Interestingly, Mneme has a retrograde orbit, which means that Mneme rotates in the opposite direction from the rotation of its planet, Jupiter. The International Astronomical Union (IAU) mandates that all retrograde orbit planets be given a name that ends in the letter “e.”
Mneme was only just discovered in 2003, when two astronomers at Hawaii’s Mauna Kea Observatory and gave it the name JXL (Jupiter XL) and S/2003 J21.
How Mneme Got Its Name
Its common name, Mneme, references the Greek god Zeus (the Romans called the same god Jupiter) and his daughters, the Muses. Mneme was the muse associated with memory.
There is some ongoing confusion about Mneme’s common name, since Zeus’s daughter Mneme and Mneme’s mother, Mnemosyne, have names that sound a great deal alike.
Size Compared to Earth’s Size
Compared with the Earth’s own Moon, Mneme is 1737.5x smaller. It also has significantly less gravity on the surface than does Earth.
Why Is Mneme Considered Part of the Ananke Group of Satellites?
Mneme is one of 12 moons orbiting around Jupiter that is considered part of the Ananke group. It is thought that this group all formed after Jupiter’s gravitational force captured a passing asteroid. This asteroid then collided with Jupiter itself. That collision then broke the asteroid up into pieces and sent it into orbit around the planet.
Jupiter’s moon Ananke is thought to be the largest single chunk of that separated portion of the asteroid and the other 11 moons, including Mneme, are the smaller separated chunks. The original asteroid is calculated to have measured about 17.4 miles (28 km) across.
All the moons in the Ananke group, including Mneme, have an irregular (rather than spherical) formation due to their small size. This also gives them each an elliptical orbit rather than the traditional circular orbit.
Mneme Is Not a Bright Moon
Mneme has an albedo rating of 0.04 (compare this with Jupiter’s albedo rating of 52). The word “albedo” basically means reflected radiation or light. The higher an object’s albedo rating, the easier it will be to spot in the night sky. This explains why Mneme was impossible to detect with earlier, weaker telescopes. At just 4 km in diameter, it is not only very small but also very faint.
Mneme Has Weak Surface Gravity
Mneme also has a very weak surface gravity of just 0.001 m/s². Compare that to Jupiter’s surface gravity of 24.79 m/s² to get a sense of just how easy it would be to float right off the surface of Mneme.
Discovered By: Scott S. Sheppard, Brett Joseph Gladman
Discovery Date: February 9, 2003
Orbit Period: 620 Earth days
Orbit Distance from Jupiter: 21,070,000 km (13,092,291 miles)
Equatorial Circumference: 6.3 km
Diameter: 4 km
Mass: 14,986,684,330,972 kg