IMPS: VERY YOUNG LUNAR MARE VOLCANISM

The enigmatic lunar landform known as Ina was first discovered in Apollo era images. Since that time scientists have puzzled over its origin but can only agree that it is somehow related to mare volcanism. One of the many amazing discoveries from the Lunar Reconnaissance Orbiter Camera (LROC) is the identification of over fifty similar landforms, known as Irregular Mare Patches, or IMPs. IMPs are defined by common landforms with distinctive textures. The smooth mounds (SM) are characterized by shallow-sloped central areas terminated by steep lobate scarps. Mounds are typically 5–20 m thick and a few 10s to 100s of m across. The SM are adjacent to a unit characterized by an uneven surface, with textures ranging from hackly to ropy to boitryoidal, exhibiting occasional patches of boulders and relatively few impact craters. Superposition relations between the SM and uneven materials (U) are ambiguous; typically mounds appear to be superimposed on the U, however in some cases the reverse is true, and in others the relation is as clear as an Escher drawing.

LROC images allow Crater Size Frequency Distributions (CSFD) down to diameters of 10 m, suitable for dating young surfaces. Absolute model ages (AMA) from the three largest IMPs range from 18 myr to 58 myr; it is here noted that these young ages are controversial because conventional wisdom dictates that lunar volcanism ceased more than 1 byr ago.

Consistent with the young ages, one IMP is interpreted to be superposed on the ejecta of Aristarchus crater (45 km diameter), which formed <200 myr ago. Additionally, the IMPS exhibit small landforms (meter-scale relief), and steep margins of the SM (>30°); such small-scale landforms will be worn down or disappear due to micro- and macro-bombardment on a <<byr scale. In summary AMAs and morphologies are consistent with anomalously young ages (<100 myr). If global thermal retention models are correct (and IMPs are ancient volcanics), then the absolute calibration of crater ages for small targets is seriously flawed, or some extraordinarily unusual property (proposed by other workers) or process is grossly altering crater retention for the IMPs (for which there is no compelling evidence). Alternately, thermal models may need revision, perhaps simply allowing for an extended tail of partial melting of mare source regions up to the present time.