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Learning objectives

By the end of this section, you will be able to:

  • Compare and contrast ideas about how lunar craters form
  • Explain the process of impact crater formation
  • Discuss the use of crater counts to determine relative ages of lunar landforms

The Moon provides an important benchmark for understanding the history of our planetary system. Most solid worlds show the effects of impacts, often extending back to the era when a great deal of debris from our system’s formation process was still present. On Earth, this long history has been erased by our active geology. On the Moon, in contrast, most of the impact history is preserved. If we can understand what has happened on the Moon, we may be able to apply this knowledge to other worlds. The Moon is especially interesting because it is not just any moon, but our Moon—a nearby world that has shared the history of Earth for more than 4 billion years and preserved a record that, for Earth, has been destroyed by our active geology.

Volcanic versus impact origin of craters

Until the middle of the twentieth century, scientists did not generally recognize that lunar crater s were the result of impacts. Since impact craters are extremely rare on Earth, geologists did not expect them to be the major feature of lunar geology. They reasoned (perhaps unconsciously) that since the craters we have on Earth are volcanic, the lunar craters must have a similar origin.

One of the first geologists to propose that lunar craters were the result of impacts was Grove K. Gilbert , a scientist with the US Geological Survey in the 1890s. He pointed out that the large lunar craters—mountain-rimmed, circular features with floors generally below the level of the surrounding plains—are larger and have different shapes from known volcanic craters on Earth. Terrestrial volcanic craters are smaller and deeper and almost always occur at the tops of volcanic mountains ( [link] ). The only alternative to explain the Moon’s craters was an impact origin. His careful reasoning, although not accepted at the time, laid the foundations for the modern science of lunar geology.

Volcanic and impact craters.

Profiles of Volcanic and Impact Craters Illustrated. At left is a terrestrial volcano. It is tall, steeply sloped with a shallow crater at the top. At right is a Lunar impact crater. Not as tall as a terrestrial volcano, nor as steeply sloped. The crater has a very wide, flat floor and a central peak.
Profiles of a typical terrestrial volcanic crater and a typical lunar impact crater are quite different.

Gilbert concluded that the lunar craters were produced by impacts, but he didn’t understand why all of them were circular and not oval. The reason lies in the escape velocity, the minimum speed that a body must reach to permanently break away from the gravity of another body; it is also the minimum speed that a projectile approaching Earth or the Moon will hit with. Attracted by the gravity of the larger body, the incoming chunk strikes with at least escape velocity, which is 11 kilometers per second for Earth and 2.4 kilometers per second (5400 miles per hour) for the Moon. To this escape velocity is added whatever speed the projectile already had with respect to Earth or Moon, typically 10 kilometers per second or more.

At these speeds, the energy of impact produces a violent explosion that excavates a large volume of material in a symmetrical way. Photographs of bomb and shell craters on Earth confirm that explosion craters are always essentially circular. Only following World War I did scientists recognize the similarity between impact craters and explosion craters, but, sadly, Gilbert did not live to see his impact hypothesis widely accepted.

Questions & Answers

why are the hyperlinks not working?
Gregory Reply
If the atmosphere blocks the view of Venus' surface, what am I seeing that looks like craters?
Michael Reply
clouds?
Michele
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters when view the planet?
Michael Reply
I don't Know! perhaps the mountains?
Michele
my question is if the surface of Venus is shining y not our earth
Tahir
when we look at venus we can't see any crater like things... check it once again
RIEM
and venus shines because of the clouds that are made up sulpher dioxide and sulphuric acid droplets. and the clouds are so dense. the case of earth is different
RIEM
also because of Venus's 70% albedo phenamenon
Shivam
and plus it reflects 70 percent of its light back into space earth don't shine because it's not a cloudy planet and its farther than the sun
americantuber
can I become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL Reply
If the surface of Venus is shrouded by white clouds, making it impossible to see the surface, what looks like craters? Are these atmospheric storms?
Michael
can a death of a massive star be the new big bang
Neeta Reply
No it might be a supernova. The big bang was much more massive.
Nick
It created the universe as we know it
Nick
the big bang is just a new beginning
Neeta
the death of a gigantic star makes it possible for a new begging
Neeta
Only in the local area of space
Nick
so many smaller universes are created and destroyed
Neeta
i think it created a supernova!
Michele
you still think? I am sure I created one supernova of my own
Neeta
I'm sure you did
Nick
neeta the death of a massive star could lead to an explosion that leads to a super nova and when the supernova explodes it becomes a nebula like messier 1
americantuber
a supernova remnant
americantuber
what is means by earthbound
Satyam Reply
you are earthbound, arnt we all earthbound? except the ISS etc.
Collins
I don't agree we may be earthbound because of our gross body/physical body...but there is also a subtle body which does not limit us to earthly existence
Neeta
good questions, as humans our habitat is earth, we are bound to this and have to alter ourselves to stay alive off land and outside the atmosphere, do you not agree? I'd guess you could also say the moon is earthbound. anything earthbound is restricted to earth. hope this helps, love your question
Collins
how do you describe astral projection
Neeta
Astral projection is spiritual. I'm assuming Satyam was talking about the physical world.
TheDirtyGhost
ah yes spiritually we are bound only to what we chose, like nothing, I like to think that way
Collins
anyone here also participating in seti@home?
dreamer
what is seti?
Mahesh
what has extraterrestrial entity got to do with this why @seti project..we are discussing human existence on earth and beyond earth
Neeta
please join me at the cosmic Diner where all your intergalactic dreams come true.
Alba
I was just thinking what if somebody ignorantly mistaken a cosmic microwave Background with a conventional microwave we be in a lot of trouble.
Alba
don't kno
dreamer
You mean the Restaurant at the end of the Universe...right we can come back and visit again but not sure of the mode of payment
Neeta
yeah I'm already heading there
dreamer
how to read stars
Amresh Reply
you don't
Max
astrology i guess
Uttam
astrologer is the study of star systems and constellations...but astronomy is the study of formation of universes multiverses birth and evolu of stars
Neeta
cool
Uttam
what is astronomy
babul Reply
study of formation evolution and the death of star star systems and galaxies
Neeta
astronomy is the study of universe
UNIVERSAL
astronomy and physics are basically the same except physics is the study of the motion and behavior of the universe
americantuber
What is 12 constellation of zodiac and why it is important to study astrology
MUHIBULLAH
And also what is main purpose of these 12 constellation of zodiac in astronomy
MUHIBULLAH
astronomy and astrology with two different things
Deja
it Is the study of universe and of life speaking generally! i think!
Michele
universe is born 13,7 miliardi of years with big Bang if i remember well! but there are also other theories for the universe, speaking generally! i remember so!
Michele
universe is actually about 13.7 billion years old
Nick
is astrology like astronomy
debjani Reply
Astrology is a lower dimension.. astronomy is much more vast and multidimensional
Neeta
yes.. astrology is about constellations only, and astronomy is about all stars, galaxies, gravity, dark matter, dark energy.. etc everything including astrology
Mahesh
can you become an astronaut without taking mathematics as a subject in 11th class
UNIVERSAL
I was reading the chapter on Cosmic Microwave Background. And, I can not seem to find it now. If anyone could help me find that portion of the Astronomy Textbook I would really appreciate it. AZ
Alba Reply
Where or which chapter discusses Cosmic Microwave Background?
Alba
29.4
Andrew
29 the big bang ch 29.4
Andrew
yes
pratham
interesting
Orlando
what's cold dark matter?
pratham Reply
How are Roche Worlds formed?
AlteredEdge Reply
In need to read some books about Astronomy so how can in get it actually leg live in Ethiopia can uh help me with that?
Mom Reply
I am sri Sharan .m .what is my best favourable numbers
Madhesh Reply
how can we know it
sruthi
😂😂
Rango
bahut hard
Rango
Why no object can travel with speed greater than light?
geometry Reply
Because the mass of light particles is zero
Asim
Can mass be negative?
geometry
No
Asim
anybody stated that the light is the only thing is speed
sruthi
geometry are you a korean or kpop fan
sruthi
in this universe anything can be faster than light because we simply can't get to an conclusion
sruthi
I am a an Army but lets not bring spam here.
geometry
But why mass can't be negative. What I mean is how can we claim an object made of strange matter has positive mass. In other words, theoretically can negative mass exist?
geometry
in our observable universe nothing is faster than the speed of light
Rango
Thanks
geometry
But theoretically, is it possible? Sorry I'm being very curious..
geometry
I don't know about dark matter but may be you should think about dark matter because dark matter are really strange
Rango
Exactly so theoretically, an dark matter lazer can be greater in speed. Also maybe quartz..
geometry
may be but i am not sure about quartz and btw techyons travel faster than light but techyons are hypothesized
Rango
there are many objects which are perceived to be faster than light
Ganeshan
The object have their own declination rate in speed,the air friction can develop higher temperature resulting the object to be disappear in molecules,not considering the Physical conditions in space.The object should be unaffected with the air friction and also speed should proportionate with fuel.
Near
the early universe expanded faster than the speed of light. Consider, at the speed of light in vacuum, time stops.
Robert
Does this mean something moving beyond the limit experiences negative time?
Robert
Under such conditions, would matter exist as wave-fronts?
Robert
When examining a cosmology it is possible to find quite unbelievable conditions. I like to imagine life at the tip of an asymptote, but to surpass it seems perilously remote.
Robert

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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