A chronology of early Mars climatic evolution from impact crater degradation

Research areas:
J. Geophys. Res.
The degradation of impact craters provides a
powerful tool to analyze surface processes in the
Martian past. Previous studies concluded that large
impact craters (20~200 km in diameter) were strongly
degraded by fluvial erosion during early Martian
history. Our goal is to study the progression of
crater degradation through time with a particular
emphasis on the craters with alluvial fans and on
the relative chronology of these craters. The
geometric properties of 283 craters of > 20 km in
diameter were analyzed in two highlands of Mars,
north of Hellas Planitia, and south of Margaritifer
Terra, both known to contain craters with alluvial
fans. Three classes were defined from morphology:
strongly degraded craters with fluvial landforms and
without ejecta (type I), gently degraded craters
with fluvial landforms and preserved ejecta (type
II), and fresh craters with ejecta and no fluvial
landforms (type III). Our main result is that the
type II craters that present alluvial fans have
characteristics closer to fresh craters (type III)
than degraded craters (type I). The distinctive
degradation characteristics of these classes allowed
us to determine a temporal distribution: Type I
craters were formed and degraded between about 4 Gyr
and about 3.7 Gyr and type II craters with alluvial
fans were formed between Early Hesperian and Early
Amazonian (about 3.7 to about 3.3 Gyr). This
chronology is corroborated by crosscutting
relationships of individual type II craters, which
postdate Late Noachian valley networks. The sharp
transition at about 3.7 Gyr suggests a quick change
in climatic conditions that could correspond to the
cessation of the dynamo.