New insights into the origin of ice: chronological implication from 14 permanently shadowed craters on Mercury

The polar regions of Mercury are characterised by areas known as permanently shadowed regions (PSRs), which provide stable thermal conditions for water ice over geological timescales. Both Earth-based radar measurements and data from the NASA/MESSENGER spacecraft identified the area encompassing the PSRs with reflectance properties indicative of both water ice and other volatiles. In this work, we employ crater counting absolute model age determination on both ejecta and crater floor materials of fourteen northern polar craters yielding cold traps and ice. Our findings unravel younger ages for the floors than for the associated ejecta, prompting further investigation into the role of ice as the mechanisms responsible for rejuvenating polar crater floors. Notably, a correlation emerges between the age of craters and the accumulation of ice within PSRs. Indeed, older craters tend to accumulate more ice within the PSRs, measured in terms of area. These correlations may be attributed to the two primary processes allowing ice continuous accumulation: micrometeorite flux and solar wind implantation. Considering the limiting factors for these processes, which are impact velocity and gardening, respectively, it is estimated that over approximately 3.7 billion years (the age of the oldest craters in this study), a substantial amount of ice would have accumulated at the poles. Two of the analyzed craters do not follow the trend and this might signify episodic delivery of ice resulting from asteroidal and/or cometary impacts. Thus, we propose that both steady-state accumulation and episodic delivery processes should be considered when analyzing the presence of ice in PSRs. • Age Correlation with Ice Accumulation : Older polar craters tend to host larger volumes of radar-bright materials, suggesting that steady-state processes, such as micrometeorite flux and solar wind implantation, constitutes an important process for ice accumulation over geological timescales. • Younger Floor Ages in Polar Craters : Crater floors with radar-bright materials exhibit significantly younger ages than their corresponding ejecta, supporting the role of ice in resurfacing and rejuvenating polar crater floors. • Episodic Ice Delivery : Two outlier craters with unexpectedly high ice content despite their younger ages may indicate episodic delivery from cometary or asteroid impacts.