The first steps on Mars?

Exploring the requirements for boots that may be worn on future missions to one of our closest neighbours.

by Gareth Littler

In the near future, there is the distinct possibility that humanity will be sending explorers to Mars – the 'Red Planet'. Mars is about half the diameter of Earth and about twice the diameter of the Moon. The Martian gravity is about one- third of that on Earth, and it has a very thin atmosphere mostly composed of carbon dioxide – CO₂ (0.636kPa surface pressure, compared to 101.325kPa – 1 'atmosphere' – at sea level on Earth). Being further away from the Sun, the average temperature is much colder than on Earth, at around -63°C. However, the temperature range is much larger – from -143°C at the pole to +35°C in the deserts during the Martian summer.

An interesting characteristic of Mars is the presence of frozen carbon dioxide. At pressures below 5 atmospheres, carbon dioxide has no liquid phase, and thus – unlike on Earth – this 'ice' forms and disappears with no flowing liquid. New facts about Mars are being discovered on almost a daily basis. As an example, recent discoveries have suggested the possibility of flowing water in some locations. Suffice to say, what we know so far suggests that Mars is a very hostile place for humans. In order to operate there, future explorers will need to have specially-designed equipment to allow them to function properly and safely during their stay. The longest manned Moon mission was Apollo 17. This mission – the last of the Apollo flights – lasted 12½ days, with just under three days spent on the moon. By comparison, a flight to Mars would take some 200-250 days. The first trip is likely to be a short stay, with just four or five days on the surface, but even this modest mission would last 18 months or more.

The North American Space Administration (NASA) has stated that putting boots on Mars by the end of the 2030s is not just a 'pipe dream'. So, what would these boots be like? What would be the nature of the footwear needed to allow explorers to make the leap to Mars? NASA has already begun prototyping Mars suits, currently called the 'Z series'. This article will focus on the kind of requirements necessary for the footwear.

Firstly, any footwear will need to be pressurised, as the atmosphere on Mars is so low – well below the so-called 'Armstrong limit'. This is the point at which the atmospheric pressure is low enough to boil water at body temperature, although this is not to say that human blood would boil, as its pressure is maintained within the circulatory system.

During testing of pressurised suits for early space missions, NASA documented the effects on humans at extremely low pressure. One individual described the feeling of 'water boiling on his tongue' shortly before he lost consciousness. This man had been taking part in early pressurised suit testing on Earth when an accident caused sudden depressurisation. This kind of event on Mars would be fatal.

It is also important to consider that dust found on the surface of Mars contains hexavalent chromium (Cr VI), which can be harmful to humans. To solve this problem, either the boots will either have to remain outside at all times, as they would be in NASA's Z series suits, or they will need a removable layer that can be cleaned after each excursion. When hexavalent chromium is inhaled in high concentrations, it can cause irritation to the nose and throat. It is also possible to develop an allergy to Cr VI that can cause asthma symptoms. If an allergic skin reaction is developed, exposure can cause swelling and rashes that may become worse over time. Direct skin contact can cause non-allergic skin irritation and contact with damaged skin can cause chrome ulcers.

With temperatures on Mars ranging from a high of 30°C to a low of -140°C, the boots will have to be designed with a proactive heating/cooling system. An ideal system to maintain foot temperature would use a liquid, especially if it has a high specific heat capacity.

Because the footwear will need to be pressurised (and, therefore, hermetically sealed), it cannot be breathable to atmosphere in the traditional sense. The material layers close to the skin will have to be absorbent and wicking in nature. This wicking will draw the moisture produced by the feet away and the absorbent material will allow the wearer to function for extended periods of time. It will be important for any sweat produced to be removed from the foot to avoid the risk of a number of immersion foot syndromes, such as 'trench foot'. In the event of an emergency, it is not outside the realms of possibility that an astronaut would have to live inside a pressure suit for an extended period of time. It would only require a time period of 48 hours before he or she would run the risk of severe foot damage if the moisture could not be managed.

Additionally, if a semi-permeable membrane were used between an insulating liquid layer and the inner parts of the footwear, unwanted moisture would move from the inner parts of the footwear to the insulating liquid layer. The SATRA TM47:2002 – 'Water vapour permeability and absorption' test method can be used to establish rates for these characteristics. The positive aspect of incorporating a semi-permeable membrane would be the reclamation of any water normally lost as a result of sweating. The negative aspect of using such a method would be that all the other substances contained within sweat would be left in the inner parts of the foot-wear, requiring the boots to be cleaned out on a regular basis.

Adjusting the boots is likely to call for a ratchet and line closure system involving metal wire, as this does not require knots to be tied. Such a system would allow the boots to be loosened and tightened while the wearer is still in a pressurised suit, which will likely be quite restrictive in nature. Using metal wire as opposed to more traditional materials will cut down on ultraviolet (UV) degradation and improve resistance to wear.

Physical effects

There is a problem in space travel that over time muscle and bone degrade as the body is exposed to a micro-gravity environment. This could cause issues with the sizing of the boots, as the astronauts' feet are likely to change shape over the course of the outward journey and during the mission on the surface. This issue could be solved by having a series of inserts made out of a compressible material that would allow the foot to change shape as it is exposed to different levels of gravity.

With regards to UV, the boots will need to be made from highly resistive materials, as the lack of atmosphere on Mars allows significantly more UV through to ground level than that on Earth. In addition, Mars lacks a magnetosphere that would block out most cosmic radiation. This would mean that the boots will have to contain a material with a high level of radiation resistance.

The sole of the boots will need to be quite stiff to accommodate any uneven ground that the astronauts will need to traverse. A 'B1' rated sole should do the job for most uneven ground covered, while also being able to accommodate 'C1' crampons should the need arise. This grading system is used by mountaineers to allow the correct boot to be chosen for different winter routes (see the box 'The Bs and Cs of climbing boots'). The grading system rises to 'B3' and 'C3', where the 'B3' boot is capable of accommodating 'C1' and 'C2' crampons. The higher the graded boot, the stiffer is the sole, and the less appropriate it is to hike over ground that is not covered in snow and ice. It would also be unsafe to wear higher graded crampons with lower graded boots, as the attachment system might fail under high load. Even with the lower gravity on Mars, this would not be worth the risk.

SATRA is capable of measuring both the sole material and the shank used to increase sole stiffness. The testing method for the sole would likely be SATRA TM3:1999 – 'Flexing index'. In addition to having a stiff sole, any rubber used will need to be quite hard to prevent wear over a short period of time. This can be tested using SATRA TM362:2014 – 'Abrasion resistance of soles – Biomechanical method'. The soles will need to be quite wide (as were the boots used in the Apollo missions), in case the astronauts have to walk along dusty or sandy ground. The sole must also be highly insulated to reduce loss of heat to the Martian ground.

SATRA TM146:1996 – 'Thermal conductivity' would be suitable to measure the sole material insulation while still in sheet form. The boots could use a system to clear out the tread, such as those used on some crampons, where a convex rubber material is compressed with each step and then pops back out again to remove any matter that would otherwise clog the crampon.

All the materials going into the construction of the shoes will need to be tested at the range of temperatures found on Mars. This is because materials like rubber and polymers will perform differently at the extreme lower temperatures then they do at standard laboratory conditions. Equally, the footwear should not be constructed with the thermal insulation required for the extreme lower temperatures, because this will create boots that would be too hot to wear at the higher temperatures found on Mars.

To conclude, much of what is necessary for boots to be suitable for exploration on Mars is similar to that of the requirements for the Moon Apollo missions. Having said that, there are additional types of terrain on Mars that were not found during the Apollo missions. For instance, much of the Martian surface would probably require winter mountaineering gear to accommodate exploration. Many of the requirements for Mars exploration boots could be tested at SATRA with established testing methods.

Mars beckons. If and when the first humans walk on the surface of the Red Planet, how close will the boots they wear be to what is being designed by current projects? Only time will tell, but one thing is certain – considerable effort is being expended to identify all perceived challenges to walking in such a hostile environment and practical solutions are currently being sought.

Publishing Data

This article was originally published on page 40 of the November 2016 issue of SATRA Bulletin.

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