Amazing Moons

When the Space Age began, explorers were eager to visit the planets of the solar system. As the years have passed, however, astronomers have realized that the moons of the solar system may be even more interesting.
Moons - also called natural satellites - come in many shapes, sizes and types. They are generally solid bodies, and few have atmospheres. Most planetary moons probably formed from the discs of gas and dust circulating around planets in the early solar system.

There are hundreds of moons in our solar system - even a few asteroids have been found to have small companion moons. Moons that begin with a letter and a year are considered provisional moons. They will be given a proper name when their discoveries are confirmed by additional observations.

Of the terrestrial (rocky) planets of the inner solar system, neither Mercury nor Venus have any moons at all, Earth has one and Mars has its two small moons. In the outer solar system, the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune have dozens of moons. As these planets grew in the early solar system, they were able to capture smaller objects with their large gravitational fields

How Moons Get Their Names

Most moons in our solar system are named for mythological characters from a wide variety of cultures. The newest moons discovered at Saturn, for example, are named for Norse gods such as Bergelmir, a giant.

Uranus is the exception. Uranus' moons are named for characters in William Shakespeare's plays so you'll find Ophelia and Puck in orbit. Other Uranian moon names were chosen from Alexander Pope's poetry (Belinda and Ariel).

Moons are given provisional designations such as S/2009 S1, the first satellite discovered at Saturn in 2009. The International Astronomical Union approves an official name when the discovery is confirmed.

Moons of the Inner Solar System

Earth's Moon probably formed when a large body about the size of Mars collided with Earth, ejecting a lot of material from our planet into orbit. Debris from the early Earth and the impacting body accumulated to form the Moon approximately 4.5 billion years ago (the age of the oldest collected lunar rocks). Twelve American astronauts landed on the Moon during NASA's Apollo program from 1969 to 1972, studying the Moon and bringing back rock samples.

Usually the term moon brings to mind a spherical object, like Earth's Moon. The two moons of Mars, Phobos and Deimos, are different. While both have nearly circular orbits and travel close to the plane of the planet's equator, they are lumpy and dark. Phobos is slowly drawing closer to Mars and could crash into the planet in 40 or 50 million years. Or the planet's gravity might break Phobos apart, creating a thin ring around Mars.

Moons of the Giant Planets

Jupiter's menagerie of moons includes the largest in the solar system (Ganymede), an ocean moon (Europa) and a volcanic moon (Io). Many of Jupiter's outer moons have highly elliptical orbits and orbit backwards (opposite to the spin of the planet). Saturn, Uranus and Neptune also have some irregular moons, which orbit far from their respective planets.

Saturn has two ocean moons-Enceladus and Titan. Both have subsurface oceans and Titan also has surface seas of lakes of ethane and methane. The chunks of ice and rock in Saturn's rings (and the particles in the rings of the other outer planets) are not considered moons, yet embedded in Saturn's rings are distinct moons or moonlets. These shepherd moons help keep the rings in line. Titan, the second largest in the solar system, is the only moon with a thick atmosphere.

In the realm of the ice giants, Uranus's inner moons appear to be about half water ice and half rock. Miranda is the most unusual; its chopped-up appearance shows the scars of impacts of large rocky bodies.

Neptune's moon Triton is as big as Pluto and orbits backwards compared with Neptune's direction of rotation.

Moons of Dwarf Planets

Pluto's large moon Charon is about half the size of Pluto. Like Earth's Moon, Charon may have formed from debris resulting from an early collision of an impactor with Pluto. Scientists using the Hubble Space Telescope to study Pluto found four more small moons.

Eris, another dwarf planet even more distant than Pluto, has a small moon of its own, named Dysnomia. Haumea, another dwarf planet, has two satellites, Hi'iaka and Namaka. Ceres, the closest dwarf planet to the Sun, has no moons.

More Moons

Scientists weren't sure if asteroids could hold moons in their orbits until the Galileo spacecraft flew past asteroid Ida in 1993. Images revealed a tiny moon, later named Dactyl.

Worlds Within Worlds

Astronomers have discovered an immense cloud of hydrogen evaporating from a Neptune-sized planet named GJ 436b. The planet's atmosphere is evaporating because of extreme irradiation from its parent star

Astronomers using Hubble Space Telescope have discovered an immense cloud of hydrogen evaporating from a Neptune-sized planet named GJ 436b. The planet's atmosphere is evaporating because of extreme irradiation from its parent star.

About 30 light years away, a Neptune-sized planetis having some of its layers peeled back.

Astronomers using 's Hubble Space Telescope have discovered an immense cloud of hydrogen evaporating from a Neptune-sized planet named GJ 436b.

This cloud is spectacular. The research team has nicknamed it The 'Behemoth.'

The planet's atmosphere is evaporating because of extreme irradiation from its parent star-a process that might have been even more intense in the past.

The parent star, which is a faint red dwarf, was once more active. This means that the planet's atmosphere evaporated faster during its first billion years of existence. Overall, we estimate that the planet may have lost up to 10 percent of its atmosphere.

GJ 436b is considered to be a Warm Neptune because of its size and because it is much closer to its parent star than Neptune is to our own sun. Orbiting at a distance of less than 3 million miles, It whips around the central red dwarf in just 2.6 Earth days. For comparison, the Earth is 93 million miles from the sun and orbits it every 365.24 days.

Systems like GJ 436b could explain the existence of so-called Hot Super-Earths.

Hot Super-Earths are larger, hotter versions of our own planet. Space telescopes such as 's Kepler and the French led CoRoT have discovered hundredsof them orbiting distant stars. The existence of The Behemoth suggests that Hot Super-Earths could be the remnants of Warm Neptunes that completely lost their gaseous atmospheres to evaporation.

Finding a cloud around GJ 436b required Hubble's ultraviolet vision. Earth's atmosphere blocks most ultraviolet light so only a space telescope like Hubble could make the crucial observations.

You would not see The Behemoth in visible wavelengths because it is optically transparent. On the other hand, it is opaque to UV rays. So when you turn the ultraviolet eye of Hubble onto the system, it's really kind of a transformation because the planet turns into a monstrous thing.

The ultraviolet technique could be a game-changer in exoplanet studies, he adds. Ehrenreich expects that astronomers will find thousands of Warm Neptunes and Super-Earths in the years ahead. Astronomers will want to examine them for evidence of evaporation. Moreover, the ultraviolet technique might be able to spot the signature of oceans evaporating on Earth-like planets, shedding new light on worlds akin to our own.

Maybe you can't judge a book by its cover, but you can judge a planet by its Behemoth.

Close Encounter with Enceladus

NASA's Cassini Spacecraft is about to make a daring plunge through one of the plumes emerging from Saturn's moon Enceladus.

Enceladus boasts an icy, ostensibly barren landscape riddled with deep canyons, dubbed tiger stripes. Underneath its icy exterior churns a global ocean, heated in part by tidal forces from Saturn and another moon, Dione, with seafloor vents expelling water at at least 194 degrees Fahrenheit. Plumes of water vapor and icy particles jettison from its surface in geyser-like spouts, hinting that there is much more to this snowy moonscape than meets the eye.

Cassini will be soaring through the jets located at the moon's south pole, only 30 miles above the surface.

Although the October 28th flyby won't be the closest we've ever been to Enceladus, it is the closest flyby over the south pole and through the plume. We'll be exploring in situ a region of the plume that Cassini has never sampled before.

So what causes these plumes, and why are they so important? Enceladus' vast, subterranean oceans may be fizzy and full of gas. When the gas and icy particles rise to the surface, they are expelled in plumes shooting from the tiger stripes. The process is similar to shaking up a bottle of soda; the gas has nowhere to go but up and out.

However, the plumes are more than just gas and water: samples show that they also contain many of the building blocks essential to Earth-like life. This lends itself to the exciting possibility that organisms similar to those that thrive in our own deep oceans near volcanic vents exuding carbon dioxide and hydrogen sulfide might exist on Eceladus. Although it is still too early to know exactly how complex potential Enceladus' lifeforms could be, scientists speculate that at the very least microbial life is a real possibility.

In the future, a different spacecraft may journey across the solar system to visit icy Enceladus. This spacecraft, unlike Cassini, could be designed to land on Enceladus' surface, near one of its tiger stripes. Such a lander would be able to take samples more directly, bypassing the plume altogether.

Ideally, it could take samples from the edge of one of the tiger stripes, speculates Spilker. This would ensure that any microbes being expelled from Enceladus' interior would be more plentiful and easier to collect.

Until then, flybys are the best we can do. And the next one should be very good indeed. Tune in on Oct. 28th!

Count Down To Pluto

Eight years after it left Earth, NASA's New Horizons spacecraft is approaching Pluto. The encounter begins less than a year from now.