Impact craters vs volcanic explosions on Mars

Question

A maar is a shallow volcanic crater with steep sides that is surrounded by tephra deposits. The tephra deposits are thickest near the crater and decrease with distance from the crater.

Source: http://geology.com/stories/13/maar/

I have looked at about 70 craters formed by this type of explosion. I have gathered several data points such as diameter, depth, and elongation. I am now trying to use this data to attempt to find this type of crater in some regions of Mars.

Earlier attempts have ended in failure. Mainly because I am mistaking maar craters with supervolcanos, and impact craters with maars, and vice versa. The craters that I have looked at are very young. Mars's volcanic activity is believed to have ceased many billions years ago.

What are some good ways to identify this type of geologic feature from other potential doppelgangers? I have no data points for old maar craters mainly because they are too eroded to measure anything, and/or have large scoria cones that have erased their once-elliptical shapes.

Answer 1

In addition to Gordons approach it might be possible to differentiate between impact and volcanic craters by comparing the characteristics of the ejecta [1]. The apparent thermal inertia (defined for example in [2]) seems to be higher around volcanic craters [3]. Unfortunately the linked papers about the method are conference abstracts about ongoing research and I did not find a paper where they try to apply the method to remote sensing data from mars.

[1] Peet, V. M., M. S. Ramsey, and D. A. Crown. "Comparison of Terrestrial Morphology, Ejecta, and Sediment Transport of Small Craters: Volcanic and Impact Analogs to Mars." 36th Annual Lunar and Planetary Science Conference. Vol. 36. 2005.

[2] Putzig, Nathaniel E., and Michael T. Mellon. "Apparent thermal inertia and the surface heterogeneity of Mars." Icarus 191.1 (2007): 68-94.

[3] Peet, V. M., M. S. Ramsey, and D. A. Crown. "Remote Sensing Analyses of Small Terrestrial Volcanic and Impact Craters: A Mars Analog for Formation, Morphology, and Erosional Processes." Lunar and Planetary Science Conference. Vol. 38. 2007.

Answer 2

I don't have a definitive answer, but here is a suggestion you might look into: A large bolide will always have a large impact velocity because of the M1 x M2 proportionality of gravitational attraction. A high velocity means that the non-directional dissipation of kinetic energy will far exceed the directional momentum, resulting in a circular or nearly circular crater. A small impactor, on the other hand, may or may not be fast. In the event of a relatively low velocity, the momentum will be relatively large compared to the kinetic energy. Hence, depending upon the angle of impact, it may leave a slightly to very elongated crater. At one extreme, some of the craters in the northern Canadian shield are almost trenches rather than craters. So concentrate on the smaller craters and do a statistical analysis of the crater's eccentricity. If they are nearly all circular then they are probably volcanic; if there is a significant fraction of elliptical craters, especially with high eccentricity, then they are probably of impact origin. Also check for strongly directional patterns of ejecta from the smaller craters.