Cosmic Perspective: Ganymede is HUGE & Enceladus is TINY

17 Mar

I’ve been looking at photos of Jupiter’s moon Ganymede and Saturn’s icy moon Enceladus for years. Still, this collage posted by Emily Lakdawalla shocked me (click to enlarge.)

Ganymede and Enceladus

Montage by Emily Lakdawalla. Data from NASA / JPL and SSI, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, and Emily Lakdawalla.

Look at the size of Ganymede (top row, second from left) compared to Mars (top row, first at left)! Maybe it’s because I’m used to seeing Ganymede as a tiny dot through my telescope beside Jupiter, but I’d never truly grasped the moon’s size until I saw it “beside” Mars.

This image also reminds us that both Ganymede and Titan (which appears larger than Ganymede due to its atmosphere) are bigger than the planet Mercury!

Tiny Enceladus

Once I’d wrapped my head around Ganymede’s girth, I was shocked again when I spotted TINY Enceladus in the bottom row, fourth from right.  Enceladus has been in the news since the Cassini space probe detected jets of water ice erupting from its south pole in 2005.

Without seeing the moon beside its kin, I’d always pictured Enceladus as being similar in size to Europa. It’s not even close! At just 500 kms across, its diameter is less than the width of Lake Superior!

Lake Superior Enceladus

At just 500 km across, Enceladus would fit “inside” this green circle.

The small size of Enceladus makes it hard to believe that it was discovered so long ago — in 1789 by William Herschel, to be exact. How could such a small moon have been noticed when Saturn is always more than a billion kilometers away? 

Enceladus Geysers

30-frame sequence of the icy plumes of Enceladus taken August 13th, 2010. (NASA/JPL/Space Science Institute)

The answer is that the surface of Enceladus is *extremely* reflective. It bounces back almost all of the sunlight that hits it, making it visible in moderate-sized telescopes when Saturn’s rings are edge-on as seen from Earth.

The Problem with Nomenclature  

In addition to showing the relative sizes of our solar system’s moons, the collage reinforces the idea that moons are “worlds”, too. In her fantastic blog post “2015 will be the Year of the Dwarf Planet, and you need to tell people about it!“, Lakdawalla points out that everyone has heard of Mercury, but few people would recognize the names Ganymede or Enceladus.

Lakdawalla writes: “Europa, Ganymede, Titan, Enceladus, and Triton: how important can they be, if they’re not on that list of eight planets? You want how much money? To fly a spacecraft to where, exactly? Enchiladas?”

The solar system is an amazing place, and it’s home to more than just planets. Let’s make sure that our arguments about what is a planet — and what isn’t — don’t cause us to overlook some of the lesser-known, but no less spectacular, planet-like places in our neighbourhood.

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