365DaysDate: May 2, 2009

Title: It’s the Season for Eclipses in the Jupiter System


Podcaster: Jason Perry

Organization: The Gish Bar Times

Description: In 2009, as it does every six years, the Jupiter system experiences equinox. While Jupiter’s low axial tilt makes the passing of season much less noticeable than on Earth, Mars, or Saturn, equinox does bring a season of eclipses for Jupiter’s moons. From Earth, astronomers can observe these eclipses as well as occultations, when one moon passes in front of another. Today, we will discuss the useful science gained from eclipses and occultations as well as a few of the eclipses coming up in the Jupiter system, focusing primarily on Jupiter’s innermost large moon, Io.

Bio: Jason Perry is an image processing specialist currently working for the Cassini Imaging Team. On the Cassini project, Jason focuses on Saturn’s moon Titan, mapping its surface and examining its geology. Previously, he has been involved with the Galileo and New Horizons missions, focusing on the best planetary body in the solar system (in his opinion), Jupiter’s moon Io. In his spare time, he writes for his Io-centric blog, The Gish Bar Times.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at

Transcript: Hello, I’m Jason Perry of the Gish Bar Times blog and this is the 365 Days of Astronomy Podcast for May 2, 2009. For today’s podcast we will take a look at the changing of the seasons for the Solar System’s two largest planets and why this year is a good time to look at their moons.

As I am recording this podcast, the northern hemisphere here on Earth just a few days ago started spring, a time of rebirth after a long, cold winter. Later this year, Jupiter and Saturn and their respective satellite systems will be going through a similar seasonal change as both planets experience vernal equinox and the start of spring in their northern hemispheres and autumn in their southern hemispheres.

The changing of the seasons can be time of change for many of these worlds, which is particularly pronounced in the Saturnian system. The rotation axis of Saturn is tilted much like the Earth’s, causing pronounced seasonal effects in the planet’s atmosphere. Since Saturn’s rings and most of its major satellites revolve around its equator, transitioning from northern winter to spring can also effect them as well. The rings become much narrower from our prospective here on Earth as we approach the ring plane. From the perspective of a spacecraft like Cassini, currently in orbit around Saturn, the shadows of gentile oscillations within the rings become more pronounced. On Titan, Saturn’s largest moon and the only moon in the solar system with a substantial atmosphere, the changing seasons can bring climatic changes as polar methane lakes fill-up or dry-up, and weather patterns shift. Already we have seen Titan’s mid-latitude clouds in the southern hemisphere form further south than they did earlier in the Cassini mission.

On Jupiter, the effects of the changing seasons are much less pronounced. Its rotation axis is nearly vertical compared to the more tilted axes of Earth, Mars, and Saturn. However, even its small axial tilt of three degrees is enough to create interesting opportunities to observe Jupiter and its four largest satellites: Io, Europa, Ganymede, and Callisto. Every six years during Jovian equinox, astronomers here on Earth can observe two types of “mutual events” in the Jovian system: occultations and eclipses. During an occultation, one moon passes between the Earth and another moon, partially obscuring our view of the latter satellite. During an eclipse, one moon passes between the Sun and another moon, partially plunging the second moon into darkness, similar to a solar eclipse here on Earth when our moon partially or completely obscures the Sun.

The current season of mutual events coinciding with Jupiter’s vernal equinox and Earth crossing Jupiter’s ring plane started in late March and continues for the next several months. Astronomers can use these mutual events to calculate the orbits and positions of these satellites by carefully timing the start and end of an occultation or eclipse.

As you might tell if you have ever visited my blog, my favorite world in the Jovian system is Io. Io is perhaps the most volcanically active world in the solar system with hundreds of active volcanoes on its surface, fueled by the heat produced by huge tides generated by Jupiter. These volcanoes release a tremendous amount of heat from Io’s interior, visible from Earth and by spacecraft as visible and infrared light. We can then monitor the type and level of activity at an individual volcano by measuring the light it emits. One way we can monitor this heat is by observing Io during an occultation. By monitoring the brightness of Io in the infrared as another moon occults it, an astronomer can determine the location of hotspots on the surface by measuring the time when bright infrared sources are occulted then later reappear. For example, University of Wyoming researcher Robert Howell measured Io’s brightness during several occultations during previous mutual event seasons in 1997 and 2003. Using these occultations, Howell could not only determine whether or not Io’s largest volcano Loki was active but also which part of its volcanic depression, or patera, was warm.

Recent developments in adaptive optics technology have greatly improved the spatial resolution of ground-based observations, improving the level of detail that is visible and seemingly reducing the need for making these types of occultation measurements as AO images now rival their ability to resolve fine-scale volcanic events on Io’s surface. However, at the present time, the amount of time dedicated to Io observations at telescopes with this technology, such as Keck in Hawaii or the European Southern Observatory in Chile, is limited to a few nights out of the year, which is not sufficient for a long-term monitoring program of the volcanic activity on Io. Occultations provide astronomers at smaller facilities, such as the Wyoming Infrared Observatory, an opportunity to make contributions to such a program with the use of a high-speed photometer.

In addition to occultations, eclipses provide an opportunity to observe volcanic activity across nearly an entire hemisphere of Io’s surface, darkened as another moon passes in front of the Sun. Observing in the infrared, astronomers could observe heat given off by erupting volcanoes without contamination from sunlight. Eclipses on Io are not uncommon, occurring every Io-day as the satellite passes behind Jupiter. However, this only allows astronomers to observe volcanic activity on one side of Io — the side facing Jupiter. With a mutual event eclipse, astronomers could potentially look at volcanic activity (without contaminating sunlight) on other parts of Io. This year, the most important eclipse on Io might be the Ganymede eclipse that occurs on July 15. This eclipse will plunge much of Io’s trailing hemisphere into darkness, providing an excellent opportunity to check up on activity at volcanoes such as Pele, a vigorously overturning lava lake that at times can produce gaseous plumes of sulfur and sulfur dioxide 300 kilometers tall. Solar eclipses such as this don’t last for very long. Totality on Io during the July 15 eclipse lasts only two minutes.

The changing of the seasons here on Earth is a great time to observe changes in the local plants and animals. For amateur and professional astronomers, the changing of the seasons on the giant planets of the solar system, provide a chance to observe phenomenon that occur once every six years, as at Jupiter, or every 15 years, like at Saturn. Even if you only have a backyard telescope, the next few months provide a great time to view Saturn as we cross the ring plane, or Jupiter as its moons continue their dance around the giant planet.

This is Jason Perry signing off for the May 2nd edition of the 365 days of Astronomy Podcast. For more news and views from Jupiter’s volcanic moon, come check out the Gish Bar Times at

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365 Days of Astronomy
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