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Impact Craters
Impact craters are the most characteristic features of planetary
bodies. They range in size from tens of meters in diameter (visible
size with the current image resolution) to hundreds or thousands of
kilometers (where the impacts create giant basins as on the Moon,
Mars, and Mercury).
Formation of Impact Craters
Three main stages are considered in the formation of an impact
craters :
- compression;
- excavation;
- post-cratering modification ;
The figures below show you these different stages.
First stage :
During this first compression stage, the meteorite strikes and
penetrates the surface, exploding and jetting material outward. The
pressure created at this moment can reach a million times the
terrestrial atmospheric pressure. Rocks are melted, vaporized or
crushed and shock waves are sent out through both the surface and the
impacting bolide.
Second stage :
This is the excavation stage when the material excavated from the
crater is distributed radially as a blanket of fragmented debris or
"ejecta". Subsequent debris falling on the surface will form
secondary impact craters.
Third stage
(Click on the image above to see how the stratigraphy is reversed
at the surface after ejecta material deposition)
The formation and transformation of the crater continues.
"Isostatic" adjustment leads to the slumping of walls and the deep
fracturing of the crater floor, allowing molten rock ("magma") to
rise to the surface. Later in the crater's history, depending on the
planet in question, rain may erode it and sediments may fill it. On
Earth, through continuous erosion and plate tectonics, the traces of
almost all the impact craters have been erased. Some young craters
remain to remind us that Earth is a target for impacts just like the
others.
The initial form and subsequent evolution of an impact crater
depend on what type of surface the asteroidal body strikes. As such,
the form that a crater takes is a good indicator of the nature of the
surface and subsurface rocks. We see striking differences in the form
of lunar and martian impact craters because those two planetary
bodies were formed with significantly different composition (the Moon
being very dry and Mars very wet).
Lunar impact crater :
Impact craters on the Moon form bright radial ejecta. The
subsurface of the Moon is dry.
Martian impact crater :
This form is characteristic of most of the martian impact craters
which display striking "lobate" ejecta -- highly suggestive that the
surface was saturated with water ice when the event took place. The
lobate ejecta vary in appearance from one crater to another and this
implies variations in the underground water-ice content.
Planetary Cratering Rates.
Craters are a most useful tool to establish the relative ages of
surfaces and thus to better understand how a planet has evolved, for
example, to learn what have been the rates of erosion and
sedimentation. Inevitably older surfaces are more heavily cratered
than young ones as they have been targets for a much longer time.
But, we have to be careful in calculating time histories because the
rate of cratering is not the same from one planet to another. We are
still learning about the differences in impact rates in different
parts of the solar system.
1)Velocity Effect : With a similar rate of cratering
between two planets, the diameter of the impact crater formed by an
asteroid of a given size will change according to the mass of the
planet in question. This is because once the asteroid come close and
is captured by the gravity field of the planet in question it begins
to accelerate under the attraction of gravity. Big planets exert a
bigger pull and a higher speed impact than small planets:
Effect Of Impact Velocity And Planetary Focusing On The Diameter Of
Impact Craters On Different Planetary Bodies. Source : "The Surface
of Mars", by Mike Carr.
Local Planetesimals Asteroids and Long
in Heliocentric Short Period Period
Circular Orbits Comets Comets
Approach Velocity (km/s)
Mercury 2.1 19 62
Venus 5.1 15 46
Earth 5.6 14 38
Moon 5.6 14 38
Mars 2.5 8.6 31
Impact Velocity (km/s)
Mercury 4.7 20 62
Venus 11.5 18 47
Earth 12.5 18 40
Moon 6.1 14 38
Mars 5.6 10 31
Impact velocity correction
factors (ratio to Moon)
Mercury 0.73 1.54 1.87
Venus 2.15 1.36 1.29
Earth 2.38 1.36 1.06
Moon 1.00 1.00 1.00
Mars 0.90 0.66 0.78
Effective radius factor
Mercury 1.28 1.26 0.25
Venus 1.27 0.38 0.26
Earth 1.26 0.42 0.27
Moon 0.29 0.26 0.25
Mars 1.25 0.33 0.26
SURFACE GRAVITY CORRECTION FACTOR
Planet Correction factor
Mercury 0.72
Venus 0.51
Earth 0.49
Moon 1.00
Mars 0.72
2) Target effect : Mars Orbit is 1.38 A.U when Mars and
Moon are at 1 A.U from the Sun. Then, the meteorites are closer from
their aphelic position when they cross by Mars (thus, they are
slower). Conversely, they are closer from their perihelic position
when they pass through Earth and Moon orbit (thus they are more
rapid). In addition, we have to considerate the respective attitude
of planets and asteroides at the time of the impact : relative speeds
could be either added or cut out according to the respective position
of the planets and asteroides.
All these effects have to be considered before establishing a
theory of impactism for a planet. It explains also why the
craterization curves of Mars and Moon are comparable but not similar.
Source : Hartmann et al., 1981
ESTIMATED CRATER PRODUCTION RATES, NORMALIZED TO THE MOON
Type of Objects Mercury Venus Earth Moon Mars
Asteroids 0.8 1 0.9 1 2
Comets 5 2 0.7 1 0.3
40%comets/60%aster. 2 1 1 1 2
Mars-crossers favored (80%) 2 1 2 1 4
Maximum Likely 5 2 2.1 1 4
Minimum Likely 0.8 0.8 0.9 1 1
Most Likely 2 1 1.5 1 2
From these differences depends our understanding of
the ages of the planetary surfaces, thus,
the history of our Solar System. The martian chronology is a relative
chronology by comparison to the lunar one which is absolute and was
calibrated by datation established on the lunar samples brought back
by the Apollo missions. We do not have yet any samples of the martian
surface and this is the reason why there are different models to
describe the martian craterization rate. Therefore, the datation of
the different types of surfaces on Mars are sumitted to a range of
uncertainties. The following tables show examples of age estimation
for the main geologic features of Mars.
Source : Hartmann et al., 1981
CRATER AGES FOR DIFFERENT SURFACE FEATURES
Crater density Estimated Crater Retention Age
Relative to (billions of years)
Average Lunar Minimum Best Maximum
Mare Likely Estimate Likely
Geologic Provinces
Central Tharsis 0.1 0.06 0.3 1.0
volcanic plains
Olympus Mons Volcano 0.15 0.1 0.4 1.1
Extended Tharsis 0.49 0.5 1.6 3.3
volcanic plains
Elyisum volcanics 0.68 0.7 2.6 3.5
Isidis Planitia 0.76 0.8 2.8 3.6
Solis Planum volcanic 0.90 0.9 3.0 3.7
Chryse Planitia 1.1 1.2 3.2 3.8
volcanic plains
Lunae Planum 1.2 1.3 3.2 3.8
Noachis ridged plains 1.3 1.7 3.3 3.8
Tyrrhenum Patera 1.4 1.8 3.4 3.8
volcano
Tempe Fossae faulted 1.6 2.3 3.4 3.8
plains
Volcanic plains on 1.7 2.6 3.5 3.8
south rim of Hellas
Alba shield volcano 1.8 2.6 3.5 3.8
Hellas floor 1.8 2.6 3.5 3.8
Syrtis Major Planitia 2.0 2.6 3.5 3.8
volcanic plains
Heavily cratered plains
- small D (< 4 km) 1.4 1.8 3.4 3.8
- large D (> 64 km) 13 3.8 4.0 4.2
Stratigraphic relationships between the different martian geologic
units coupled with the craterization statistics helped to obtain a
picture of what could have been the martian geological chronology
geological chronology. The following table
gives you the description of this chronology.
Source : Tanaka (1986) : N = number of craters > (x) km
diameter/106km2
Series N(1) N(2) N(5) N(16) N(4-10)
Upper Amazonian <160 <40
Middle Amazonian 160-600 40-150 <25 < 33
Lower Amazonian 600-1600 150-400 25-67 33-88
Upper Hesperian 1600-3000 400-750 67-125 88-165
Lower Heperian 3000-4800 750-1200 125-200 < 25 165-260
Upper Noachian 200-400 25-100 > 260
Middle Noachian > 400 100-200
Lower Noachian > 200
Larger basins on Mars
Argyre Basin as imaged by the Viking orbiter.
BASINS OF MARS (Source : Wood and Head (1976), Schultz et al.
(1982), Frey and Schulz (1988), McGill(1989a), and Schultz and
Frey (1990).)
Name Latitude Longitude Diameter
Acidalia 60°N 30° 1950
Al Qahira 20°N 190° 1034
Al Qahira A 13°N 184° 1994
Amazonis 6°N 168° 800
Antoniadi 22°N 299° 400
Aram Chaos 3°N 22° 550
Arcadia A 37°N 167° 600
Arcadia B 32°N 167° 1925
Argyre 50°S 42° 1850
Borealis 50°N 190° 7700
Cassini 24°N 328° 930
Cassini A 14°N 324° 1204
Chryse 22°N 47° 4600
Daedalia A 26°S 125° 1800
Daedalia B 15°S 127° 3960
Deuteronilus A 44°N 342° 280
Deuteronilus B 43°N 338° 201
Elysium 33°N 201° 4970
Gale 5°S 222° 150
Galle 51°S 31° 220
Hellas 43°S 291° 420
Herschel 15°S 230° 290
Holden 25°S 32° 580
Huygens 14°S 304° 460
Isidis 13°N 273° 1900
Kaiser 46°S 340° 200
Kepler 47°S 218° 210
Ladon 18°S 29° 1700
Liu Hsin 55°S 121° 135
Lowell 52°S 81° 190
Lyot 50°N 330° 200
Mangala 0°N 147° 570
Memnonia 22°S 166° 2065
Molesworth 28°S 210° 180
near Columbus 25°S 164° 145
Nilosyrtis Mensae 33°N 282° 380
North Tharsis 11°N 98° 4500
overlapped by Newcomb 22°S 4° 800
overlapped by Schiaparelli 5°S 347° 560
overlapped by South Crater 73°S 213° 680
Phillips 67°S 44° 175
Ptolemaeus 46°S 157° 150
Schiaparelli 3°S 344° 470
Scopulus 5°N 278° 2700
Sirenum basin 44°S 166° 1548
southeast of Hellas 58°S 273° 500
southeast of Ma'adim Vallis 30°S 180° 1000
South Hesperia 32°S 255° 1255
south of Hephaestus Fossae 10°N 233° 1000
south of Lyot 42°N 322° 570
south of Renaudot 38°N 297° 600
South polar 83°S 267° 850
Utopia 48°N 240° 4715
west of Le Verrier 37°S 356° 430
West Tempe 56°N 78° 830 ?
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