Future human explorers of Mars can leave their umbrellas back on Earth, but
perhaps they shouldn’t forget their Geiger counters! A NASA experiment en
route to the Red Planet aims to find out.
Take Mars, for example. A morning weather report on the Red Planet might
sound like this:
“Good morning, Martians! It looks like another solar storm heading our way.
An X-class solar flare exploded this morning and proton counts have soared
1000-fold. More of the deadly particles are en route, so don’t leave shelter
today without your radiation suit!”
“Coming up next, the sunspot report, right after this word from our sponsor:
Levi’s Relaxed Fit LeadPants.”
It doesn’t sound much like the forecasts we hear on Earth, which feature
rain and the daily pollen count. On Mars — a world that’s desert-dry,
Antarctic-cold, and possibly lifeless — human colonists will have a
different set of weather concerns.
The Red Planet is substantially exposed to the harshest elements of space
weather. Unlike Earth, which sits inside a protective magnetic bubble called
the magnetosphere, Mars does not have a global magnetic field to shield it
from solar flares and cosmic rays. Scientists aren’t sure why, but Mars’
internal magnetic dynamo turned off about 4 billion years ago. After that,
the solar wind gradually eroded the martian atmosphere until, today, it is
less than 1% as thick as Earth’s.
No global magnetic field and a very thin atmosphere — those are the two
factors that render Mars vulnerable to space radiation.
Does such exposure mean Mars is lifeless? Not necessarily, say scientists.
Indigenous life forms could be radiation resistant, like the terrestrial
microbe Deinococcus radiodurans. Tiny Martians might also live in rocks or
soil, substances that provide natural protection against radiation.
Nor is Mars necessarily uninhabitable for humans. If we learn how to shelter
ourselves from the planet’s unique brand of weather, humans can explore and
perhaps even live on Mars. That’s why NASA is sending a radiation monitor to
the Red Planet … to find out how much protection we humans might require.
MARIE, the Mars Radiation Environment Experiment, blasted off April 7th with
the 2001 Mars Odyssey spacecraft. MARIE is one of three scientific
instruments on board — the other two will search for signs of water and
interesting minerals on Mars. If all goes as planned, MARIE (along with the
rest of Odyssey) will arrive in October and spend at least two years
circling the Red Planet.
“MARIE can detect charged particles — electrons, protons, and atomic nuclei
— with energies between 15 MeV and 500 MeV,” says Gautam Badhwar, the
experiment’s principal investigator at the Johnson Space Center. “There have
never been any measurements of this kind from Mars orbit,” he added. (Note:
1 MeV equals one million electron volts.)
Space radiation can be electromagnetic, like x-rays and gamma-rays, or
particulate, like protons and electrons. Particulate radiation poses the
greater threat to humans.
Most charged particles in our solar system come from two sources: solar
flares, which produce a rain of dangerous protons, and distant supernova
explosions, which accelerate atomic nuclei –called “cosmic rays”– to
nearly light speed.
“Both can be hazardous, but from the standpoint of crew health, solar flares
are the greater concern,” says Badhwar. Solar flares produce particles with
relatively low energies (~70 MeV). “Such protons lose energy in tissue at a
much higher rate than faster-moving particles like cosmic rays” he added.
Cosmic ray nuclei, carrying typically 300 to 500 MeV per nucleon, zip
through the human body so quickly there’s not enough time to dump their
energy into the surrounding tissue.
Solar protons passing through humans ionize molecules along their tracks.
“The ionization creates free radicals,” explains Badhwar, “which can be very
damaging.” Sometimes protons will modify or even break DNA strands within
cells. If the cell survives it can become cancerous — a long-term health
risk of radiation exposure.
Mars’ thin atmosphere does little to protect the planet from energetic
protons. The air density at martian “sea level” is roughly equivalent to
that of Earth’s atmosphere at 70,000 feet altitude! Fortunately, astronauts
can find the protection they need indoors; shelter walls made of lightweight
materials provide adequate shielding. But future explorers won’t want to
spend all their time inside shelters. They’ll need to know how to handle
radiation levels outdoors in the “martian wilderness” — an environment
MARIE will probe from Mars orbit.
Although MARIE won’t reach Mars for another six months, the instrument is
already hard at work.
“We turned it on last week,” says Badhwar. “All the engineering data look
good.”
By monitoring radiation levels during Odyssey’s cruise phase, Badhwar and
colleagues will discover what sorts of hazards await travelers in transit
from Earth to Mars.
Radiation hazards … tissue damage … broken DNA. Space sounds like a
dangerous place! Nevertheless, MARIE is an optimistic experiment. Its
underlying assumption is that humans will eventually cross the divide
between our planet and Mars. Thanks to MARIE and future experiments like it,
Mars explorers will know how to survive and prosper when they get there.
