Meteorites

Definition

A meteorite is a body of space debris that enters the atmosphere of a planet and survives the friction with surrounding atmospheric gases to impact the surface energetically. Impact causes planetary cratering, ejecta and dust (forming a layer of regolith on planets with low geological activity such as Mars). This poses an obvious risk to a Martian settlement, and due to the tenuous atmosphere of Mars, smaller debris have the greater chance to impact the surface than on Earth.

Risk and Mitigation

• Small meteorites are best handled by a thick layer or regolith. Mars One planed to cover their inflatable greenhouses with at least 2 meters of it. Most meteorites are small. self-healing puncture protection for space suits and facilities should be installed.

• Larger meteoroids are rare, but they can happen. Meteorites a few centimeters in diameter would be fatal. Little can be done to fend them off, the kinetic energy is much too high.
• There are possible precautions. One is to modularize the settlement with redundant facilities. In the case of an impact, the affected module may be damaged, but the remaining modules allow the surviving settlers to continue the settlement process.
• For even bigger impacts, the required distances between the redundant modules may be larger. Independent settlements should be equipped to work autonomously, but cables and pipes between them can help them support each other in emergency situations, e.g. to supply oxygen and electricity. A railroad, airlock or tunnels between them , depending on distance, would allow material and personnel transportation. However this type of redundancy may be more appropriate for internal accidents, such as fires or structural failure, than any eventual asteroid impact.

The frequency of meteorites and their dimension are known.

The chance of a direct hit is very low, but debris may damage the colony and will effect a much larger area. Settlements in Lava Tubes will be largely immune from meteor impacts, except for large ones which are so close to cause cave ins. (Since many of these lava tubes have lasted tens or hundreds of millions of years, short lived humans will justifiably ignore such dangers.)

In practice, settlements are likely to ignore the risk of large impacts as too rare to worry about.

Dust (or smaller) Meteorite impact frequency on Mars

Diameter interval		diameter, average	mass, average	impacts per year		Impact energy
um	um	um	kg	max imp/yr/m2	min imp/yr/m2	J
6	11	9	6.9e-13	23 000	1 100	3.9e-8
12	26	19.5	7e-12	7 800	370	3.14e-7
27	57	42.5	7.2e-11	1 800	84	3.26e-6
58	124	91	7.1e-10	460	21	3.2e-5
125	268	196.5	7.2e-9	93	4.3	3.2e-4
269	576	422.5	7.1e-8	7.8	0.36	3.2e-3
577	1240	908.5	7.1e-7	0,42	0.016	3.2e-2
1241	2680	1960.5	7.1e-6	0,009	0,00041	0.32
2681	5757	4219	7.1e-5	0,000096	0,0000045	3.2

• Average impact velocity : 300 m/s
• Average density: 1800 kg/m3
• For reference, the impact of a pistol bullet is about 400 Joules and the impact from a rifle shot 2700 joules.

• From the table we can see that a large 1 km2 settlement might expect to receive between 4 and 96 impacts of 3,2 joules per year. Each of these might require some kind of repair, however, an underground settlement would be impervious to such small impacts. As the rate of impact decreases roughly by 100 for every mass interval of 10, impacts with the energy of a bullet, with 100 times more energy (400J vs 3.2), should be about 10 000 times less frequent. So the same 1 km2 settlement should be hit by a meteorite with the energy of a bullet every 100 years or more.

Observations of Meteorite Impacts on Mars

Mars Global Surveyor images of the same site in 1999 and 2006 - Impact from meteorite is obvious.

On January 9, 2006 the Mars Global Surveyor MOC science operations team came to the realization that their camera (used primarily to map the Martian surface) might be able to locate and characterize fresh impact craters on the surface of Mars. Such a survey would provide useful information about the current meteorite impact rate. This survey would be the first of its kind ever carried out on a Solar System body (including the Earth-Moon system) due to the unprecedented number of high resolution cameras inserted into Mars orbit.

In results gathered on January 6, 2006, the MOC had acquired a new feature on the Martian surface in Arabia Terra in one of its images. The feature was circular and very dark. At the time, the camera was capturing images at a resolution of 240 meters/pixel, so there were some ambiguities as to what the blurred feature was. The team began exploring the possible scenarios, but after proving shadows of the two moons, Phobos and Deimos were not to blame, they quickly realized that something in the area was new when compared with images taken by Mariner 9 (in 1971) through to the Mars Express mission (in 2003). There was still the possibility that the dark circular object may have been caused by the removal of sand and dust due to high winds in the region so better observations had to be carried out.

To increase the resolution in the images, a technique known as Roll-Only Targeted Observation (ROTO) was employed. This massively improved the images for analysis, the new resolution registered at 1.5 meters/pixel allowing the team to see the main impact crater and several smaller craters arcing away from the large dark spot. Another observational technique – compensated Pitch and Roll Observation (cPROTO) – was used to improve the images further until the evidence was indisputable. A fresh impact crater had been discovered.

Mars Odyssey's THEMIS instrument and Mars Express' High Resolution Camera (HRSC) were able to provide supplementary observations of the area to constrain the impact date to some time between November 12, 2004 and January 6, 2006. Since this first discovery in January 6, 2006, another 20 new impact craters have been discovered by the MOC.

Martian meteorites on Earth

A significant source of knowledge about Mars are the Martian meteorites that were found on Earth.

See also

• Meteors
• Meteoric iron
• Martian Meteorite List