/usr/share/eccodes/definitions/mars/grib.ukmo.s3.def is in libeccodes-data 2.6.0-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # 360 year dates
meta dayOfTheYearDate g1day_of_the_year_date(centuryOfReferenceTimeOfData,yearOfCentury,month,day) : dump;
alias mars.date = dayOfTheYearDate;
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