Mars

Mars is the fourth of the nine twelve  eight ...

Mars is the fourth planet from the sun located within Earth's solar system.

Headmasters
Having harnessed the plasma energy released in the destruction of Cybertron, Decepticon Headmaster leader Zarak had the Terrorcons plant bombs charged with the energy at strategic points beneath the surface of Mars, intending to generate more energy by destroying the planet. Their activities were stumbled across by Spike, Daniel, Wheelie and the Trainbots, who were on the planet conducting a geological survey at the time and summoned the Autobot Headmasters to help. However, Fortress Maximus was rendered inactive by a plasma energy blast, and Zarak took the opportunity to activate his newly-constructed Transtector and head to the planet, pummelling Maximus before ordering the planet's destruction. Two clusters of bombs at the north and south poles were simultaneously detonated, beginning a chain reaction that started to tear the planet apart, allowing Zarak and the Decepticons to escape while the Autobots were consumed by the inferno. Fortress Maximus, however, was able to absorb some of the energy released in the early stages of the conflagration and escaped as Mars exploded.

The destruction of Mars thankfully had minimal impact on the other planets of the solar system, but its absence did disrupt the gravimetric balance of space bridge travel, destabilizing it.

''The (hilarious) Malay-English dub of Headmasters includes a throwaway line at the end where the Autobots promise to 'rebuild Mars,' which they apparently did. This does not appear in the Japanese version.''

Victory
Ore acquired Mars, coupled with ores from Earth and other planets, was used the creation of Schaeffer Energy. Jan happened to be visiting the Schaeffer Energy plant in America as a ship carrying a payload of Martian ore returned from its mission.

[Note: When the origins of Schaeffer Energy are related, it is implied that Mars still exists.]

Armada
Jetfire and Starscream once travelled to Mars in seach of the Mini-Con panel containing Firebot. Upon hearing this, the kids asked the two Transformers to buy them something from the souvenier shop. Sadly, as Mars was uninhabited by either human or alien lifeforms, there were no shops to be found. Starscream did however find a mysterious glowing green rock and gave it to the children. They were so happy.

Joke's on them - its probably radioactive.

Energon
Ten years after the defeat of Unicron, the Cybertronian Alliance and the Earth Federation installed a facility on the surface of Mars to mine the potent naturally-occuring ore Energon. Unfortunately, this facility was decimated by the initial Terrorcon attack, leading Carlos to aptly remark "Holy frijoles".

A little while later, the survivors of the first attack, Autobot, Decepticon and Mini-Con, made a valiant last stand against a second Terrorcon attack, this time being personally led by Scorponok.

Cybertron
Thunderblast, Ransack, Crumplezone and Thundercracker left Earth on a rickety rocket, attempting to forge the New Decepticon Army. Their rocket made it as far as Mars before crashing and stranding them on the planet.

Movie
On the planet Mars, an blury Decepticon destroys the Beagle 2 rover. Oddly, in this continuity Mars is depicted as having a blue sky and looks suspiciously like the American southwest.