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It's a Moon Thing

Studying the Moons of the Outer Planets

My Very Excited Mother Just Sewed Uncle Ned's Pants.

It is strange to think this very sentence helped many of us in grade school remember the order and names of the planets in our solar system.

Order of the planets in the Solar System
The order and names of the planets in our solar system.

But now, things are constantly a-changin. Despite the unexpected demotion of Pluto to a mere "dwarf planet," our understanding of the mysterious worlds of the outer planets in the solar system has grown substantially throughout the 50 year history of NASA's space exploration.

Even throughout the past decade, discoveries from Galileo, Voyager 1 and 2 and the Cassini Mission have unlocked many mysteries surrounding the major outer planets including Jupiter, Saturn, Uranus, Neptune and yes, Pluto. Jupiter's Great Red Spot, for example, is actually a hurricane that has been raging on this gas giant for more than 400 years. There have been compelling discoveries about the majestic ringed planet Saturn, and Uranus held its own secrets, which the Hubble Space Telescope has in the past two years revealed to be an unstable system that contains many satellites and even faint rings.

Up until recently, studying the outer planets of the solar system was one of the most fundamental goals for scientists to understand the origins of the universe. But now it seems there is a shift in the way scientists are studying these outer planetary systems -- by specifically targeting their unique and mysterious moons. With the discovery of geysers spewing out of Saturn's moon Enceladus, organic lakes on Saturn's Earth-like moon Titan, and possibly an underwater ocean teeming with possibilities on Europa, what scientist can resist creating a more sturdy comparison of these moons to Earth and the possible origins of life in the universe?

Titan: The Mars of the Outer Solar System

Everything that makes Mars interesting -- its landscape, meteorology, connections to the origin of life -- applies in equal measure to Titan. As a body with a thick 'earth-like' atmosphere, 'moon-like' gravity, and the possibility of large expanses of surface liquid, Titan is already known to be a giant organic chemistry laboratory. Further, it might become an excellent physical laboratory for the study of terrestrial processes. These processes include oceanography, meteorology, and even geology. To take an interactive tour of Titan, click here.

Earth-like, but bizarre

Thanks to the work of the Cassini spacecraft and Europe's Huygens probe of Titan, this enigmatic moon deserves further exploration. Because of its pre-biotic Earth-like atmosphere, Titan has many similarities to Earth, but also exhibits bizarre differences.

Artist rendition of a balloon landing on Saturn's moon, Saturn
This artist's conception shows Titan's surface with Saturn appearing dimly in the background through Titan's thick atmosphere of mostly nitrogen and methane.

Ralph Lorenz, a scientist with the Lunar and Planetary Laboratory at the University of Arizona in Tucson, points out that Saturn's Titan appears set to challenge even Mars as a priority target of exploration.

Lorenz is an advocate for a Titan airborne platform. It would cover vast distances of Titan's terrain, flying over possible lakes and seas of liquid methane and ethane. A new vista would be surveyed by the aerial platform every day. The airship would offer up, down, and side-way sensing methods.

Under autonomous control, the robotic airship might even deploy sample devices, even mini-robots lowered by tether or balloon onto Titan. This would provide the crucial data that scientists crave to understand this mysterious moon.

Europa: "The Ocean Notion"

For the past several years, scientists have become keen on resolving the quandary of Europa's "ocean notion." That is, whether or not an ocean truly exists underneath Europa's icy facade.

Jupiter's icy moon Europa is slightly smaller than the Earth's moon. Like the Earth, Europa is thought to have an iron core, a rocky mantle, and a surface ocean of salty water. Unlike on Earth, however, this ocean is deep enough to cover the whole surface of Europa, and being far from the Sun, the ocean surface is globally frozen over.

Europa orbits Jupiter every 3.5 days and is phase locked -- just like Earth's moon -- so that the same side of Europa faces Jupiter at all times. Tidal forces raise and lower the sea beneath the ice, causing constant motion and likely causing the cracks we see in images of Europa's surface from visiting robotic probes.

This "tidal heating" causes Europa to be warmer than it would otherwise be at its average distance of about 780,000,000 kilometers (485,000,000 miles) from the sun, more than five times as far as the distance from the Earth to the Sun. The warmth of Europa's liquid ocean could prove critical to the survival of simple organisms within the ocean, if they exist.

A top science goal of the Europa Orbiter, for example, would be to confirm or rule out the presence of a subsurface ocean on this intriguing moon. Additionally, a comprehensive study would be done of Europa's icy crust, to scout out possible zones where liquid might exist.

According to Dr. Bob Pappalardo, a key Jet Propulsion Laboratory scientist and expert on Europa, "such a mission would be able to confirm the existence of a subsurface ocean and provide important information about its habitability. If the findings indicate that Europa is a habitable environment, a future lander might search for evidence of past or existing life."

"If those results are positive," Pappalardo noted, it would be a turning point for humanity."

The spacecraft would also search for organic and inorganic material on Europa providing potential for astrobiologists to set the stage for further probing of the moon. For more information about Europa, please refer to a detailed feature, Taking Another Look at Europa, written by Dr.Pappalardo.

Enceladus: Galactic Geysers

On March 12, 2008, the Cassini spacecraft made a historic close flyby (within 50 km) of Saturn's most-talked about moon, Enceladus. The venerable Cassini spacecraft revealed a geyser-like plume of ice particles shooting up from Enceladus' South Pole region. This means there's a water source on the moon, and of course, water on another body in our solar system is an intriguing mystery that we want to take a closer look at.

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus.

Could microbial life exist inside Enceladus, where sunlight virtually cannot reach, photosynthesis is impossible and no oxygen is available? To answer that question, searching exotic ecosystems here on Earth is necessary to determine whether life exists or could have existed on Saturn's geyser moon. It is this tantalizing potential that warrants scientists to return to Enceladus for further study.

In recent years, life forms have been found on Earth that thrive in places where the sun doesn't shine and oxygen is not present because no photosynthesis takes place. Microbes have been discovered that survive on the energy from the chemical interaction between different kinds of minerals, and others that live off the energy from the radioactive decay in rocks. The ecosystems are completely independent of oxygen or organic material produced by photosynthesis at Earth's surface. These extraordinary microbial ecosystems are models for life that might be present inside Enceladus today. To take an interactive tour of Enceladus, click here

According to Bob Mitchell, Project Manager for the Cassini-Huygens Mission to Saturn and Titan... "One thing we'd like to understand better is just what's going on inside of Enceladus, what's causing, creating these geysers? Is there liquid water in there? What are the conditions, are there conditions in there that could be conducive to life? Now we're a long ways from saying that we have found life on Enceladus. But it's rather tantalizing based on what we've got so far to realize that these conditions could be very conducive. For Titan, of course, we found methane lakes up around the north pole, this from the radar instrument."

The Cassini flyby of Enceladus was monumental in the fact that the spacecraft dove directly into the geysers coming within 50 kilometers (30 miles) of the moon. Scientists were thrilled with the results of this crucial flyby and created a blog space for discussion about this daring encounter. To read NASA's blog entries about Enceladus, click here.

In summary, the ability to study these moons in detail would shed significant light on the phenomena of the universe as a whole as well as our place within it.

Titan, a complex, Earth-like system abundant with organics, should be the next scientific target in the outer Solar System. Titan shares features with both icy satellites and the terrestrial planets. It is subjected to tidal stresses, and its surface has been modified tectonically to form mountains and likely also by cryovolcanism, where liquid water and methane make their way to the surface from the interior. Titan has the largest concentration of organic material in the Solar System aside from Earth, and its active hydrological cycle is analogous to that of Earth, with methane replacing water. Titan's clouds, rain, flash floods, and greenhouse and anti-greenhouse effects may provide important lessons for Earth.

An orbital mission to Europa continues to be a top priority for exploration, as recommended by the 2007 NASA Science Plan, the 2006 Solar System Exploration Roadmap, and the 2003 planetary sciences "Decadal Survey." This high priority arises from the very strong indications that Europa has the "ingredients" necessary for life: a vast subsurface ocean, energy sources, and the elements from which organic molecules can be constructed. With its recent geological activity and potential surface-ice-ocean exchange of watery material, Europa is the archetype for understanding the habitability of icy satellites.

And finally, based on existing knowledge of Enceladus and 2003 Decadal Survey goals, the Science Definition Team (SDT) have developed challenging and exciting science goals for studying Enceladus and identified the possible mission configurations that could meet those goals. Orbiters, as well as a single flyby spacecraft, were considered with the added possibility for sample return and various lander-type options.

In essence, it is the study of these outer planet's moons that is creating the paradigm by which NASA missions might be designed in the decades to come. Stay tuned. I know we certainly are.

Last Updated: 10 February 2011

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Last Updated: 10 Feb 2011