NASA’s Juno mission has provided its first science results on the amount
of water in Jupiter’s atmosphere. Published recently in the journal Nature
Astronomy, the Juno results estimate that at the equator, water makes up about
0.25% of the molecules in Jupiter’s atmosphere — almost three times that of the
Sun. These are also the first findings on the gas giant’s abundance of water
since the agency’s 1995 Galileo mission suggested Jupiter might be extremely
dry compared to the Sun (the comparison is based not on liquid water but on the
presence of its components, oxygen and hydrogen, present in the Sun).
An accurate
estimate of the total amount of water in Jupiter’s atmosphere has been on the
wish lists of planetary scientists for decades: The figure in the gas giant
represents a critical missing piece to the puzzle of our solar system’s
formation. Jupiter was likely the first planet to form, and it contains most of
the gas and dust that wasn’t incorporated into the Sun.
The leading
theories about its formation rest on the amount of water the planet soaked up.
Water abundance also has important implications for the gas giant’s meteorology
(how wind currents flow on Jupiter) and internal structure. While lightning — a
phenomenon typically fueled by moisture — detected on Jupiter by Voyager and
other spacecraft implied the presence of water, an accurate estimate of the
amount of water deep within Jupiter’s atmosphere remained elusive.
Before the Galileo probe stopped
transmitting 57 minutes into its Jovian descent in December 1995, it radioed
out spectrometer measurements of the amount of water in the gas giant’s
atmosphere down to a depth of about 75 miles (120 kilometers), where the
atmospheric pressure reached about 320 pounds per square inch (22 bar). The
scientists working on the data were dismayed to find ten times less water than
expected.
Even more
surprising: The amount of water the Galileo probe measured appeared to be still
increasing at the greatest depth measured, far below where theories suggest the
atmosphere should be well mixed. In a well-mixed atmosphere, the water content
is constant across the region and more likely to represent a global average; in
other words, it’s more likely to be representative of water planetwide. When
combined with an infrared map obtained at the same time by a ground-based
telescope, the results suggested the probe mission may have just been unlucky,
sampling an unusually dry and warm meteorological spot on Jupiter.
“Just when we
think we have things figured out, Jupiter reminds us how much we still have to
learn,” said Scott Bolton, Juno principal investigator at the Southwest
Research Institute in San Antonio. “Juno’s surprise discovery that the
atmosphere was not well mixed even well below the cloud tops is a puzzle that
we are still trying to figure out. No one would have guessed that water might
be so variable across the planet.”
Measuring Water From Above
A rotating, solar-powered spacecraft, Juno launched in 2011. Because of
the Galileo probe experience, the mission seeks to obtain water abundance
readings across large regions of the immense planet. A new kind of instrument
for deep space planetary exploration, Juno’s Microwave Radiometer (MWR)
observes Jupiter from above using six antennas that measure atmospheric
temperature at multiple depths simultaneously. The Microwave Radiometer takes
advantage of the fact that water absorbs certain wavelengths of microwave
radiation, the same trick used by microwave ovens to quickly heat food. The
measured temperatures are used to constrain the amount of water and ammonia in
the deep atmosphere, as both molecules absorb microwave radiation.
The Juno science
team used data collected during Juno’s first eight science flybys of Jupiter to
generate the findings. They initially concentrated on the equatorial region
because the atmosphere there appears more well-mixed, even at depth, than in
other regions. From its orbital perch, the radiometer was able to collect data
from a far greater depth into Jupiter’s atmosphere than the Galileo probe — 93
miles (150 kilometers), where the pressure reaches about 480 psi (33 bar).
“We found the
water in the equator to be greater than what the Galileo probe measured,” said
Cheng Li, a Juno scientist at the University of California, Berkeley. “Because
the equatorial region is very unique at Jupiter, we need to compare these
results with how much water is in other regions.”
Northward Bound
Juno’s 53-day orbit is slowly moving northward, as intended, bringing
more of Jupiter’s northern hemisphere into sharper focus with each flyby. The
science team is eager to see how atmospheric water content varies by latitude
and region, as well as what the cyclone-rich poles can tell
them about the gas giant’s global water abundance.
Juno’s 24th science
flyby of Jupiter occurred on Feb 17. The next science flyby takes place on
April 10, 2020.
“Every science
flyby is an event of discovery,” said Bolton. “With Jupiter there is always
something new. Juno has taught us an important lesson: We need to get up close
and personal to a planet to test our theories.”
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