When an asteroid or comet impacts a planetary body, it releases a tremendous amount of energy.  Except for objects smaller than a few meters, the impacting asteroid or comet is obliterated by the energy of the impact.  The impactor material is mixed with the target material (the rock on the planet's surface) and dispersed in the form of vapor, melt, and rock fragments.

    During the impact, sulfur in the impactor or in sulfur-containing target rocks can be injected into the atmosphere in a vapor-rich impact plume.  In some impact events, such as Chicxulub, the rocks hit by the impactor contain sulfur.  Sedimentary rocks hit by an impactor sometimes include large amounts of evaporites.   Evaporites are rocks that are formed with minerals that precipitated from evaporating water, such as halite (rock salt) and calcite (calcium carbonate).  Two other very common evaporite minerals are gypsum (CaSO4 + H20) and anhydrite (CaSO4), both of which contain sulfur (S).

    Projectiles also contain sulfur-bearing minerals, particularly the mineral troilite (FeS), which is obliterated in an impact event. This material releases its sulfur, which is then injected into the stratosphere.  The amount of sulfur injected into the stratosphere depends partly on the composition of the projectile, which can vary from one crater to another.  Using chemical traces of the projectiles left at impact craters, scientists can determine the type of meteoritic material involved.  Using this data, scientists can then calculate the amount of sulfur each specific impact injects into the stratosphere.  The amount of this sulfur can be substantial, because meteoritic materials contain up to 6.25 weight percent sulfur.  Consequently, even if the asteroid or comet does not hit a S-rich target, it can still cause dramatic increases in the total amount of atmospheric sulfur.

  Once vaporized, this sulfur can react with water to form sulfate (or sulfuric acid) particles.   These particles can greatly reduce the amount of sunlight that penetrates to the surface of the earth for a period of up to several years.  Over time, the sulfate will settle out of the stratosphere (upper atmosphere) into the troposphere (lower atmosphere) where they can form acid rain which can have additional environmental and biological effects.

FAQ About The Table Below:
- What are the projectile types and how are they determined?
- What does enhancement mean?
- What does Ir in ejecta mean?
- What are the Eltanin and Australasian impacts events?
    Do they have craters associated with them?

    The table below shows calculations of the abundances of sulfur added to the atmosphere during known large impact events.  These calculations are based on the amount of sulfur in the projectile only, and do not take into account the sulfur present in the target rocks.

Impact event
Age (Ma)
Ir in ejecta (g)
Type of projectile
Mass of 
projectile (g)
S added to stratosphere (g)
Enhancement **
Australasian 0.7 8.9x108 (chondrite) - 1013-1014 50-500
Botsumtwi 1.3 ± 0.2 - iron 4 x 1013 - 2 x 1014 1 x 1010 - 2 x 1012 0.05-10
New Quebec 1.4 ± 0.1 - chondrite 2 x1012 - 9 x 1012 3 x 1010 - 5 x 1011 0.15-2.5
Eltanin ~2.3 6 x 107 mesosiderite - 1012 - 1013 5-50
Popigai 35 ± 5 - chondrite 1 x 1017 - 6 x 1017 2 x 1015 - 4 x 1016 10000-105
Wanapitei 37 ± 2 - LL (chondrite) 1 x 1013 - 6 x 1013 2 x 1011 - 2 x 1012 1-10
Chicxulub* 65 2 x 1011 - - 1014 - 1016 500-105
Kamensk 65 ± 2 - chondrite 1 x 1015 - 5 x 1015 1 x 1013 - 3 x 1014 50-1500
Kara 73±3 - chondrite 3 x 1016 - 1 x 1017 5 x 1014 - 8 x 1015 2500-40000
Ust-Kara 73 ± 3 - chondrite 1 x 1015 - 5 x 1015 1 x 1013 - 3 x 1014 50-1500
Lappajarvi 73.3 ± 0.4 - chondrite 2 x 1014 - 1 x 1015 4 x 1012 - 7 x 1013 20-350
Boltysh 88 ± 3 - chondrite 1 x 1015 - 5 x 1015 1 x 1013 - 3 x 1014 50-1500
Obolon 215 ± 25 - iron 1 x 1014 - 6 x 1014 4 x 1010 - 6 x 1012 0.2-30
Clearwater East 290 ± 20 - CI 5 x 1014 - 3 x 1015 7 x 1012 - 2 x 1014 35-1000
Ilyinets 395 ± 5 - iron 2 x 1012 - 8 x 1012 5 x 108 - 9 x 1010 0.0025-.45
Brent 450 ± 3 - L or LL (chondrite) 3 x 1012 - 1 x 1013 5 x 1010 - 3 x 1011 0.25-1.5
Saaksjarvi 514 ± 12 - chondrite 3 x 1012 - 2 x 1013 5 x 1010 - 9 x 1011 0.25-4.5
Table adapted from Kring, Melosh, and Hunten, Earth and Planetary Science Letters 140 (1996).

Projectile types:
 The majority of geological information about asteroids comes from meteorites, which are their associated rock-type fragments. Meteorites, and asteroids by association, are classified based on their chemical composition.

Given below are descriptions of the various meteorite types:

Ordinary Chondrites (H,L,LL):
These stony meteorites are the most common meteorites.
They are composed mostly of silicate minerals (olivine, pyroxene, plagioclase) and represent undifferentiated primitive material from the solar nebula, dating back over 4.5 billion years.  Chondrites are characterized by small, globular, millimeter-sized inclusions called chondrules.  If you could remove chondrules from the meteorites, they would roll across a table like a marble. These meteorites also contain several percent metal. Both the chondrules and the metal content of chondrites can be seen in the photo below. The sulfur in chondrites is primarily in the form of troilite (FeS) - a sulfide mineral.

(Above) The ordinary chondrite - Dos Cabezas.

The S-type asteroid 243 Ida (NASA).
While the link between specific types of meteorites and asteroids is uncertain, some scientists have suggested that S- type asteroids like Ida are composed of ordinary chondrite material.

Iron meteorites are composed of a nickel-iron alloy along with trace amounts of non-metallic minerals and sulfides.  Some iron meteorites are thought to be fragments of the iron core of a differentiated asteroid.  The sulfur in iron meteorites is primarily in the form of troilite (FeS) - a sulfide mineral.

(Above) The iron meteorite - Bagdad.

Shape model rendering from radar data of the M-type asteroid 216 Kleopatra (NASA/JPL).
The refelectance characteristics of M-type asteroids like Kleopatra suggest
that they may be composed of iron-nickel which hints at a possible source for iron meteorites.

Carbonaceous Chondrite (CI, CM, CV, CO, CK, and CR):
Carbonaceous chondrites are very rare and primitive meteorites.
These meteorites contain organic compounds as well as hydrous silicates (water bearing minerals).  The sulfur in carbonaceous chondrites can take the form of sulfide minerals such as troilite (FeS), elemental sulfur, or water soluble sulfate. The Allende meteorite (CV) shown below is approximately 2.1% sulfur by mass, while CI carbonaceous chondrites have ~6.25% sulfur.


(Above) The carbonaceous chondrite - Allende.

The C-type asteroid 253 Mathilde (NASA/JHUAPL).

While the link between specific types of meteorites and asteroids is uncertain, some scientists have suggested that C- type asteroids like Mathilde are composed of carbonaceous chondrite material.

Stony Iron - Mesosiderite:
This is an unusual type of meteorite that is composed of nearly equal amounts of metals and silicates. Breccia is a rock type that contains broken rock fragments welded into a finer grained matrix.  Mesosiderites probably represent the shattered regolith of an asteroid that has been the target of several asteroid-asteroid collisions. Pieces of this regolith can be blasted off the surface of a larger body, and eventually reach the Earth as meteorites, or if large enough, as impacting bolides.  The sulfur in mesosiderites is primarily in the form of troilite (FeS) - a sulfide mineral.


(Above) The mesosiderite - Clover Springs

The S-type asteroid 951 Gaspra (NASA).
While the link between specific types of meteorites and asteroids is uncertain, some scientists have suggested that S- type asteroids like Gaspra are composed of stony-iron meteoritic material, possibly including mesosiderite material.

What does enhancement mean?

The enhancement value listed in the table is a calculation of how many times greater the overall sulfur content of the stratosphere would be following a large impact event. The baseline for this calculation is the background sulfur present in our current atmosphere.  This number likely fluctuated to a small degree over geologic time, especially following periods of extreme volcanism.

What does Ir in ejecta mean?

This is the amount of the rare trace element iridium (Ir), a platinum-group mineral, sampled in an impact crater's ejecta.  The importance of iridium in impact ejecta comes from its very low concentration in the Earth's rocks, and its relatively high concentration is meteorites, comets, and asteroids.  Anomalously high levels of iridium in a thin clay layer at the Cretaceous-Tertiary boundary are what led Luis Alvarez, (a Nobel Prize-winning physicist) and his son Walter (a geologist), to propose the impact hypothesis for the K-T mass extinction.

What are the Eltanin and Australasian impacts events? Do they have craters associated with them?

The Eltanin event occurred over two million years ago when a 1-4 km asteroid impacted in the Southern Ocean between the southernmost tip of South America and Antartica. The impact evidence for Eltanin stems from iridium anomalies in ocean drilling cores (see above). Although the impact was very large, no submarine crater has been found.

The Australasian impact is inferred from the huge number of tektites found over thousands of kilometers of southeast Asia and Australia. Tektites are small teardrop or button-shaped rocks that are formed by the solidification of molten droplets. The droplets were terrestrial rocks and dirt that were superheated during the impact, ejected from their source crater, and then rained down on the land and sea.  The source crater for the 700,000 year-old Australasian tektite strewn field has not been found.

- Return to Environmental Effects Main Page

This web site is based on information originally created for the NASA/UA Space Imagery Center’s Impact Cratering Series.
Concept and content by David A. Kring and Jake Bailey.
Design, graphics, and images by Jake Bailey and David A. Kring.
Any use of the information and images requires permission of the Space Imagery Center and/or David A. Kring (now at LPI).