The Crestone Eagle • July, 2021

Skies Over Crestone: July 2021

by Kim Malville

July 5: The earth is at aphelion, its farthest distance from the sun, moderating very slightly the temperature of our northern summer. We are a mere 3.4% farther from the sun than in January.

July 11: The two-day old moon appears in the evening on a line with Venus and Mars, low on the west-northwest horizon.

Ganymede photographed by Juno spacecraft on June 7.
photo courtesy pf NASA

July 12: Look for a conjunction of Venus and Mars about 45 minutes after sunset, when these two planets come with one-half of a degree from each other.

July 29: Another conjunction of Mars, about 30 minutes after sunset on the western horizon. This time it is with Regulus, the brightest star of Leo, the Lion. The red mars hangs briefly above wihite Regulus. Binoculars will help separate the two object close to the horizon.

Life and death on Ganymede

On June 7, the spacecraft Juno made a spectacular and potentially dangerous fly-by past Ganymede, the largest moon of Jupiter and the largest moon of the solar system. It came within 645 miles of its surface, flying 432,000 miles per hour. It is utterly amazing what the spacecraft controllers can accomplish over a distance of over 4 billion miles and achieve “razor sharp” photographs in the process.

Ganymede is an amazing moon, larger than the planet Mercury and the dwarf planet Pluto. It is, quite astonishingly, the ninth-largest object in the solar system including the sun. If it were orbiting the sun, it would be identified as a planet.  Yet, I suspect that few school children or even adults know what a remarkable object it is.  

Comparison of Earth, Ganymede, and Earth’s moon. photo courtesy of NASA

On January 7, 1610, Galileo Galilei used a telescope to observe what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and another object that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but he had seen each of the moons before this date at least once. By January 15, Galileo concluded that these stars were bodies orbiting Jupiter. 

Interior of Ganymede. photo courtesy of NASA

Some say that this evening was when Galileo realized that the earth was revolving around the sun, tossing the geocentric model to a heliocentric picture of the solar system, leading Freud to comment:  “Humanity has in the course of time had to endure from the hands of science two great outrages upon its näive self-love. The first was when it realized that our earth was not the center of the universe”; The second, as you can imagine was …”when biological research robbed man of his peculiar privilege of having been specially created…” Sigmund Freud, Introduction à la psychanalyse.

Ganymede has a molten metallic core that produces a magnetic field.  It is the only moon with a magnetic field, stronger than anything possessed by Mercury, Venus, or Mars. The magnetic field causes auroras, which are ribbons of glowing oxygen in regions circling the moon’s north and south poles. Its magnetic field is like that of the Earth trapping electrons and protons which collide oxygen atoms in its thin atmosphere. Jupiter rotates at an astonishing rapid rate of slightly less than 10 hours. It carries its magnetic field with it sweeping through space.  The result is that its magnetic field, carrying charged particles, smashes into the slower-moving magnetic field of Ganeymede, which takes slightly more than 7 days to circle Jupiter. The effect is like what happens when Earth’s magnetic field is hit by bursts of the solar wind, known as coronal mass ejections. The result is the glorious aurora of the polar regions. 

Polar aurora on Ganymede.
photo courtesy of NASA

The Hubble Space Telescope was the first space probe to discover evidence of a thin layer of oxygen-rich atmosphere trapped in Ganymede’s icy surface. Although this atmosphere is much too thin to support life as we know it, there is enough oxygen to produce an aurora.  In the Earth’s atmosphere, the brightest light of the aurora is a yellow green emission produced by electrons hitting oxygen atoms in the upper atmosphere. There is probably the same yellow green flicking glow in the atmosphere of Ganymede as there is in the skies over Alaska and Norway, although it would be much closer to the surface. It is not a place for us to enjoy, however.  Constantly bombarded by the incoming fast electrons of Jupiter’s magnetosphere and its own modest magnetosphere, Ganymede is not a person-friendly place. Its Chernobyl-like environment is such that radiation would cause severe illness or death in any human living on its surface for two months.  

But below the flickering blue-green aurora, below its 95-mile-thick crust of ice, the ocean of Ganymede may be more friendly to life. Its saltwater ocean is 60 miles thick, 10 times deeper or thicker than the earth’s ocean, and contains more water than all of Earth’s oceans or, for that matter, than anywhere in the solar system. Its molten hot core may raise Ganymede near the top of the list of places in the solar system favorable to the formation of life. At the depths of its dark cold ocean, fumaroles may be bubbling away, providing energy for exotic forms of life. But life, so very inaccessible to human exploration, 60 miles deep in dark water, locked beneath an icy crust, enveloped in an environment of lethal radiation. This is a grand scientific adventure for the distant future.  

Ganymede has polar caps, likely composed of water frost. The frost extends to 40° latitude. Theories on the formation of the caps include release of water vapor near the poles due to bombardment of the ice by the same electrons and protons that produce its aurora. 

As you can observe in the photograph taken by Juno, Ganymede has large, bright regions of ridges and grooves that slice across older, darker terrains. These grooved regions are a clue that the moon experienced dramatic upheavals in the distant past resulting in a buckling of the icy crust. Groove ridges, as high as 2,000 feet, may run for thousands of miles across Ganymede’s surface. The grooves have relatively few craters, evidence of which was probably erased by melting of surface ice or eruption of water from cracks in its icy armor. The dark regions on Ganymede are old and rough, and its dark, cratered terrain contains memories of bombardment that occurred billions of years ago. 

What will future explorers find on this young and ancient moon? Will they dare venture forth? “Optimism is a precondition for scientific work” says Loeb in his new book Extraterrestrial.

Long ago, Heraclitus predicted “If you do not expect the unexpected, you will not find it.”

And finally, Einstein’s comments seem appropriate to a search for life on Ganymede: 

“If at first the idea does not seem absurd, there is no hope for it.”