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 :

  1. compression;
  2. excavation;
  3. 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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