Scientists are guessers.  They guess how our world works.  They guess why the sun shines.  They guess why things fall the way they do.  They guess why some kids have blonde hair while others have brown hair.  They look at the world and guess, and then they look for evidence -- stuff that will prove their guesses to be true or false.  Some of the hardest things for them to guess are the things that happened long, long ago.  Such as why did the dinosaurs die?  Let's look at some of the best guesses and what they learned that back those guesses up.

Every year animals disappear from our world.  Parts of our world change.  They get warmer or colder.  More people move to these places or too much rain falls and this causes frogs, birds, spiders and fish to die out.  When one kind of animal dies out never to return, we call it extinction.  This affects everything from frogs, birds, spiders, and fish.  So what happens when the whole world changes?  Think of all of the dinosaurs going away.  One year they are there, the next year, they are not.  Maybe it happened slowly, over many years.  Maybe it happened quickly over weeks or months.  Fossils show us many bodies from around the same time, around 65 million years ago.  Mass extinction means many kinds of living things die forever.  So, we have the bodies.  We know something big must have killed them.  Now, as scientists, we have to guess what might have caused it.

Think of a volleyball hitting the beach.  Even though the ball is small, if it's moving fast enough, it can make a big dent and kick up a lot of sand.  Thirty years ago, scientists found a crater or big hole in the ocean near Mexico.  This hole was 112 miles across.  They figured out that the only thing that could make a hole that big would have to be a rock 6 miles across and moving over 40,000 miles an hour.  A meteor is a rock from outer space that enters the Earth's atmosphere.  The earth dented just like the sand after being hit with the volleyball!  This hole brings up more questions than it answers.  What really killed the dinosaurs?  Was it the many rocks that broke off the giant rock, raining like fireballs?  Was it the heat from it striking the earth?  Was it the dust that the rock kicked up?  Was it something we have not thought of?

This volleyball may have smashed a couple sand fleas. Something much bigger must have led to the end of the dinosaurs.

Juanedc from Zaragoza, España, CC BY 2.0 , via Wikimedia Commons

When the volleyball hits the sand, a big wave of sand shoots up, splashing sand in everyone's eyes.  What might have happened when that huge meteor hit the Earth.  It could have kicked up enough dust to fill the air across the globe.  Why would that be so bad?  What can dust do besides get in your eye?  Dusty air is not good for animal lungs, but that would not be the biggest problem.  These great big clouds of dust would not let the sun shine through.  Scientists guess that this dust cloud might have covered Earth for weeks, maybe even months.  Climate change is when the weather becomes hotter or colder across the Earth.  Think if the sun were blocked out for a whole summer.  This would change the weather.  It might grow a lot colder, freezing some of the animals.  With no sun, the plants would die out and other animals would starve.  Dust can be scary.

Cold weather?  No food?  A rock the size of a city crashing into the earth?  Dust that covers the sun?  Whatever wiped out most of the dinosaurs, it had to have been BIG.  Big enough to kill eight out of every ten animals on Earth.  A catastrophe is a sudden change that causes a lot of damage.  When scientists make guesses about how all the dinosaurs and other living things of that time died, they have to think of things that would change the whole world.  Anything smaller would not have been a big deal.  It's sad to think that all of those animals died, but if it had not happened, people might not have had their time in their sun.  Or if we did, we might be running away from dinosaurs . . .

Smoke and ash from a volcano can cause a catastrophe. When Mt Vesuvius erupted in 79 AD, it buried the Italian city of Pompeii.

McGimsey, Game, Public domain, via Wikimedia Commons

Scientists are still trying to figure this one out.  In the desert, they blow up giant bombs, seeing how that big rock would have changed things.  The bursts make big clouds of dust that cover everything they can see.  They make guesses, take notes, and then they blow it up again.  These tests give them the evidence they need to decide if a meteor hit the earth and what it did to the earth.  Being a scientist is easy.  You guess and search and then guess again.  It's putting it all together that's the hard part.

References:

Britannica. "K-T extinction." Encyclopedia Britannica, 2012. <http://www.britannica.com/EBchecked/topic/1314796/K-T-extinction>

UCMP. "The K-T extinction." Berkley, 1999. <http://www.ucmp.berkeley.edu/education/events/cowen1b.html>

Scientific American. "A Theory Set In Stone: An Asteroid Killed the Dinosaurs, After All." Scientific American, 2010. <http://www.scientificamerican.com/article.cfm?id=asteroid-killed-dinosaurs>