An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

An object having an estimated mass between 1.0 × 10¹⁵ and 4.6 × 10¹⁷ kg, which struck the Earth at the → Cretaceous-Tertiary event about 65 million years ago. It was probably an → asteroid 10 km in diameter with a velocity of roughly 20 km per sec at an angle of just under 60°. The collision created the → Chicxulub crater. The event was responsible for eliminating approximately 70 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods.

→ Chicxulub crater; → impactor.

A model for → Moon formation (initially put forward by Hartmann and Davis, 1975, Icarus 24, 504), according to which the → proto-Earth suffered a collision with another → protoplanet near the end of the → accretion process that ejected material into a → circumterrestrial disk, out of which the Moon formed. Also called → canonical model. The giant impact hypothesis is the leading theory for lunar formation. There are, however, some key observations that cannot be explained using this model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is difficult to get enough mass into orbit to form such a massive Moon. Second, the Moon has a similar bulk composition to the Earth, but it is missing large amounts of more → volatile elements. The model does not properly explain Moon's distinctive composition. Finally, Earth and the Moon share virtually the same → isotopic ratios. It is therefore expected that the body that hit the Earth, often called → Theia, would have had a different isotopic ratio than the proto-Earth. In the canonical model, most of the mass of the Moon comes from Theia and so the Moon should have a different isotopic fingerprint than Earth, but it does not. The type of impact that formed the Moon in the canonical model is dictated by a very strong constraint, the → angular momentum of the Earth-Moon system. It is assumed that the angular momentum of the Earth-Moon system immediately after the Moon formed was the same as it is today. This assumption limits the velocity of the impact, the mass of the impacting bodies, and the angle at which the two bodies collided. It was found that only a grazing impact with a Mars-mass impactor at near the escape velocity can put enough mass into orbit to potentially form a lunar-mass Moon. This is why the canonical model is such a specific type of impact. However, the angular momentum of the Earth-Moon system could have been reduced over time by competition between the gravitational pull of Earth, the Moon and the Sun. Therefore, the Moon-forming collision could have been much more energetic than the canonical impact.
Simon Lock and Sarah Stewart (2017, J. Geophys. Res. Planets, 122, 950-982) have shown that such high-energy, high-angular momentum impacts can produce a different type of planetary object, → synestias. High-energy impacts vaporize a substantial fraction (~ 10%) of the rock of the impacting bodies and the resulting synestias can be huge, with equatorial radii of more than ten times that of the present-day Earth. Because the impact-produced synestia was so big, the Moon formed inside the vapor of the synestia surrounded by gas at pressures of tens of bars and temperatures of 3000-4000 K. Fragments of molten rock from the impact collided together and formed a lunar seed orbiting within the vapor of the synestia. The surface of the synestia was hot (2300 K) and the body cooled rapidly. The loss of energy led to the condensation of rock droplets at the surface of the synestia, and a torrential rock rain fell towards the center of the synestia. Some of this rain was revaporized in the hot vapor of the synestia, but some encountered the lunar seed, and the Moon grew. As the synestia cooled, more of the vapor condensed and the body contracted rapidly. After ten years or so, the synestia shrank inside the orbit of the Moon and the nearly fully-formed Moon emerged from the vapor of the synestia. The synestia continued to cool and became a planet within a thousand years or so of the Moon emerging from the structure. Without the tight constraint of the angular momentum, impacts that form synestias can put a lot more mass into the outer regions of the synestia than can be put into the disk in the canonical impact. This makes forming a large, lunar-mass Moon much easier. Moreover, because the Moon formed within the synestia, surrounded by hot vapor, it inherited its composition from Earth but only retained the elements that are more difficult to vaporize. The more volatile elements remained in the vapor of the synestia. When the synestia cooled and contracted inside the Moon's orbit, it took all the more volatile elements with it. This model can also help explain the isotopic similarity between Earth and the Moon. The Moon inherited its isotopic fingerprint from the vapor that surrounded it in the outer regions of the synestia. Energetic impacts that form synestias tend to efficiently mix material from the two colliding bodies, and the outer portions of the synestia in which the Moon formed would have had an isotopic composition that was similar to the rest of the synestia. Earth and the Moon therefore share a similar isotopic fingerprint which is made by a mixture of the isotopic compositions of both the bodies that collided.

→ giant; → impact; → hypothesis.

A collision between two bodies. In the case of solar system objects, when one is much smaller than the other (like a meteoroid colliding with the Earth), a crater may be produced on the larger body.

From L. impactus, p.p. of impingere "to drive into, strike against," from → in- "in" + pangere "to fix, fasten."

Barxord, verbal noun of barxordan "to collide, clash, dash against each other," from bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + xordan "to hit, strike," originally "to eat, drink, devour," and by extension "to destroy," from Mid.Pers. xvardan "to eat, enjoy (food)," Av. x^var- "to consume, eat;" Laki dialect hovârden "to eat;" Proto-Iranian *huar- "to consume, eat."

A depression produced by the collision of a meteorite, asteroid, or comet with the surface of a planet or a satellite. Impact craters are the most characteristic surface features of solar system rigid bodies. They range in size up to hundreds or thousands of kilometers (where the impacts create giant basins as on the Moon, Mars, and Mercury).

→ impact; → crater.

An → atmospheric escape mechanism that occurs where atmospheric gases are expelled en masse as a result of large body impacts, such as the cumulative effect of asteroids hits (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

→ impact; → erosion.

A collision between two celestial objects, specially solar system bodies, with considerable consequences. Impact events involve release of large amounts of energy. Some examples are the 1908 Siberian → Tunguska event by a → comet, the → Barringer Crater, and the collision of an → asteroid with Earth 65 million years ago, which is thought to have led to the extinction of the dinosaurs and other species of the → Cretaceous-Paleogene period.

→ impact; → event.

The danger of collision with Earth posed by solar system small bodies that pass near our planet. These objects include → near-Earth asteroids and nuclei of → comets. See also: → near-Earth object, → impact crater, → Torino scale, → Palermo scale, → Space Situational Awareness.

→ impact; → hazard.

The loss of orbital electrons by an atom of a crystal lattice which has undergone a high-energy collision.

→ impact; → ionization.

1) A measure of the distance by which a collision fails being frontal.
2) In → gravitational lensing, the distance of closest approach of the light path to the → lensing object.
3) In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.

→ impact; → parameter

The enormous drop in temperature and the related effects of the shrouding of Earth with soot and dust particles after the planet is struck by a sizable comet or asteroid. Such a phenomenon is believed to have killed off the dinosaurs 65 million years ago.

→ impact; → winter.

A general term used for all rocks affected by, or produced by, the → shock waves and other processes generated by hypervelocity → meteorite → impact events. Impactites occur in and around the → impact crater, typically as individual bodies composed of mixtures of melt and rock fragments, often with traces of meteoritic material.

→ impact; → -ite.

A natural impacting body, such as a comet, asteroid, or planet. It can also be a space probe designed to collide with an astronomical body in the solar system.

Impactor, from → impact + -or a suffix forming agent nouns.

Barxordgar, from barxord, → impact, + -gar agent suffix, from kar-, kardan "to do, to make" (Mid.Pers. kardan, O.Pers./Av. kar- "to do, make, build," Av. kərənaoiti "makes," cf. Skt. kr- "to do, to make," krnoti "makes," karma "act, deed;" PIE base k^wer- "to do, to make").

A striking of a meteorite against another body, especially the solar system planets or satellites.

→ meteoritic; → impact.