Crashing comets not likely the cause of Earth's mass extinctions: new research

July 30, 2009 by Vince Stricherz
A long-period comet called 2001 RX14 (Linear) turned up in images captured in 2002 by the Sloan Digital Sky Survey telescope in New Mexico. Credit: Mike Solontoi/University of Washington

( -- Scientists have debated how many mass extinction events in Earth's history were triggered by a space body crashing into the planet's surface. Most agree that an asteroid collision 65 million years ago brought an end to the age of dinosaurs, but there is uncertainty about how many other extinctions might have resulted from asteroid or comet collisions with Earth.

In fact, astronomers know the inner has been protected at least to some degree by Saturn and Jupiter, whose gravitational fields can eject comets into interstellar space or sometimes send them crashing into the giant planets. That point was reinforced last week (July 20) when a huge scar appeared on Jupiter's surface, likely evidence of a comet impact.

New University of Washington research indicates it is highly unlikely that comets have caused any mass extinctions or have been responsible for more than one minor extinction event. The work also shows that many long-period comets that end up in Earth-crossing orbits likely originate from a region astronomers have long believed could not produce observable comets. A long-period comet takes from 200 years to tens of millions of years to make a single orbit of the sun.

"It was thought the long-period comets we see just tell us about the outer Oort Cloud, but they really give us a murky picture of the entire Oort Cloud," said Nathan Kaib, a University of Washington doctoral student in astronomy and lead author of a paper on the work being published July 30 in Science Express, the online edition of the journal Science.

The Oort Cloud is a remnant of the nebula from which the solar system formed 4.5 billion years ago. It begins about 93 billion miles from the sun (1,000 times Earth's distance from the sun) and stretches to about three light years away (a light year is about 5.9 trillion miles). The Oort Cloud could contain billions of comets, most so small and distant as to never be observed.

There are about 3,200 known long-period comets. Among the best-remembered is Hale-Bopp, which was easily visible to the naked eye for much of 1996 and 1997 and was one of the brightest comets of the 20th century. By comparison, Halley's comet, which reappears about every 75 years, is perhaps the best-known comet, but it is a short-period comet, most of which are believed to originate in a different part of the solar system called the Kuiper Belt.

It has been believed that nearly all long-period comets that move inside Jupiter to Earth-crossing trajectories originated in the outer Oort Cloud. Their orbits can change when they are nudged by the gravity of a neighboring star as it passes close to the solar system, and it was thought such encounters only affect very distant outer Oort Cloud bodies.

It also was believed that inner Oort Cloud bodies could reach Earth-crossing orbits only during the rare close passage of a star, which would cause a comet shower. But it turns out that even without a star encounter, long-period comets from the inner Oort Cloud can slip past the protective barrier posed by the presence of Jupiter and Saturn and travel a path that crosses Earth's orbit.

In the new research, Kaib and co-author Thomas Quinn, a UW astronomy professor and Kaib's doctoral adviser, used computer models to simulate the evolution of comet clouds in the solar system for 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets that eventually cross Earth's path.

By assuming the inner Oort Cloud as the only source of long-period comets, they were able to estimate the highest possible number of comets in the inner Oort Cloud. The actual number is not known. But by using the maximum number possible, they determined that no more than two or three comets could have struck Earth during what is believed to be the most powerful comet shower of the last 500 million years.

"For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth," Quinn said. "We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir."

With three major impacts taking place nearly simultaneously, it had been proposed that the minor extinction event about 40 million years ago resulted from a comet shower. Kaib and Quinn's research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.

"That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so showers are probably not likely causes of mass extinction events," Kaib said.

He noted that the work assumes the area surrounding the solar system has remained relatively unchanged for the last 500 million years, but it is unclear whether that is really the case. It is clear, though, that Earth has benefitted from having Jupiter and Saturn standing guard like giant catchers mitts, deflecting or absorbing comets that might otherwise strike Earth.

"We show that Jupiter and Saturn are not perfect and some of the comets from the inner Oort Cloud are able to leak through. But most don't," Kaib said.

More information:

Source: University of Washington (news : web)

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2.6 / 5 (5) Jul 30, 2009
The Oort Cloud could contain billions of comets, most so small and distant as to never be observed.

Thus it would not be wrong to name the matter in the Oort Cloud "dark matter".
2.6 / 5 (5) Jul 30, 2009
The Oort Cloud could contain billions of comets, most so small and distant as to never be observed.

Thus it would not be wrong to name the matter in the Oort Cloud "dark matter".

5 / 5 (5) Jul 30, 2009
We always do need to be extremely careful when forming conclusions based on computer simulations. Bias is easily introduced during the development of the simulation. Phrases such as "we have shown... .." must be rejected. The only thing 'shown' is that the mathematical model constructed did what it did.

This is not to condemn simulations but is an appreciationthat they are an 'economic' approximation of waht the developers sought to represent: they are not more.
2.3 / 5 (4) Jul 30, 2009
Interesting. There seems to be increasing evidence that many extinction events, including the demise of the dinosaurs, are directly linked to flood basalt eruptions. The most famous is of course 'the great dying' 250 million years ago, directly correlated with the massive Siberian flood basalt eruptions. 65 million years ago there was a huge eruption in India and around 55 million years ago, at the time of the famous 'temperature maximum' there was high levels of CO2 probably caused by big eruptions in the area of Iceland.
3.5 / 5 (4) Jul 31, 2009
"Some of the comets from the inner Oort Cloud are able to leak through. But most don't"... I mostly have to agree with tkjtkj.. On these lines, I suspect Jupiter and Saturn nudge just as many comets back in toward the inner planets as they spend nudging them away.. If there were any significant protection from the outer planets, there wouldn't be so many routine and predictable comets passing thru here (watched over thousands of years)..
1 / 5 (3) Jul 31, 2009
They need to specify the assumptions of the simulation. Gravity and electrical charge need to be part of it. They seemed to model only the Oort cloud and not the Kuiper belt.
4.3 / 5 (3) Jul 31, 2009
Dark Matter is a very specific term used by cosmologists to explain the 'missing mass' of the universe posited by some cosmological theories. Dark Matter may not even be matter at all, we won't know until we find it. I think it would be confusing to call stuff which is simply too small to see 'dark matter' as well - it's not dark, just too small and far away for us to see any light reflected from it here on earth. Unlike real (or posited) Dark Matter, the Oort material can bee 'seen' with other wavelengths.
1 / 5 (3) Jul 31, 2009
Perhaps this is a bit more evidence to support the exploding planet hypothesis. Based on how solar systems form, we appear to be missing a few planets. They may have exploded long ago and fragments revisit the inner solar system periodically.
1.3 / 5 (3) Jul 31, 2009
First, the 'guardians' Jupiter and Saturn are very big and do have gravitational fields far greater than we can imagine, but ... lets try.

They both circle the sun, and are both in the same night sky for three months a year. So they are 'guarding' that side of the sky, (of the solar system), but not the other.

They do not have so much gravity that they attract each other and are getting closer together, so there is a gap between them that is safe enough to fly through.

Jupiter, being closer to the sun (and the orbit of Mars) than Saturn, has been accused of cleaning up many of the loose rocks in the Asteroid Belt between Mars and Jupiter. No observation evidence exists, and the belt of rocks varying in size from dust to small moons continues to maintain a reasonably stable ring around the Sun, (but it is fair to say that some of those rocks actually have got eccentric orbits and that others may have found Jupiter hard to resist, ending up thrown into an orbit that eventually took them to the Sun, out of the solar system, or into Jupiter itself). Hence, for the purpose of this exercise, we can say that the gravitation influence of Jupiter diminishes (long) before it reaches the Asteroid Belt, and that distance could be considered the radius of Jupiter's gravitational influence, (in all directions around Jupiter).

You could then place that 'sphere of influence' on a huge imaginary circle around the Sun, and (very generously) say that Jupiter can 'guard' 10% of that circle at any one time.

You could repeat the exercise for the smaller and much more distant Saturn, and suggest that the two planets protect 15% of the ecliptic plane (the imaginary disk in/on which all the solar system's planets orbit the Sun).

This would leave 85% of the ecliptic un'guarded', and obvious does not consider the area of the sky that is not in the ecliptic plane.

For those (many) of you who already know this, and that Jupiter and Saturn in fact 'guard' substantially less that this, consider also the fact that the Comets from the Oort Cloud are travelling so fast that they can pass through the orbit of Jupiter in a matter of days, if not hours, and so the possibility of intersect with the planet is very slim indeed. Further, Jupiter is not actively 'guarding' anything: it could just as easily place an approaching object on more dangerous course for the inner solar system, as on a safer course for outer space.

We have witnessed two impact events on Jupiter in the past fifteen years ... let us hope that in fact the second was a small and dark remnant of the first, and that a piece of Shoemaker-Levi 9 (should we call it Shoemaker-Levi 10 now?) almost escaped Jupiter and has been on the run for over a decade in an eccentric elliptical orbit around Jupiter which permitted its existence to be missed by Jupiter watchers during all that period. It is certainly possible, though many of you will be thinking 'improbable'.

The alternative is that these two events are unconnected, and that the increase in Jupiter observation has shown a empirical frequency for previously theoretic and underestimated phenomenon: that the number of objects crossing the Jupiter orbit is high enough to present two collisions with Jupiter in fifteen years, and that we should expect to see many more in our lifetimes.

You might as well buy a telescope ... now that you are not going to be able to sleep at night ;-)

That 'buying a telescope' particularly goes to those theoretical physics mathematicians who make wild claims about 'Dark Matter' being 95% of the Universe and being hidden by fascinating (expensive to research) phenomena at the event horizon or deep space inter-galactic boundaries. I have little doubt that those phenomena are both possible and very interesting to prove ... but 95% of the Universe?

Point a telescope at Pluto once a month, and tell me what is hidden in space! Then get your estimated number and mass of the Oort Cloud accurate, and find similar spherical solar 'clouds' around every star in just our own galaxy. Then consider the objects that float between the galaxies that have no extraordinary origins ... perhaps your 95% can be reduced a little. Perhaps after a few years at a telescope, you would not find that 5% is such a bad figure for the amount of 'matter' that we can see and measure, (even those of you that know that 5% is a grossly generous percentage).
5 / 5 (3) Jul 31, 2009
Ray Cherry, brant and frajo, even assuming that(best case scenario) every star in all galaxies possess Oort clouds, Kuiper belts, asteroids , plutoids, etc. (remember that there are many more stars in galaxies with a lower than solar mass than extra-solar mass), the total combined mass is still a mere fraction of what is expected to comprise Dark Matter. These 'dark' souces of mass, as well as MACHOs are well accounted for in current DM models, aided by decades of measurements of their contributions. Even if every star and brown dwarf possess these dim objects, nowhere near enough are present to totally account for the observed DM fraction of our universe.
5 / 5 (3) Jul 31, 2009
You all listen to YYZ (great song btw)

The Oort cloud doesnt has mass enough to form a new Jupiter, if put together.

If every star in the universe has something similar to an Oort cloud, that would answer for like 0.1% of the total mass of KNOWN matter...

That means that if Oort Clouds were dark matter, now instead of 99% of the universe mass missing, now we have 98.9% of the mass missing.

Stop talking nonsense guys. Oort clouds ARE NOT dark matter.
2.3 / 5 (3) Aug 01, 2009
Well then does it not stand to reason that if the equations that supposedly govern galactic motion are so far divergent of the point they're trying to hit, why would anyone be led to believe they're absolutely right, there just has to be an unequalable. extremely massive. virtually undetectable substance? The common response might be "because there's evidence of halos around galaxies." And while that might be an argument for dark matter, does it exclude the possibility that it could be the effect of another theory that more "naturally" explains observation?
not rated yet Aug 04, 2009
Ok chaps and chapesses ... I have been trying to find some useful Oort Cloud data/information for general and more academic reading here to try (pushing that rock back up the hill) to illustrate how little we know of the mass of each solar system ... so here is a link (I know, I know, but I can't simply copy the text here, ok?):


which provides a good overview of the Öpic/Oort/Hills theory/phenomenon and the current status of the quantification of the cloud, (how many objects; what sizes they may be; what materials they may be made of; their density and mass)

As for the size of such objects, we have found (very recently) a minor-planet sized object, albeit about the size of Pluto, but in an orbit three times as distant as Pluro's. Some of you will have heard of it as 'Sedna' ... no, no, no, it is not Planet X, chill out ... but it is an indication of how little we know about 'our neighbourhood'.

Perspective: Earth is at one Astral Unit (AU) from the Sun, Jupiter is at 5AU, Saturn at twice that, Pluto at around 25AU, Sedna at its closest point to the Sun is more that 75AU and could only be discovered with todays best technology. It is considered to be a very close Oort Cloud Object because at its furthest distance from the sun during a single orbit is currently thought to reach 900AU.

Good job that we saw it while it was close!

The Inner Oort Cloud is thought to be in the range of 2000AU to 10000AU and most of this is in (approximate) alignment with the eliptic plane of our familiar planets. The Spherical Oort Cloud is considered to be in the range of 10000AU to 50000AU, (think of the magnification required to see a Jupiter sized object at this range?) and even more, the Outer Oort Cloud is up to 200,000AU (are we up to three light years yet?) and probably intersects with the Oort Cloud of the nearest star, Alpha Centauri.

Ok ... interesting. So what?

Everything I read recently about the Oort Cloud gives estimates of this, theories of that, sporadic and rare confirmations of basic concepts put forward in the past hundred years about a population of 'ice' planetoids/comets in a spherical cloud (in every direction) around our sun.

Estimates that start in the millions of millions of objects, with average size of Haley's Comet, with water ice compositions ...

but ... nobody knows how many; some of the objects found so far are certainly not 'ice' as we know it and are more like rocks; and the theories say that the Oort Cloud Objects started life near to the sun and got 'kicked out' by the gravity of the 'bully/guardian planets' of 'our solar system'.

So ... back to basics.

Nobody knows how much mass exists around any star (if we can see a star, it has already shed much of its mass during ignition). Nobody knows how many ojects are floating around our sun, (even the asteroid/main belt between Mars and Jupiter is only now moving firmly from 'estimate' to 'count' of objects above 100m diameter - and they are just 2AU away from Earth), and nobody knows if those Oort Cloud Objects are fluffy dust balls, ice cubes or minor planets. Further, in my humble opinion, nobody knows for certain that accretion/coagulation has not taken place out in the Oort Cloud under circumstances that are highly unusual, but not impossible. Giant, or even SuperGiant Planets may well exist in interstellar space, but in 'loose' orbit around our sun (not necessarily in alignment with the eliptic), and we have not yet discovered them because we spend our telescope time looking too close or too far.

Considering that Oort Clouds are thought to exist around most stars, and that ours is thought to intersect with that of Alpha Centauri's, I have yet to see a dual star Oort Cloud model anywhere on the Internet, and any thoughts of the exchanges of solar system materials between two or more stars.

Finally, considering the Oort Clouds intersection, isn't it probably that 'loose orbital' material gathers in the interstellar space directly between two stars, so that the Outer Oort Cloud forms disks around the outside of the spheres ... and the loose eliptical orbit to become a figure of eight between the two stars?

Dark, cold, faraway and moving at unimaginable speeds between the two gravity swings. Imagine a hundred Jupiters on that conveyor belt ... invisible to us, and yet holding so much 'missing mass'.

Twinkle, twinkle little star ;-)
not rated yet Aug 04, 2009
@ Ray Cherry, the AU stands for the Astronomical Unit, defined as the distance from the center of the Sun to the center of the Earth. See the wiki page here: http://en.wikiped...cal_unit .

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