Titan and Enceladus: Saturn's Most Intriguing Moons

Saturn has a large and growing family of known moons, but two of them stand out above the rest.

Titan is a giant, haze-shrouded world with weather, rivers and seas.

Enceladus is a small, brilliant white moon that sprays water from a hidden ocean out into space.

Both were transformed from faint points of light into real places by the Cassini mission, which orbited Saturn from 2004 to 2017.

Here is what makes each moon so remarkable,

what spacecraft have discovered and why both rank among the most interesting places in the solar system to look for the ingredients of life.

Two Moons, Two Kinds of Mystery

At first glance, the two could hardly be more different:

• Titan is about 5,150 km (3,200 miles) across, Saturn's largest moon and the second largest in the solar system. It is wider than the planet Mercury and has a thick atmosphere.
• Enceladus is only about 500 km (310 miles) across, small enough to fit comfortably within many countries on Earth. It has no real atmosphere, but its icy surface is one of the most reflective in the solar system.

What unites them is that both are far more active than anyone expected,

and both may hide liquid water beneath their icy crusts.

Titan: A World Wrapped in Orange Haze

Titan was discovered by the Dutch astronomer Christiaan Huygens in 1655, but its surface stayed hidden for more than three centuries.

The reason is its atmosphere, the only thick atmosphere of any moon in the solar system.

Titan's air is mostly nitrogen, like Earth's, with a few percent of methane.

At the surface, the pressure is about one and a half times what we feel at sea level.

High above, sunlight breaks methane apart, and the fragments recombine into complex organic molecules that form a dense orange smog.

That haze blocks the view of ordinary cameras, so spacecraft used radar and special infrared cameras to see the ground.

Titan is also bitterly cold, around −179 °C (−290 °F) at the surface.

Combined with low gravity, about one seventh of Earth's, and thick air, this makes for a very strange environment.

Walking would feel light and bouncy, and falling would be slow.

Rain, Rivers and Seas of Methane

Titan has something no other place besides Earth is known to have: stable bodies of liquid on its surface.

It is far too cold for liquid water, so the role of water is played by methane and ethane,

compounds that are gases on Earth but liquid at Titan's frigid temperatures.

A Landscape Like Earth's, Built From Ice

Titan has a weather cycle much like Earth's water cycle.

Methane evaporates, forms clouds, falls as rain and flows across the surface in rivers that carve valleys.

Near the north pole, Cassini's radar revealed large seas and hundreds of lakes, some of the seas hundreds of kilometers across.

Elsewhere lie fields of dunes made of dark organic grains, mountains and channels.

The "rock" in these landscapes is water ice, frozen so hard that it behaves like stone.

Landing on Titan

In January 2005, the European Huygens probe, carried to Saturn by Cassini, parachuted through Titan's atmosphere and landed on its surface.

It was the first landing on any world in the outer solar system.

Its images showed dark drainage channels running down hillsides and, at the landing site,

rounded pebbles of ice resting on a damp, sandy plain, smoothed as if by flowing liquid.

Beneath all of this,

measurements of how Titan flexes under Saturn's gravity were long read as a sign of a deep ocean of liquid water,

though a later reanalysis of the same data suggests layers of slushy ice with pockets of water instead.

Enceladus: The Moon That Sprays Its Ocean Into Space

Enceladus was long considered too small to be interesting.

Cassini changed that in 2005, when it spotted plumes of water vapor and ice grains erupting from the moon's south pole.

The jets rise from four long, parallel fractures nicknamed "tiger stripes," which are warmer than the frigid terrain around them.

The material they spray into space is so abundant that it feeds Saturn's broad, faint E ring.

Further measurements showed that Enceladus wobbles slightly as it orbits,

which is best explained by an ocean of liquid water lying beneath its entire icy shell.

Tidal heating from Saturn's gravity is thought to keep that ocean from freezing.

The plumes also explain why Enceladus looks so bright.

Ice grains that fall back onto the surface coat it like fresh snow, so it reflects most of the sunlight that reaches it.

Its surface tells a story of activity, too: some northern regions are heavily cratered,

while the terrain around the south pole has almost no craters at all,

a sign that it is constantly being renewed.

What Cassini Tasted in the Plumes

Because Enceladus sprays its ocean into space, Cassini could sample it simply by flying through the plumes.

What it found was remarkable:

1. Salty water, with salts suggesting that the ocean is in contact with a rocky seafloor.
2. Tiny grains of silica, which on Earth can form where hot water interacts with rock, hinting at hydrothermal activity on the seafloor.
3. Molecular hydrogen, a potential source of chemical energy for microbes like those living around deep-sea vents on Earth.
4. Organic molecules, including some complex carbon-based compounds.
5. Phosphates, detected in ice grains, supplying phosphorus, an element essential for life as we know it.

Could Either Moon Support Life?

No evidence of life has been found on Titan or Enceladus.

What makes them exciting is that they appear to have some of the conditions life needs, but whether life actually exists there is unknown.

Enceladus

Enceladus seems to offer liquid water, chemical energy and many key elements in one place.

That combination makes it one of the leading candidates in the search for life beyond Earth.

Scientists stress, however, that having the ingredients does not mean life ever arose there.

Titan

Titan's surface is rich in organic chemistry,

making it a natural laboratory for studying the kinds of chemical steps that may have come before life on the early Earth.

Some scientists have speculated about exotic life that uses liquid methane instead of water, but that idea is purely hypothetical.

Water hidden deep inside Titan, whether a buried ocean or pockets within slushy ice, is another possible, though very hard to reach, habitat.

Seeing Them From Earth

Titan is easy to spot with a small backyard telescope.

It appears as a faint star near Saturn, changing position over its 16-day orbit, and you can follow it from night to night.

Enceladus is much fainter and lies close to Saturn's glare,

so it is a challenge that usually requires a larger telescope, steady skies and a detailed chart.

Worlds Worth Returning To

Titan and Enceladus overturned the old assumption that the outer solar system is cold, dead and dull.

One has a weather cycle and seas; the other has an ocean it shares with space.

When you find Titan beside Saturn in a telescope, remember that you are looking at a world with rain, rivers and shorelines of its own.