Background
The planets in our Solar System are all in roughly circular orbits.
Other objects in our Solar System, such as many asteroids and comets
are in much more elongated orbits, which means they sometimes end up on
collision courses with a planet. All planets in our Solar System are
struck from time to time by meteoroids ranging in size from small
pebbles up to several hundred meters across.
The inner planets, Mercury, Venus, Earth and Mars, as well as our
Moon and the moons of Mars, all show evidence of many impacts with
meteoroids. Our Moon has over 30,000 impact craters, while Earth has
fewer than 200. Mercury has more impact craters than the other inner
planets, and Earth has the fewest. The motion of the tectonic plates
that constantly change the Earth’s surface cover up impact craters over
time. The Earth’s atmosphere also stops many meteoroids before they
reach the surface. Mars and Venus show evidence of having had more
active surfaces in the past, but have far more craters than the Earth
because impact craters are not erased by this motion as they are
on Earth.
The gas giants also collide with meteroids and comets from time to
time, but because their surfaces are not solid, there are no impact
craters on these planets. Instead, the meteoroids burn up as they enter
the planet, and the increase in temperature and change in chemical
composition can be viewed as one or more dark spots for many months.
Many of the moons of the gas giants have impact craters on
their surfaces.
Craters
Impact craters have different features depending on their size. Impact
craters smaller than 10 km across are called simple craters. They are
bowl shaped, and their depth is about 20% of their diameter. Craters
between 10 and 150 km across have outer walls that have slumped into
the the crater pit. They often have a central peak caused by recoil
during the impact from the rocks beneath the crater. Craters over 150
km across normally contain concentric rings of mountains, created by
ripples as the material from the center rebounded and solidified after
the impact.
Craters are normally at least 15 times bigger than the impacting
meteoroid, which normally vaporizes on impact. Craters are almost
always nearly circular, because the shape is created by a shock wave
that spreads out from the point of impact. So unless an object imacts
from an extremely shallow angle, the crater will be circular no matter
what the trajectory was.
The activities on the next pages take you through using Google Earth to find, measure, and analyze impact craters on Earth, the Moon and Mars.
