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Lightning in the Jovian Clouds

The longer we can keep a mission going in an exotic place, the better. Sometimes longevity is its own reward, as Curiosity has just reminded us on Mars. After all, it was only because the rover has been in place for six years that it was able to observe what scientists now think are seasonal variations in the methane in Mars’ atmosphere. Thus the news that Juno will remain active in Jupiter space is heartening, and in this case necessary. The mission is now to operate until July of 2021, an additional 41 months in orbit having been approved. More time on station allows Juno to complete a primary science mission that had appeared in jeopardy.

The reason: Problems with helium valves in the spacecraft’s fuel system resulted in the decision to remain in the present 53-day orbit instead of the 14-day ‘science orbit’ originally planned, and that has extended the time needed for data collection. Thus the lengthening of operations there not only allows further time for discovery but essentially enables the spacecraft to achieve its original science objectives. NASA has now funded Juno through FY 2022, allowing for the end of prime operations in 2021 and data collection and mission close-out carrying into 2022.

“This is great news for planetary exploration as well as for the Juno team,” said Scott Bolton, principal investigator of Juno, from the Southwest Research Institute in San Antonio. “These updated plans for Juno will allow it to complete its primary science goals. As a bonus, the larger orbits allow us to further explore the far reaches of the Jovian magnetosphere — the region of space dominated by Jupiter’s magnetic field — including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. We have also found Jupiter’s radiation environment in this orbit to be less extreme than expected, which has been beneficial to not only our spacecraft, but our instruments and the continued quality of science data collected.”

In its present 53-day polar orbit, Juno moves as close as 5,000 kilometers from the Jovian cloud tops and backs out as far as 8 million kilometers. It’s an orbit that minimizes exposure to Jupiter’s radiation belts even as it allows the craft to study the planet’s entire surface over the course of its time there. The latest work on data collected during these orbits comes in two papers, one in Nature, the other in Nature Astronomy, that look at Jovian lightning and how it is produced.

The first analysis draws on data from Juno’s Microwave Radiometer Instrument (MWR), which can record emissions at a wide range of frequencies. Because lightning discharges emit radio waves, Juno can keep an eye on lightning activity on the gas giant. Jovian lightning has also been detected by optical cameras aboard spacecraft as localized flashes of light. Shannon Brown (JPL), lead author of the paper on this work, points out that until Juno, the radio signals spacecraft have detected all came from the Galileo probe, Cassini and the two Voyager flybys, but these were all found in the kilohertz range of the radio spectrum, despite attempts to find signals in the megahertz range. The reason for the discrepancy has been a mystery.

After all, terrestrial lightning emits a broad signal over the radio spectrum up to gigahertz frequencies. Juno is helping to resolve the discrepancy, detecting Jovian lightning ‘sferics’ (broadband electromagnetic impulses) at 600 MHz. That implies that the planet’s lightning discharges are not fundamentally distinct from the lightning we experience on Earth. During Juno’s first eight orbits of Jupiter, the spacecraft detected 377 sferics, finding them prevalent in the polar regions and absent near the equator, with the most frequent occurring in the northern hemisphere at latitudes higher than 40 degrees north.

”We think the reason we are the only ones who can see it is because Juno is flying closer to the lighting than ever before,” says Brown, ”and we are searching at a radio frequency that passes easily through Jupiter’s ionosphere.”

But what would account for the fact that Earth’s lightning activity is highest near the equator, while Jupiter’s is most frequent in the polar regions? Brown and company suggest that Jupiter’s poles allow more warm air to rise from within because there is less upper-level warmth from sunlight. Possibly the heating from sunlight at Jupiter’s equator can stabilize the upper atmosphere to inhibit warm air rising from below as it does at the poles. If this is the case, we would expect the polar regions to experience the convective forces that lead to lightning.

From the paper’s abstract:

Because the distribution of lightning is a proxy for moist convective activity, which is thought to be an important source of outward energy transport from the interior of the planet, increased convection towards the poles could indicate an outward internal heat flux that is preferentially weighted towards the poles. The distribution of moist convection is important for understanding the composition, general circulation and energy transport on Jupiter.

Image: This artist’s concept of lightning distribution in Jupiter’s northern hemisphere incorporates a JunoCam image with artistic embellishments. Data from NASA’s Juno mission indicates that most of the lightning activity on Jupiter is near its poles. Credit: NASA/JPL-Caltech/SwRI/JunoCam.

What scientists now have to explain, as this JPL news release points out, is why the north pole is so much more active than the south. Our understanding of energy flow and circulation on Jupiter is clearly a work in progress, something the Juno data trove may help us untangle. Meanwhile, Ivana Kolmašová (Czech Academy of Sciences, Prague) and colleagues have offered what NASA is calling ‘the largest database of lightning-generated low-frequency radio emissions around Jupiter (whistlers) to date.’ The dataset includes more than 1600 signals collected by Juno’s Waves instrument, 10 times the number recorded by Voyager 1.

We’re not only further along in detection technology than in the Voyager days, with advances in microwave and plasma wave instruments to sense lightning amidst Jupiter’s emissions, but we’re also dealing with a spacecraft that has come closer to Jupiter than any other craft in history, allowing a vast increase in signal strength. The knowledge that Juno will now be able to proceed through its entire primary data collection mission is thus a cause for celebration.

The papers are Brown et al., “Prevalent lightning sferics at 600 megahertz near Jupiter’s poles,” Nature 558 (2018), 87-90 (abstract); and Kolmašová et al., “Discovery of rapid whistlers close to Jupiter implying lightning rates similar to those on Earth,” Nature Astronomy 6 June 2018 (abstract).

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{ 12 comments… add one }
  • ljk June 8, 2018, 16:13

    Why Are the Galilean Moons So Massive?

    By: Alexander Hellemans | June 4, 2018

    Scientists propose that Saturn’s meddling helped create the four giant Galilean moons orbiting Jupiter.

    http://www.skyandtelescope.com/astronomy-news/why-galilean-moons-so-massive/

  • Brett Bellmore June 9, 2018, 9:06

    I continue to believe that we really need a long duration atmospheric probe for Jupiter. Maybe an isotopically heated hot ‘air’ dirigible, or some form of glider.

    There’s no reason such a probe couldn’t be active in the Jovian atmosphere for years on end, taking readings and photographs, and transmitting the data to a relay in orbit.

    • J. Jason Wentworth June 10, 2018, 13:07

      I agree–for meteorological, geophysical (“joviophysical”), as well as biological purposes (the sinkers, floaters, and hunters from “Cosmos” could exist there), we need to place long-life automatic interplanetary stations (as the Russians call probes) in Jupiter’s atmosphere–and imagine the camera views! It’s possible that an aerostat of some kind (perhaps a powered one, such as a blimp) might use electric ducted fan propulsion. If the vehicle alternately descended and ascended either vertically or at slants (not too low & hot, or too high & cold), the ducted fans might double as generators to recharge the batteries, and:

      Jupiter’s day is only about ten hours long, so the heated hydrogen envelope might be made dark to absorb heat from the sunlight and from below. In order to stay aloft, the aerostat, whether it was self-propelled or a free balloon, could be made like the MIR–montgolfière infrarouge–scientific balloons, see: http://www.google.com/search?source=hp&ei=g1cdW6KpB8ea0gLq4KfYBg&q=montgolfière+infrarouge&oq=montgolfiere+infra&gs_l=psy-ab.1.0.0i22i30k1.1263.8432.0.11673.18.18.0.0.0.0.185.1965.7j11.18.0..2..0…1.1.64.psy-ab..0.18.1962…0j0i131k1j0i10k1.0.qFVuj27Oi3s ), which operate–in the thermal sense–rather like greenhouses, in order to stay aloft day and night (MIR balloons have circled the Earth several times).

      • Brett Bellmore June 11, 2018, 6:05

        Just my opinion, but the solar heated balloon concept is a bad idea for Jupiter. Jupiter has *clouds*. You don’t want a probe which irreversibly goes down if shaded too long, especially when you might want to enter those clouds to sample them.

        I picture it using the waste heat from an isotopic generator to heat the gas bag, so that you have vertical control even in the dark, and a reliable source of power. You could design it so that a control failure caused it to go up, rather than down, and thus salvage the mission.

        If the isotopic power supply is too expensive or politically fraught, a glider could stay aloft for a very long time if it stuck to one of the upwelling regions, and still extract a bit of power by means of a prop connected to a small generator. But the failure modes here are not as forgiving.

  • Erik Landahl June 10, 2018, 0:21

    “a spacecraft that has come closer to Jupiter than any other craft in history,”

    Galileo got pretty close. :)

    • Paul Gilster June 10, 2018, 11:38

      Close indeed!

      • J. Jason Wentworth June 10, 2018, 13:10

        I guess they meant, “…and survived to report on its close encounter with Jupiter.” We’ll find out how close that will be for the upcoming “solar scraper” Sun-orbiting probe…

  • J. Jason Wentworth June 10, 2018, 5:58

    I presume (the JPL press release didn’t mention the difference) that they’re referring to Jupiter’s geographic (joviographic?) north pole, which is actually the planet’s magnetic south pole (a magnet on Jupiter would point south, as Pioneer 10 found). Also:

    The illustration of Juno’s distant-perijove, highly-eccentric elliptical orbit in *this* http://spaceflightnow.com/2018/06/08/nasa-approves-three-year-extension-for-juno-mission-orbiting-jupiter/ article raises a question. Since Juno might pass within worthwhile observation distances (and perhaps very close) of Galilean *and* outer satellites (and possibly Amalthea, Metis, etc., too), are there any plans for “targets of opportunity” satellite flyby observations?

    • ljk June 11, 2018, 9:42

      If they want a “target of opportunity” then Juno should be used to this during its closest passes – which would have been more frequent had the original orbiting plan taken place:

      https://www.centauri-dreams.org/2009/02/25/edwin-salpeter-and-the-gasbags-of-jupiter/

      Quoting from the above:

      “The paper also noted that the two Voyager space probes scheduled to make flybys of Jupiter in 1979 had cameras powerful enough to just resolve the floaters if such beings existed at that world and were high up enough to be spotted. Whether the images of Jupiter’s swirling, colorful face that were later returned by the twin robot explorers were ever scrutinized for any floaters remains unknown.”

      Juno has superior cameras and is hanging around the gas giant world, even if it is often further than expected. The space probe team wasn’t even going to include a camera at first, an amazing lack of vision and public relations in this day and age. Since the JunoCam is often run by citizens, I say some of them should start scrutinizing the planet’s clouds for the equivalent of floaters.

  • Michael June 10, 2018, 23:26

    I would not be surprised if Io was involved, there is a large current conduit between them.

  • ljk June 13, 2018, 11:57

    How Jupiter Is Helping the Hunt for Habitable Alien Worlds

    By Nola Taylor Redd, Space.com Contributor | June 11, 2018 07:00 am ET

    DENVER — Jupiter itself isn’t a great candidate to host life [oh, really?], but the gas giant is playing a significant role in the search for habitable worlds.

    In a planetary-science first, astronomers used their knowledge of Jupiter’s magnetic field to model what kinds of radio signals might be emitted naturally by the fields of smaller, rocky worlds.

    “If we can get a handle on how to find direct radio emissions from large exoplanets, we can then eventually use these same techniques to study Earth-sized planets and determine which ones have magnetic fields,” Jake Turner, a doctoral student in the Department of Astronomy at the University of Virginia, said in a statement.

    https://www.space.com/40839-search-alien-life-exoplanets-jupiter-magnetic-field.html

  • ljk June 13, 2018, 12:05

    Are There Enough Chemicals on Icy Worlds to Support Life? – Universe Today

    https://www.universetoday.com/139399/are-there-enough-chemicals-on-icy-worlds-to-support-life/

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