Astronomy

Why doesn't star size distribution conform to space rock size distribution?

Why doesn't star size distribution conform to space rock size distribution?


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The number of space rocks is exponentially related to the size of the rocks. There are more small space rocks than big ones. Stars are most commonly the size of the sun, big and smaller stars are rare, and giant stars are the rarest. Stars that are 0.5 solar masses should also be very bright and visible, but they are less numerous, when they should be more frequent than the sun.

What about planets? There should be many unaccompanied planets wandering in the darkness of space, without a star, and they should be a lot more common than stars themselves, becaues of the size distribution of space rocks and stars should be roughly parallel.

What am I missing?

graph of star size distribtion.

graph of space rock size distribution.


You are comparing distributions in a way that they are not easily comparable and the eye is misled: watch the scaling of your axes of the plots you compare! In order to compare, you want to make sure that you use similar, either log-log for both graphs or linear-linear or something else - but identical in both graphs.

Mind also that the size distribution of the small solar system objects is NOT an exponential, but a power law distribution. A power law distribution is a sloped straight line in a log-log plot and the exponent of the power law is found in the slope in such plot.

The stellar size distribution usually is found under the keyword "initial mass function". And when you look at that (see the plot therein), you will find, that the size distribution of stars also follows a similar power law as does the size distribution of smaller bodies as found in our solar system. This ignores the evolution of stars - but that is what you want to do in order to compare the sizes of objects as they are created.

The slopes of the size distributions course differ as different processes play a role in formation of stars and the objects forming around a star-being-born.


NASA Will Need Your Help Mapping Asteroid Bennu

Your mission, should you choose to accept it, is simple — click on the boulders you see in a detailed image of a distant space rock.

That straightforward task could steer NASA's OSIRIS-REx mission toward a successful sample collection from an asteroid named Bennu. The team in charge of the spacecraft will have just six weeks between producing an extremely high-resolution map of Bennu's surface and choosing where precisely to gather a sample from that rock.

"It is a massive task to actually map every single boulder on the surface, and we just didn't know a better way to do it in the amount of time," Carina Bennett, an imaging processing engineer for the mission based at the University of Arizona, told Space.com. "We basically really, truly need the help."

So Bennett and her colleagues turned to CosmoQuest, a platform for crowdsourcing space science, for the extra eyes they need to complete the mapping in time. The Bennu mapping project will go live on the CosmoQuest site beginning in late April or early May. That's when asteroid buffs will be able to help NASA out, tagging the edges of boulders in detailed images of Bennu's surface.

The exact launch date for the project depends on precisely how long it takes the OSIRIS-REx team to compile the detailed global mosaics that will reveal the boulders in need of counting. (The spacecraft is currently working on the detailed survey that will produce the many different angles of imagery scientists need to build that global mosaic.)

Once the mosaic is complete, the clock starts ticking, with just six weeks to identify each boulder on the surface and produce a detailed hazard map of Bennu's surface. That information will be passed along to the team in charge of selecting where OSIRIS-REx will attempt to grab a sample so they can reduce the odds of the sampling apparatus floundering on boulders.

That's particularly important because Bennu has turned out to be a more complicated target than scientists thought before the spacecraft's approach. "It was significantly more rocky and bouldery than we expected," Bennett said.

So the team needs to be careful to avoid hazardous sites. A boulder 8 inches (21 centimeters) across, for instance, could clog the spacecraft's sampling mechanism. The apparatus works by blowing compressed air at the surface, so if it touches down off-kilter, it could be stuck at a tilt and simply blow the targeted sample away.

That's why the OSIRIS-REx team can't choose their sampling location until they consult with a comprehensive boulder tally — and with a team of amateurs from around the world. "I think the most interesting part is that you're actually contributing to a decision," Bennett said. "You can actually go and watch when we sample and potentially see some of the same areas and images that you might have actually counted."

But the mapping project doesn't end when the OSIRIS-REx team selects the sample site. There's science work to be done with the images as well. The work CosmoQuest volunteers do will help the OSIRIS-REx team study the size distribution of boulders, for example, and how light or dark they are.

With that data in hand, scientists should be able to draw conclusions about how Bennu works. Maybe lighter rocks are smaller, suggesting they're made up of material that breaks apart more easily. Maybe the direction boulders point toward suggests that the asteroid is shaking them around its surface.

The team also wants to be able to compare these pre-sampling images with those that the spacecraft will gather after it snatches a piece of Bennu to bring home.

"We're taking in a massive amount of data," Bennett said. "Just eyes on images is incredibly valuable."


THE BIG BANG THEORY IS WRONG

A top scholar says there are big holes in the big bang theory.

Big Bang scientists extrapolate a hypothetical scenario from a few facts. Yes, some galaxies are expanding, moving further away, but this is not the case with the entire universe. There are galaxies in the universe running perpendicular to the rest of the galaxies. That’s contrary to Big Bang. If Big Bang really occurred, there should be a uniform distribution of gasses.

This uniform distribution of the gasses would have made sure that the gasses would not have coalesced, due to gravitational attraction, into planets and stars. The hypothesis of dark matter providing enough gravitational force has been recently discredited.

“The (galactic) structures discovered during the past few years, however, are so massive that even if CDM (Cold Dark Matter) did exist, it could not account for their formation” (Dr. Duane T. Gish, “The Big Bang Theory Collapses”). Furthermore, an explosion cannot explain the precise orbits and courses of thousands of billions of stars in thousands of billions of galaxies.

Some evolutionary astronomers believe that trillions of stars crashed into each other leaving surviving stars to find precise orderly orbits in space. Not only is this irrational, but if there was such a mass collision of stars then there would be a super mass residue of gas clouds in space to support this hypothesis. The present level of residue of gas clouds in space doesn’t support the magnitude of star deaths required for such a hypothesis. And, as already stated, the origin of stars cannot be explained by the Big Bang because of the reasons mentioned above. It is one thing to say that stars may decay and die into random gas clouds, but it is totally different to say that gas clouds form into stars.

Most people don’t realize how much disagreement there is among evolutionary scientists concerning their own theories. The media doesn’t report those details, at least not to any substantial extent.

Read the author’s recent collage of creationist evidences: The Science Supporting Creation


The crowded solar system

The first step to stopping a killer asteroid is finding it. “There are literally hundreds of thousands of asteroids out there, and we want to separate out those we should keep a closer watch on and monitor over time,” says Lindley Johnson, NASA’s planetary defense officer. So far, he says, there are 2,078 potentially hazardous asteroids in the catalog.

Traveling at nearly 20,000 miles an hour, 1998 OR2 will pass within four million miles of our planet this week, or roughly 16 times farther than the moon. While this distance is no cause for concern, 1998 OR2 will continue on its 3.7-year orbit around the sun, venturing into the asteroid belt beyond Mars and circling back inside Earth’s orbit with each lap. On its next approach to our planet in 2078, it will be much closer, swinging within about a million miles of Earth. After a few hundred years, astronomers can’t calculate exactly where 1998 OR2 will be.

NASA classifies anything over 140 meters (about 459 feet) wide that passes within five million miles of Earth as a potentially hazardous asteroid. “Five million miles comes from how much orbits can change over time, and a little bit of a margin put on it, of course, to be sure that we capture anything that might be a potential impact hazard in the future,” Johnson says.

In just seven years, another huge asteroid called 1990 MU, nearly two miles wide, will pass within three million miles of Earth.

“We don’t want to get hit by something that big,” Johnson says. “Our most important task is finding them and getting a fuller catalog of everything that’s out there, so we don’t get surprised.”

In 1998, the U.S. Congress directed NASA to detect and characterize at least 90 percent of potentially hazardous asteroids measuring one kilometer (about 3,200 feet) across or larger. Seven years later, the space agency was directed to find 90 percent of nearby asteroids that are 500 feet wide or larger.

The bigger asteroids, including 1998 OR2 and 1990 MU, could devastate life across the planet if they hit. “It is estimated that asteroids of one kilometer or more would result in continent-wide devastation, and the dust injected into the atmosphere would cause drastic cooling and the possibility of global crop failures for at least a few years,” says Jay Melosh, a geophysicist at Purdue University.

We’ve found roughly 900 of these larger objects, or 95 percent of the estimated total population. None are even remotely likely to hit the planet in the next several centuries. But of the smaller group, which could still destroy cities, we’ve only detected roughly 30 percent of the estimated 25,000 objects, according to a report from the National Science and Technology Council.

“These smaller ones—the sub-global sizes that are capable of causing regional problems—we still have a lot of work to do,” Mainzer says. “Searching for these gray or black rocks against the blackness of space—it’s just a hard problem.”

Even space rocks smaller than 500 feet wide can be extremely dangerous. Some meteors explode in the sky with the strength of nuclear bombs, such as one that burst over Chelyabinsk, Russia, in 2013. At only 66 feet wide, this fireball meteor caused a shock wave that hit the city, shattering glass and resulting in about 1,500 injuries. No one saw it coming.


Venus

I would love to explore the planet Venus as it is the second planet from sun and is of almost similar size ,mass and composition as earth and is the brightest object in the sky and is close to our planet earth .It is named after the Roman Godess of Love and Beauty .It is the brightest object in the sky after moon and can cast shadows and on rare occassions it is visible even in the day time .

With a rotation period of 243 earth days, it takes longer to rotate about its axis than any other planet in the Solar system by far , and does so in the opposite direction to all but Uranus .HOW & WHY ?

Venus does not have any moons ,a distinction shared only with Mercury .WHY ?

Venus has a thick toxic atmosphere filled with carbon dioxide and it is perpetually shrouded in thick ,yellowish clouds of Sulphuric Acid that trap heat , causing a runaway greenhouse effect .It is the hottest planet in our solar system ,even though mercury is closest to the sun .Venus has crushing air pressure on its surface , more than 90 times that of the earth. WHAT LEAD TO THIS RUNAWAY GREENHOUSE EFFECT ? WHY VENUS 's atmosphere is not yet stripped off its surface .Venus has not lost its atmosphere yet .What is making that possible ?

Venus was the first planet to be explored by a spacecraft - NASA's Mariner 2 which successfully scanned and photographed on 14th December 1962 the cloud cover world so close to our home planet earth and so identical in so many ways .Since then numerous spacecraft from US and various other agencies have explored venus , including NASA's Magellan but only Russia could land their spacecraft on the surface of Venus but it did not survive the immense atmospheric pressure of Venus and its hostile environment .

Another interesting discovery is the discovery of Phosphine parts-per-billion levels could be a bio-marker .This raises the possibility that biological processes could be responsible .Phosphine is a toxic flammable gas with a characteristic smell of garlic or rotting fish .Phosphine is highly reactive and so survives for only a very short time .There could be an airborne ecosystem at temperate altitudes in Venus 's thick atmosphere .

Venusian atmosphere is highly acidic .So any form of life would need to be able to cope with this in someway ,perhaps coated with a thick protective layer or perhaps exploit the acidity .

The Planetary geologists recently concluded that Venus may have supported liquid water for many billions of years, until as recently as 700 millions of years ago .It then suffered a catastrophic greenhouse effect , which left it with a surface temperature hot enough to melt.

Understanding what went wrong will be crucial in preventing a similar catastrophe on earth .That's the reason why NASA ,ESA and other agencies must turn their attention urgently to Venus .

Catastrophe

Approaching asteroid? Is this THE one?

Generally, a pretty good summary.

One point:
"Understanding what went wrong will be crucial in preventing a similar catastrophe on earth ."

I think we already know and may be running out of time to avoid it.

Dragrath

Catastrophe

Approaching asteroid? Is this THE one?

Catastrophe

Approaching asteroid? Is this THE one?

Amongthestars931

Generally, a pretty good summary.

One point:
"Understanding what went wrong will be crucial in preventing a similar catastrophe on earth ."

I think we already know and may be running out of time to avoid it.

Trevize62

Dragrath

Hmm you are right that there are many mysteries here but a few problems first the Sun was dimmer early on which has to be factored in and the state of the atmospheric system is strongly dependent on the way the system evolves. It is true we don't know whether Venus had oceans but we do have good evidence for it once having water.

The role of the Magnetosphere on Mars is complicated its biggest problem is that Mars has a weak remnant magnetization induced magnetic field from when the planet had a real magnetic field. This field is weak and decentralized so it connects directly to the solar wind i.e. its field lines are an open configuration. The consequence is any particles in Mars's atmosphere at any altitude that gets ionized or has intrinsic polarity is accelerated away along the magnetic field lines directly into the solar wind. This effect becomes extremely pronounced during a Martian global dust storm as the denser medium allows more water vapor to get into the air and allowing charge imbalances to build up. Once a molecule is charged it is then funneled on a one way trip out of the solar system.

Then there is the complicating factor that Mars lacks the gravity to prevent molecular nitrogen, water, argon, molecular oxygen, carbon monoxide, methane etc. from drifting away in significant amounts due to the temperatures of the upper Martian atmosphere meaning a significant upper tail of the velocity distribution exceeds the escape velocity of Mars. Add the ionization effect from remnant magnetization and they are gone. During a Martian dust storm residency times for water vapor are measured in hours to days as the natural buoyancy combined with the polar nature of the molecule and the magnetic field lines conspires to accelerate molecules away from the surface of Mars into the solar wind. It is quite tragic but Mars is the main culprit for the loss of its atmosphere as it effectively gives its atmosphere to the solar wind for free. There are also atmospheric gravity buoyancy balanced waves which propagate strongly up from the surface of Mars in a one way flow away from the planets surface further accelerating the loss of atmosphere in a manner like how a river flows downhill gases on Mars flow up and away thanks to the combination of these factors particularly during Martian Dust storms.

The point is that research on Mars's atmosphere has revealed that a weak magnetic field is worse than no magnetic field at all couple that with the impacts of the gravity of Venus and Mars and Venus's lack of remnant magnetization from the planet contributing to its induced magnetosphere and the difference is substantial. Surprisingly the water loss rate from Venus appears to be virtually identical to the water escape rate of Earth Mars on the other hand expels as much water as there is water vapor to expel.

Whatever process wiped Venus's magnetosphere did so cleanly where as Mars and the Moon for that matter have local remnant magnetization which actively funnels charged particles away from the surface into the solar wind. And even a magnetic dynamo isn't enough if the field is much weaker than the solar wind field as is seen with the not quite dead magnetic dynamo of Mercury where the solar wind is so dense it pushes the field lines away from the planets surface.

Weak magnetospheres are quite simply awful for maintaining an atmosphere so the best case for losing one is having any remnant magnetization get quickly erased else you become like Mars where literally any physical process that could effect atmospheric escape seems to be compounding to create an astonishing loss rate. Basically any dreams of terraforming Mars are hopeless and will only result in a planetary mass comet nucleus spewing out volatiles about as fast as you can throw them in unless you want to go through the effort to eliminate each and every contributing factor. Unfortunately the need to remove/destroy Mars's remnant magnetization is a bit problematic as it kind of requires melting Mars and then there is the need to block radiation from reaching Mars the need to add an artificial magnetosphere large enough to prevent the solar wind from wrapping back around to the planets surface. It is kind of Ugly. Venus at least still has most of what would be needed to terraform it water is the main thing lacking that and the need to sequester that dense atmosphere into rock (as removing it would be getting rid of the planets valuable reserves of Carbon, Nitrogen Sulfur and Phosphorous pretty much the only significant source of said elements in the inner solar system aside from the Sun and Earth but I digress.


Why were some ancient galaxies so bright? Supercomputer probes mystery

This image shows a snapshot of the gas density distribution at one moment in time of the model starburst galaxy, which stretches roughly 650,000 light years across. Extreme star formation in the central galaxy is fueled by significant amounts of gas flowing in, rendering it extremely bright.

Not all astronomy is about gazing at stars. By creating a galaxy inside a powerful supercomputer, scientists say they’ve developed a model that may explain how some of the brightest galaxies in the early universe came to be.

The findings, described in the journal Nature, could help solve a long-standing mystery about the origins of these luminous objects in the early cosmos.

The brightest denizens of the cosmos today literally pale in comparison to the behemoths that populated the early universe just 3 billion years after the Big Bang.

“During that epoch, known as cosmic noon, the average star-formation rate across the cosmos was 100 times higher than it is at present, and individual galaxies were growing commensurately rapidly,” Romeel Davé of the University of the Western Cape, who was not involved in the study, wrote in a commentary. “This was illustrated by the surprising discovery, more than a decade ago, of galaxies whose star-formation rates during that era were 1,000 times the Milky Way’s current output — no such galaxies are seen in the present-day universe.”

Even though these monstrous galaxies are bursting at the seams with bright stars, astronomers didn’t even know they existed until relatively recently because the visible light that’s being generated by the stars is actually absorbed by the massive amounts of dust surrounding the galaxy and re-emitted at longer, “redder” wavelengths — rendering them essentially invisible to optical telescopes.

“Usually, young stars are covered with a veil of dust, but in this case, the dust is covering the whole galaxy itself,” said study coauthor Dusan Keres, an astrophysicist at UC San Diego. “So it’s an unusual situation.”

But once astronomers built radio telescopes that could pick up longer wavelengths of light, these giants started popping up around the night sky.

Why did the early cosmos have such active, monstrous galaxies — called submillimeter galaxies, for the wavelengths of light at which they emit most strongly — which aren’t seen at all in the universe today?

Scientists have argued over two main theories. Perhaps these enormous galaxies were the result of violent galactic mergers, when two gas-rich galaxies crashed into each other. After all, that’s how many of the brightest galaxies in the universe today come to be (even though they’re admittedly much dimmer than their predecessors from more than 10 billion years ago).

On the other hand, perhaps these galaxies simply are long-lived galaxies that grew to their impressive size and brightness by steadily gathering more and more gas over a much longer period, perhaps a billion years or so.

Unfortunately, it’s hard to tell which one is true just by looking at the galaxies themselves, Keres said.

“They were observed with these radio telescopes that have relatively poor resolution, so we only see them as a smudge in the sky … so we couldn’t see much detail in there,” Keres said.

Instead, the researchers ran a sophisticated simulation of the dynamics of one such galaxy, using a powerful supercomputer at the Texas Advanced Computing Center. They took into account the emissions from the stars themselves, the obscuration by the dust and the re-radiation of the starlight in longer wavelengths.

“We have much more detailed modeling of the star formation and what the star formation is doing to the galaxy itself,” Keres said.

The model showed that, in fact, these galaxies really could grow steadily over long periods, without the need for a violent galactic smashup.

“Our model is showing that what’s powering submillimeter emission of these galaxies is constant bombardment by smaller galaxies and lots of gas that they are eating from their surroundings,” Keres said.

This was made much easier by the fact that the universe was much denser in the past than it is today, with more gas and dust squeezed into a smaller volume. A galaxy that thrived some 3 billion to 2 billion years after the Big Bang, for example, would have inhabited a cosmos that was roughly 20 to 50 times denser than it is today, Keres said. Because the universe is expanding, it gets sparser with every passing eon.

This doesn’t mean that a galaxy merger could not possibly create submillimeter galaxies — but they would probably be in the minority, Davé said.

“What is particularly encouraging is that the authors did not tune the simulations so as to reproduce [submillimeter galaxies]: rather, they simply used a state-of-the-art galaxy-formation model and ran it at the highest currently feasible numerical resolution — and a plausible [submillimeter galaxy] emerged,” Davé said.

Of course, this is just a simulation of a single galaxy, the scientists pointed out. More simulations would need to be done to show whether this could really be the rule among this population of bright, ancient galaxies.

And newer radio telescopes like ALMA (short for Atacama Large Millimeter/submillimeter Array) could finally shed clearer light on what might be happening in these distant objects.

“There are still a lot of unanswered questions,” Keres said.

In the meantime, Davé said, the authors of the study “have presented the first impressively viable model of [submillimeter galaxy] formation, allowing us a tantalizing glimpse behind the mask of these behemoths of deep space.”

Star-struck? Follow @aminawrite for more intergalactic science news.


Why are planets spherical? Are there any that aren't?

It may not be a perfect sphere, but it seems like most (maybe all?) planets are relatively spherical. Why?

The first thing to understand is that all matter attracts all other matter by gravity. Over time, atoms floating freely in space will clump together into objects.

Smaller objects can be irregularly-shaped, such as many asteroids and comets. This is because while their mutual gravitational attraction is strong enough to bring them together, it is not strong enough to smooth the distribution of matter into a spheroidal shape around the center of mass more quickly than other factors continue to deform it (e.g., new mass accumulating, or impacts blasting it into new irregular shapes).

But as an object becomes more massive, the pull of gravity will tend to distribute the matter more evenly relative to the center of mass. Earth, for instance, is so smooth - and things like mountains and canyons are so trivial compared to its radius - that the planet is actually smoother than a cueball.

An object that is relatively smooth like this is said to be in "hydrostatic equilibrium," and is one of the criteria developed by the International Astronomical Union (IAU) for the definition of a planet. An object with too little gravity to become smoothed like that is therefore excluded as a different kind of object.

However, higher gravity is not the only determinant of how spherical something is, because another factor can deform the shape of even an object with very high gravity: Rotation. Saturn, for instance, is very oblate - it's bloated into an oblong shape along its equator because its rotational momentum increasingly counterbalances gravity the closer you are to the equator. Image showing Saturn's oblateness:

Oblateness due to rotation even occurs in black holes, since angular momentum of a stellar corpse is conserved even when it collapses into a black hole. In that case is manifests in the shape of the event horizon, even though that's just a region of space rather than an actual material environment (as far as we know).


Entropy is Probability (not disorder)

The Myth of Entropy-and-Intuition: The popular myth is that the Second Law is easy to understand and apply, so nonscientists can simply think about a process and conclude that "this is (or isn't) consistent with the Second Law." The everyday analogies used by some young-earth creationists — like "a tidy room becoming messy" due to increasing entropy — are not used in scientific applications of the Second Law, because entropy is about energy distributions and associated probabilities, not macroscopic disorder, and our psychological intuitions about "entropy as disorder" are often wrong. Scientists can develop science-based intuitions about entropy, but this requires understanding and careful thinking.
Because this idea is so important, and misunderstanding is so common, I'll share excerpts from Section 1 in my longer page which emphasize that entropy is not about disorder, and it's not always intuitive:
The correct formulations by scientists (not by writers who are science popularizers) never say "with time, things become more disordered." And the everyday analogies used by some young-earth creationists — like "a tidy room becoming messy" due to increasing entropy — are not used by experts in thermodynamics, because thermodynamics is not about macroscopic disorder. These everyday analogies, which depend on human psychological intuitions about disorder and complexity, are often wrong. .
In his excellent website about thermodynamics, Frank Lambert, a Ph.D. chemist and a teacher whose ideas about entropy have been published in The Journal of Chemical Education, says: "Discarding the archaic idea of ‘disorder’ in regard to entropy is essential. It just doesn't make scientific sense in the 21st century. [because] it can be so grievously misleading. . Judging from comments of leading textbook authors who have written me, ‘disorder’ will not be mentioned in general chemistry texts after the present cycle of editions. " .
If disorder is not a central concept in thermodynamics, why is it used in some descriptions of the Second Law? The reasons can be historical (due to the inertia of tradition), dramatic (in sloppy writing by science popularizers who either don't understand the Second Law, or have decided that entertaining readers with colorful analogies is more important than scientific accuracy), epistemological, or heuristic: . < These ideas are explained in more detail in my longer page. >
Even though "disorder" is not a central concept in thermodynamics, young-earth creationists imply that disorder is THE central focus of the Second Law. For example, Henry Morris states that the Second Law "describes a situation of universally deteriorating order." [ Morris and others also illustrate entropy increase with everyday analogies. ]


Why Big Cities Matter More than Ever

T he information revolution, we used to hear, would break the shackles of geography and make cities irrelevant. Thanks to e-mail, the Internet, and an ever-widening array of technological devices, you would be able to work just as effectively in South Podunk as in the Big Apple. A new, post-metropolitan era would open in which creative and flexible firms could locate their operations anywhere. The age of the big city would come to an end.

But that hasn’t happened. Big cities have continued to grow. In rich nations today, urbanization levels are on the order of 80 percent or higher. China and India are urbanizing at breakneck speed, with Shanghai and Bombay racing each other to become the world’s largest metropolitan area and eclipse Tokyo (currently 33 million strong). Why is it that cities have lost none of their powers of attraction, despite the new freedom that information technology brings individuals and firms? The economic advantages of cities—of urban “agglomeration,” in the language of the people who study these things—are difficult to measure precisely and not the same for all firms. But they are quite real, and we can capture them in what I call the Seven Pillars of Agglomeration.

L et’s start with the most basic pillar, one that has historically supported the great manufacturing economies of big cities: economies of scale in production. That is, as the scale of production increases, unit costs fall. That basic rule of economics makes it profitable for firms to manufacture goods in just a few large factories, rather than in many smaller ones. And if you’re going to have just one or two big plants, it makes sense to locate them where you can find a lot of workers: densely packed urban areas. This logic explains the growth of large manufacturing cities like Detroit in the earlier part of the twentieth century. Nowadays, though, it applies most readily to midsize cities, because real estate in larger cities often costs too much to build big factories (see the sidebar below).

Smaller Cities Matter, Too

Consider a remarkable fact: over the long run, the growth rates of American cities in various size groups tend to be about the same. That is, over the last century, the country’s distribution of small, midsize, and big cities has remained stable. This stability intrigues economic geographers. Why aren’t all cities converging toward one ideal size—whether large or small?

Part of the answer is that the attributes that make big cities attractive to some industries make them less attractive to others—and every industry gravitates to locations where its comparative costs are lowest. For example, providing environments for rapid, diverse, face-to-face contacts is a comparative advantage of big cities, while physical space is more readily available in smaller ones. If the demand for face-to-face contacts increases, real-estate and wage costs will rise in the largest cities, crowding out industries for which those costs carry—comparatively—less weight. We understand intuitively that Manhattan isn’t a good location for manufacturing airplanes or laptops. But it’s excellent for management consulting and producing operas. So urban systems have a self-regulating nature: certain industries emerge (or centralize) in the largest cities others move to smaller ones.

So small cities, no less than large ones, fill an essential economic need. The more attractive big cities become for some industries, the more alluring small cities will be to others.

The second pillar, however, tends to push firms back to larger cities: economies of scale in trade and transportation. Just as larger factories lead to lower unit costs by making manufacturing more efficient, fully loading a truck, an airplane, or a cargo ship leads to lower unit costs by making delivery more efficient. And filling up those trucks, planes, and ships—both coming and going—is generally easier if they’re delivering to the largest ports, airports, and other distribution centers. That means the bigger urban areas.

Reinforcing this tendency is the third pillar of agglomeration: falling transportation and communications costs. Throughout history, transportation costs—not only monetary outlays but also lost time and the frustration that can come with trading with distant partners—have been a barrier to market expansion. It follows that a fall in transportation costs will stimulate trade, enabling the lowest-cost producers to improve market share. And the steeper the drop in transportation costs and the greater the weight of scale economies in production, the greater the potential for centralizing production in one or two places. Henry Ford could locate automobile production in Detroit because paved roads and railways allowed him to reach the entire American market.

Indeed, if scale economies are infinite and transportation costs are close to zero, all production will be centralized in one place, with the first (lucky) producer to have arrived on the scene. Such an extreme case probably doesn’t exist in the real world, but the film industry comes close. Scale in the industry is vitally important, with its enormous sound studios and vast budgets. It costs little to ship a film—and even less if done electronically. The centralization of the film industry in Southern California is the result. With transportation costs removed as an economic factor, competitors had no way to match the scale economy that Hollywood had established early in the twentieth century.

What about the argument that falling communications costs actually undermine urban concentration? For example, didn’t the existence of e-mail encourage Silicon Valley companies to outsource computer programming to Bangalore, India? The truth is that this shift did foster urban concentration—in Bangalore. Think of communications costs as tariffs: competition intensifies when they fall. If one city is already more efficient at producing a particular good and then the barriers are removed, that city’s market share will expand accordingly.

The centralizing influence of technology is consistent with history. The advent of the telegraph—as revolutionary in its time as the Internet is today—not only failed to slow the growth of London and New York it enabled financial firms and corporate offices in those cities to extend their reach. The arrival of radio and television in the twentieth century replaced a lot of locally produced entertainment with programs produced in New York or Los Angeles.

S cale economies are only part of the urban-expansion story. Most city dwellers work not in massive plants but in small and midsize firms in a wide array of industries: legal services, shirt-making, financial counseling, and on and on. Why should these companies set up shop in cities? Pillar four—the need for proximity with other firms in the same industry—provides part of the answer.

Proximity brings numerous advantages. To name just one: face-to-face contacts remain essential for the most valuable and sensitive information. Finance, among the most spatially concentrated of industries, is an obvious example. Trust must be constantly renewed millions of dollars will be committed based on a brief encounter. The greater the risks and sums involved, the greater the need for relationships built on something more than e-mail exchanges. Body language, facial expressions, and eye contact are among the signals that financial workers use to judge others.

Personal contact is also crucial in industries where creativity, inspiration, and imagination are vital inputs. For firms working in these rapidly evolving industries—high fashion, say, or computer graphics—the surest way to stay on top of the latest news is to locate near similar firms. The more that information can be transmitted electronically, it seems, the more valuable becomes information that cannot be so transmitted. Electronic and face-to-face communications tend to be complements business travel, for example, has accelerated since the advent of the Internet. The more people communicate, the more they want to meet in the flesh.

Lower recruitment and training costs are additional advantages of proximity, particularly in highly specialized fields. A firm clearly benefits if it can hire from a pool of available workers with relevant training acquired at previous employers. The chances of finding a first-rate, experienced screenwriter will be a lot better in Los Angeles than in Baton Rouge.

C ompanies that require a wide array of talents, across a broad range of industries, will be drawn to big cities as well. Thus pillar five: the advantages of diversity. Consider advertising, a field whose products are constantly changing and come with no blueprint. Successful ad firms must rapidly assemble dizzying combinations of expertise and talent according to various clients’ needs. Each ad campaign, after all, is unique: one may call for animation, another for symphonic music, a third for trained chimpanzees. Where better to find the necessary components than in big cities, with their myriad industry clusters? Of the world’s top ten advertising agencies, it is no surprise that three are in New York, three in Tokyo, and two each in London and Paris. The entertainment industry, publishing, and many other fields feel the same pull.

Firms—above all, general-service businesses, for which customer access is important—naturally want to locate in the geographic center of their markets, which brings us to pillar six: the quest for the center. What economic geographers call “centrality” varies by industry, however. For companies with low-scale economies—a gas station, for instance—a central location can simply be a busy street, where the potential number of customers driving by is sufficient to ensure profitability.

But the centrality principle also holds at the national and international levels, and it makes large urban agglomerations particularly appealing. The performing arts are a good example. Broadway, the largest cluster of theaters in America, is in New York City not only because of the sizable local population but also because of all the potential theatergoers within a manageable distance of the city. Greater Philadelphia, with more than 5 million residents, is a mere 90-minute drive away and don’t forget Gotham’s many rail, air, and bus links to other cities (including distant ones), which bring in countless more potential theatergoers. Centrality is often a legacy of history. Paris, shaped by many centuries of investment in roads, rail, and other transportation modes converging on the capital, remains the undisputed center of the French market.

Finally, there’s pillar seven: buzz and bright lights. Talented and ambitious people benefit from being in a big city, just as firms do—in part because the companies can hire talented and ambitious workers. Some people move to cities not just because they need to make a living (though being in a metropolis does offer all the advantages of a diverse labor market) but also because they want to be where the action is. Ambition, dreams, the need for recognition—all are powerful forces in human behavior. Many a young man or woman will ask: Where are my chances best of meeting the right people and doing exciting things? The answer, for good reason, will often be the big city. Why, indeed, are some people willing to spend small fortunes for apartments on Fifth Avenue or homes in Beverly Hills if not to feel that they are truly at the center?

C ities face many challenges in the coming years: municipal debt, onerous taxes, the cost of living, and crumbling infrastructure, among others. But whatever the genuine threats to urban prosperity, human contact is more important than ever in the age of information technology, and people will continue to seek places where they can share ideas, make transactions, and pursue their dreams. There’s nowhere better to do these things than big cities.

Mario Polèse is a professor at the Centre Urbanisation Culture Société at Montreal’s Institut National de la Recherche Scientifique and holds the Senior Canada Research Chair in Urban and Regional Studies. He is the author of The Wealth and Poverty of Regions: Why Cities Matter.


Mars meteorite controversy continues

The meteorite ALH84001. Credit: NASA

The most illustrious meteorite in history continues to inspire heated debate. Does it carry microbial fossils from Mars or are its strange features just the product of some unique geochemistry? After almost 20 years, dueling papers are still coming out, and the opposing parties are no closer to a resolution.

Most scientists agree that the meteorite ALH84001 is the oldest meteorite ever found to have come from Mars.

"The meteorite is so old that if Martian life existed back then, it probably floated by the rock at some point," says Timothy Swindle of the University of Arizona. "But did it leave any record?"

In 1996, one research group claimed yes, sending shock waves through the scientific community and beyond. President Bill Clinton made a special address on the apparent discovery, and the media widely broadcasted the scientists' images of what appeared to be dead "bug" remains from the rock. Had we finally met our neighbors?

The iconic meteorite became the grist for many imaginations. The TV show The X-files depicted an ALH84001 look-a-like with live bugs in it, and a Dan Brown novel imagined a conspiracy to cover-up extraterrestrial evidence from a space rock.

Biopic of a falling star

The meteorite made its debut in 1984, when it was picked up by a geologist team riding snowmobiles through the Allan Hills region of Antarctica. It took 10 years for researchers to realize this 4-pound specimen likely came from Mars.

The general consensus now is that the original rock formed 4 billion years ago on Mars. It was eventually catapulted into space by an impact and wandered the solar system for millions of years before landing on Earth 13,000 years ago.

Over 50 other meteorites have been identified as coming from Mars, but ALH84001 is by far the oldest, with the next in age being just 1.3 billion years old.

"That alone makes ALH84001 a very important sample," says Allan Treiman of the Lunar and Planetary Institute. "It's our only hope to understand what Mars was like at this time period."

The first thing that struck researchers examining the meteorite was the presence of 300-micron-wide carbonate globules that make up 1% of the rock. Dave McKay from NASA's Johnson Space Center and his colleagues determined that the carbonate most likely formed in the presence of water.

Although evidence for a wet ancient Mars has accumulated in the subsequent years, the claim that ALH84001 once sat in water was pretty revolutionary at the time, says Kathie Thomas-Keprta, also from the Johnson Space Center.

Inside the ALH84001 carbonates, McKay spotted odd features that resembled very small worm-like fossils, so he asked Thomas-Keprta to look at them more closely with electron microscopy.

A few of the orange-colored carbonate globules found in ALH84001. Credit: NASA

"I kind of thought he was crazy," she says. "I thought I would join the group and straighten them out."

In the end, she helped the team characterize the biomorphic features, as well as unusual grains of the mineral magnetite found in the meteorite. In a 1996 Science paper, these two phenomena – along with the chemical distribution in the globules and the detection of large organic molecules – were taken collectively as signatures of biological activity occurring long ago on Mars.

The storyline unravels

However, skeptics began to pick apart the four lines of evidence presented in the 1996 paper.

Groups of geologists and chemists proposed alternative ways that the carbonate globules and the organic molecules could have formed without the need of Martian microbes.

The supposed fossil shapes were so small they could only have been the remains of hypothetical "nanobacteria." A more plausible explanation, according to other researchers, was that the tiny artifacts are uneven patches in the coating used to prepare the samples for electron microscopy.

That left the magnetite grains as the strongest case for a biologic imprint in ALH84001.

"The focus of the last 10 years has been the magnetite," says Thomas-Keprta.

A chain of magnetite crystals, "like a string of pearls,” within meteorite ALH 84001. Arrows indicate the ends of the chain. Credit: NASA

Magnetite (Fe3O4) is a common mineral found on black sandy beaches, in iron-rich sediments and even in interplanetary dust. The majority of this magnetite forms in geologic processes, where many elements mix together and iron often gets replaced with iron-like elements such as magnesium and chromium.

However, the magnetite grains found in the carbonate globules of ALH84001 have very few of these sorts of substitutions.

"I had never seen magnetite as chemically pure as this before," Thomas-Keprta says.

But when she looked through the literature, she realized that chemically pure magnetite is known from biology. So-called magnetotactic bacteria create a chain of magnetite grains to help orient themselves in their search for nutrients. Iron makes for a stronger magnet, so the bacteria are very selective when they form their magnetite compasses. They also build grains of a uniform size (roughly a tenth of a micron) that optimizes the magnetic response.

"The size and purity of the magnetite is controlled by the organism to be the best magnet it could be," Thomas-Keprta says.

In 2001, she and her colleagues showed that many of the same properties in biologically-derived magnetite are reproduced in the grains from ALH84001. The conclusion was that Martian microbes once used magnetite for the same purpose as terrestrial ones do.

Treiman agrees that the ALH84001 magnetite is unlike geologically-produced magnetite found on Earth. "But everything else about this meteorite is unique," he argues. "There comes a point where being unique is not unique."

It's improbable that Martian microbes deposited magnetite grains directly in the rock, so Thomas-Keprta and her colleagues have to argue that the magnetite formed outside of the rock and washed in. They also have to assume that Mars had a much stronger magnetic field in the past so that building an intracellular magnetic compass would be an advantage.

Treiman and others argue that the magnetite could be explained more easily with some sort of shock event that heated the carbonate enough to allow magnetite grains to form. Thomas-Keprta says these abiotic models are fatally flawed. The problem is in the cooling time. If the rock cools too fast, the magnetite ends up full of impurities. Too slow and the surrounding carbonate becomes too uniform.

"They are looking for a single event that can account for all the magnetite," Thomas-Keprta says. "But no natural or laboratory synthesized analogs proposed have yet to reproduce the chemical and physical properties observed in the ALH84001 carbonate-magnetite assemblages."

She and Treiman went head to head at a recent Lunar Planetary Society Conference. Neither side has relented.

"Naysayers are always going to be naysayers," Thomas-Keprta says. "But I hope people on the fence will look at the evidence."

Polling the community

Researchers believe that the Allan Hills meteorite was blasted out of Eos Chasma on Mars, near the far horizon in this Mars Express image. The canyon feeds into the larger Valles Marineris canyon. Credit: ESA

Treiman thinks that the issue is probably settled for most of his colleagues. "I am one of the few holdovers still arguing about it," he says. "I can't move on."

The debate may not be settled anytime soon. Treiman isn't sure how one could ever entirely rule out that Martians might have had a hand in forming ALH84001. "Nature is infinitely complicated," he says. "It is always surprising us."

However, he believes the alternative explanations from geology and chemistry are simpler, since they don't require inventing the whole new science of Martian biology. Scientists are trained to pick the simplest explanation.

An informal poll of more than 100 scientists by Swindle in 1997, right after the first announcement of possible biological relics in ALH84001, showed that most of the community was already hedging their bets. The typical response gave about even odds that Mars once had life but said that there was just a 1-in-5 chance that McKay's group had found the smoking gun.

A few years later, Swindle tried to do the poll again but couldn't get enough respondents to form a representative sample. He thinks most people had made up their mind that ALH84001 did not carry biosignatures from Mars. But that doesn't mean that sifting through the meteorite hasn't been worth it.

"It was good science," he says. "It challenged people to really think about what would count as evidence of life on Mars."