Data from Cassini Provides Clues to the Climate and Seasons of Planets

While learning about the climate of other planets, what can we learn about Earth’s climate?

Liming Li, an assistant professor of physics in the University of Houston’s College of Natural Sciences and Mathematics, began his career in meteorology in China, concerned with weather conditions on Earth. When he came to the U.S. for his Ph.D., he turned his interests to planetary science, studying meteorology on other planets.

In recent years, Li has been actively involved in ongoing space missions exploring the giant planets in our solar system.

“The weather is so different among the various planets,” Li said. “By studying the weather systems on planets, we can get a wide perspective for how the climate changes on Earth.”

Now, through two new projects awarded by NASA’s Planetary Science Division and funded for $709,000, Li has the opportunity to study data collected by three instruments on board the Cassini spacecraft. Cassini is an international mission exploring Saturn’s systems and supported by NASA, the European Space Agency and the Italian Space Agency.

Li is leading a team which includes scientists from NASA’s Jet Propulsion Laboratory and the University of Wisconsin-Madison. The team hopes their data analysis will shed light on the climate of Jupiter, Saturn and its largest moon, Titan.

Cassini Mission Provides Years of Data

Fortunately, Cassini is a long-term mission – gathering data for more than 10 years,” Li said. “Every year, every day, we are getting beautiful data from the spacecraft.”

The Cassini-Huygens unmanned spacecraft launched aboard a rocket in 1997. Once deployed on its mission, it passed Jupiter and gathered data from that planet in 2000 and 2001. Reaching Saturn in 2004, the craft included the Cassini orbiter and Huygens lander, which landed on Titan in January 2005.

With 12 data-gathering instruments aboard Cassini, scientists have access to an unprecedented amount of data. The primary mission, scheduled to end in 2008, was so successful that NASA extended it several times. It is now slated to end in late 2017.

Revealing Saturn’s Seasons

“For the first time, we will be able to learn about the seasonal changes of Saturn. The Saturn year is roughly 30 Earth years, so you need long-term observations to learn about the seasons,” Li said.

Earth and Saturn have roughly the same rotational angle, which Li says means similar seasonal changes. “We would expect a strong seasonal change for Saturn, but before this mission, we didn’t have the data,” he said.

With data from 2004 to 2017, observations will cover half a Saturn year providing data from 2-3 seasons.

For an atmospheric scientist, seasonal change is an important topic. It offers the opportunity to look at the climate change in a short-term scale.

“Saturn’s atmospheric systems differ with the seasons,” Li said. “For example, in spring and summer there are giant storms. Through Cassini, we observed probably the biggest storm in our solar system. It was 100,000 km wide, which is more than 62,000 miles. That is much bigger than a storm on Earth, and actually, bigger than Earth.”

Energy Budget Data Impacts Understanding of Planetary Climate and Evolution

With observations from Cassini, Li and his team are calculating the energy budget for Jupiter, Saturn and Titan. The energy budget accounts for how much energy comes into a planet’s climate system from the Sun and how much is emitted.

“On Earth, the incoming energy is about the same as the outgoing energy. That means the temperature doesn’t change dramatically, even with greenhouse effects,” he said. “Saturn and Jupiter are emitting more energy than absorbed; they are generating some type of internal heat. Earth and Titan are similar, with no significant internal heat.”

Knowing whether the energy budget is balanced or imbalanced and how the energy budget changes with time is important for understanding climate change and the evolution of a planet, Li says.

“Scientists think that Titan’s atmosphere is like the ancient atmosphere on Earth. By studying Titan’s atmosphere, we can learn what has happened in the past to Earth’s atmosphere,” he said.

Data Provided by Three Cassini Instruments

Li is analyzing data from three instruments. The Composite Infrared Spectrometer provides information about the chemical component of the planet’s spectrum. From this data, they can determine the temperature.

With Cassini’s Imaging Science Subsystem, Li has access to images of the planet at different visible wavelengths. The system’s two cameras provide a narrow angle, high resolution view of a fixed area and a wide angle, low resolution image of larger areas. The wide angle images help them identify interesting areas that can be studied using the higher resolution images.

The third instrument, the Visible and Infrared Mapping Spectrometer, includes a camera and a spectrometer, providing real-time temperature and a visual at the same time. “The spectral resolution of the spectrometer isn’t good, but if we see something interesting, we can access images of that area taken by the camera,” Li said.

In addition to the project team, a postdoctoral fellow and several students are involved. Participating from Physics are Li’s postdoctoral fellow Daniel Liang, graduate students Yefeng Pan and Aaron Studwell, and undergraduate Joseph Hernandez. Earth and Atmospheric Sciences graduate student Justin Trammel is also involved in the research project.

Through this data analysis, Li hopes to learn more about the processes operating in the atmospheres of giant planets and their natural satellites.

“By studying other planets in different stages of their life, we can learn about the past and future of our Earth,” Li said.

- Kathy Major, College of Natural Sciences and Mathematics