it is not so--a fairly narrow range of temperatures (high enough that certain essential chemical processes can take place, but low enough to avoid breaking complex and somewhat fragile organic-molecular bonds), an abundance of certain elements (oxygen for organic energy generation, carbon as the basis of organic chemistry, and water or, just perhaps, ammonia in liquid form--at least those things are necessary for any but some wildly exotic (and highly improbable, though not fully impossible) non-organic life form to exist. A brief review of the four inner planets shows how narrow the possibilities are.
(We discuss the possibilities of life on Mars, and elsewhere, at much more length on a separate page of this site.)
Little Mercury is so close to the Sun, and so small (meaning it has a weak gravitational pull), that whatever atmosphere it may have had long ago boiled away into space; its surface experiences wild swings of temperature, from -300°F to +800°F. Venus, long thought of as a "twin to Earth" (they are about the same size and mass), possibly even swamp-like, turned out to be no such thing, but rather a hellishly hot (750°F.), wind-scoured desert of a world--owing largely to a runaway "greenhouse effect" from the large amounts of carbon dioxide in its atmosphere (Venus probably once had about as much water as Earth, but as it evaporated into the atmosphere, solar radiation split it into its constituent hydrogen and oxygen, with the hydrogen then boiling away into space leaving the free oxygen to combine with with elemental carbon to form that carbon-dioxide layer and thus heat the planet).
Our own Earth we know to be hospitable to life (though we seem determined to artificially make our own greenhouse CO2 layer, and otherwise disrupt ecological processes and balances). We have a reasonably stable surface temperature in the right zone for organic chemistry to thrive; we have an oxygen-rich atmosphere to power organic chemical reactions; we have generous supplies of carbon and nitrogen, essentials of organic chemistry; and we assuredly have a large expanse of liquid water in which such reactions can readily proceed.
When we look out past Earth to Mars, we see a planet that today is less than ideal for life: no expanses of liquid water, a thin atmosphere, and somewhat low temperatures. But, as many have noted, none of the parameters are such that life--considering the weird varieties we are constantly finding on Earth in the strangest places (deep ocean, volcanoes, and so forth)--is outright impossible or even wildly improbable. What is improbable is that life would initially arise under such conditions--but conditions were not always thus, and it is on that fact that those hopeful of discovering a life form that has developed independent of Earth, on another world, stake their hopes. We will see more fully how and why life might have evolved on Mars--or in some other places in the Solar System--on another page of this site; but before we finish up here, let's look at those other places where life might exist.
A "moon" is just an astronomical body orbiting any other astronomical body that isn't a star--else it would be a planet or an asteroid or a comet or some such thing. (There do not seem to be any moons with moons of their own, chiefly because the gravitational field of a moon's "primary"--what it orbits--would tug any such body out of orbit in a relatively brief time; but it's not impossible by definition, and a few asteroids actually have tiny "moonlets".)
One tends to think of moons as small because they are, naturally, smaller than their primaries; but "large" and "small" are relative terms. Earth's Moon, a rather large one for the Solar System, is significantly bigger than Pluto, which long was (and by some still is) considered a planet. A couple of Jupiter's moons are about as large as or larger than Mercury. Indeed, a fair number of moons in our solar system would, if placed in appropriate orbits around the Sun, easily qualify as "planets": those about as large as or larger than Pluto--besides our own Luna--are Jupiter's Io, Europa, Ganymede, and Callisto; Saturn's Titan; and Neptune's Triton.
In general, small planets seem to have few (or no) moons, while gas giants roll in them: Mercury and Venus have no moons, Earth one, and Mars two (and those are scarcely more than hills in space--one seven miles across, the other thirteen); Jupiter has at least 63, Saturn at least 35, Uranus at least 27, and Neptune at least 17--and new space probes always seem to find a few more for each. Of this plethora of astronomical bodies, several are of interest as having been plausibly considered as possible homes to extraterrestrial life. (In fact, of all the suggested possibilities in the Solar System, Mars is the only one that is not a moon.) Leading candidates (at least relatively) are Titan (the only moon known to possess a significant atmosphere, and which may also possess a liquid ocean, now under an ice sheet); Europa (which may once have had a liquid ocean); Ganymede; and Saturn's tiny Enceladus (which has a thin atmosphere, but one of water vapor, and which thus may even now have liquid water beneath its surface).
Many of the Solar System's moons--probably most--formed in the same way as did the planets: condensation out of the dust-and-gas disk surrounding the early Sun; that is probably why gas giants have so many moons: their large mass could gravitationally "capture" other bodies forming in their vicinity. But there are also some moons believed to have different origins--some may be gravitational captures from farther out, possibly the Asteroid Belt, the Kuiper Belt or even the Oort Cloud; some are likely fragments of once-larger bodies broken up by either collisions with other objects or gravitational stresses; some may be chunks that would have formed a larger object save that their primary's gravity interfered. Our own Luna is perhaps the most interesting case of all moons: it is now believed to be a chunk of the Earth blasted off rather early in the history of the Solar System by a titanic collision between the young Earth and a proto-planet about the current size of Mars. If so, what an awesome event that must have been!
In short, though, moons are not structurally different from planets of about the same size and distance from the Sun, and are no less (or no more) likely to harbor life than they would were they true planets.
To see how likely it is that Mars, or any other known or supposed astronomical body, might harbor life that has arisen independent of life on Earth, go to the site page about life on Mars.
(Why not look in at Is it a blog yet?)
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Site Directory:
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(introducing this site) |
Introductory Material: | ||
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Please do take a moment to look over the Introduction page--it's helpful. |
Front Page:
· a quick site overview and some mechanical details |
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(the hard-core Mars information) |
Scientific Mars: | ||
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You can read these pages in any order, but the way they're listed here is clearest. |
Planets:
· what they are and where they come from |
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Life on Mars:
· the possibilities there, and elsewhere |
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Exploring Mars:
· who did what when, and what's next |
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(Mars in human perspective) |
Human Mars:
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No technical information about Mars here, but lots of interesting Mars-related things. |
Mars in History and Culture:
· a history of beliefs about the heavens and the objects in them |
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What If We Find Life on Mars?:
· speculations on the consequences of proven extraterrestrial life |
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Science and Religion:
· a brief look at how they do or do not conflict |
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Further Mars Resources:
· where to find more Mars-related information |
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NASA Online Resources:
· there's so much, it needs a page of its own |
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(about buying books on Mars and space) |
Mars-Related Books: | ||
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Buying Books New:
· about buying books from Amazon · searching for new books at any Amazon division |
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Buying Books Used:
· about the Advanced Book Exchange |
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The late Brian Keith, regrettably and amazingly, has no star on the Hollywood Walk of Fame; click to find out how you can help get a Star for Brian.