Europa’s Biological Mystery
Discovery News - Europa is thought to possess a vast sub-surface ocean beneath its thick icy crust, kept in a liquid state via tidal interactions with the gas giant. Possibly containing three times the volume of water held in Earth’s oceans — and because on Earth, where there’s water, there’s life — astrobiologists hypothesize that Europa’s ocean might be quite a cozy place for biology to gain a foothold.
“Europa’s ocean, to the best of our knowledge, isn’t that harsh of an environment,” said astrobiologist Kevin Hand, JPL’s Deputy Chief Scientist for Solar System Exploration, at a special JPL “Icy Worlds” media event on Monday.
Although Europa’s ocean may be up to 100 kilometers (62 miles) deep, the conditions at the bottom of that monstrous abyss may be akin to the environment at the bottom of Earth’s comparatively shallow Mariana Trench, the deepest region of the Pacific Ocean, which is 11 kilometers (6.8 miles) deep. Complex biology has evolved in Mariana’s cold, dark environment, so it’s not such a stretch to think that if there is life in Europa’s ocean, it may also be thriving, extracting energy not from the sun (via photosynthesis), but from chemosynthesis near hydrothermal vents.
Europa’s deep ocean owes its potential habitability to the moon’s size. It’s only the size and approximate mass of Earth’s moon and therefore has comparable gravity, ensuring ocean pressures are not too extreme for biology to evolve. It’s possible that, through the constant tidal heating of Europa’s core, the moon will also have hydrothermal vents spewing the heat and chemicals needed for Europan life.
What’s more, the icy crust of Europa would shield the ocean from the powerful radiation above.
“The radiation is stopped in the upper 10′s of centimeters to a meter” of icy crust, said Pappalardo, who is principal investigator for the Europa Clipper concept.
But this radiation isn’t all bad; the high-energy particles trapped in Jupiter’s magnetosphere trigger chemical reactions in Europa's surface ice layers, producing nutrients. Previous studies have shown that there may be an icy equivalent to plate tectonics continually refreshing Europa’s surface. Subduction zones may drag the collected nutrients below, supplying any hypothetical biosphere.
Europa's Ocean Currents Shape Potential Biological Life Zones
Europa’s life-giving potential is exciting — it has liquid water, a heat source and possible nutrient cycling — but the JPL scientists are keen to point out that the Europa Clipper concept will not be a life-hunting mission.
“The way we framed the Europa mission science objectives is not to specifically look for life, but to understand habitability; the ingredients for life,” said Hand. To search for life, argues Hand, a surface mission would be required, a technological feat that is currently out of our scope.
The instrumentation to be chosen to fly on the Europa Clipper spacecraft will observe Europa’s finest scales of a few feet, a scale that we currently know nothing about. Of particular interest will be what the famous reddish veins across Europa’s icy crust are composed of and whether they contain any organic compounds. Also, as the spacecraft will fly close to the moon, it could ‘sniff’ Europa’s possible water-rich geysers that the Hubble Space Telescope recently detected.
Like Saturn’s moon Enceladus, it’s possible that Europa has vents in its ice that eject subsurface water into space, leaving forensic evidence of the salts and other compounds it contains for any flyby spacecraft to collect and analyze.
Radiation Threat
For the Europa Clipper to explore the moon at such close proximity would require some tough shielding and clever orbital planning to protect the spacecraft’s sensitive electronics.
“It’s a jungle out there around Jupiter! It’s a jungle of radiation,” said Sara Susca, Europa Clipper payload systems engineer.
Jupiter’s magnetic field acts as a powerful particle accelerator, blasting anything within its magnetosphere with particles traveling at 50-200 meters per second. So when these particles hit things, they can be lethal for spacecraft.
To mitigate this problem, Susca envisions a highly elliptical orbit that will take the Clipper deep into Jupiter’s radiation belts for short periods of time, amassing the lowest possible dosage of radiation per orbit. In addition, the Clipper concept will have a “vault”; a section of the spacecraft with heavy shielding that will act like a ‘skull’ to the orbiter’s ‘brains.’
Overall, “the spacecraft will be quite big,” added Susca. “It will have two large solar panels both about 29 feet by 4 feet.” The main body of the vehicle will be about 18 feet (5.5 meters) tall, approximately the size of school bus.
NASA’s Galileo mission, which orbited Jupiter from 1995 to 2003, had a height of 17 feet (5.2 meters) and measured 36 feet (11 meters) from spacecraft body to the end of the magnetometer boom. As Galileo was powered by a radioactive power source, it didn’t use solar panels, so the solar powered Europa Clipper would dwarf Galileo. NASA’s incoming Juno mission, however, will be bigger than the Clipper, with three 29 x 8.9 feet (8.9 x 2.7 meters) solar arrays — the first mission to the outer solar system using large, efficient solar panels to collect the weak sunlight. Juno will arrive at Jupiter orbit in 2016.
Should the Europa Clipper spacecraft be ready for launch in the next decade, there could be a wonderful opportunity for a fast-track to Europa. Currently under development is NASA’s Space Launch System (SLS), a powerful rocket that could send the Clipper from Earth to Jupiter orbit in “under 3 years,” according to Pappalardo. The alternative, sending the spacecraft via a number of gravity assists around the solar system, could take 7-8 years.
Although Monday’s good budget request news is only the start of a long road to Europa, it takes the Europa Clipper from concept to planning, so hopes are high that the Europa Clipper will follow NASA’s Juno spacecraft as the next big mission to explore Jupiter’s enigmatic moon.
“We are really looking forward to next spring when, hopefully, we’ll become another flagship mission,” added Susca.
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