Monday, September 26, 2022

The Planets That Were Not


Animation of Jupiter's three nearest moons over 2.5 hours; Credit: Purnendu Gupta; Sep 26, 2022 

It was the night of January 7, 1610. A man in his mid forties stood in the back garden of his three story home in Padua, Italy. In his hand was his home made telescope pointed at the planet Jupiter.  He was the chair of mathematics at the university of Padua. Though well known by then, he had not quite become the icon that he would go on to be. Most of his remarkable achievements still lay in the future. But one that he would make over the next few days would etch his name in the history of astronomy forever.

Four Nights in Padua

The man was Galileo, The design of the device he had crafted, was simple. A long tube with two curved pieces of glass at both ends which offered about 20 times magnification. It was based on the Dutch invented spyglass which had so far been used for terrestrial viewing - such as for ships on the ocean. Galileo would be the first to use it to observe the night sky.  

Source: English Translation of Sidereus Nuncius
by Albert Van Helden
©1989 by The University of Chicago
From: reed college 
On that night, he observed three "stars" next to Jupiter, which were invisible to the naked eye due to their smallness. The stars lay on a straight line along Jupiter’s equator. On the following night all three dots had shifted to one side of Jupiter and were closer together. Over the following evenings, the stars shifted position but all moved with the planet in a manner fixed stars would not. On the night of January 10th one of them seemed to disappear from view. Galileo deduced it was being hidden behind Jupiter. 

By January 11, he had concluded that these objects revolved around Jupiter in the same way Venus and Mercury revolve around the Sun. Since no one had heard of planets other than the earth having moons, Galileo decided to 

call the group the "Medician planets". This was in honor of his future benefactor, the ruling Medici family of Tuscany. He documented this in a booklet called “Sidereus Nunicus” using an open circle for Jupiter and a star symbol for the “planets”. Little did he know that one day these were to be renamed after him as the discoverer. 

The discovery caused quite a stir since this was the first record of "planets" that went around other planets instead of the earth. The finding flew in the face of Aristotle's geo-centric model. It was part of a growing body of evidence in favor of Copernicus' heliocentric model and a stance that would famously put Galileo against the church and cost him his freedom.  

At the same time in 1610, a German astronomer Simon Marius independently discovered the three closest moons of Jupiter. They are called Io, Europa and Ganymede as named by him.

Worlds of Fire and Ice

Four hundred and twelve years later, the garden variety backyard astrophotographer can detect the motions in one night over the span of two and a half hours.  The animation above is a composite of four individual stills, each made using the "lucky imaging" method to get around the roiling currents in the earth’s atmosphere. Each of the four frames is derived from a video clip at 30 frames per second, running for about a minute each. Special software then culls the highest quality ones and creates a master frame by adding them up.

Earth based imaging aside, thanks to several spacecraft missions in the past 50 years, we know a lot about Jupiter and the Galilean moons. 

A Galileo spacecraft image shows volcanic activity on Io
Source: NASA/JPL. 1999
 
Io, the closest one is pulled and pushed by the enormous gravity of Jupiter and the other moons leading
to "tides" in its solid surface up to 300 feet high, generating enough heat to give rise to volcanic activity. In fact Io has one of the highest volcanic activity in the entire solar system. 

Ganymede is the largest moon in the solar system (larger than Mercury), and is the only moon known to have its own internally generated magnetic field. 

Callisto’s surface is extremely heavily cratered and ancient — a record of events from the early history of the solar system. Ganymede and Callisto, like Europa, have oceans, but they are deeper and less accessible. Their seafloors are believed to be covered with thick layers of ice  - which when formed under pressure, sink rather than float

 Europa surface shows domes and ridges, as well as
a region of disrupted terrain including crustal plates
which are thought to have broken apart and
"rafted" into new positions. Source; Nasa/JPL

The most intriguing of the moons is perhaps Europa. It's surface is mostly water-ice. The icy crust is believed to cover a global water ocean. This moon according to our expert estimates has the highest potential for having a habitable ocean very much like Earth’s. 


Clipper of (Alien) Clouds

NASA is currently preparing the Europa Clipper mission targeted for launch in 2024. It is expected to arrive by 2030. While it is not a life detecting mission, it will be in orbit and perform close flyby-s of this icy world looking for evidence of life supporting environments.

The spacecraft will the largest ever that NASA has built for a planetary mission. Its solar arrays will span 100 feet, wider than the length of a basketball court. With fuel, it will weight over 6 thousand kilograms - the weight of one and a half average American car. An array of nine science instruments that include high resolution cameras, magnetometers, thermal imagers, mass spectrometer, ice penetrating radar, surface dust mass analyzers will help it probe Europa. 

Will its findings be as revolutionary as that of a middle aged man from four centuries ago armed with a home made gizmo and a beautiful mind?   

We hope to know in another 8 years.

Galileo's Telescopes at the Galileo Museum, Florence, Italy
Photo Credit: Purnendu Gupta (Aug 18, 2022)

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