I came across this interesting poster of Tintin and his friends in Antarctica.
It seems that in the early 1980s, the environmental group Greenpeace bought a ship called the "Sirius". A member of Greenpeace, who was also a Tintin fan, contacted Herge and asked if it was the actual ship that he had used as a model for "Red Rackham's Treasure". Herge denied this, but it started a correspondence and he agreed to making a poster of Tintin involved with Greenpeace.
Herge died before finishing the poster which was completed by Bob de Moor. Apparently, "It sold faster than croissants on a Saturday morning in a Paris bakery." :)
I believe that the ship in the background is the Greenpeace "Sirius". It does look a bit like the "Sirius" on page 12 of "Rackham".
According to a
Spanish newspaper, the poster was part of Greenpeace's campaign to keep Antarctica free of exploitation.
Antarctica includes a lot of oil and mineral reserves. Greenpeace's campaign aimed to prevent the exploitation of Antarctica which culminated in the Madrid Protocol of 1991 by which many nations agreed not to exploit the natural resources and to keep it as a "natural reserve" until at least 2048.

Tintin poster from the website of the Spanish El Independiente newspaper.