The first measurement of radiation at Mars' surface has implications for a human mission to the Red Planet, as well as for where Mars' missions might find traces of Martian life – if it was ever there.
Radiation has never been kind to Mars. If life ever existed on the Red Planet, radiation might have finished it off. It might then have erased all trace of that life ever having been there. And it will now make putting humans on the planet a dangerous and difficult mission.
Two new papers in the journal Science address how radiation will factor into a future manned mission to Mars and into searches for signs of life there. Both papers are published as part of a package of six articles on the Curiosity rover’s most recent findings at Mars’ Gale Crater.
One of the papers presents the first direct measurement of the radiation level on the Martian surface, a figure that has significant implications for NASA’s proposed manned trip to Mars. Along with another paper that presents the first radiometric dating on another planet, it also proposes where future Mars missions might find traces of life on the Red Planet – if it was ever there to begin with.
“This is an important milestone,” says Don Hassler, Science Program Director at the Southwest Research Institute in Boulder, Colorado and the lead author on the first paper.
On Aug. 6, 2012, the Curiosity rover landed at 4.4 km MOLA (Mars Orbiter Laser Altimeter) altitude in Mars’ Gale Crater. The 96-mile-wide crater dates to about 3.7 to 3.5 billion years ago, either at the very end of Mars’ Late Noachian or the very beginning of its Early Hesperian period, the first two of the three periods in the Martian geological time scale. At the crater’s middle is Mount Sharp, and the rest of the valley is filled with features named in evocation of Earth’s landscapes: Yellowknife Bay, Gillespie Lake sandstone, Sheepbed mudstone.
These are generous names, though. Scientists have known since 1964, when the Mariner 4 probe skirted Mars and sent back images of a brittle, dust-caked planet, that if the Red Planet looks like Earth, it looks like a post-apocalyptic version of it, after a giant went through and ripped the trees from the cliffs and guzzled all the water from the valleys.
Evidence suggests that Mars was once wrapped in a thick atmosphere. But, sometime during the Noachian period, it lost it. At that time, Mars, now in its peculiarly named Amazonian period, also lost its water and its warmth. Lost, too, was its robust protection against radiation.
While Earth’s atmosphere protects it from most of the cosmos’ constant barrage of radiation, Mars’ slight one lets lots of that radiation in. Mars also lacks the radiation-deflecting magnetic field that Earth has. Scientists have long expected that Mars’ sparse protection against radiation will be a major barrier to putting humans on the planet. So too will be the long travel time to and from Mars, during which astronauts will be subjected to even stronger radiation in empty space. NASA has identified assessing the radiation threat to Mars-travellers as among its top priorities before sending a manned mission to Mars sometime in the 2030s.
When the Curiosity craft launched on Nov. 11, 2011, aboard it was the Radiation Assessment Detector (RAD), a device designed to monitor how much radiation the rover was exposed to during its 253-day journey to Mars and its subsequent time there.
The RAD data from Curiosity’s flight to Mars was reported earlier this year in the journal Science. Based on data showing an average daily radiation dose of 1.8 thousandths of a Sievert (Sv), the paper’s authors estimated that astronauts would be exposed to a dose of 0.66 Sv during a six-month, round-trip to Mars.
For comparison, the average American gets a total dose of about .003 Sv of radiation from all sources in a year. A worker responding to the 2011 Fukushima disaster was allowed to accumulate a dose of no more than .25 Sv of radiation.
The data reported in this week’s paper comes from RAD’s measurements on the Martian surface. For 300 Martian days, or Sols (one Martian Sol is equivalent to 24 hours 39 min), Curiosity was zapped with cosmic rays, receiving a radiation dose of about .64 thousandths of a Sievert per day, the data shows.
The team then combined the figures from the flight to Mars with those on the surface of Mars to estimate the total radiation does an astronaut would receive during NASA’s template mission to Mars: 360 days travel time and 500 days surface time. That number comes to 1.01 Sieverts (Sv), or about 10 times the radiation dose an astronaut receives during a six-month mission on the ISS.
“We now have a more complete picture of what the radiation exposure might be for an astronaut on a trip to Mars,” says Dr. Hassler. “Making these first ever measurements is very significant.”
Radiation is a concern for good reason: a dose of one Sievert of radiation is associated with a five percent increase in a person’s risk of developing fatal cancer. It is also associated with long-term damage to the eyes and lungs, as well as to the gastrointestinal system. Much is also still unknown about how it wastes the body, including its effects on the central nervous system; understanding how radiation might affect an astronaut’s cognitive performance during a long-term mission is critical before such a mission is launched, NASA has said.
NASA does not put an absolute limit on the dose of radiation its astronauts can accumulate over time, but it does put a career limit on the percentage increase that it will allow its astronauts to increase their cancer risk. For astronauts in Low Earth Orbit, that limit is three percent. Depending on age and gender, that puts the radiation limit between one and four Sieverts.
The space agency has not yet determined the limit for Mars-bound astronauts. The National Academy of Medicine is reviewing the issue as both a medical and ethical one and is expected to release its recommendations in April 2014.
Meanwhile, NASA is investigating possibilities for minimizing astronauts’ radiation exposure during a Mars mission, says Chris Moore, deputy director of advanced exploration systems at NASA.
“Radiation protection will be a significant challenge,” says Dr. Moore.
One option is to insulate a Mars-bound spacecraft from radiation using absorbing materials like hydrogen or polyethylene, which is also used in the outer layers of the International Space Station, he says.
Still, put too much extra material on a spaceship, “and at a certain point it becomes too heavy to launch to Mars,” says Moore. So, NASA is also reviewing technological possibilities for shortening the travel time to and from the Red Planet, he says. Another possibility is to configure the spaceship layout such that hydrogen-rich food and water supplies are kept in the crew’s sleeping quarters, as a protection measure, he says.
Though Curiosity’s measurements are a major step in understanding the radiation exposure for Mars-bound astronauts, much is still unknown about how radiation will factor into a manned mission to the Red Planet, says Moore.
That’s because all of RAD’s data was collected during an unusually weak solar minimum, with no big pops or spurts from the sun, he says. So, RAD registered little of the kind of cosmic radiation produced in a solar maximum’s storms, called Solar Energetic Particles (SEPs). Instead, almost all the radiation it absorbed was from Galactic Cosmic Rays (GCRs), which tend to be stronger when SEPs are weaker, and vice versa.
Understanding when is the best time for travel to Mars will require data from a big solar storm, says Moore.
“It will probably be a while before we’re able to go to Mars,” he says, “and by that time we’ll have learned a lot more.”
Curiosity’s principle goal is to find evidence that Mars once had the proper conditions to host life – not that it did, in fact, have life. But a kingpin find, nevertheless, would be to uncover evidence suggesting that life did indeed make a home out of the once Not-So-Red Planet, before its atmosphere was shredded up. That would require finding bio-signatures, or organic material that suggests life’s previous presence there.
But, so far, scientists have found no such evidence. That leaves two options, says Kenneth Farley, a geochemist at the California Institute of Technology and an author on the second paper: “Either Mars never had life, or it was there, and the evidence was destroyed by cosmic rays.”
Indeed, it’s possible that constant, extreme radiation didn’t just kill whatever life might have been on Mars, but also might have erased all trace of it ever having been there, he says. And if any suggestion of life is left at all (were life ever there), scientists will have to look in just the right spots.
In their paper, Dr. Farley and colleagues present the first radiometric dating of the Martian surface, a feat that, beyond demonstrating “that these incredibly complex calculations can be made on another planet,” hints at where Mars missions should scout for bio-signatures that radiation has not yet wasted to nothing, says Farley.
The team’s dating showed that the scarp ringing Gale Crater’s Yellowknife Bay is being laterally eroded as a southwest-blowing wind hurls sand at the cliffs. So, if the Curiosity rover were still at Yellowknife, the best place to look for bio-signatures would be at the southwestern base of the scrap, where the rock was most recently exposed and anything inside it has not been exposed to surface radiation for long.
As Curiosity travels to Mount Sharp, the findings offer another bio-signature-hunting strategy to the rover’s toolkit. At the moment, Curiosity looks for bio-signatures either by digging down into the Martian surface or sampling from recent craters, where sediments are freshly exposed. But digging is difficult, and plumbing only newly created craters is limiting, since these craters don’t always have “the most interesting rocks,” says Farley.
“We don’t know exactly what lies ahead,” says Farley, “but we will find other scraps. And we now understand, going forward, how missions on Mars might take advantage of what nature is already doing.”
Mars’ might not have any evidence left of past life. Life might have never even been there at all. But that hasn’t stopped Earthlings from asking the romantic question: could there be living Martian life?
If there is, it would have to be underground, the first paper suggests.
That’s because rock cover provides shielding from the constant barrage of radiation that would over time be fatal to organisms. At one meter deep in the Martian rock, the radiation dose per year is about half what it is aboveground. At three meters deep, it is about 98 percent less than what it is at the surface (it’s for that reason that Mars One, the private mission to put a human colony on Mars in the 2020s, says that its settler habitat will be buried several meters deep in Martian dirt).
Based on RAD data, the researchers estimate that life would have to be buried at least one meter underground to survive. Even there, to still be alive, microbial colonies would need mechanisms to withstand the radiation that does reach depths of a meter or more, including abilities to self-repair radiation damage. Without such adaptive abilities, even the most radioactive resistant organisms buried several meters deep would be killed off within a few million years, the researchers found.
“We know that life on Earth has adapted to pretty much every extreme environment,” says Jen Eigenbrode, an astrobiologist at NASA Goddard Space Flight Center and an author on both the papers. If this principle is applied to Mars, “perhaps life has found a way of surviving in this highly irradiated environment,” she says.
Still, “it’s just pure speculation at this point,” she says. “We’re entering an area we just don’t understand.”
And Curiosity’s mission goal has never been to find actual life on Mars, says Dr. Eigenbrode. Instead, Curiosity is charged with furnishing a better understanding of a planet millions of miles from Earth, a planet that had in its beginnings seemed as if it might become something like Earth – wet and warm – but instead went a different course, to become a strange environment that humans don’t yet understand.
“This is exploration,” says Eigenbrode. “One day, we will address the life question, but I also hope both the public and scientists will embrace the journey as we approach that answer.”