Mark 1 Marine Armor: "Thunder Suit" 

Mark 1 armor, as it would later be known, was created during the Age of Strife. When the Emperor began his conquest of Earth, his troops were equipped with gear similar to that of the techno-barbarian warriors that dominated the planet at the time. The first Space Marines were already a part of his force. And while they were given the finest quality equipment, it was really no more advanced than that of other humans on the planet. Although this armor is considered Powered Armor, it is in reality only partly powered. The main portion of the armor is contained within the torso. Under the chest plate lie synthetic fiber bundles that transmit power to the wearers upper extremities. This had the effect of increasing his fighting abilities by 300 to 400 percent. The suits legs receive no power at all, instead consisting of simple padded breeches covered with armored grieves and boots. On the Marines back, he wears a backpack which is the suits power source. Its bulk is considerable because of the necessary cooling equipment needed to keep the power unit from overheating. As you can see, Mk.1 armor is not enclosed. Life support functions were not necessary in the early days of the Imperium when fighting was restricted to Earth, Luna and Mars since all have breathable atmospheres. The thunder-bolt and lightning decoration on the chest and legs of this suit was the personal emblem of the Emperor at the time, giving it its other name - "Thunder Armor". 

Mark 2 Marine Armor: "Crusade Armor" 

As the conquest of the Terran system began to wind down, the Emperor directed that a new, more sophisticated type of armor be designed. With the new acquisition of Mars and her advanced technology, Martian factories began production of a second type of powered armor. And with the newly installed administrators of Mars, the Adeptus Mechanicus directing efforts, the entire Space Marine Corps were rapidly re-equipped Crusade Armor (as it would later be known,) is the first fully enclosed Powered Armor. And is capable of combat in hostile environments such as space. The armor consists of a number of concentric bands which now cover the entire body and yet allow for more freedom of movement for the wearer. More efficient coolers allow the design to cover most of the chest cables, while the addition of powered leg armor necessitates the exposed cabling on the suits upper leg area. The Marine wears a backpack which looks similar to the earlier design. However it is now much more efficient and allows for the inclusion of life-support and other related functions. In addition, the new helmet design includes automated sensory devices which register audio and visual stimuli in a way that mimics hearing and sight. By buffering the sensors through a microprocessor directly to the Marines mind via a neural connector, he is able to see and hear normally. Also, the helmet allows for vision into the ultra-violet and infra-red range, and hearing into extremely high and low frequencies. 

Mark 3 Marine Armor: "Iron Armor" 

When the Emperor began to move his forces out toward the galactic core, not all of the planets contacted were happy with the idea of galactic reconquest from Earth. Many of these included Squat Homeworlds, where the fighting would be done in cramped tunnels or onboard spacecraft. Early fighting that consisted of a frontal assault, where cover was almost non-existent had shown a need which was filled with the Mk.3 armor. Mk.3 armor was designed with an assault role in mind, and was not intended as a replacement for Mk.2 armor. Designers simply increased the frontal armor protection of the Mk.2 and reduced the rear protection to compensate. The Helmet was also slightly reshaped, giving it a more wedge shaped front intended to deflect incoming shots. 

Mark 4 Marine Armor: "Imperial Maximus Suit" 

The Great Crusade lasted approximately two-hundred years. And after a time, the original armor suits manufactured on Mars began to wear out. It had been decided that a new design was needed to replace the old ones. The main design change would be from using separate armor plates to fabricating large inflexible armor casings with flexible joints. This change would allow for easier manufacture and repair with only a slight loss in flexibility. Knowledge gained from conquering innumerable worlds enabled the Tech-Priests of Mars to increase this suits protection while at the same time reduce its weight. And increased armor to the power cables allowed for the placement of the main chest and arm supplies outside the chest plate once again. New materials and recovered technology allowed for a decrease in the size and number of exposed cables. The Mk. 4 suit was designed as the ultimate type of Marine armor, and the Martian factories were dedicated to its production. 

Mark 5 Marine Armor: "Heresy Armor" 

Space Marine Chapters were in the process of being re-equipped with the new Mk. 4 armor when the Horus Heresy began. With such a great disruption to Imperial shipping, the entire supply program was thrown into confusion. The new suits were being used equally by both sides, sometimes Loyalists in older armor would be fighting Traitor Marines who had just been given the new style armor. The Techmarines had barely gotten used to working with the new design. Many of them were not yet able to maintain it or fabricate replacements as was the original intent. Such a large scale conflict as the Heresy was not expected, and the high need for spares had not been anticipated. The new armor quickly became unusable, even with minor battle damage. This was partly caused by the fact that the new, exotic materials that were used to make the Mk. 4 were unavailable to Chapters. With this problem identified, production of Mk. 4 armor was halted and almost over-night a new suit was designed to replace it. The Mark 5 armor was designed with the battlefields of the Heresy in mind. It would use as many pre-Mk. 4 components as was possible, since all of the Loyalist Chapters still had large stock-piles of them and were familiar with their use. The chest plate is of the new style for example, but the chest, arm and leg cables are of the older design. Since the older cables were larger, they did not fit under the new chest plate and had to be placed on the exterior of the armor. These cables were to prove a major weakness in the design, which led to the addition of improvised armor plates added over them on the battlefield. One of the most noticeable features of Mk. 5 suits is its studded armor plates. This was an extra skin plate molecularly bonded to the armor as reinforcement. Due to the addition of so much weight, especially if an extra chest plate was added, the energy consumption was beyond that for which the power pack was rated. As a result, the wearer could turn up the packs power level and suffer intolerable heat, or leave the power level as it was and accept reduced performance. The helmet design is taken from the Terminator development program, which had been recently completed. It contains the same type of auto-senses as contemporary Terminator armor. Although the Mk. 5 was rushed into service, it proved to be remarkably durable and easy to fabricate and maintain. During the Heresy vast numbers of the suit were shipped out to Marine Chapters. Many of the shipments were captured by traitors for their use, as well as the scavenging of the armor from fallen enemies. 

Mark 6 Marine Armor: "Corvus Armor" 

At the same time the Mk. 5 armor went into production, its replacement was already being designed. The Heresy suit was only intended as a stop-gap measure, and not for long term use. Although research had not been completed on the Mark 6 design, it was rushed into production as the Treacher Legions closed in on the Terran System. The development laboratories were still being dissassembled when Marines began fighting in the new armor. Eventually the production factories were overrun and traitor Marines began to manufacture them as well. (If you ask me, this is a contrivance whose sole purpose is to allow Chaos Marines to wear Mk.6 armor. Anyone with half a brain would have destroyed the production facilities before Horus even reached Mars. Or at least would have detonated buried nuclear devices under the facilities as the enemy got into blast range.) The Mark 6 suit, compared to it's predecessors, has a very clean appearance. This is mainly due to the once again rehousing of the chest cabling under the armor plates. In addition, the visible chest and arm cables are duplicated inside the torso of the armor as a secondary back-up system. If the main system were damaged, this isolated set would automatically take over functionality. A distinguishing feature of the Mk. 6 suit is the studded left shoulder armour. This pad was the same as the right shoulder pad with an extra skin plate molecularly bonded to the armor as reinforcement. This was done for economical purposes, since the Marines left side would need to be better protected as he fired his weapon, the right side could be more easily replaced with slightly less effective plates. At the time, economizing in this way was necessary. After the Heresy the studded pad became associated with the Siege of Terra, and the extraordinary heroism displayed by Loyalist Marines. And so it became a traditional emblem of those days. The helmet is based on the Mk. 4 version, with suitable upgrades taken from both Terminator research and the Mk. 5 production run. Early lower leg designs consisted of a single shin/knee plate with basic joint articulation. After the Heresy, the Adeptus went back and refined the knee joint, separating the knee pad armor from the rest of the lower leg armor. 

Mark 7 Marine Armor: "Eagle Armor" 

As the final battle of Mars was underway, the Adeptus began duplicating the Martian production lines back on Earth. They realized Mars would fall to the Traitors. And so the armor development teams were transfered, with all their research, as well. From their new locale on Earth, they were able to continue the development program and incorporate their continued research into the Mark 7 armor. By the time Mars was captured Mark 7 armor was in production. Space Marines on Earth and Luna began receiving the new suits immediately. However, the Mark 6 armor was such an effective design, that both types have widespread use. Some Chapters have even chosen to continue with the older Mk 6 armor rather than adopt the Mk 7. Mark 7 Powered Armor represents a further refinement of the design started with the Mk 6. The greatest change in the armor is the new chest plastron, which was designed to encase most of the arm and chest cabling within the torso. This piece commonly bears a version of the Imperial Eagle, giving it its alternate name of Eagle Armor. The only exposed cables on the body of the armor now were those located in the abdominal area, which received heavy reinforcement. In addition, improved power usage via a redesigned back pack allowed for both shoulder pads to be replaced with larger (and uniform) pieces for increased protection. Improvements made in later Mk 6 knee joints were officially incorporated into the Mk 7 design. And the Mk 7 helmet design is based on the earlier Mk 5 helmet, however it can freely be exchanged with the Mk 6 helmet. Indeed, parts from Mk 6 and Mk 7 armor are readily interchangeable with each other. 

Mark 8 Marine Armor: "Errant Armor" 

The newest Mark of powered armor seems from outward appearances to be not much more of a jump than the Mk 7 was. Mark 8 Powered Armor appears to represent further refinements of the design started with Mk's 6-7. The suit itself is a highly modified Mark 7. The addition of fixed armour plates to the torso now enclose all of the suits cables, which were most vulnerable to area effect weapons, and a new armored collar protected the helmets respirator. The addition of an armored collar also fixed the vulnerability of the neck joint. On rare occasions the chest and helmet of the older styles would act as a shell trap. Sometimes a round would hit the armor in such a way as to cause it to skim the surface of the armor upward, under the helmet and into the neck joint. The collar precipitated a change to the helmet, and unfortunately this caused a lack of backward compatibility with the earlier helmets. Mk 7 helmets could be used, however considerable modification would be required. Mark 8 armor has only seen limited production thus far, and is seen in limited numbers on the battlefields of the 41st millennium. Often, it is warn by officers or squad leaders as a symbol of rank.