It has a dense atmosphere as well as wind, rivers, lakes of hydrocarbons like methane and a liquid sea of water. Understanding its atmosphere can help us find life on other planets.
Hence the excitement this July, when a rare opportunity was available to study Titan from here on Earth. At 8:05 pm on July 18 (WAST, West Australian time), Titan passed before a faint star, as observers in much of Australia have seen.
This event known as occultation lasted only a few minutes 2% of the star's light was blocked by the Titan atmosphere.
The effect was so small that large telescopes and a special camera were required to record them. But the data collected should have profound implications for our understanding of an atmosphere in another world.
Examining the Titan's atmosphere
Scientists have developed a very clever technique to study the Titan's atmosphere with the help of stellar coverings. When Titan enters and leaves an occultation, the star's light would illuminate the atmosphere from behind but be blocked by the moon itself.
The scientists then record subtle changes in the brightness of the star over a few minutes, which is a profile of the density of the atmosphere at altitude.
This method was used to study the Titan atmosphere in front of a star cover in 2003.
But in 2005, when Cassini's Huygens lander arrived at Titan and sank to its surface, the atmospheric profile measured by his instruments did not match that derived from the 2003 coverage. This led to the question of how variable the state of the titanium atmosphere is.
Since the Cassini mission ended in 2017, NASA's Karsten Schindler showed great interest in new atmospheric observations by cover: "Occultation remains the only means of studying Titan's upper atmosphere and its development for the foreseeable future."
How were the last observations made and how was the data collected?
From the air, the plan for the veiling on 18 July was through a camera mounted on a telescope of the Stratospheric Observatory for Infrared Astronomy (SOFIA) aboard a Boeing 747.
That's right: a telescope mounted in a modified passenger plane that maps an object more than 1 billion kilometers away! SOFIA would fly above the clouds between Australia and New Zealand.
From the ground, several facilities throughout Australia should try to capture the cover.
The University of Western Australia's Zadko Telescope, located approximately 80 km north of Perth (see map below), has been identified by NASA as a ground facility sensitive enough to contribute to the project.
The The most obvious deal breaker was the weather, July is one of the wettest months on the Zadko telescope, but, as we found out, there were other unforeseen challenges.
Three Days to Be Covered
Karsten Schindler of NASA arrived at the UWA Research Center in Gingin on Monday, July 16, armed with a suitcase full of cameras, cables, and electronics.
The camera was the key to recording the event. The current Zadko telescope camera can not record fast enough to capture the fast brightness changes of the hidden star.
The Zadko telescope was equipped with a fast shot (every few seconds a frame), NASA camera, more like a movie camera than a standard astronomical camera. After hours of installation, the new imaging system had to be tested.
Unfortunately, the observatory could not open due to a faulty sensor. No assembly test, but hey, we had Tuesday to test the system? Engineers on site tried to fix the sensor for Tuesday.
Two Days to Obscure
On Tuesday, I received the following SMS from the site. "23:07 clock: Rain sensor works, but cloudy … Cheers Arie. So no chance to test the camera and weather forecast for Wednesday, was bleak."
The Day of Covering
Despite the cloud and almost constant rain showers, team members (Karsten, Arie and John) were on-site ready to set up the telescope and activate the imaging.
"It was still raining until 10 pm," Karsten told me the next morning. "Then a miracle happened."  Less than an hour before the event, and he said the weather changed.
"The clouds seemed to evaporate, leaving a completely cloudless sky with 100% visibility and I have never seen anything like it."
The team stepped into action and aimed the telescope at the target star, focusing the camera , At the fixed exposure time at 23:05 Karsten pressed the image acquisition button, so that the camera could take hundreds of pictures within a few minutes.
Eager to see if the data contained the signature of a trigger, the team conducted a pre-investigation by analysis in minutes. Yes, there was a clear signature of the irradiance, a strong decrease in the brightness of the star at exactly the predicted time of triggering.
The next morning I was told that SOFIA had also captured the event.
The data recorded by the Australian ground stations and SOFIA will be analyzed and published in journals in the coming weeks.
But one thing the journals do not emphasize is the excitement of observation and the tremendous effort some people have made to get that data, which hopefully would give us a better understanding of Titan's atmosphere.
SOFIA to study the southern sky in New Zealand