In Our Skies: Jupiter rides majestically across our southern sky

Last week’s column discussed the various bright planets of our solar system that are presently inhabiting our nighttime skies.

Perhaps none of those worlds dominates our nighttime sky as much as our solar system’s largest planet, Jupiter; having been at opposition, i.e., the point directly opposite the sun in the sky, less than two weeks ago.

Jupiter is already well up in the southeast by the end of dusk and rides majestically high across our southern sky during the remainder of the night.

Among Jupiter’s many wonders are its four large moons: Io, Europa, Ganymede, and Callisto. Collectively these are referred to as the Galilean moons, so named after the Italian astronomer Galileo Galilei, who discovered and observed them with his primitive telescope in the early 17th century. Via his observations, Galileo could demonstrate that these objects orbited around Jupiter rather than the Earth, and this provided one of the strongest pieces of evidence against the geocentric, or Earth-centered, view of the universe that humanity held at that time.

Although the Galilean moons can be detected with binoculars, even in large telescopes they usually appear as little more than points of light. It wasn’t until the twin spacecraft Voyagers 1 and 2 passed through the Jupiter system in 1979 and captured close-up photographs and other information of these objects that they came to be viewed as separate and unique worlds.

While the label of most interesting Galilean moon is certainly a matter of subjective definition, a rather strong argument can be made for ascribing this status to Europa. It is the second-closest in of the four, and orbits around Jupiter every 3.55 days; it is also the smallest of the four, being slightly smaller than our own moon.

The images of Europa captured by the Voyager spacecraft showed a blueish and very smooth world, crisscrossed by various dark lines. Analysis of these images and other data revealed that Europa’s surface is solid ice. However, various calculations, including studies of the tidal stresses that Europa undergoes due to its proximity to Jupiter, rather strongly suggested that Europa’s subsurface temperature is warm enough for water to exist in its liquid form. Thus, was the idea born that, underneath Europa’s icy crust, there exists a worldwide ocean of liquid water.

Such a scenario conjures up the possibility of life existing in such a subsurface ocean, and indeed this has been examined in both scientific discussions and in fiction, for example, in a couple of the novels in Arthur C. Clarke’s Odyssey series. The evidence for such an ocean, however, remained circumstantial for a large part, and required additional observations to be confirmed.

Many of these observations came from the Galileo spacecraft, which orbited Jupiter from 1995 until 2003, and which made various flybys of the Galilean moons in the process, and especially of Europa during the extended mission phase of its investigations.

The images and data acquired by Galileo were quite intriguing, and pointed to large fractures in the icy crust, the origins of the dark lines imaged by the Voyagers, possible floating icebergs, and the possibility that, in some places, the ocean is only a few miles below the surface. However, while the evidence for a subsurface European ocean is very strong – and most scientists do believe it is there – it cannot quite be said to be confirmed yet.

Meanwhile, over the past decade the Hubble Space Telescope has detected evidence of plumes coming off Europa’s surface, similar to the water geysers that the Cassini spacecraft imaged erupting off Saturn’s moon Enceladus. Inspired by these, a team led by University of Michigan researcher Xianzhe Jia examined old Galileo data with newly-developed computer analysis software and has just announced that Galileo detected some of these plumes and apparently even flew through one. Analysis of the plume data rather strongly indicates that they were composed of water.

The case for a global subsurface Europan ocean is thus now stronger than ever, although it still isn’t completely solid yet. That will probably have to wait until the arrival of NASA’s Europa Clipper mission, presently under development and scheduled for launch during the early- to mid-2020s. Europa Clipper is designed to make repeated close flybys of Europa’s surface – some as close as 16 miles – and examine the surface with a variety of instruments, including ice-penetrating radar.

Additional missions to Europa, including a possible landing mission that could launch in the not-too-distant future, are being studied. If there really is indeed a subsurface Europan ocean, that would be, by far, the most likely place we might find life. The discovery of such life – especially, as would seem likely, the determination that that life arose independently of life on Earth – would constitute the greatest discovery in the history of science.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere.