The Mars Sight is the variant of the Red Dot Sight for the TAR-21. It is featured in Call of Duty: Modern Warfare 2 and Call of Duty: Modern Warfare 2 Campaign Remastered.
Call of Duty: Modern Warfare 2
In Call of Duty: Modern Warfare 2’s multiplayer, the Mars replaces the Red Dot Sight on the TAR-21. In multiplayer, the Mars sight is automatically identified as Red Dot Sight. The Mars has the same ADS time, turning speed, and accuracy as the standard Red Dot Sight. Sixty kills with this attachment on the TAR-21 will unlock the Holographic Sight.
On the Create-A-Class menu, it incorrectly names the MARS as the Red Dot Sight, possibly because it is essentially the Red Dot with a different skin. A common misconception is that the F2000 uses the MARS. This, however, is incorrect as the F2000 uses its own Red Dot Sight.
Many people dislike this sight, and instead choose the Holographic Sight once it is unlocked. This is because the Mars sight can be foggy and it gets sun glare in multiplayer, which can be very annoying for placing a long distance head shot. However, the Heartbeat Sensor is visible while aiming down the sight when equipped with Bling and the Mars sight.
In the campaign, it is possible to find a TAR-21 with a labeled Mars Sight, used by the Ultranationalists, especially in "Second Sun". The Internal Troops use the TAR-21 with Mars sight in "No Russian". It's also possible to obtain a TAR-21 labeled Mars Sight in the Museum through killing a NPC.