Because Mars has such a weak atmosphere, the inhabitants are not a separate species in their own right. They are the same Homo apotelesmatis that predominates in the asteroid belt and makes an uneasy coexistence with Homo sapiens in the atmosphere of Saturn.
At a distance the most visible feature of H. apotelesmatis is their short stature. From the origin of their species in the space industries of Namibia and Zaire, those able to construct space habitat the fastest are those who are smallest. As this pressure has been relaxed on Mars, the planet has gradually seen a resurgence of somewhat larger-bodied individuals. Though it is politically quite controversial, due to planetary populism the inability to comfortably inhabit standard space habitats is no longer viewed as a medical condition requiring treatment during adolescence.
As one approaches, their color is also distinctive, being much darker than that of Africans or any others on Earth. For obvious reasons, melanin is produced by all epithelial cells in abundance, and in its abundance serves beside filaggrin as a chemical cross-linker to toughen the skin. Their skin also shows what H. sapiens often deride as an unpleasant waxy sheen, as the production of glycolipids has also been greatly amplified to improve the barrier against water loss.
Deeper changes are found in the dermis, where several proteins incorporate Grignard (organomagnesium) amino acids, which are identified in the sequence by a SECIS-like mechanism. The proteins are engineered to synthesize, degrade, and remodel a cytoskeleton of carbon nanofilaments, linked by heavily reinforced desmosomes at the membrane. The resulting dermis, which has been altered to increase its tension at some points of the body, very effectively prevents injury, rupture, or fluid pooling due to vacuum exposure.
Resistance to radiation is not at all special to H. apotelesmatis; like nearly all humans, even from many of the more conservative indigenous groups, they have been modified with an extensive set of p53 paralogues modelled on the elephant genome's effective cancer-suppression mechanisms. Together with some additional repair enzymes for radiation resistance, modelled on Deinococcus radiodurans, these date back to the Third World War.
For spacefaring humans, the most important but also the most difficult of all advances has been the transition to electronic respiration. Hydrolyzers in the lungs split water into hydrogen and oxygen, which are circulated to the rest of the body. Where nutrition is needed, the hydrogen is used to inject electrons into the electron transport chain. Mars dwellers passionately enjoy their planetary cuisine, however, and their biochemistry vents excess hydrogen from the lungs. The lungs are generally kept at a pressure well above Martian surface pressure by the closed glottis, and the Martians exhale a mixture of unused hydrogen and carbon dioxide produced from foodstuffs. When their water intake is low, however, the action of vasopressin reduces the level of exhalation. The same hormone stimulates water salvage enzymes in the colon, which secretes tightly coiled strands of carbon, recovering nearly all of the metabolic water originally present.
Despite this, Martians still need to drink aqueous solutions somewhat regularly, as the function of the kidneys remains essential. Martians do not trust the ability of any biological recycling mechanism to remove all potential hazardous compounds from the kidney filtrate, and demand external water processing and recycling.
Members of H. apotelesmatis who are presently using electronic respiration are commonly referred to as Torced, even though the use of magnetic induction to transfer current has long been obsolete. The system of nanocarbon fibers in the dermis permits modification to open up conductive channels almost anywhere in the body, and the system rapidly and reliably conducts this to electrical storage organs in the mediastinum. (These are accommodated by a partial reduction in the space occupied by the lungs) Inflow of the current is controlled by adjusting the conductive properties of the melanin polymers of the skin, permitting efficient uptake from reasonable energy sources while protecting the internal network from unintended shock.
Originally, storage was done simply by implanted batteries, but over time biological solutions have been found. These rely on the capacitance implicit in multiwalled carbon nanotubes. The nanotubes are produced much like the reinforcing fibers of the dermis, but the keloid scar tissue structure induced during involution of branches of the lung buds take a different pathway, producing tight arrays of blind-ended MWNTs with immense capacitance.
As their skin will extract electric current from virtually any pattern of positive and negative electrodes over a moderate range of voltages, the Martians have developed a wide range of fashions to facilitate carrying extra storage. These can be quite large, permitting respiration and optionally providing nutrition for at least as many days as a comparably sized package of food. In space habitats, such accessories are frowned upon due to the need to move in cramped quarters, and as an antisocial and distrustful hoard of resources, so Martians have come to view them as a cherished part of their local culture.
As we speak of Martian clothing, we should also recall the mythology of the cold Martian climate. Earth dwellers, comparing the temperature of the Martian surface to that of Antarctica, imagined the planet as a frigid wasteland. However, the thinness of the atmosphere (0.6% the pressure of Earth) correspondingly reduces its ability to chill the traveller. H. apotelesmatis, being black-skinned, radiates heat exceptionally well - nonetheless, the starry sky of Mars is little colder than that of Benson, Arizona. The body radiates heat at a fixed rate, and what returns from above the horizon is minimal in either case. The result is that natives of Mars experience only a mild chill, unless they come into direct contact with soil or unheated structures. To help them in that instance, the electrical storage harnesses they wear have an appearance of shining jewelry or armor, reflecting thermal radiation very effectively, even though they conduct heat very poorly. The Martians do not need to swaddle in clothing like natives of cold Earth regions, but expose much of their skin, particularly in the chest region, to allow effective thermoregulation. The Martians are a physically active culture, and many of them have additional genetic enhancements that tend to require the effective dissipation of waste heat.
The eyes and ears of Martians remain a sore point to this day. The thickness of protection required at the tympanic membrane inevitably reduces Martian hearing, even when they visit Earth to experience primordial environments with the breathing of oxygen. It is easily corrected by rudimentary external aids, but this can make Martian pilgrims from some of the major religions feel like their experience is inauthentic. And Martian eyes ... well, there is no genetic engineering program known that can make them beautiful. The sclera is black and waxy, the cornea covered under a cold diamond layer of hard secretion, the ensemble lubricated with high molecular weight hydrocarbons. Martians have a sense of smell, as the vapor pressure of volatile compounds is the same as on Earth, and with that comes pheromones, so they become conditioned to regard the others on their planet with mathematically average features as beautiful. Yet their poets and their singers speak of many lovely features, and almost never of eyes.