Towards a Standard Definition of Mercury Years

The comparison of observations of Mercury acquired from space missions and ground-based facilities requires a consistent temporal framework. While such a standard exists for Mars in the form of Martian Years (e.g. Clancy et al., 2000; Piqueux et al., 2015), no equivalent system is currently adopted for Mercury. This limits the ability to intercompare datasets obtained at different epochs and from different observing platforms.

We propose the definition of a Mercury Year (MeY) numbering system based on a fixed reference epoch corresponding to a physically meaningful dynamical configuration. Specifically, we suggest defining the start of Mercury Year 1 (MeY1) as the instant when Mercury reached a mean true anomaly (MTA) of 0° prior to the first Mariner 10 flyby flyby of Mercury. This corresponds to 11 February 1974 at 01:40 UTC. This epoch provides a well-defined and historically anchored reference point associated with the beginning of the era of in situ exploration of Mercury.

Using this reference, Mercury years can be counted sequentially with a duration equal to Mercury’s orbital period around the Sun (TMercury≈87.9691 days). This approach enables consistent time tagging of observations, facilitates comparisons between datasets, and aligns spacecraft and ground-based observations within a common chronological framework.

As an illustration of this system, the first perihelion after orbit insertion of the BepiColombo mission (Benkhoff et al., 2021) will occur during the transition to Mercury Year 222 (MeY222). The establishment of a Mercury Year standard would align Mercury science practices with those of Mars and other planetary bodies, improving data interoperability and fostering coordinated analysis across the community.

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