Web based GIS database on craters that you can explore.

Controls on crater morphology:

- impact velocity.
- impact size (mass, radius) and composition.
- impact site composition.
- impact angle.

What produces impactors?

What is a fractal geometry, a fractal distribution?

A theoretical distribution of broken blocks.

Actual distributions of fragmented materials (see handout).

**Conclusions**:
a) impactors may follow a fractal distribution, and b) there is
likely more than one population of impactors, each population
with its own fractal distribition.

Good background reading: Turcotte, Chapt. 3, Fragmentation, in Fractals and chaos in geology and geophysics, Cambridge Press.

Excellent intro web document by Hartman.

ISOCHRONS FOR MARTIAN CRATER POPULATIONS OF VARIOUS AGES by William Hartman - Detailed description of Mars crater distributions and the construction of a temporal framework. http://www.psi.edu/projects/mgs/chron04a.html

The epochs of Mars.

- Amazonian: 0 to 1.8 Billion Years Ago
- Hesperian: 1.8 to 3.5 Billion Years Ago
- Noachian: > 3.5 Billion Years

'geological' epochs | Ncum (>2 km diameter) | Ncum (>5 km diameter) | Ncum (>16 km diameter) |

Amazonian | <400 | <70 | -- |

Hesperian | 400-1200 | 70-200 | -- |

Noachian | >1200 | >200 | >25 |

Numbers from http://web.pdx.edu/~ruzickaa/G456/labs/Mars2.htm.

Summary from Hartman, http://www.psi.edu/projects/mgs/chron04a.html

- Late/mid Amazonian, log N (craters/km2) = -2 log D (km) - 3.947 (1 km < D < 4 km)
- Mid/early Amazonian, log N = -2 log D - 3.373 (1 km < D < 4 km)
- Amazonian/Hesperian, log N = -2 log D - 2.947 (1 km < D < 4 km)
- Late/early Hesperian, log N = -2 log D - 2.674 (1 km < D < 16 km)
- Hesperian/Noachian, log N = -2 log D - 2.470 (1 km < D < 16 km)
- Late/mid Noachian, log N = -2 log D - 1.742 (4 km < D < 16 km)
- Mid/early Noachian log N = -2 log D - 1.441
(4 km < D < 16 km)

Diagram showing size versus crater density isochrons for Mars, Hartman (2004).

Web based GIS database on craters that you can explore.

Arthur, D. W. G. 1954. "The Distribution of Lunar Craters." Journal of the British Astronomical Association, 64 (February), pp. 127-132.