NASA's Juno mission has released its first scientific results on the amount of water in Jupiter's atmosphere. Data, published recently in the journal Nature Astronomy, shows that at the equator, water makes up about 0.25% of the molecules in Jupiter's atmosphere – nearly three times that of the Sun.
An accurate estimate of the total amount of water in Jupiter's atmosphere has been on planetary scientists' wishlists for decades: the figure in the gas giant represents a critical missing piece in the formation of our solar system. Jupiter was probably the first planet to form, and it contains most of the gas and dust that was not included in the Sun.
Leading theories about its formation are based on the amount of water that the planet has absorbed. The abundance of water is also important for the gas giant's meteorology (as wind currents flow around Jupiter) and internal structure. While lightning – a phenomenon commonly caused by moisture – discovered on Jupiter by Voyager and other spacecraft implied the presence of water, an accurate estimate of the amount of water deep in Jupiter's atmosphere remained elusive.
Before the Galileo probe stopped transmitting data in 1995, it transmitted spectrometric measurements of the amount of water in the gas giant's atmosphere to a depth of about 120 kilometers, where atmospheric pressure reached about 22 bar. Scientists working on the data were alarmed to find that the water was ten times less than expected.
Even more surprising, the amount of water measured by the Galileo probe appears to still increase at its greatest measured depth, well below the level where theories suggest the atmosphere should be well mixed. In a well-mixed atmosphere, the water content in the region is constant and is likely to represent the global average. In other words, it will be a more accurate indicator of the availability of water on the planet. Combined with an infrared map obtained at the same time by a ground-based telescope, the results indicated that the probe's mission could simply be unsuccessful, hitting an unusually dry and warm meteorological spot on Jupiter.
“Just as we think we've figured out something, Jupiter reminds us how much we still have to learn,” said Scott Bolton, principal investigator for the Juno mission at the Southwest Research Institute in San Antonio. “Juno’s surprising discovery is that the atmosphere was not well mixed even under the cloud tops, which is a mystery that we are still trying to solve. No one would have guessed that water could be so volatile on this planet. '
The solar-powered spacecraft Juno was launched in 2011. Through experience with the Galileo probe, the mission seeks to obtain readings of the abundance of water in vast regions of the vast planet. A new kind of instrument for exploring planets in deep space, the Juno Microwave Radiometer (MWR) observes Jupiter from above using six antennas that simultaneously measure air temperature at different depths. The microwave radiometer takes advantage of the fact that water absorbs certain wavelengths of microwave radiation. The measured temperatures are used to limit the amount of water and ammonia in the deep atmosphere, since both molecules absorb microwave radiation.
The Juno science team used data collected during the first eight scientific explorations of Jupiter to produce new results. They were initially concentrated in the equatorial region, because the atmosphere there appears to be more well mixed even at depth, unlike other regions. From its orbit, the radiometer was able to collect data from much deeper into Jupiter's atmosphere than the Galileo probe – 150 kilometers, where the pressure reaches 33 bar.
As a result, scientists found out that there is much more water at Jupiter's equator than it was believed according to the data of the Galileo probe. Now the main task is to compare the obtained results with the analysis of water availability in other regions of the planet.
