Crater counting

Last updated October 03, 2024

Crater counting is a method for estimating the age of a planet's surface based upon the assumptions that when a piece of planetary surface is new, then it has no impact craters; impact craters accumulate after that at a rate that is assumed known. Consequently, counting how many craters of various sizes there are in a given area allows determining how long they have accumulated and, consequently, how long ago the surface has formed. The method has been calibrated using the ages obtained by radiometric dating of samples returned from the Moon by the Luna and Apollo missions.^[1] It has been used to estimate the age of areas on Mars and other planets that were covered by lava flows, on the Moon of areas covered by giant mares, and how long ago areas on the icy moons of Jupiter and Saturn flooded with new ice.

Crater counting and secondary craters

The crater counting method requires the presence of independent craters. Independent craters represent the primary impact point on a planets surface, while secondary craters represent the second impact on the surface of a planet.^[2] Secondary craters ('secondaries') are craters formed by material excavated by a primary impact that falls back to the surface seconds or minutes later.^[2] A way to distinguish primary and secondary craters is to consider their geometric arrangement; for example, large craters often have rays of secondary craters.^[2] Secondaries can sometimes also be recognized by their particular shape different from primary craters; this is due to the fact that the excavated material is slower and impacts at a lower angle than asteroids that arrive from space to create the primary crater.^[2]

The accuracy of age estimates of geologically young surfaces based on crater counting on Mars has been questioned due to formation of large amounts of secondary craters. In one case, the impact that created Zunil crater produced about a hundred secondary craters, some more than 1000 km from the primary impact.^[3] If similar impacts also produced comparable amounts of secondaries, it would mean a particular crater-free area of Mars had not been "splattered by a large, infrequent primary crater", as opposed to suffering relatively few small primary impacts since its formation.^[4] High speed ejecta generated from independent craters generates secondary craters which can resemble independent craters as well, contaminating counting processes as the secondary craters appear more circular and less cluttered than typical secondaries.^[5] Secondaries will inevitably contaminate independent crater counts leading to some who may question its effectiveness (see criticism heading for further information).

History

The earliest scientist to study and produce a paper using crater counting as an age indicator was Ernst Öpik, an Estonian astronomer and astrophysicist.^[6] Ernst Öpik utilized the crater counting method to date the Moon's Mare Imbrium to be approximately 4.5 billion years of age, which was corroborated by isotopic samples.^[6] The method was also utilized by Gene Shoemaker and Robert Baldwin, and further improved by Bill Hartman.^[7] Hartman's work includes dating the Lunar Mare to be approximately 3.6 billion years old, an age that was in accordance with isotopic samples.^[7] In later years, Gerhard Neukum advanced the method by proposing a stable impacting population over the period of 4 billion years due to unchanged shape of crater size-frequency distribution.^[8] More recent work has seen the transition from Lunar surface to Martian surface cratering, including work done by Neukum and Hartman.^[9] Within the past ten years, the Buffered Crater Counting approach has been used to date geologic formations.^[10] The calibration provided by the Lunar samples brought back during the six Apollo missions between 1969 and 1972 has remained invaluable to further refining and advancing the crater counting method to this day, but new work is being done to computerize the crater counting technique using Crater Detection Algorithms which uses high resolution imagery to detect small impact craters.^[11]^[12]

Criticism

While crater counting has been refined in past years to be an accurate method of determining surface age of a planet despite a lack of isotopic samples, there is dissension in the planetary scientific community concerning the acceptance of crater counting as a precise and accurate form of geochronology. This method is influenced by assumption that at time zero of a planet, the surface had no craters and the craters which followed time zero are spatially and temporally random. It can only be applied with accuracy to planets which have little to no tectonic activity, since constant resurfacing (like on Earth) would distort the true number of craters over time. Shallow surface mechanism such as aeolian deposition, erosion, and diffusional creep can also alter crater morphology, making the surface appear younger than it truly is.^[13] Planets heavily covered by water or dense atmosphere would also impede the accuracy of this method, since observational efforts would be hampered. Planets with dense atmospheres will also cause incoming meteors to burn up due to friction before impacting the surface of the planet.^[14] The Earth is bombarded with approximately 100 tons of space dust, sand, and pebble particles every day; however, most of this material burns up in the atmosphere before ever reaching the surface of the planet.^[15] This is common for space material that is smaller than 25 meters, burning up due to friction in the atmosphere.^[15] While resulting observational values dating the Lunar surface from Hartman and Öpik do illustrate ages that correspond to isotopic data, they are potentially hampered by observational bias and human error. New advances continue to improve upon the original method.

Application

Below is a list of studies which utilize or concern crater counting:

Dating very young planetary surfaces from crater statistics: A review of issues and challenges^[13]
- Using small craters for age analysis and the challenges and limitations associated such as illumination and imagery issues, crater modification (such as resurfacing), and impact rates.
Comparison of different crater counting methods applicated to Parana Valles^[16]
- Testing the reliability of the crater counting method using Parana Valles and comparing results to other methods using fine scale age determination of small diameter craters. Results conclude that the buffered crater counting method is more reliable in large areas than the basin method.
Crater size-frequency measurements on linear features buffered crater counting in ArcGIS^[17]
- Buffered crater counting analysis using the CraterTools software in ArcGIS. This tool allows for accurate data collection in areas which might have poor image quality or limited area of measurement.
Crater counting lab exercise^[18]
- Teaching exercise which allows for age determination of Lunar surface taken from Apollo mission images and extend this practiced method to Mars and Earth.
The Importance of Secondary Cratering to Age Constraints on Planetary Surfaces^[19]
- Discusses small craters caused by interplanetary debris in comparison to secondary craters formed by ejecta of primary impact. Results indicate that small craters can provide meaningful data to aid in age determination.

External links

Related Research Articles

<span class="mw-page-title-main">Moon</span> Natural satellite orbiting Earth

The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth. The Moon's gravitational pull—and, to a lesser extent, the Sun's—are the main drivers of Earth's tides.

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. It is named after Phobos, the Greek god of fear and panic, who is the son of Ares (Mars) and twin brother of Deimos.

The lunar geological timescale divides the history of Earth's Moon into five generally recognized periods: the Copernican, Eratosthenian, Imbrian, Nectarian, and Pre-Nectarian. The boundaries of this time scale are related to large impact events that have modified the lunar surface, changes in crater formation through time, and the size-frequency distribution of craters superposed on geological units. The absolute ages for these periods have been constrained by radiometric dating of samples obtained from the lunar surface. However, there is still much debate concerning the ages of certain key events, because correlating lunar regolith samples with geological units on the Moon is difficult, and most lunar radiometric ages have been highly affected by an intense history of bombardment.

<span class="mw-page-title-main">Lunar meteorite</span> Meteorite that originated from the Moon

A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon.

Utopia Planitia is a large plain within Utopia, the largest recognized impact basin on Mars and in the Solar System with an estimated diameter of 3,300 km (2,100 mi). It is the Martian region where the Viking 2 lander touched down and began exploring on September 3, 1976, and the Zhurong rover touched down on May 14, 2021, as a part of the Tianwen-1 mission. It is located at the antipode of Argyre Planitia, centered at 46.7°N 117.5°E. It is also in the Casius quadrangle, Amenthes quadrangle, and the Cebrenia quadrangle of Mars. The region is in the broader North Polar/Borealis Basin that covers most of the Northern Hemisphere of Mars.

Stickney is the largest crater on Phobos, which is a satellite of Mars. It is 9 km (5.6 mi) in diameter, taking up a substantial proportion of the moon's surface.

The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by lava flowing into ancient impact basins. They were dubbed maria by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich composition, and hence appear dark to the naked eye. The maria cover about 16% of the lunar surface, mostly on the side visible from Earth. The few maria on the far side are much smaller, residing mostly in very large craters. The traditional nomenclature for the Moon also includes one oceanus (ocean), as well as features with the names lacus ('lake'), palus ('marsh'), and sinus ('bay'). The last three are smaller than maria, but have the same nature and characteristics.

A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena, plural catenae, as specified by the International Astronomical Union's rules on planetary nomenclature.

<span class="mw-page-title-main">Ejecta blanket</span> Symmetrical apron of ejecta that surrounds an impact crater

An ejecta blanket is a generally symmetrical apron of ejecta that surrounds an impact crater; it is layered thickly at the crater's rim and thin to discontinuous at the blanket's outer edge. The impact cratering is one of the basic surface formation mechanisms of the solar system bodies and the formation and emplacement of ejecta blankets are the fundamental characteristics associated with impact cratering event. The ejecta materials are considered as the transported materials beyond the transient cavity formed during impact cratering regardless of the state of the target materials.

Secondary craters are impact craters formed by the ejecta that was thrown out of a larger crater. They sometimes form radial crater chains. In addition, secondary craters are often seen as clusters or rays surrounding primary craters. The study of secondary craters exploded around the mid-twentieth century when researchers studying surface craters to predict the age of planetary bodies realized that secondary craters contaminated the crater statistics of a body's crater count.

The presence of water on the terrestrial planets of the Solar System varies with each planetary body, with the exact origins remaining unclear. Additionally, the terrestrial dwarf planet Ceres is known to have water ice on its surface.

The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized astronomical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypothesis, during this interval, a disproportionately large number of asteroids and comets collided into the terrestrial planets and their natural satellites in the inner Solar System, including Mercury, Venus, Earth and Mars. These came from both post-accretion and planetary instability-driven populations of impactors. Although it gained widespread credence, definitive evidence remains elusive.

The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. The absolute age of the Noachian period is uncertain but probably corresponds to the lunar Pre-Nectarian to Early Imbrian periods of 4100 to 3700 million years ago, during the interval known as the Late Heavy Bombardment. Many of the large impact basins on the Moon and Mars formed at this time. The Noachian Period is roughly equivalent to the Earth's Hadean and early Archean eons when Earth's first life forms likely arose.

The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today. The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the Late Heavy Bombardment and probably corresponds to the start of the lunar Late Imbrian period, around 3700 million years ago (Mya). The end of the Hesperian Period is much more uncertain and could range anywhere from 3200 to 2000 Mya, with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth's early Archean Eon.

Hesperia Planum is a broad lava plain in the southern highlands of the planet Mars. The plain is notable for its moderate number of impact craters and abundant wrinkle ridges. It is also the location of the ancient volcano Tyrrhena Mons. The Hesperian time period on Mars is named after Hesperia Planum.

The geological history of Mars follows the physical evolution of Mars as substantiated by observations, indirect and direct measurements, and various inference techniques. Methods dating back to 17th-century techniques developed by Nicholas Steno, including the so-called law of superposition and stratigraphy, used to estimate the geological histories of Earth and the Moon, are being actively applied to the data available from several Martian observational and measurement resources. These include landers, orbiting platforms, Earth-based observations, and Martian meteorites.

<span class="mw-page-title-main">Amazonian (Mars)</span> Time period on Mars

The Amazonian is a geologic system and time period on the planet Mars characterized by low rates of meteorite and asteroid impacts and by cold, hyperarid conditions broadly similar to those on Mars today. The transition from the preceding Hesperian period is somewhat poorly defined. The Amazonian is thought to have begun around 3 billion years ago, although error bars on this date are extremely large. The period is sometimes subdivided into the Early, Middle, and Late Amazonian. The Amazonian continues to the present day.

<span class="mw-page-title-main">LARLE crater</span> Class of Martian impact craters

A low-aspect-ratio layered ejecta crater is a class of impact crater found on the planet Mars. This class of impact craters was discovered by Northern Arizona University scientist Professor Nadine Barlow and Dr. Joseph Boyce from the University of Hawaii in October 2013. Barlow described this class of craters as having a "thin-layered outer deposit" surpassing "the typical range of ejecta". "The combination helps vaporize the materials and create a base flow surge. The low aspect ratio refers to how thin the deposits are relative to the area they cover", Barlow said. The scientists used data from continuing reconnaissance of Mars using the old Mars Odyssey orbiter and the Mars Reconnaissance Orbiter. They discovered 139 LARLE craters ranging in diameter from 1.0 to 12.2 km, with 97% of the LARLE craters found poleward of 35N and 40S. The remaining 3% mainly traced in the equatorial Medusae Fossae Formation.

Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics, and interactions such as impact cratering, space weathering, and magnetospheric physics in the solar wind, and possibly biology, via astrobiology.

Northeast Syrtis is a region of Mars once considered by NASA as a landing site for the Mars 2020 rover mission. This landing site failed in the competition with Jezero crater, another landing site dozens of kilometers away from Northeast Syrtis. It is located in the northern hemisphere of Mars at coordinates 18°N,77°E in the northeastern part of the Syrtis Major volcanic province, within the ring structure of Isidis impact basin as well. This region contains diverse morphological features and minerals, indicating that water once flowed here. It may be an ancient habitable environment; microbes could have developed and thrived here.

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