Ep. 80: Craters

Shownotes

Types of meteorites:
• Iron
• Stony
• Stony-Iron
• Chondrites — these rocky meteorites do actually hit the ground frequently, as about 80-90% of meteorites found on Earth are chondrites.  The chances of reaching the ground intact are lower for a chondritic asteroid than an iron-nickel of a small size, but a sufficiently large enough chondrite will easily reach the surface of Earth intact.  And as for bodies without atmospheres, there’s no difference.
• Arizona State University site on meteorites

Parts of a crater:

• Rim
• Floor
• Wall
• Ejecta
• Rays
• Central Uplifts

All About Craters

• All about craters — Hawaii Space Grant Consortium
• Pamela’s Star Stryder post about the crater sessions at the Lunar and Planetary Conference
• Lunar Impact Craters Geology and Structure — Lunar and Planetary Institute
• Craters are not always round –if the angle of impact is ~<5°, then an ellipsoidal crater will form. See examples at the Planetary Blog (Mars), and Enchanted Learning (moon)
• Earth Impact Database website (Search for craters by continent, age, diameter and name)
• Earth Impact Database FAQ’s
• Earth’s 10 Most Impressive Impact Craters — Universe Today
• Recent impact crater in Peru — Peruvian Meteorite May Rewrite Impact Theories — Universe Today
• Lake Titicaca
• Meteorites from Mars — NASA
• Abstract:  Shock Magnetism and Demagnetism
• Impact Theory for the origin of the Moon — Planetary Science Institute
• Chicxulub Crater
• Asteroid Impact Simulator — Universe Today
• Determining the age of a planetary surface using craters — Malin Space Science Systems
• PowerPoint from Lunar and Planetary Institute about Surface Dating using Craters
• Fluvial Processes on Mars — HiRISE
• Dichotomy of Mars’ Hemispheres Possibly Explained by Giant Impact — Universe Today
• Water Ice on the Moon? — NASA
• Water Ice on the Moon? — Universe Today
• Possible Telescopes at Lunar Poles — Universe Today
• Movie of Landing Near a Crater Rim on the Moon — Universe Today

Books:

• Impact Cratering: A Geologic Process by H.J. Melosh
• See a list of more books on impact craters from Amazon

Transcript

Download the Transcript

Astronomy Cast Episode 80:

Craters

Fraser Cain: Welcome to Astronomy Cast our weekly fact-based journey through the
cosmos. Pamela is in Houston, Texas.
Dr. Pamela Gay: Fraser, it’s been a sad conference without you here with us.
Fraser: There you go but someone’s got to hold the fort down back in Vancouver.
Pamela: Keeping Canada safe.
Fraser: Right, from space. But how’s the conference been going?
Pamela: My brain is full. It’s been an amazing week of all sorts of content. My
background is astrophysics, it’s not geophysics and I have learned more in the
past week than I think I’ve read reading journal articles in the past several
months. It’s just been an amazing experience of well science concentrate.
Fraser: Can you talk the planetary lingo now?
Pamela: No. But I at least know what the people who can speak the lingo are saying
most of the time.
Fraser: So you’re able to translate a little better. That’s good. Now the results of all your
work is being posted to astronomycast.com/live
We have pictures and audio and text and video, interviews, and all kinds of stuff
so if you haven’t already, go to astronomycast.com/live which is where all of
the coverage for this event is.
Pamela: And it’s not just me. We had help from Emily Lakdawalla of the Planetary
Society; Rebecca Bemrose-Fetter our producer has been doing a lot of blogging
and photography. We also got a special guest correspondent.
One of my students, Scott Miller, went to see STS-123. He went to see the last
night launch of the shuttle program and we have his Geeks Pilgrimage
documented over on astronomycast.com/live.
Fraser: Awesome. I saw some video of the launch and I really want to see that. Have
to see that launch before it stops launching. Let’s get on with the show.
As you know, Pamela is attending the 39th Lunar and Planetary Sciences
Conference in Houston, Texas. That means the moon and planets. When you
think of the moon, you think of craters.
In fact a big theme this week at the planet is craters. Pamela has taken it as an
inspiration so this week is the week we drive the show into the crater. Pamela,
why don’t you give us the basic explanation of how we get a crater. Although I
think we can kinda guess.
Pamela: Well you start with a rock, although rocks have more words than I ever knew
existed. You can have a rock that is from the moon or Mars or you can an
asteroid and these can have all sorts of different names.
Most typically the ones that end up hitting planets they were calling chondrites.
You get iron meteors that hit planets. When they impact they can come in at all
sorts of different angles. The angle of impacts affects what direction the ejecta
travels. This is the cloud of material that gets thrown out of the ground and
spewed in different directions.
But you always get a round crater. That’s one of the cool things that you can
knock a planet and you can do it from all sorts of crazy angles and the crater is
always going to be a nice happy little perfect circle.
Fraser: Why?
Pamela: It’s just the way the physics works.
Fraser: Right, like the rock just gets consumed when it strikes the ground and it’s
always in a circle?
Pamela: There are a lot of really complicated processes that go into this. There’s
conservation of energy; conservation of momentum. You hit downward and
material gets flung upwards as the energy is being released. It just ends up
leading to this nice happy little perfectly round crater.
You can get craters that end up with multiple rings of material around them. You
end up with craters that have neat layers layered through them with all sorts of
different morphologies depending on where you hit.
In some cases you can actually create an instant lake if you hit someplace on the
planet Earth and the crater happens to break through to the water table. This
actually happened last September. In Peru there is a crater formed about 15
meters across when a fairly good-sized rocky asteroid in this case, which we
didn’t think those could actually reach the ground, which is a bit troubling.
But, a rocky asteroid came crashing through our atmosphere. People saw it
while they were hanging out on the roof of a hotel looking up when the asteroid
turned meteor turned meteorite hit the side of a riverbank. It poked through to
the water table and within minutes this crater turned into a little tiny watering
hole.
3
Fraser: Wasn’t it making people sick?
Pamela: That was actually one of the stories that were in the news. Several different
teams of scientists a few weeks and a few months later went down to study it.
They interviewed the people involved, took pictures, documented everything.
It appears that it was a psychosomatic reaction. There was a lot of fear when
this happened that it was actually a missile from a neighboring country or just
some other country that for whatever reason decided to throw a missile at the
middle of Peru.
This was a fairly remote area near the Bolivia border and somewhat near Lake
Titicaca. There was a lot of fear and paranoia and no it was just a rock.
Fraser: Why don’t you walk me through the steps nice and slow like for as a rock
strikes the ground – space to crater.
Pamela: Okay. The most typical case is you have what starts as an iron asteroid. A
happy little friendly object on an orbit around the sun minding its own business,
but its orbit happens to intersect the orbit the Earth. It can intersect it at all
different angles.
You can end up with an asteroid that hits our atmosphere head on where its path
is going straight from space straight down towards the earth. That’s kind of
rare. Most of the time you’re at some crazy angle.
Just probability says that its’ more likely you’re going to be somewhere
between zero and 90 degrees than at 90 degrees when you hit the atmosphere.
Depending on the angle that it hits the atmosphere it’s going to deal with
differing amounts of slowing down from the atmosphere.
Different frictional effects as it passes through the atmosphere will slow it down,
heat it up, dissipate some of its energy and dissipate some of its mass through
all sorts of different burning up processes. This is what you see when you look
up in the sky and see this really bright streaking object. It is melting,
evaporating, ionizing all these things depending on its composition.
So it’s getting smaller and smaller and so you might start off with something
that is several meters in size that ends up the size of a football by the time it hits
the surface of the earth. Then when it hits all that energy is transferred into the
ground.
You can end up with instantaneous melting. You can end up with shocks, but
the basic result is you end up with this great big splash of materials that shoot
straight up.
4
Fraser: Does the asteroid always hit? Can’t they explode in the air?
Pamela: No. It doesn’t. They can explode in the air, or they can vaporize in the air. The
vast majority of stuff in the air that hits our atmosphere is pebble and dust grain
size. Things like that aren’t going to make it to the surface of the earth. It’s
only the larger objects and how large is required really depends on the angle
that the object impacts on our atmosphere and the difference in velocity
between them.
You can imagine that you have this asteroid that is on an orbit that causes it to
just hit our atmosphere and is going just fast enough that it is mostly gravity
sucking it in. You could also end up with an asteroid that is perhaps going
around the sun in the opposite direction so it could hit the Earth’s atmosphere
with the exact same angle.
Because of the difference in its’ orbital velocity and our orbital velocity where
we have basically a head-on collision, it’s screaming in at tens of kilometers per
second and this huge extra velocity ends up making it a much more dangerous
object.
Fraser: Okay so the rock has transferred its energy to the ground. What happens to the
ground?
Pamela: It depends on how big the object is. There are some really cool models that
have been done at Los Alamos National Lab where they have a super
computing facility. In some of these models, there are simulated scenarios of
large object hits the ocean and the splash of the ocean water is so great that a
column of water goes through our atmosphere.
So we’re actually if we get hit by something large enough and hopefully this
would never happen because the tidal waves would be devastating and as would
many other things. If you hit our oceans just right, you can splash ocean water
into orbit basically. That’s just really cool.
You can do the same thing if you hit land but it would be much more
devastating. This is actually how we end up getting Mars rock hitting the Earth
as meteors and being found as meteorites all over the planet.
At some point in Mars past it was hit by something big and chunks of Mars was
sent into space on orbits that carried them to Earth where they passed through
our atmosphere, survived and landed somewhere on the planet just waiting for
some geophysicist or some farmer to find it.
5
Anyone can find a meteor. They are all over the planet. They are easiest to find
in deserts and in Antarctica. But you could find one in your back yard if you’re
lucky.
Fraser: Now either you’re on a tangent or you’re avoiding my question. I think you’re
on a tangent.
Pamela: It’s also after midnight where I am and also very late where Fraser is.
Okay, so the rock hits the planet and you can get dirt or water thrown into the
air depending on where it hits. The energy as it propagates through the soil can
take a chunk of the soil and actually flip it over.
You can end up with inverted layers stacked up on top the soil, dirt, or glacier
around wherever the impact occurs.
Fraser: You mean like dinosaurs on top, then newer rock, then finally topsoil?
Pamela: That’s exactly what happens, it’s a complete flip. If there is enough energy, it
can liquefy as it hits. Soil is made up of silicates, organic materials, because we
have earthworms here on our planet. Organic materials form just about
everywhere but they don’t always have earthworms and microbes in them.
An organic material is just something that has carbon atoms and molecules. But
you take this stuff and if it has silicate in it that basically melts to glass, which is
cool and you can melt it and get these fascinating structures around it.
There is also all of the material that is in the meteor that has now turned
meteorite and that can shatter on impact and you end up with ejecta fields that
are filled with quartz crystals. You can end up with all these various blobs of
shiny glass strewn all around where the central crater is located.
You also get the pieces of the meteor if it chooses to shatter, which can be
chunks of metals. So you have this ejecta field around a crater rim that includes
inverted materials.
In some cases depending on what you hit, if you’re hitting something that’s rich
in metals, you might actually in the process be able to whack them hard enough
or melt them just right that you end up creating magnetic fields within the
materials that you’ve just knocked really hard
Fraser: All right, I know that if you hit a piece of iron really hard you can give it a
temporary magnetic field, right? Because you are aligning all the little jiggled
up iron atoms so that they’re all pointing in essentially the same way and they
get that magnetic field going. So a large enough rock can do that to dirt and to
iron ore in the ground?
6
Pamela: Exactly. This is actually something that we think has happened in some cases
on the moon. If you look at the lunar craters there are some amazing maps of
the moon’s magnetic fields from the lunar Prospector.
When you look at these maps, there is a little bit more magnetic field in one
place than another and is coincident with the centers of craters where there is
little upwellings of material which sometimes happens for reasons that we’re
still trying to figure out.
It is thought that these magnet fields are either induced through shock, like
hitting it as you would hit a nail with a hammer, which is something anyone can
try. Go get a real metal nail and whack it a few times with a hammer really
hard and you can use it to pick up paper clips.
Either that or perhaps in some really ancient cases there was an intrinsic
magnetic field around and by heating the material, you randomize the atoms in
it if it is material capable of becoming a magnet.
As those heated up atoms cool they align along any magnetic field that happens
to be around. This is why in different parts of the planet Earth we can actually
figure out the history of the Earth’s magnetic field by looking at the way the
magnets are aligned, natural ferromagnetic materials.
With the moon we have these neat little lumps and bumps of magnetic fields
that are coincident with craters. And that’s just cool.
Fraser: Right, you get some lava that pours out of the ground or is created in an asteroid
strike and it is liquid enough that all of its atoms can align while it is cooling in
the magnetic field.
Then the magnetic field is cooled and they are locked in place and maintain a
record of the magnetic field that was there at the time.
Pamela: In some cases if you whack something hard enough with an object that is large
enough you can even do things like create the Earth’s moon.
Our planet once upon a time was hit by another object and we go into this in our
“How the Moon was Created” episode.
Fraser: That’s like a big crater there, mighty big.
Pamela: Actually, it’s more like an ejecta. So the splash of material I told you about can
sometimes make it up through the atmosphere, that would be our moon.
Fraser: A really big collision.
7
Pamela: It’s a really big collision. But in these really big collisions you don’t always get
moons. In fact, it looks like both Mercury and Mars have giant basins. In
Mars’ case one that is about half the planet – the whole hemisphere.
Both Mercury and Mars appear to have been clobbered by something of double
digit percentages of their own size at some point in their past. This had huge
morphological affects on the entire planet.
Fraser: Now what good have craters done for us?
Pamela: Well, they probably got rid of the dinosaurs which some would argue allowed
mammals to evolve on the planet Earth and thus reign supreme and destroy the
planet in new and interesting ways.
At the same time, it’s a way of distributing material around the solar system and
there also a tool that geophysicists can uses to measure the ages of other objects.
For instance on the moon you can look around and there are lava fields on the
moon. The moon actually had a much more liquid core in its past and lava was
able to escape through various different types of dyke features.
There are also all sorts of very neat little underground effects of lava going
underground where we could see it and creating neat geographical formations.
We can date different features on the moon such as the highlands, the mare
based on how many craters there are in different areas. We could also measure
the depth of lava using the craters.
The way this works is you look at an area and count how many craters there are
in different areas and we can also measure the depth of lava using craters. The
way this works is you look at an area and count how many craters of different
sizes there are within that area.
You can make a plot of number of craters versus size of craters and you’ll end
up with a bazillion little tiny craters and very, very few giant craters and you
can fit a pretty much straight line to those relationships.
Now in an area that has a ton of craters, where you end up with the entire line
shifted so that it intersects the Y-axis is really high number. That is a really old
surface, one that has been around for along time getting whacked with rocks
from space.
Fraser: So you just count the number and the size of craters in some region and then you
consult some geophysicists chart somewhere and it tells you how old that region
is. I guess you might have regions that are right at almost the beginning of the
solar system while other places might just be a few million years old.
8
Pamela: They actually define different geological periods based on the crater number.
This gives us the relative age. Trying to get at the actual age of a planet, a
moon, whatever requires you to actually go out, grab a rock and do radioisotope
counting. Look at what different elements have had a chance to decay in that
particular rock.
We can’t do that with Mars yet. But we sent Apollo astronauts to the moon and
they landed in different places. By taking those rocks and looking at the
radioisotopes in them and what has decayed and what is left, we’re able to say
this part of the moon has this age; this part another age; and use that to scale our
understanding, at least with the moon, this crater rate corresponds to this date in
the past. That’s kinda cool.
With Mars, we’re not at a point yet where we can do the radioisotope work and
say we know exactly how old this part of the surface is. Although we have
some fair guesses based on our understanding and on the Rovers we have sent
there so far.
However, with Mars we do the same thing. We age different surfaces and also
age things like stream beds, filuvial systems cut out by liquid we think and they
look like deltas and we’re able to say this section is older or younger than this
other section based on how craters are layered on top of or not layered on top of
based on these filuvial systems.
Fraser: Now we look in the sky and we see the moon just wracked by craters and yet
here on Earth, I think there are meteor crater for well-known crater? Why isn’t
the Earth as hammered as the moon?
Pamela: It rains. We are hammered just as much as the moon. The difference is that as
you look at the surface of the Earth you’re not seeing any rocks that are 3.5
billion years old unless it’s a rock you happen to find and pick up and test and
just happened to survive.
There are a few places on the Earth where we find old rocks but it’s not the
whole surface of the planet. When we look at Mars; when we look at the moon,
we’re looking at surfaces that have rocks on their surfaces that are billions of
years old and they haven’t been eroded by rain or dust and the plate tectonics on
both the moon & Mars are much less.
Mars has the Tharsis Bulge, it has Olympus Mons, it has all these volcanoes that
are problematic. They raised a whole chunk of the surface. To understand
those parts of the surface we have to do all sorts of crazy other stuff. In general
the surface of both of these worlds haven’t been rained on.
Already with the crater that recently formed just last September in Peru, 15-
meter diameter crater is almost gone. It rains, and the rain washes soil in,
9
flattens things back out. Anyone who has ever dug a hole in their back yard
knows the hole is going to fill itself back in rather quickly.
Our planet erodes. It kinda sucks.
Fraser: So you just wonder how many enormous craters are just gone.
Pamela: There is actually thought that things like the Yucatan are crater edges. As we
look more and more at satellite images we’re finding more and more giant
craters from their rims all over the planet.
Fraser: That’s right, there’s like the latest satellite missions are able to measure the
contours of the Earth such precision that they are able to find these enormous
craters just by the tiny little difference in the height of the rim.
It’s been eroding for a hundred million years but there is still just enough of the
rim remaining that they would know there was a crater there, that there was a
huge collision there. Only just now they are able to discover these craters.
Pamela: The other way that we’re finding it is using gravity measurements. This is one
of the coolest things. I learned about this a few years ago. I thought this was
really cool and I don’t geek out too much about gravity.
There is the ability to measure gravity with some instruments so precisely that
you can tell the difference in gravitational acceleration between someone’s foot
and their head using this instrumentation.
It’s possible to go around with gravity detectors and if you know your distance
from the center of the planet, which you can get from GPS systems, and you
measure the acceleration of gravity at that altitude from the center of the planet,
you can roughly figure out the amount of material that has to be between you
and the center to cause that acceleration. Doing this, they go out and find things
like petroleum reserves.
But working in South America, a group of geophysicists actually found a big
impact basin this way because the densities versus shape of the terrain just
didn’t make sense for any other process. Gravity allowed them to find a hidden
crater.
Fraser: They’re using that technique to measure ice loss from glaciers to see how the
ground such as in Canada is bouncing back after the last Ice Age. They can
measure how the ground is moving back up after the Ice Age. It’s pretty
amazing.
Pamela: It was actually using these gravity measurements that we’ve recently been able
to figure out that the reason that you get this weird dichotomy between the
10
northern and southern hemispheres of Mars is because Mars got whacked by a
really big object. We didn’t know about what had happened for a long time
because of the volcanic system that spews volcanic material all over the
boundary between the highlands and the lowlands. We had to figure out how
does that boundary move beneath the lava flows.
A group of geophysicists combined topographical maps that show the altitude
of the terrain with gravity maps that were extremely precise. They were able to
determine that if we assume that the crust of Mars has this density and the lava
flows have this density, what do the boundaries between these have to look like
in order to get the gravity we observe and the altitude of the land that we
observe.
When they did this, they could basically peel off all the volcanoes and see what
the crust looked like beneath them. They were able to find that the boundary
between the highlands and the lowlands is basically a perfect ellipse around
Mars. You pretty much can only get that shape if you whack Mars and you
make an impact basin.
There are ways that you can work really hard with other models and twist
parameters and other things and jump through lots of hoops and ignore
Ockham’s razor and get this to happen other ways. But the easiest way to
explain those results is to say that Mars got whacked with something over 2,000
kilometers in diameter and didn’t end up producing a moon in the process but
instead created this dichotomy between the highlands and lowlands.
Fraser: Wow, so a couple more things. One was just to talk a bit about how the gravity
and structure of what gets hit changes the nature of the crater itself.
Pamela: It’s not just the gravity, it’s also the surface that’s getting hit. So if you impact
on top of a bunch of ices and say you dig up the soil beneath the ices and spread
them out on the top of the ice.
Then you can get these really neat plateau craters where over time the ice
around the crater in this ejecta blanket might vaporize or sublimate away and as
this ice goes straight to gas where the soil has been dug up and plopped down
onto the ice it can’t do that. You end up with that layer of ice basically
protected by the ejecta that’s on top of it and everything else around it gets
lower and lower and vaporizes into the atmosphere.
You also have depending on the gravity the amount of stuff that gets flown up is
going to differ. If you hit a really big heavy object with a rock, the gravity of
that object is going to hold on to the material and make it not fly out quite so
much.
11
But if you hit something much smaller you launch rocks from Mars to Earth.
So the height of the crater walls is going to be a function of the density of the
material you’re hitting and the gravity of whatever it is that you’re hitting. It all
plays together.
Fraser: I know we mentioned when we were doing our tour through the solar system
some of the bizarre crater formations in some of Saturn’s moons.
Pamela: The death star.
Fraser: Yeah, where it almost looks like it’s hitting a real spongy material and the
material is just collapsing. I think there’s one last really good use for craters
that we haven’t talked about which is when the astronauts return back to the
moon one of the things that they will be looking at is the craters at the southern
pole of the moon which are in some cases eternally in sunlight and in other
cases eternally in shadow and may even hold water ice.
Pamela: One of the things that is amazing that I just learned this week is the difference in
altitude between the base and the rim of some of these craters is like four
kilometers. That’s about two miles.
We talk about Denver, the mile high city. Imagine standing on the edge of
something twice the altitude or more of Denver looking down. There is an
amazing movie they showed us of a little tiny sad little lander craft coming in
and just perching on the very edge of one of these craters.
Fraser: Nancy, one of the writers did a story about that on Universe Today. We’ve got
the video on the site. It’s kind of scary.
Pamela: The reason that we’re looking to do this is when you land at the equator of the
moon, in daylight you’re several hundred degrees. When in darkness, you’re at
minus a couple hundred degrees. Pick Fahrenheit or Celsius it really doesn’t
matter it’s really huge swings in temperature either way you go.
Once you get down into one of the craters in constant darkness the temperatures
stay constant. One of the things about electronics is they don’t really like to
have their temperature messed with. We can engineer things that work at a
couple hundred degrees and we can engineer things that work at a negative
couple hundred degrees. It’s hard to engineer things that can survive huge
temperature swings.
If we go to one of the poles of the moon you can stick your habitats down in the
shaded part and just keep people warm and stick your solar panels straight up
and they will be in constant daylight. So you have constant power and thermal
regulation. It’s just a lot easier to function that way.
12
Fraser: You could have the sunlight just a few tens of meters away from your habitat
and still be safe.
Pamela: One of the ways they phrased it was you can be down in a constant darkness
area and just raise your hand and the simple act of raising your hand that is
above your head, your hand will be in perpetual sunlight.
Fraser: I can’t wait until that exploration starts happening.
Pamela: Just a few more years.
Fraser: Just a few more years. The missions are going to be launching just within the
next year and it seems like it will start steamrolling from there.
Pamela: We have LCROSS and LRO are launching on the same rocket in I think
October of this year. LCROSS is this really cool mission that they’re going to
basically plow objects into the surface of the moon making artificial craters,
making our own space craft into meteorites and see what dust gets chewed up
into the air. It’s going to be neat work.
Fraser: All right, well I think we’ll be covering that as we go for a much, much future
show.
Pamela: It was a great experience and next year you need to come with us Fraser.
Fraser: Will do.

This transcript is not an exact match to the audio file. It has been edited for clarity.
Transcription and editing by Cindy Leonard