The last time we saw Jupiter up close, it was 16 years ago,
when a probe from NASA’s Galileo spacecraft took a death plunge through the
cloud tops and radioed back tantalizing data that all but screamed, “To be
continued...”
Now NASA is headed back to the big planet, looking for the clues to help answer
pressing questions about the early days of the solar system. Because whatever
was in Jupiter at the beginning — more than 4.5 billion years ago, when the
solar system was formed — is still there, scientists say, hiding in a mysterious
gas giant made up of dust and gas left over by the Sun.
A spacecraft named Juno (after Jupiter’s wife in Roman mythology) is scheduled
to lift off Friday morning from the Cape Canaveral Air Force Station in Florida,
embarking on a five-year trip. On July 4, 2016, as determined by planetary
mechanics, not American patriotism, Juno will pull into orbit around Jupiter and
spend a year there, making scientific observations of gravity, magnetic fields
and the wetness of the Jovian atmosphere.
And then scientists may learn more of the secrets of Jupiter, which has twice as
much mass as the rest of the planets in the solar system combined.
“Jupiter holds the history of the solar system,” said Scott Bolton, director of
the space science department at the Southwest Research Institute in San Antonio,
and the principal investigator for the Juno mission. “If you want to understand
that first step of how you went from forming a sun to forming the planets, you
have to understand what went into Jupiter and how it was made.”
Although the wait will be long, scientists are excited enough about what they
learned from Galileo — which was sent into Jupiter’s atmosphere in 2003, lest it
crash into one of the moons and contaminate the environment with bacteria from
Earth — that they have great expectations for Juno. The trip is costing $1.1
billion.
The biggest question is the water, because Galileo’s atmospheric probe found
hardly any.
“There’s a missing piece that turns out to be important,” Dr. Bolton said.
Astronomers have the big picture of the origins of the solar system. A cloud of
hydrogen, much like interstellar hydrogen clouds seen elsewhere in the galaxy,
collapsed to form the Sun. As the cloud collapsed, it began to spin, like a
figure skater pulling in the arms. That produced a flattened disk of leftovers
orbiting the newborn Sun, and those leftovers coalesced into the planets.
What exactly was in those leftovers is not known, however, which is why many of
the spacecraft crisscrossing the solar system are looking for pristine remnants
in comets, asteroids and — soon — Jupiter. While scientists can explain the
hydrogen and the helium, it is the smidgen of heavier elements like carbon,
oxygen, iron and nitrogen that are key elements for almost everything on Earth.
“Life is in the balance here,” Dr. Bolton said, “and the things that make us up
— that everything we’re looking at, breathing, touching — is all more or less
unexplained.”
There are a few reasons that Jupiter holds particular interest for scientists.
For one thing, it probably formed first, before the other planets. And its
gravity is so strong that once anything got sucked into it during those
formative years, it never got out again.
Even before the Galileo mission, astronomers had measurements indicating that
Jupiter contained higher concentrations of heavier elements than the Sun — a
surprising finding, because both bodies were formed out of the same hydrogen
cloud. But they came up with a plausible explanation: Among the solar system
leftovers, they speculated, were water ice crystals. After all, hydrogen is the
most common element in the universe, oxygen is third and water molecules — two
hydrogens and one oxygen — should be common. Ice traps heavier elements, and
thus the icy bits that gathered into what became Jupiter could have skewed the
concentrations higher.
The Galileo probe did measure enhanced concentrations, as expected, but not in
the pattern predicted by the ice explanation. “In fact, in that one measurement,
all the theories of how planets were made were proven wrong,” Dr. Bolton said,
“and we were like, ‘Oh no, what now?’ Nature threw us a curveball.”
Even more perplexing was the paucity of water. In the post-mortem analysis, many
scientists surmised that, by unlucky chance, the probe descended into a
particularly hot and dry spot on Jupiter.
But that was only an educated guess. The obvious next step would have been to
send more probes beneath the cloud cover of Jupiter — especially ones that could
survive to greater depths and temperatures — but that would have been
prohibitively expensive.
Juno is taking a different approach. The heat of Jupiter emits microwaves, and
water absorbs microwaves. By simply measuring the strength of the microwaves
radiating from Jupiter, scientists will be able to figure out how much water is
in the clouds.
“Once we get all those ingredients, we’ll see if we can figure out how to bake
the cake, so to speak,” Dr. Bolton said.
To make the measurements, Juno will travel along a squashed elliptical orbit,
swooping to within 3,100 miles of the cloud tops. Over the course of 33 orbits
during the mission, Juno will get a global view of the interior. Unlike
Galileo’s orbit, Juno’s will pass over Jupiter’s north and south poles, allowing
the first close-up looks at the bright auroras there.
To survive the intense radiation around Jupiter, its instruments are housed
inside a titanium vault. Eventually, the radiation will destroy the electronics
and the craft will be sent crashing into the planet.
The gravity and magnetic field measurements could provide evidence of metallic
hydrogen — at the crushing pressures inside Jupiter, hydrogen is expected turn
into a liquid metal — and a core of heavier elements.
At a news conference on Wednesday, Dr. Bolton revealed that Juno would be
carrying a commemorative plaque of Galileo Galilei, the scientist who first
looked at Jupiter through a telescope, as well as three aluminum Lego figures:
of Galileo, Juno and Jupiter carrying his thunderbolts.
