# How do I dress for a hike on a hot summer day on Titan?

Titan has an atmosphere with a surface pressure of around 150 kPa. It has an average surface temperature of around 94 K, around -183°C. On a nice, sunny, calm, summer day near the equator, it may feel nearly as pleasant as Condition One weather on Antarctica (less than -73°C and sustained winds exceeding 30 m/s). Fortunately, it's usually quite calm on Titan, and who knows how far above 100K the temperature may rise in a heat wave¹?

Provided I bring plenty of oxygen for breathing, how would I dress for a hike on a hot summer day in Titan? With a surface gravity of only 1.45 m/s², I'm not too worried about carrying a 50 kg backpack (although I'm feeling queasy about carrying an RTG in my pack).

• @Jack I wanted to ask it there! But I think they will not have the right expertise and balk at my 50 kg backpack proposal, as they're infiltrated by ultralight folks. – gerrit Aug 7 '18 at 13:58
• ultralight? Titan has 0.85 Moon's gravity accoding to Wikipedia. – J. Doe Aug 7 '18 at 14:05
• @J.Doe I mean that some of the The Great Outdoors crowd think anyone trekking with more than 8 kg on their back is crazy, so I'd need to explain them that 50 kg on Titan has the same gravity as 7 kg on Earth, although it still has the same inertia and will still be bulky, so I don't know :) I'll post it in their chatroom. – gerrit Aug 7 '18 at 14:07

The ESA Huygens probe very accurately characterized the conditions at its landing site on Titan's surface and verified measurements made by the Voyager 1 Radio Science (VRS) investigation nearly 25 years earlier. It measured a surface temperature of 93.8K (VRS: 94.0 ± 0.7 K), a surface pressure of 1467 mb (VRS: 1496 ±20 mb), a methane abundance of 5.65 ± 0.18 % by volume in otherwise almost pure nitrogen, and near-surface winds in the 0.5-1 m/s range, in good agreement with detailed global circulation models. The atmosphere's composition, temperature, and pressure make for a mass density at the surface of ~4 times the density of Earth's atmosphere at sea level.

On Earth, planning a hiking trip requires at least a little forethought. Will the hike be a daytime hike in the Sahara or a winter hike at the south pole, or somewhere between? The huge range of conditions possible at Earth has a big effect on the kinds of gear you'd need to take.

Not so at Titan. In some ways Titan is like Venus: its dense atmosphere is very efficient at moving heat from sources such as solar energy all over Titan. Like Venus, anywhere you go on Titan, locations at the same elevation show very little temperature difference. The Wikipedia article on Titan's climate says "Titan is about 12% closer to the sun during the southern hemisphere summer, making southern summers shorter but hotter than northern summers." Hotter?? All the observations indicate that the largest temperature difference to be found at Titan, from the subsolar latitude in summer to the winter pole, is ~3.5 K! So one suit will do wherever you go on Titan.

And suit it would be, as @peterh says. Not a full space suit, since it doesn't need to maintain increased pressure inside, but you just can't let any part of your body be exposed directly to Titanian air at 94 K.

The suit also needs to supply breathable air, and here there's a bit of a twist. You simply can't plan on breathing pure oxygen at nearly 1.5 bars pressure. In addition to the extreme fire hazard, it's also a health hazard. And you probably don't want to try to maintain the pressure inside the suit at ~1/3 bar so you could use pure oxygen. Running such a pressure difference would make the suit much heavier and stiffer. So in addition to carrying oxygen, you need some way to provide an inert gas to make up the difference in pressure. You can't just bring in Titanian air, heat it, and mix oxygen with it. It's not that methane is toxic, to the contrary it's fairly benign in that regard. It's that 5+% methane mixed with oxygen is a combustible mixture, so it would be an accident just waiting to happen. If you use Titanian air for the background gas, you have to take most of the methane out of it, and that takes energy.

Fortunately, removing methane by condensing it would require cooling the Titanian air by less than 20K. This is not particularly difficult and could be done with a Peltier cooler. Cooling exhaled gas to ~195 K will condense out CO2 for rebreathing of what's left, notably the oxygen. No cooling mechanism is needed for that, just something akin to a car's radiator to reject heat to the Titanian atmosphere. This would also condense out water, which you'd want to return to the scrubbed (of CO2) air. Having two different temperatures in the scrubber is probably the best approach, the warmer one to remove (for subsequent return) most of the water but not CO2, the colder one to scrub CO2 for disposal.

If you're not going to use Titanian air then you have to bring your own background gas, whether nitrogen, helium, or whatever. It wouldn't have to be in a separate tank, but the contents of a single tank would be only ~15% oxygen. If you used scrubbed Titanian air, your tank could contain 100% oxygen, allowing longer durations before needing replenishment.

The suit needs to keep you warm. My friend and colleague Julian Nott, the accomplished balloonist who has analyzed the potential for balloon flight at Titan, has looked into how much insulation would be required to maintain comfortable temperatures inside a suit using only body heat. His (unpublished) result: the suit gets pretty thick, and might hinder movement. You could make the suit thinner by using a suit heater, but that takes more energy. That's a trade yet to be made: do you make the suit thicker, to use less energy for heating, or thinner with a suit heater, to ease mobility?

There's another item the suit will need. Regardless of which options are chosen, electric power will be needed, so some kind of battery, or fuel cell, or some other source of electric power will be needed. It won't be a solar array! Light levels at noon on Titan's surface are roughly 1/1000 of full sunlight on Earth. Titan's clouds and haze make the sunlight diffuse, so you can't make an effective concentrator. So much for solar for relatively power-hungry activities on Titan! But battery technology has come a long way since the Apollo days, so a relatively light battery (especially in Titan's ~1/7 g!) could supply enough power for the suit.

None of the challenges outlined here appear to be show-stoppers. The biggest challenge is in getting to Titan in the first place!

Once there, in that low gravity and thick atmosphere, you could don your suit, strap some relatively small wings onto you arms, flap your arms—and fly! (with reference to Robert Zubrin's 1999 book) I would recommend some training before you try it, though.

• I wonder if I would feel any noticable difference between standing in the sunshine or in the shade. – gerrit Aug 8 '18 at 8:39
• Note that 5% is the low end of Methane's flammability limit (en.wikipedia.org/wiki/Flammability_limit). 4% methane won't burn (although I am not a chemist so don't quote me on that :)). You don't have to scrub out all or even most of the methane to avoid the combustion issue. – Jay Lemmon Aug 8 '18 at 11:15
• Availability of methane provides the energy solution. – joojaa Aug 8 '18 at 14:56
• @joojaa The stoichiometric ratio for the complete oxidation of methane (thus getting all the energy you can from the reaction) requires that for every gram of methane you react, you have to provide four times that mass of oxygen, whether it's using combustion, or a fuel cell—whatever. For a very short stay at Titan, lugging the oxygen along might not be too much of a burden. But for a long stay you'd need a lot of oxygen, and you'd probably wind up being better off with a nuclear electric source to charge secondary (rechargeable) batteries or to hydrolyse the local water (ice) for oxygen. – Tom Spilker Aug 8 '18 at 18:43
• There is another advantage to using Titan's methane as part of the inert gas mixture: Flatulence in the space suit will pretty much go unnoticed. Some might consider that a disadvantage, actually. – dotancohen Aug 9 '18 at 7:31

A -183 C nitrogene-methane mix doesn't make you an icecube on the spot, but it still causes quickly frozen wounds. Your only way to avoid it if your whole body is protected, no $cm^2$ may remain open.

You need also oxygen supply. Here comes another problem: methane with oxygen supply becomes an explosive substance. The methane concentation on the Titan's surface is around 5% (ref), which is surely a strong fire hazard, it might be even an explosive mix.

Combining them together, the result won't be too far away from a spacesuit. Maybe the only difference is, that spacesuits need to resist pressure, while it becomes a non-issue because it can be easily handled by over-pressurization.

But this suit need to be extreme safe against the smallest oxygen leaks. In ordinary space suits, a little leak is not a problem (if it is with orders below the astronaut's oxygene consumption).

The largest problem is that your exhaled air still contains 10-16% oxygen, thus you can't simply allow it to leave. Or you risk that it will collect at your feet (oxygen is more dense as methane), and then explodes.

So

P.s. @imsodin says in excellent comment:

Couldn't that [the burning oxygen in the methane atmosphere] be used for heating up?

P.s.2. @called2voyage's excellent comment:

So you'd have to wear a suit on Titan, but it would be more like a wetsuit and scuba gear than an EVA suit.

• Atmosphere of Titan is 1.4% methane. Is that enough to make the mixture of 10% oxygen added to 1.4% methane explosive? – gerrit Aug 7 '18 at 15:09
• Even if it is, just use a rebreather like used in diving. – imsodin Aug 7 '18 at 15:16
• @imsodin Right! The majority of the Titan atmosphere is nitrogene and not methane. But the wiki says, the methane concentration in the lowest 8km of the atmosphere is nearly 5%, in the stratosphere is it 1.4%. Even if it doesn't explode, it surely can burn. – peterh Aug 7 '18 at 15:19
• You're also going to be significantly hotter than your surroundings. Does the spacesuit need to be deliberately designed to avoid melting the terrain beneath you? – notovny Aug 7 '18 at 18:29
• You need a suit that keeps you warm. Luckilly, that bit is easy: wear the equivalent of the Liquid Cooling And Ventilation Garment... bring oxygen, then take in the Titanian atmosphere and have it react in a fuel cell. Presto: electricity for your on board travel computer and warm water to circulate in your Titan EVA suit. About the fire hazard... you basically just have the same kind of issue that you have on Earth when it comes to moving flammable gasses around, now you just treat the oxygen as the flammable gas. – MichaelK Aug 8 '18 at 10:17