Although not exactly the same this reminds me of:
Space: 1889 which is available from DriveThruRPG. It’s a Victorian-era space-faring RPG and there are source books that cover the inner planets, including Mars in some detail. Interestingly Mercury is described as:
…a tidally locked world, with one side always facing the Sun, the other the void of outer space. Between the cold of the dark side and the heat of the light side, there is a narrow 100-mile wide temperate zone that circles the globe. All around the zone runs the World River, linking the various lakes and small seas, its flow driven by Coriolus effects.
Giving it a somewhat Riverworld feeling.
Less well known is Marcus Rowland’s Forgotten Futures RPG and specifically the source book Planets of Peril. This has a cosmology based on Stanley G. Weinbaum’s 1930s science fiction stories so in some ways it’s more advanced than Space 1889. Again there’s lots about Mars but this time Mercury is described as ‘not worth visiting’! A footnote about Mercury’s ‘wobble’ kills off the world river:
Mercury’s orbit is very eccentric, and its rotation sometimes lags behind its orbital position, and is sometimes ahead of it. As a result of this eccentricity there is no permanent twilight zone, as on Venus; all parts of the border between the bright and dark sides experience extremes of warmth and cold at some point in the planet’s short year.
But even that's not right!
Interestingly I discovered from Wikipedia that the idea of a twilight zone was disproven in 1965 when
radio astronomers discovered that Mercury rotates three times for every two revolutions, exposing all of its surface to the Sun
And that you can thus divide stories about mercury between ones with the twilight zone and ones that don’t.
The Wikipedia article also includes this 1978 quote from Carl Sagan-
A clement twilight zone on a synchronously rotating Mercury, a swamp‐and‐jungle Venus, and a canal‐infested Mars, while all classic science‐fiction devices, are all, in fact, based upon earlier misapprehensions by planetary scientists.
However on the positive side Pluto exists and is the lair of pirates!
Another good thing about Forgotten Futures is that it’s all available on the Authors website for free. However he does ask for donations to support the site and also 10% goes to the charity Cancer Research UK.
I’ve always fancied running something set in this world but up to this point I know absolutely no one who has even the remotest interest in this sort of setting.