10th Anniversary of Galileo's I27 Encounter

I almost completely forgot about this, but today (for another 2 minutes here) is the tenth anniversary of the Galileo spacecraft's I27 flyby of Io.  This encounter occurred on February 22, 2000 and was the third of three flybys of Io that took place over a four-month period between October 1999 and February 2000.  The closest approach altitude was 198 kilometers.  It was the fourth Io flyby overall by Galileo (including the one right before orbit insertion) and took place during the spacecraft's 27th orbit of Jupiter.  Galileo went on to perform three more flybys of Io between August 2001 and January 2002.

Compared to the first two flybys of this "Io campaign" section of the Galileo Europa Mission (the spacecraft's first extended mission), when spacecraft safe-mode events and instrument malfunctions limited the science return, the I27 flyby was relatively problem free.  All of the planned Io observations were acquired.  However, this did caused problems when it came to playing all that data back, as more images were planned than could be downlinked with the expectation that some data might be lost due to some spacecraft malfunction, like the other two encounters.  In which case, mission planner carefully selected which data to playback, in some cases skipping entire frames or only playing back a portion of others.  That was the case for the CHAAC_01 observation, a very high-resolution mosaic across Chaac Patera (a portion of which is shown above).  Another good example is the SOPOLE01 observation, a four-image, medium resolution mosaic that covered the southern mid-latitudes of the anti-Jupiter hemisphere, where only the first frame over Telegonus Mensae was played back in close to its entirety, while two others had 100 lines out of 800 played back.  These kinds of limits were seen in many of the SSI observations from this flyby, and that's even with three months between I27 and the next flyby, this time of Ganymede, G28.

For the other instruments on Galileo, I27 was a very exciting, with high resolution, Near-Infrared observations of Pele, revealing the location of the majority of the volcanoes thermal emission (which would be targeted by the camera in October 2001), coverage over the Chaac Patera and Camaxtli Patera region, which led to a confirmation of the connection between dark patera floor terrain and thermal emission as well as a patera covered in nearly pure sulfur dioxide ice (Balder Patera), and observations of the Prometheus and Amirani flow fields, showing the detailed thermal structure of these features.

For the camera, check out my I27 image page to see the images Galileo took during the flyby.  Among the highlights include the highest resolution images acquired of Io (covering layered terrain in Bulicame Regio, broken up by a dark promontory, a jagged fissure, boulders, and sapping channels), a 12-frame mosaic over the Chaac-Camaxtli region, and a high-resolution mosaic across the Prometheus flow field and the hummocky terrain that surrounds it. Mosaics of the Amirani and Prometheus lava flow fields showed changes since they were imaged in October 1999, providing the first direct measure of the lava coverage rate at these two prominent features.  So even though many images were only partially played back or not played back at all, an amazing array of gorgeous terrain is seen in the images that were returned.

With that, I want to leave you with a nice simulation from Celestia showing an animation of the I27 flyby (note the fly over of the Zamama plume):



Link: Galileo I27 Images [pirlwww.lpl.arizona.edu]

Animation of the Jupiter Europa Orbiter's Four Io Flybys

Yesterday, while finishing up work on the Exploration of Io article on Wikipedia and looking for a graphic for the Jupiter Europa Orbiter (JEO), I noticed that a SPICE trajectory file for JEO had been posted online. The Jupiter Europa Orbiter is NASA's portion of the Europa/Jupiter System Mission approved last year. I can use the SPICE kernel to display the position of the spacecraft at a given time in Celestia.  The trajectory file covers the Jupiter tour portion of the current mission baseline, though obviously a number of factors between now and the arrival of JEO at Jupiter will cause changes to this baseline, including changes in the launch date (assumed as February 2020 with an arrival at Jupiter in December 2025).  So, as of right now, these provide more of an example of the types of flybys JEO can perform at Io.

I've created a little video and uploaded it to Youtube with animations from Celestia simulating each of the four Io flybys in the current tour.  You can see some of the highlights from each encounter, though note that no science is planned for the first encounter (Io-0) as it takes place right before JEO's all-important, Jupiter Orbital Insertion burn.  For Io-1, on July 9, 2026, JEO passes almost directly over the Amirani plume, and depending on how high gases from the volcano reach, it could directly sample the composition of it.  JEO can also image the north polar region of Io from an oblique angle. However, Amirani's plume is ~75 km tall, while the altitude for Io-1 is 300 km.  For Io-2, on September 3, 2026, JEO should be able to image the Pele volcano and south polar region at high resolution. In the latter case, this is the section not covered by Voyager 1 at medium resolution.  The Zamama, Marduk, Prometheus, and Pele plumes will also be visible along the bright limb near C/A.  Finally, for Io-4, JEO will pass directly over Tohil Mons providing a chance to obtain laser altimetry over a mountain we have pretty good stereo coverage already, providing a useful comparison.

I wrote up a more detailed article on the potential science from each of these encounters last February.



I hope you all enjoy!  I should point out that this video was uploaded at 720p so you can view it in high-definition, and this definitely works better if you view it full screen.

Link: Animation of the Jupiter Europa Orbiter's Four Io Flybys [www.youtube.com]

Video of Lava Pond Draining Event/Haiti

A few weeks ago the USGS's Hawaiian Volcano Observatory released an incredible video of lava draining out of a lava pond at Kilauea's Halema`uma`u vent (an embedded Flash version can be found on the Big Island Video News website).  At the beginning of the video, lava mostly fills the pond (a depression filled with mostly molten lava that is NOT connected directly to the underground source of the lava, unlike a lava lake), but as the lava drains out to the right, more of the shelf surrounding the pond becomes visible until lava starts cascading over that shelf.  Known lava ponds on Io include Pillan Patera during the 1997 eruption, when lava from a fissure eruption 75 kilometers to the north of Pillan flowed over the edge of the patera, and covered its floor.

While we ooo and ahh at the powerful forces of nature, like this beautiful lava pond, these same geologic forces on Earth, volcanism and tectonism, can cause tremendous suffering for the planet's inhabitants.  This can be clearly seen this week in the island nation of Haiti, where a simple break along a major strike-slip fault on the boundary between the Caribbean and North American Plates has caused a massive loss of life and tremendous suffering for the survivors in the area around Haiti's capital of Port-au-Prince.  I encourage my readers to donate some of their resources to the charity of their choice that they feel can make a difference down in the earthquake zone.  One important thing to remember that in the days and weeks to come, the survivors of this quake will be facing a second disaster, one of disease and infection as a result of a complete loss of what sanitation and infrastructure they did have and the injuries they suffered from falling debris during the quake and its aftershocks.  My favorite charity, Doctors without Borders will be working on the ground there in Haiti to try to mitigate that potential second disaster.  Consider a donation to that wonderful organization, or the International Red Cross/Red Crescent, or check out the CNN.com website for a list of worthwhile and trustworthy charities.

Thanks!

New Videos Showing Topography in the Jupiter System

Paul Schenk has posted some new, high-definition videos on Youtube showing the topography of various surface features across the four Galilean satellites.  These videos were created for a conference/celebration going on in Padua starting today, the IAU Symposium 269: Galileo's Medicean Moons - Their Impact on 400 Years of Discovery.  Schenk's videos can be found online at his Youtube channel, GalSat400.  These videos are based on photoclinometry and stereo imaging derived from Galileo images.  Photoclinometry uses changes in the topographic shading of a features to determine local slopes, and thus topography.  Stereo imaging uses pairs of images taken at different viewing angles to determine topography.  The latter example are often depicted as stereo pairs or anaglyphs.  For Io, these videos include flyovers of Tohil Mons based in part on a high-resolution mosaic of mountain from Galileo's flyby of Io during orbit I32, Hi'iaka Montes from I25 data, and the Pillan flow field from I24 data (not posted yet but I definitely look forward to Paul getting that online).  For your convenience, those videos are also below.  Paul has also post videos using high-resolution Galileo mosaics for the other Galileans.  Of these other videos, I kinda like the flyover of Manannan crater on Europa.  This relatively small impact crater looks crater-ish from a distance, but up close, when you look a the topography of the feature, you can see that the thin ice shell and ductile layers of that shell have mangled that crater out of all recognition.

Definite check these videos out!


Tohil Mons, Io (high-resolution)


Liftoff from Hi'iaka Montes

Link: Youtube - GalSat400's channel [www.youtube.com]
Link: Galileo and Padova - 1610-2010 Celebration (New Videos) [stereomoons.blogspot.com]

Animation of Galileo's I24 Flyby of Io

Over the last few days we've been looking back at the Galileo's October 11, 1999 flyby of Io that occurred ten years ago this past Sunday.  We've looked at the planning that went into the science observations as well as some of the issues encountered during flyby.  Today, I wanted to present a video I created using Celestia and edited in Adobe Premiere Pro.  This combines a simulation of the flyby with some of the actual data that was returned during each observation.  And by ALL MEANS, full-screen this video!

The music used in this video is by _Ghost from ccMixter.com.  The two samples used, Ice and Chilli and Low (Ghostrust Reflection), are available under the Creative Commons NonCommercial Sampling Plus 1.0 license.



I had hoped to have this animation done yesterday, but what are you going to do? I hope you all enjoy!

Link: Galileo's I24 Flyby of Io [www.youtube.com]

July 22 Eclipse of Io by Ganymede

Later this morning, Io's trailing hemisphere will experience a total solar eclipse when Ganymede passes between Io and the Sun. The eclipse runs roughly from 13:34 to 13:39 UTC (14:08-14:14 UTC as seen from Earth). The video below was created in Celestia and shows the eclipse both from above Io, showing the shadow of Ganymede cross Io's surface, and from the surface of Io, showing Ganymede pass in front of the Sun.


Computer Animation of the total eclipse of the Sun by Jupiter's moon Ganymede over the trailing hemisphere of Io on July 22, 2009. First half shows a view from 4500 miles above Io's trailing hemisphere. Second half zooms in on the sun from an unnamed volcanic pit showing the total eclipse. The animation runs from 13:30 to 13:40 UTC on July 22, 2009.

EDIT 07/22/2009 10:06 AM: Fixed the title of the article, changing Jupiter to Ganymede. Obviously, a Jupiter eclipse is nothing special. A Ganymede one is.

Ganymede Eclipse on Io Wednesday Morning

Tomorrow brings Io's most interesting eclipse this mutual event season as most of Io's trailing hemisphere (51° West-231° West) is plunged into darkness by Jupiter's largest moon, Ganymede. This is the culmination of a series of weekly eclipses by Ganymede on Io. With each weekly eclipse, the center of Ganymede's shadow appears further south on Io. Tomorrow, the center of Ganymede's shadow passes just north of Io's equator. The eclipse takes place tomorrow morning, July 15, between 10:45 and 10:50 UTC (3:45-4:50 MST) on Io. If you have a good telescope and want to try to observe this event, from Earth the penumbral shadow of Ganymede will reach Io at 11:19 UTC, totality will run from 11:21 to 11:25:28 UTC, and end of the eclipse comes at 11:27 UTC. The peak of the eclipse, as observed from Earth, comes at 11:23:14 UTC. The timing of this eclipse should make it a good observation target for observers in the western United States, western South America (like the European Southern Observatory), and Hawaii.

During the eclipse, Ganymede will appear 13' 17.2'' across in Io's sky (compared to our moon, which appears around 30' across in Earth's sky). The sun will appear 6' 19.9". Therefore, it is unlikely that the sun's corona would be seen during the eclipse except near the beginning and end. At its peak near Tol Ava Patera, the eclipse will last 1 minute and 55 seconds long.

For this eclipse, I've created a little fancier video using Celestia and Adobe Premiere. I think I am starting to get along with that latter software package...


Computer Animation of the total eclipse of the Sun by Jupiter's moon Ganymede over the trailing hemisphere of Io on July 15, 2009. First half shows a view from 4500 miles above Io's trailing hemisphere. Second half zooms in on the sun from east of Ra Patera showing the total eclipse. The animation runs from 10:40 to 10:55 UTC on July 15, 2009.

I have also created a nice map showing the area on Io that will experience this eclipse. You can download a full-res version here.

My post last year on this mutual event season should help provide some information on the science of these types of eclipse as well as occultations.

30th Anniversary of the Voyager 2 Flyby of Jupiter

Thirty years ago today, on July 9, 1979, the Voyager 2 spacecraft encountered Jupiter from a distance of 650,000 kilometers about its cloudtops, marking the second Voyager project flyby of the planet. The encounter provided an opportunity to see the anti-Jupiter hemispheres of Ganymede and Callisto, to monitor changes on Jupiter and Io since the Voyager 1 encounter four months earlier, and to observe Europa up close for the first time.

Back in March, we took an extensive look at the Voyager 1 encounter with Jupiter and Io. The Voyager 1 flyby provided a revolution in our understanding of the giant planet and turned the four Galilean satellites from mere points of light we were only beginning to understand into four separate worlds, each with their own unique geologies. In particular, during the Voyager 1 encounter, active volcanism was observed on Io as well as a narrow ring around Jupiter.

For Io, the second Voyager encounter did not provide the same revolution in our understanding of that world; the clear star of the July 9 encounter was the cracked world of Europa. While Voyager 1 flew within 20,000 kilometers of Io on March 5, 1979, Voyager 2's trajectory kept the spacecraft outside Europa's orbit and, with Io on the other side of Jupiter during closest approach, Voyager 2 never came close than 1,128,000 kilometers of Io. However, the discovery of active volcanism on Io by Voyager 1 necessitated a change in the schedule of observations for the second encounter, including a 8-hour long sequence of images of a narrowing crescent Io as Voyager 2 receded from Jupiter. This prolonged observation sequence allows Voyager 2 to monitor volcanic plumes along Io's limb, including those at Amirani, Maui, and Loki. From observations such as these, Voyager 2 found that most of the plumes first observed by Voyager 1 were still active during the July 1979 encounter. Only Pele appeared to have shut down, though later observations by Hubble, Galileo, and other spacecraft seem to suggest that the Pele plume is intermittently active. Volund was not observed near the limb of Io during the Voyager 2 encounter, so it could not be determined if that plume was still active.

Earlier, Voyager 2 had observed Io while it was more illuminated by the Sun, like the image at left. The goal of these observation was to search for surface changes on Io as the result of volcanic activity between the two Voyager encounters. Among the changes observed were two new plume deposits surrounding Surt (probably active during a short, intense eruption in June 1979, but Surt was not viewed near the limb by Voyager 2 to see if it was still active) and Aten Patera. The presence of these changes from large plumes but without the observation of the plumes associated with them have led to the conclusion that large, Pele-type plumes tend to be short-lived compared to more persistent, smaller dust plumes like at Prometheus. Additional surface changes included a larger dark area within Loki Patera, the result of overturning of more surface area of the lava lake that covers much of Loki, and a change in the shape of the Pele plume deposit, from a heart shape to an oval. Subtle changes in the shape and intensity of the Pele plume deposit were also observed by Galileo in the late 1990s and early 2000s.

Of course, Io wasn't the only world Voyager 2 observed during its encounter 30 years ago. The star of the show was Europa. Europa was poorly observed during the previous Voyager encounter so this one really provided a great leap in our understanding of this icy world. Like Ganymede, Europa's surface was dominated by tectonic structures, ridges and dark, linear bands that criss-cross the surface. Unlike Ganymede, Europa's surface was found to be quite young with very few impact craters, though enough to show that the satellite's surface was much older than Io's. Spectral measurements suggesting a water ice surface, mass estimates between that of Io and Ganymede, and youth surface age soon led to the suggest that just beneath Europa's ice shell lay a liquid water ocean that today is a main focal point for exploration in the Jupiter system.

Voyager 2 never got the same amount of attention that the earlier Voyager 1 encounter did. During the same week, Skylab was slowly approaching its destruction over Australia, dampening press interest in the encounter, along with the perception that this encounter was covering similar territory as the previous one. But Voyager 2 provided an opportunity to follow up on the discoveries made by Voyager 1 by allowing for an adjustment to the observation plan, such as to monitoring Io's volcanic plumes and Jupiter's narrow ring system as Voyager 2 receded from the giant planet. Voyager 2 also allowed imaging scientists to fill out the global map of Ganymede and Callisto by observing their anti-Jovian hemisphere and providing the first close-up look of Europa. The Voyager 2 encounter unfortunately also began a 17-year gap in close-up spacecraft imaging of the Jupiter system. But Voyager 2 went on to bigger and better things, including doing followup observations of the Saturn system in August 1981 as well as our only encounters of Uranus and Neptune in 1986 and 1989, respectively.

For this post, I have posted some movies on Youtube created in Celestia showing the geometry of this encounter:

• Voyager 2 Trajectory through the Jupiter System
• Animation of Io as seen from Voyager 2 in July 1979
• Animation of Io as seen from Voyager 2 in July 1979 - Short Version
  

Over at UMSF, Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston, Texas, shared some of his memories of the Voyager 2 encounter.

Today's Eclipse of Io by Ganymede

Sorry I haven't covered the last two eclipses (June 24 and July 1), but taking a look at today's eclipse of Io by Ganymede we see as Ganymede's shadow is now reaching further south across Io's trailing hemisphere. The eclipse will take place in about 25 minutes from the time I am typing this, but you can still see an animation of it in the Youtube video I uploaded and embedded below:


Computer Animation of the total eclipse of the Sun by Jupiter's moon Ganymede over the trailing hemisphere of Io on July 8, 2009. First half shows a view from 7,317 km above Io's northern trailing hemisphere. Second half zooms in on the sun from the floor of Loki Patera showing the total eclipse. The animation runs from 07:30 to 08:20 UTC on July 8, 2009.

Today's Eclipses of Io by Callisto

Here are two videos showing today's total eclipses of the Sun by Callisto on Io's south polar region:


Computer Animation of the first of two total eclipses of the Sun by Jupiter's moon Callisto over the south polar region of Io on June 20, 2009. First half shows a view from 7,317 km above Io's southern trailing hemisphere. Second half zooms in on the sun from near Kurdalagon Patera showing the total eclipse. The animation runs from 04:10 to 04:56 UTC on June 20, 2009.


Computer Animation of the second of two total eclipses of the Sun by Jupiter's moon Callisto over the south polar region of Io on June 20, 2009. First half shows a view from 7,317 km above Io's southern trailing hemisphere. Second half zooms in on the sun from near Svarog Patera showing the total eclipse. The animation runs from 08:40 to 09:20 UTC on June 20, 2009.

An earlier partial eclipse occurred on June 19 over Io's southern leading hemisphere as well.

Eclipses of Io by Ganymede on June 16

Tomorrow (June 16), Io will experience three solar eclipses by its larger neighbor, Ganymede. Like those that took place last week, these eclipses are mostly visible over the north polar region of Io, though totality could be observed much further south than those last week (I say could because, obviously, no one is there to experience them). Luckily, thanks to software like Celestia, we don't have to let a little thing like a few hundred million kilometers stop us. Below is a video of an eclipse over the northern part of Io's anti-Jovian hemisphere that will take place in about three hours (1:08 am MST). Yes, like the nerd I am, I will watch it live on Celestia...

And completely off topic for a second, I know I have a few Iranian readers. All I want to say is good luck and stay safe! The world is pulling for ya!



In addition to this eclipse, there was an eclipse a few hours ago (5:52 pm MST) over the northern leading hemisphere and there will be one later on June 16 (3:39 pm MST) over the northern trailing hemisphere.

Another Total Eclipse by Ganymede Tomorrow

I completely forgot in my last post that there are TWO total solar eclipse by Ganymede tomorrow, in addition to the one around 08:50 UTC. The second, to occur around 18:50 UTC, will also cover the north polar region a little further to the west than the earlier eclipse, over the trailing hemisphere side of the north pole. Below is an animation of that eclipse:


Computer Animation of a second total eclipse of the Sun by Jupiter's moon Ganymede over the north polar region of Io on June 9, 2009. First half shows a view from 7,317 km above Io's northern trailing hemisphere. Second half zooms in on the sun from near the north pole showing the total eclipse. The animation runs from 18:30 to 19:15 UTC on June 9, 2009. This eclipse occurs nine hours after another total eclipse by Ganymede over a region to the east.

30th Anniversary of the Voyager 1 flyby of Io

30 years ago today, the Voyager 1 spacecraft encountered the first planet in the Voyager programs decade-long epoch tour of the outer solar system. While Pioneer 10 and 11 were the first space probes to reach Jupiter, proving passage through the asteroid belt was possible, Voyager 1 opened up the worlds of Jupiter system to humanity, turning these moons from mere points of light into fully-realized worlds.

Voyager 1 flew within 19,000 km of Io, making it the best resolved world during the spacecraft's passage through the Jupiter system. The world Voyager 1's cameras revealed was unlike anything planetary scientists. Instead of being dead with numerous impact craters, like our own moon, they found a world with strange pits, flow terrain, an incredible diversity of albedo markings, and mountains 10-20 km tall. Global mosaics revealed a dark, horseshoe shaped pit, an albedo marking shaped like a hoof-print (in keeping with the myth of Io, a lover of Zeus who was transformed into a cow to protect her from Zeus's wife, Hera).

Animation of the Voyager 1 Flyby of Io on March 5, 1979

Voyager 1's highest-resolution images covered the southern pro-jovian hemisphere of Io, an area not seen again at high resolution (Galileo's best images of the region were on the order of 10-20 km/pixel). You can see my attempt to piece together Voyager 1's highest resolution mosaic, though I can't seem to find where I put the original project files... These images revealed numerous large, volcanic pits, low scarps that cross Io's generally flat terrain, and large mountains. But not a single impact crater, unlike any other solid-body world in the solar system imaged to that point.

Recent volcanic activity on Io would explain the lack of impact craters on its surface. Within a short time after a crater forms, the feature is filled in with lava and pyroclastic material. An explanation for these amazing images came in the form of a paper published in the journal Science only three days before the Voyager encounter by Stan Peale, Patrick Cassen, and R. T. Reynolds. They suggested that the varying tidal pull on Io by Jupiter, created by the orbital resonances with the other Galilean satellites, would heat up and partially melt Io's interior. This internal heat would then be released in the form of volcanic activity.

This only left the question of whether Io was still active.

Voyager 1 provided a wealth of information about not just Io, but the other moons in the Jovian system as well as Jupiter itself. Images revealed three additional small moons of Jupiter (later named Metis, Adrastea, and Thebe) orbiting near Amalthea, as well as narrow ring of material confirming a prediction made by Acuña and Ness 1976 based on Pioneer charged particle data. Voyager 1's cameras also revealed in detail the cratered and grooved surface of Ganymede and the ancient surface of Callisto. The spacecraft never came very close to Europa and could only hint at the amazing surface features seen by Voyager 2 four months later and Galileo 16 years later. Voyager imaging scientists did note a lack of obvious impact features as well as dark, linear streaks criss-crossing Europa's surface.

The Voyager 1 flyby of the Jupiter system 30 years ago today truly opened up the moons of the outer planets to humanity. The robotic explorer transformed the four Galilean satellites from points of light in the sky to worlds with geology and amazing vistas. The discoveries made at Io, of a world of abundant active volcanism, were perhaps the most amazing of a truly incredible encounter. Later this week, we'll look back at the discovery of active volcanism on Io and on the post-Voyager view of Io's interior and how it compares to our view now.

Astronomy on Io

With this being the International Year of Astronomy, I thought this would be a good time to do a post on Astronomy on Io. Obviously, there are no astronomers on Io (not yet anyway), but perhaps it would be insightful to take a look at a simulated view of Io's night sky and to explore the wonders one could see with the naked eye and through a telescope if one were on the satellite's surface (assuming one were protected from the elements).

Our make-believe observatory is located with in the Gish Bar Patera caldera, just south of the primary flow field, in an elevated area on the patera floor. Locally, the ground is flat and yellowish in color with 500-meter tall cliffs visible to the south curving around to the west and east. A 9-km-tall mountain would be visible to the north, reaching five degrees into the sky. Hopefully, though, for most of your observing, you wouldn't just be standing around outside, with all the radiation...

Jupiter

The largest object in our simulated sky is the planet Jupiter, a scant 350,000 km away. Because Io is tidally locked to Jupiter, much like our own Moon, Jupiter remains motionless in the sky, always resting on the eastern horizons from our observation point at Gish Bar Patera. Jupiter subtends almost 20° in Io's sky, 40 times the size of the Moon in Earth's sky. To give you an idea, an outstretched fist measures about 10° across, so Jupiter would appear 2 fists across. The great size of Jupiter in the sky would make Io an excellent platform to watch Jupiter's cloud formations as Jupiter rotates a little more than four times over the course of an Ionian day.

Like our own Moon, Jupiter goes through phases. At Gish Bar, "New Jupiter" occurs at daybreak. In fact, daybreak is delayed by more than an hour because Jupiter eclipses the Sun when the Sun would be rising above the horizons. Also, because of the presence of Jupiter on the eastern horizon, sunrise at Gish Bar would seem remarkably like sunrises on Earth, minus the beautiful colors in the sky. The sun's rays would be refracted through Jupiter's upper atmosphere as the day's eclipses draws to a close, bathing the landscape in reddish light. "First quarter Jupiter" would occur near noon at Gish Bar, "Full Jupiter" would occur just before sunset, and "Third quarter Jupiter" would occur near local midnight.

Io's orbit is not a perfect circle, so like our Moon in Earth's sky, Jupiter would very subtly grow and shrink in Io's sky over the course of a day. Currently, Io's closest point in its orbit around Jupiter (called perijove) occurs shortly after sunset at Gish Bar, when Jupiter is just past full phase. Apojove, when Io is furthest from Jupiter, occurs shortly after sunrise (again, presuming Jupiter weren't there to block the actual sunrise). The difference in distance between perijove and apojove amounts to a 1% difference in the size of Jupiter over the course of an Ionian day. In addition to these size differences, libration over the course of an Ionian day causes Jupiter to appear to rock slightly. This "rocking" is on the order of 4°.

Because of the size of Jupiter in the sky, and the brightness of its cloudtops, it would be easier to observe Io's night sky when Jupiter is at a high phase angle, when only a crescent is visible or less, such as during the middle of an eclipse, when the ground would also be dark from the lack of sunlight. While there would be no atmospheric scattering to prevent you from seeing the stars, moons, and planets in the sky, light from the Sun and light reflected from the ground and Jupiter would make it difficult to dark adapt your eyes, making all but the brightest of stars invisible to the naked eye.

Moons

While Jupiter is known to have at least 63 natural satellites, only eight of these would be visible to the naked eye from Io's surface: Metis, Amalthea, Adrastea, Thebe, Europa, Ganymede, Callisto, and Himalia.

Four satellites orbit Jupiter inside the orbit of Io. These moons - Metis, Amalthea, Adrastea, and Thebe - are all much smaller than Io and would appear as bright points of light in the sky. Amalthea, the largest of these inner moons, is big enough that the keenest eyes might be able to see its elongated shape while it transits across Jupiter. Each moon would rise in the east already transiting across the face of Jupiter and would set behind Jupiter several hours later. Because these moons orbit interior to Io, they would never stray very far from Jupiter, only 7° in the case of Metis and Adrastea, 15° for Amalthea, and 20° for Thebe.

Europa, the next moon out from Io, is the second largest object in Io's sky when it is closest to Io. From Gish Bar Patera, Europa would appear to rise above Jupiter shortly after perijove and would set one Ionian day later. At its furthest - just after conjunction and it rises above Jupiter - Europa is more than 1.075 million km away from Io and is only 10 arcminutes across in the sky, one-third the size of the Moon in Earth's sky. At its closest - at opposition before setting - Europa is only 256,000 km away and is 41.5 arcminutes across, one-third larger than the Moon in Earth's sky. Currently, Europa rises and sets shortly after sunset, though this shifts in time over the course of a Jovian year. Currently, Europa appears as a very thin crescent when it is at its largest but in three years, Europa will be nearly full at opposition. For the most part, from Gish Bar, the sub-Jovian hemisphere would be seen, though a bit of the anti-Jovian hemisphere (such as the ray crater Pwyll) would be visible when Europa was high in the sky.

Ganymede is the next largest object in Io's sky when it is closest to Io. From Gish Bar Patera, Ganymede would appear to rise above Jupiter and sets about 26 hours later. Because of the 4:1 resonance between their orbits, Ganymede rises and sets to a fairly predictable pattern. For example, on January 6 at 12:15 UTC, Ganymede will rise from behind Jupiter shortly after Io has reached the closest point in its orbit to Jupiter. At this point, Ganymede is 1.478 million km from Io and is 12 arcminutes across in Io's sky, about 2.5 times smaller than our Moon in Earth's sky. This occurs shortly after sunset on Io and Ganymede is just past full. Ganymede sets 26 hours later nearing half phase during the late morning hours at Gish Bar. The largest satellite in the solar system would be 644,000 km away and would appear 28 arcminutes across, slightly smaller than the Moon in Earth's sky. 28 hours later, a crescent Ganymede rises above Jupiter, and 26 hours later, at Io's perijove, a thin crescent Ganymede sets shortly after sunset. Another 30 hours later, on Jan. 11 at 05:09 UTC, a half-phase Ganymede rises above Jupiter shortly before noon and 26 hours later Ganymede sets a little past half phase. 30 hours later, on Jan. 13 at 13:30 UTC and a full Ganymede after it began, the cycle repeats as a nearly full Ganymede rises above Jupiter. Interestingly enough, 20 minutes later, an Ionian observer would be able to watch Ganymede's north polar region darken as it experiences a total solar eclipse by Europa.

Callisto, the furthest of the Galilean satellites from Jupiter, doesn't display such regular cycles as Ganymede and Europa as Callisto is not in an orbital resonance with Io. Callisto rises above Jupiter at a distance of 2.309 million km from Io and would appear 7.2 arcminutes across in Io's sky, about a quarter of the size of the Moon in Earth's sky. Callisto sets a little more than 22 hours later near opposition when Callisto is 1.472 million km from Io and is 11.2 arcseconds across, or about 3/8ths the size of the Moon in Earth's sky. Among the features that would be targets of interest for an Ionian observer would be the Valhalla impact basin on the western limb and the bright palimpsets Lofn and Heimdall on Callisto's south polar region.

Himalia, the largest of Jupiter's outer irregular satellites, would appear as a barely visible "star" near 5.5 mag. at its brightest. Jupiter's other irregular satellites, would be telescopic targets too faint to be visible to the naked eye.

Planets and the Sun

From Io, five additional planets would be visible to the naked eye most times of the year, with Uranus a naked eye target during opposition with that planet. Like Venus and Mercury from Earth, the terrestrial planets would stick fairly close to the Sun, with Mars appears as far as 17° from the Sun. The sun itself would appear much smaller than it does from Earth, only 6.5 arcminutes across, compared to 30 arcminutes from Earth.

Jupiter-Ganymede-Io Animation

Mike Salway posted a cool animation of images he took of Jupiter, Ganymede, and Io over at the Cloudy Nights forum. These images were acquired on March 19. You can see a rotating Io, Io on the near-side of Jupiter orbiting off to the left, and Ganymede on the far-side of Jupiter orbiting off to the right. Note that you can see Ganymede emerging from an eclipse in the first frame.

Definitely worth checking out.

Link: Jupiter + GRS + Ganymede + Io + Animation [www.cloudynights.com]

Jupiter and Io animation

Welp, Io still exists as of Monday...

Mike Salway posted on the Cloudy Nights Telescope Reviews forum a nice animation created from images he acquired using his 12" Newtonian from New South Wales, Australia. Definitely worth checking out at the link below. Once at the forum thread, just click on animation.

The ALPO-Japan website has a nice image taken about 27 minutes before the start of Salway's animation by Anthony Wesley from Murrumbateman, Australia. It shows Io against Jupiter's North Tropical Zone (I think that's right, you can correct me in the comments if I am wrong).

Link: Jupiter + Io (+animation) [www.cloudynights.com]