If Humans Go To Mars, Where's The Best Place To Land?

NASA is taking suggestions this week

Gale Crater

Gale Crater

Looks nice.NASA

In The Martian, a stranded astronaut has to survive a hostile Red Planet. This week, scientists will hold the first workshop to identify sites on Mars where astronauts might actually land one day.

Arguments may turn fierce when the workshop starts today as to which of the more than 50 sites under analysis might be the best real estate for life on the distant world — say, an underground home in the subterranean caves of Hebrus Valles, or a base near one-time hot springs in Gusev Crater that might have possessed life. And a number of scientists going to the meeting hope to go to Mars someday themselves.

"I'm picking landing sites that I may be visiting in the future," says planetary scientist Zachary Gallegos of the University of New Mexico, one of the final 100 candidates for the Mars One mission, a proposed one-way trip to Mars.

"It is my dream to be the first person to walk on Mars someday," adds aspiring planetary scientist and Mars explorer Alex Longo, a 10th-grade student at Cardinal Gibbons High School in Raleigh, North Carolina.

Landing Spot

The first thing Mars explorers will need is a safe place to land. The goal is a flat area roughly 25 kilometers large suitable for landing many supply vessels and crew ships over a number of missions, Gallegos says.

"In addition, you don't want something that's super-rocky, since boulders are hazardous to landing and make roving difficult," says planetary scientist Fred Calef at NASA's Jet Propulsion Laboratory. "But you don't want something super-soft either. Some places on Mars have pockets of dust that are several meters deep, and you wouldn't want to land on fluffy powder."

Low altitudes could also offer better landing sites "since you'll have more atmosphere above you," Calef says. Having more air makes it easier to land safely with parachutes or other braking mechanisms.

Local Resources

Life for astronauts on Mars will depend largely on making use of resources already on the Red Planet: what space exploration calls "in situ resource utilization." The most important resource that astronauts hope to collect on Mars will likely be water, which is useful not only for drinking, but also as radiation shielding and as fuel when it is split into hydrogen and oxygen. "The cost of bringing water from Earth to Mars is quite expensive," Calef says.

There may be five sources of water on Mars — sheets of water ice; water-rich hydrated minerals; underground aquifers; seasonal flows of water technically known as recurring slope lineae; and atmospheric humidity. "There's going to be a vigorous debate about which of these is the best source of water," says Richard Davis, Jr., assistant director for science and exploration at NASA's science mission directorate.

The best sources of water on Mars might be sheet ice and hydrated minerals, Davis says. At first glance, the most obvious choice for water might be sheet ice, but Mars can be very cold, "and the colder ice gets, the harder it gets; it will take a lot of energy to dig rock-hard ice out of the ground and melt it," Calef says. Still, getting water involves dredging sand or rock and baking it until the water comes out, and the energy needed for this is generally considered to be higher than melting sheet ice. More research is needed to see which option for water is best, Davis says.

Other important resources would-be Martians might need are gravel, sand, and rocks for construction material. "Dirt can be piled on habitats as radiation shielding," Gallegos says. "Cobbles and larger rocks can be used to construct and maintain service roadways. Metals and silicon can be refined from rocks and minerals to be used for various purposes."

Great Views

Of course, astronauts will not simply want to land and live on Mars, but they will also want to check out local attractions as well — that is, places where scientific discoveries might be made. While it took everything Mark Watney had just to survive on Mars in The Martian, actual astronauts on the Red Planet might find look for signs of past or even present life there.

The main scientific focus of astronauts on Mars will likely be water. "NASA wants to follow the water, follow the organics, to go places where you might find signs of current or ancient life," Calef says. "You want to visit a past habitable environment, like remnant pond sediments or an old river delta, where water collected and might have preserved organic material."

Astronauts will not only focus on ancient bodies of water, but also present water activity, such as recurring slow lineae, water ice fog, and possible signs of underground aquifers, which "would be of high interest to the search for extant life on Mars," Carr says.

Geological features such as craters, outcrops, canyons and dune fields promise to shed light on the the geological history of Mars.

Geological features such as craters, outcrops, canyons and dune fields promise to shed light on the the geological history of Mars. Although such features may be challenging for astronauts to navigate, "these are usually the kind of features planetary scientists want to study," Gallegos says.

Astronauts on Mars will not be limited to making discoveries only near their landing sites. "We expect crews to have significant mobility, to go up to 100 kilometers from where they land," says Ben Bussey, chief exploration scientist at NASA's human exploration and operations directorate.

Equator Or Elsewhere?

One of the main dividing lines between scientists at the workshop will be whether they propose human missions either near the equator or at higher latitudes.

"One of the primary advantages of landing at a high-latitude site is easy access to water, especially in the northern plains," Viola says. "We know that there is water ice within the uppermost meter of the surface, which means that astronauts won't have to dig deep into the subsurface. Since this ice is fairly pristine, it also means that they won't need to exploit a large region to get enough water to support a human crew."

"Furthermore, I think that water ice is potentially an important science target," Viola says. "It may preserve a record of the Martian climate from the time when it was deposited, sort of like how we can use ice cores on Earth to learn about our planet's climate history, and it might even contain evidence for either past or present life."

However, higher latitudes "would be an extremely challenging place to live," Longo says. "During the winter, an astronaut crew wouldn't see the sun for months, causing a negative effect on crew morale, and the base would have to be completely nuclear-powered because you would lose solar panels as an energy source." The colder temperatures seen during the winters at high latitudes can also prove hard on equipment.

"An equatorial exploration zone will have, just like on Earth, more direct sunlight per year," Gallegos says. "NASA's concept for human Mars exploration utilizes nuclear energy as a main power source, but many small outposts and devices will need to be solar powered."

Finally, unless astronauts are on a one-way trip to the Red Planet, as the Mars One candidates are, they will want a good place to blast off from to return to Earth. "The best place on Mars to launch from is the equator — planets spin faster there, giving a velocity boost to launch vehicles, so you won't need as much rocket fuel," Calef says. (Such boosts are why Europe's spaceport is located in French Guiana near Earth's equator.)

Still, while launches are easier at equatorial sites, "we should not be limited to such sites just for that reason," Davis says. "Cape Canaveral and Vandenberg Air Force Base are both U.S. launch sites, and they're not located all that near the equator. We should go where the science and the resources take us."

Gusev Crater
Gusev CraterNASA

Best Sites

The best places for humans to land on Mars are ones that rovers already scouted beforehand, Calef argues. "Everyone knows the rover landing sites are flat and safe. We've gathered large volumes of scientific information about those sites," Calef says. "We have much less guesswork there about where to go and where to drill to learn more about Mars; we know what the rocks are like there, and where extra investigation is needed."

Of these previously explored sites, Longo argues that Gusev Crater, which the Spirit rover wandered across, is the most exciting possibility for human exploration. "In my view, an ancient hydrothermal environment like that found by Spirit in Gusev Crater is the single most important science target for any future mission," Longo says. "Hot springs on Earth are abodes for life, and could also provide an ideal locale where life could have begun."

Another possibility is Gale Crater, which the Curiosity rover explored. "There you can see a major transition in Mars' geological history, between wet Mars and dry Mars," Calef says. "It's also low altitude, so there's more of an air cushion there, giving an advantage to landings."

Although Gallegos also thinks Gale Crater is a compelling choice for human exploration, "a disadvantage to returning to a previous area is not studying a new area," he says. "We have a whole planet to study, so some might say we should visit different places." Davis also notes that a lot of rover-explored sites tend to be dry, potentially complicating attempts at habitation.

Both Gale and Gusev Craters are equatorial sites. Another possible equatorial site that planetary scientist Gennady Kochemasov at the Russian Academy of Sciences suggests is Acidalia Planitia, which is where Watney lived in The Martian. Still another is Vallis Marineris, which "has been proposed as a landing site for decades," Gallegos notes. "This enormous canyon stretches over 4,000 kilometers across the planet, and offers a glimpse into Mars' geologic and hydrologic past." Vallis Marineris has recurring slope lineae that hint at liquid saltwater below the surface, says scientist and engineer Christopher Carr at MIT.

A site on the eastern rim of the giant meteor crater Hellas Planitia possesses ancient Martian crust and large pockets of water ice.

At higher latitudes, Protonilus Mensae is part of a complex network of mesas and valleys that "has been shown to contain large amounts of water ice in the form of glaciers," Gallegos says. "I have proposed an exploration zone in the Protonilus Mensae region, directly to the east of Moreux Crater. It provides access to ancient Mars crust as well as the rock units that record the transitions between the different chemical regimes and time periods on Mars." Similarly, he notes that "Mesopotamia," a site on the eastern rim of the giant meteor crater Hellas Planitia, possesses ancient Martian crust and large pockets of water ice useful for human explorers.

Then again, the best site to live on Mars might not be on the surface at all. Instead, astrobiologist Alberto Fairén at Cornell University and his colleagues suggest that subterranean caves could make perfect homes.

"Because Mars rotates around the sun every two Earth years, the time between two launch windows from Earth to Mars and back will be two years; therefore, any exploration party would necessarily stay two years on the planet," Fairén says. "For a crew to survive two winters on Mars, it will be necessary to provide very serious shelters against radiation and extreme temperature fluctuations on the surface. The only way to achieve these requirements at reasonable costs, energy and effort levels is going down and hiding below the surface."

Ancient catastrophic floods helped create networks of caverns in Hebrus Valles. The area may hold subterranean water ice and fossils of life. "It was a big surprise realizing that our proposal was the only one out of dozens suggesting using subsurface environments such as caves or lava tubes as a base to build shelters for human exploration," Fairén says. "The subsurface seems to be the only possible choice to start human exploration of Mars."

This workshop will not be the last word on where humans might land on Mars. "It's just awesome that we're really taking such a big first step in that decision," Calef says. "Once you start to pick a destination, this goes from a pie-in-the-sky vision to becoming real."