Mars Star

The Mars Star is one of the Elemental Stars, small round gem-like items each aligned to a particular element. The Mars Star appears as a small, perfectly round gleaming red ball, somewhat resembling a precious stone, though it may or may not be one. The Mars Star is the source of Mars Psynergy. Since the time when Alchemy was sealed away from the world, the Mars Star has been kept in an inner room at the centre of Sol Sanctum in Mount Aleph, along with the other Elemental Stars.

The Mars Star does not have a gameplay use in Golden Sun. Like other story essential items such as the Shaman's Rod and Jupiter Star, it is unable to be dropped or sold by the player, and simply takes up an item slot it your inventory that could theoretically be put to better use. The Mars Star only comes into play in storyline-centric moments during the plot.

Story
In the opening act of Golden Sun, the player is forced to collect the Elemental Stars from the various statues that hold them in the room at the centre of Sol Sanctum (they are carried in Mythril Bags given to the player by Kraden). When the first three Stars are handed to Saturos and Menardi (who are holding hostage Jenna and Kraden at the time, to ensure the player's cooperation), the player returns for the Mars Star. When the Mars Star is removed from its statue, Mount Aleph begins to erupt, forcing Saturos, Alex, Felix and Menardi to flee with Jenna and Kraden to save their collective lives. Thus, Saturos and company are not able to obtain the Mars Star. Isaac and Garet escape also, and so the Mars Star is held throughout Golden Sun by Isaac and his companions, though they are not able to prevent Saturos and co. from lighting Mercury and Venus Lighthouses.

Nearing the finale of Golden Sun: The Lost Age, the Mars Star is handed over to Felix and his companions after Isaac and his companions are defeated at the hands of Agatio and Karst near the top of Jupiter Lighthouse. Felix's party proceeds to use the Jupiter Star, which they have had since the very beginning of the game, to light Jupiter Lighthouse. Karst then steals the Mars Star from the party saying that they are no longer of use. She explains that because the Mars Lighthouse is in Prox, she and Agatio can light it themselves. After Felix and co. fire the Jupiter beacon, they head back down to join Isaac and his companions. After the joining of the two parties, and the explanation to Isaac of why the Elemental Lighthouses must be lit, both parties head for Mars Lighthouse.

In Mars Lighthouse, after being recovered from Karst, the Mars Star is used to "activate" the lighthouse and renew its fiery energies. At the summit of Mars Lighthouse, after defeating the Doom Dragon, the eight companions decided that the lighthouse must still be lit, and they throw the Mars Star into the lighthouse and a great orange-red-yellow beacon appears. The lighting of this last lighthouse heralds the return of Alchemy and the end of the Mars Star.

Trivia

 * It is interesting to note that in the first game, the Mars Star can be given to Ivan as soon as he joins your party. This means that when he temporarily leaves your team before rejoining at Goma Range, Ivan takes the Mars Star with him which would be a disaster to the storyline if he never returned.