Bonnie J. Buratti

After so many close flybys of Enceladus, we’re starting to feel as if this little moon of Saturn is an old friend. But during the encounter planned for Nov. 2, 2009, we are going to get up-close and personal. Cassini is going to take its deepest dive yet into the plumes spewing out from the moon’s south pole to try to learn more about their composition and density.

The spacecraft is going to approach within about 100 kilometers (62 miles) of the surface. We’ve been closer before (25 kilometers or 15 miles), but we’ve never plunged quite so deeply into the heart of the plume.

To get a better sense of our flyby, watch the animation created by my colleague Brent Buffington. This is the seventh targeted flyby of Enceladus, so we sometimes refer to it as “E7.” The video starts out with our approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Then at around 7:40 a.m. UTC (Coordinated Universal Time), we do our long-anticipated flyby through the plumes. The passage will be quick: traveling at about 8 kilometers per second (about 5 miles per second) - fast enough to go from Los Angeles to New York in less than 9 minutes - we’ll spend only about a minute in the plume.

This animation shows Cassini’s approach to Enceladus, rotating through the various instruments scanning Enceladus for data. Image credit: NASA/JPL.
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Then, we zoom away from the plumes and Cassini turns on an infrared instrument (red rays in the animation) to take the temperature of the south-pole fissures known as “tiger stripes” where the plumes originate. A few minutes later, Cassini uses an ultraviolet instrument (purple rays in the animation) to measure the plumes against the background of the peach-colored Saturn. The infrared instrument then gets another turn to examine Enceladus. For more details, see the mission description.

The focus of this flyby is to analyze the particles in the plume with instruments that can detect the size, mass, charge, speed and composition. Instead of using its eyes (the cameras), Cassini is going to use its senses of taste and smell. But we’re going to get some pretty good pictures too, including some impressive shots of the plumes from far away.

So far, we have detected water vapor, sodium and organic chemicals such as carbon dioxide in the plumes that spew out from the tiger stripes, but we need more detail. Are there just simple organic molecules, or something more complex? This is the first time we’ve found activity on a moon this small (the width of Arizona, 500 kilometers or 310 miles in diameter). We really want to understand what’s driving the plumes, especially whether there is liquid water underneath the surface. If we can put the pieces together - a liquid ocean under the surface, heat driving the geysers and the organic molecules that are the building blocks of life - Enceladus might turn out to have the conditions that led to the origin of life on an earlier version of Earth.

So if this is all so interesting, why did we wait so long to travel into the plumes? One reason is the plunge is tricky. We wanted to make sure we could do it. We worried that plume particles might damage the spacecraft. We did extensive studies to determine that it was safe at these distances. We also wanted to have the right trajectory so we didn’t use an excessive amount of rocket fuel. We are going very fast through this sparse plume; so to play it safe, we’re using Cassini’s thrusters to keep it stable through this flyby.

We’ll be monitoring the thrusters closely because we don’t want to have to use them on another flyby through the plumes planned for April 28, 2010. In the future flyby, we plan on tracking the spacecraft very closely with the radio instruments on Cassini and on Earth so we can measure how the spacecraft wobbles as it passes near Enceladus. These measurements should tell us more about the interior of the moon, including whether it really does have a liquid subsurface ocean. With the thrusters on, we won’t be able tell if the motion of the spacecraft comes from the gravity of Enceladus or the thrusters. We’d like to know whether we can rely on other kinds of attitude control equipment.

We’re all eager to pore over the results of this flyby. Stay tuned. In the meantime, feast your eyes on this map of the surface of Enceladus that the Cassini imaging team has updated and released today. The tiger stripes are located in the lower middle left and lower middle right of the image.

Today, JPL Earth scientist Hui Su joins thousands of other bloggers in more than 130 countries around the world for the Blog Action Day ‘09 Climate Change.

Blog Action Day is an annual event that unites the world’s bloggers in posting about the same issue on the same day, with the aim of sparking discussion around an issue of global importance. The theme of this year’s event, climate change, affects us all and will be the topic of international climate negotiations taking place in Copenhagen, Denmark, this December.

As a world leader in studying Earth’s climate, NASA researchers play a vital role in shaping our understanding of global change. In today’s post, Su discusses the critical role clouds play in climate, and why learning more about them is a key to predicting how our climate will change in the future.

For more information on Blog Action Day, visit: http://www.blogactionday.org .

Hui Su

Clouds are among the most fascinating natural phenomena and have inspired countless works of literature and art. Their ever-changing forms make them a great challenge to atmospheric scientists working to predict how our climate will change in the future in response to increasing greenhouse gases such as carbon dioxide.

Clouds occur at many different heights in our atmosphere and take many different forms. There are three main types of clouds: stratus, cumulus and cirrus. Stratus clouds are low clouds, usually within 2 kilometers (7,000 feet) above the surface. They look like a gray blanket, extending thousands of kilometers across the sky. Cumulus clouds look like puffy cotton balls and extend vertically for large distances. The third type is wispy and feathery-looking cirrus. Cirrus clouds are usually high in the sky, about 7 kilometers (23,000 feet) above the surface. These three types of clouds have different impacts on Earth’s climate due to their unique abilities to reflect sunlight and trap heat radiated from Earth’s surface.

Artist’s concept of NASA’s CloudSat spacecraft, which is providing the first global survey of cloud properties to better understand their effects on both weather and climate. Image credit: NASA/JPL

Su et al. (2008, Journal of Geophysical Research) suggested that cirrus clouds increase as sea surface temperature becomes warmer, further enhancing surface warming. Image credit: NASA/JPL/Caltech
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Stratus clouds can effectively block sunlight from reaching the surface; therefore, they act as an umbrella that cools Earth. Cirrus clouds are relatively transparent to sunlight but can trap terrestrial radiation, JUST AS carbon dioxide does, so they have a net warming effect on Earth. Cumulus clouds can block sunlight and also trap terrestrial radiation. Their net effect varies greatly depending on their actual heights and thicknesses.

Climate scientists have long struggled to quantify how different types of clouds change when global warming occurs. For example, an increase in stratus clouds may cool Earth’s surface, compensating for global warming; while an increase in cirrus clouds may further warm Earth’s surface, exacerbating global warming. Up to now, scientists have not been able to come to a consensus as to whether stratus, cumulus or cirrus clouds will increase or decrease as global temperatures increase.

A key advancement in cloud studies in recent years has been the availability of global satellite observations of clouds, especially the measurements of clouds at different heights provided by NASA satellites like CloudSat, managed by NASA’s Jet Propulsion Laboratory (JPL). These observations are allowing scientists to better simulate clouds in climate models, which are the primary tools climate scientists use to predict future climate change. Up till now, the dynamic nature of clouds has made them very difficult to simulate in current climate models. But by applying space data, we at JPL are working closely with modelers to improve cloud simulations and thereby improve predictions of future climate change.

To learn more about JPL’s research in this field and the CloudSat mission, visit:
http://cloudsat.atmos.colostate.edu/home .