Capturing the shadow of Saturn’s moon Titan from right here on Earth

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NASA’s Cassini spacecraft captures Saturn’s largest moon, Titan, passes in front of the planet and its rings.
NASA/JPL-Caltech/Space Science Institute

David Coward, University of Western Australia

Titan is Saturn’s largest moon, and it is more like a planet than a moon in many respects.

It has a thick atmosphere as well as wind, rivers, lakes made of hydrocarbons such as methane, and a liquid water ocean. Understanding its atmosphere may help us in the search for life on other planets.

Hence the excitement this July when a rare opportunity was available to further study Titan, from right here on Earth. On July 18 at 11:05pm (WAST, Western Australian time) Titan passed in front of a faint star, as seen by observers across most of Australia.

Read more:
The secrets of Titan: Cassini searched for the building blocks of life on Saturn’s largest moon

This event, known as an occultation, lasted only a few minutes and about 2% of the star’s light was blocked by Titan’s atmosphere.

The effect was so small it required large telescopes and a special camera to record it. But the data gathered should have profound implications for our understanding of an atmosphere on another world.

Saturn’s moon Titan compared (by diameter) to the Earth and its Moon.
Wikimedia/The Conversation

Examining Titan’s atmosphere

Scientists have developed a very clever technique to examine Titan’s atmosphere using stellar occultations. As Titan enters and exits an occultation, the star’s light would illuminate the atmosphere from behind, but be blocked by the moon itself.

Scientists then record subtle changes in brightness of the star over a few minutes, which represents a profile of the atmosphere’s density with height.

This method was used to study Titan’s atmosphere before, during a stellar occultation in 2003.

Artist’s concept of Cassini’s June 4, 2010, flyby of Saturn’s moon Titan.

But in 2005, when Cassini’s Huygens lander arrived at Titan and descended to its surface, the atmospheric profile measured from its instruments did not match that derived from the 2003 occultation. This fuelled the question of how variable is the state of Titan’s atmosphere.

Composite of Titans surface taken by Huygens at different heights.
ESA/NASA/JPL/University of Arizona

Since the Cassini mission ended in 2017, NASA’s Karsten Schindler said there was keen interest in any new atmospheric observations from occultations:

Occultations remain the only means to study Titan’s upper atmosphere and its evolution for the foreseeable future.

Countdown to the July occultation

So how were the latest observations made, and how was the data gathered?

From the air, the plan was for the July 18 occultation to be recorded by a camera mounted on a telescope of the Stratospheric Observatory of Infrared Astronomy (SOFIA) on board a Boeing 747 aircraft.

SOFIA takes off from Christchurch International Airport in 2017.
SOFIA/ Waynne Williams

That’s right: a telescope mounted inside a modified passenger plane imaging an object more than 1 billion kilometres away! SOFIA would fly above the clouds between Australia and New Zealand.

From the ground, several facilities across Australia were to attempt to record the occultation.

The University of Western Australia’s Zadko Telescope, located about 80km north of Perth (see map, below), was identified by NASA as a ground facility sensitive enough to contribute to the project.

The most obvious deal breaker was the weather. July is one of the wettest months at the Zadko telescope site. But, as we found out, there were other unforseen challenges.

Three days to occultation

NASA’s Karsten Schindler arrived at the UWA research site, at Gingin, on Monday July 16, armed with a case filled with delicate cameras, cables and electronics.

The camera was the key to record the event. The current Zadko telescope camera cannot record fast enough to capture the rapid changes in brightness of the occulted star.

The Zadko Telescope was fitted out with a fast shooting (a frame every few seconds), NASA camera, more like a movie camera than a standard astronomical camera. After hours of installation, the new imaging system needed to be tested.

Ground occultation team: John Kennwell, Arie Verveer, Karsten Schindler with the Zadko Telescope in the background.

Unfortunately, the observatory roof would not open because of a faulty sensor. No Monday test, but hey, we still had Tuesday to test the system? Onsite engineers scrambled to fix the sensor ready for Tuesday.

Two days to occultation

On Tuesday, I received the following text message from the site.

11:07pm: Rain sensor working but clouded out … cheers Arie. So no chance testing the camera and weather forecast for Wednesday was bleak.

The day of occultation

Despite the cloud and nearly constant rain showers, team occulation (Karsten, Arie and John) were on site ready to start pointing the telescope and activate the imaging.

“Up to 10pm it was still raining,” 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 vaporise away, leaving a totally cloudless sky with 100% visibility. I have never seen anything like it.”

The team swung into action, pointing the telescope at the target star, focusing the camera. At the designated occulation time 11:05pm, Karsten hit the image acquisition button, enabling the camera to take hundreds of images over a few minutes.

Read more:
What Cassini’s mission revealed about Saturn’s known and newly discovered moons

Eager to see if the data contained the signature of an occulation, the team performed a preliminary analysis within minutes. Yes, there was a clear occulation signature, a big dip in the brightness of the star at exactly the predicted time of the occulation.

Next morning I was informed that SOFIA had also captured the event.

The data recorded from the Australian ground stations and by SOFIA will be analysed over the coming weeks and published in peer reviewed journals.

The ConversationBut one thing the journals won’t highlight is the excitement of the observation, and the enormous effort by a few individuals who helped acquire this data that should hopefully give us a better understanding of the atmosphere of Titan.

David Coward, Associate professor, University of Western Australia

This article was originally published on The Conversation. Read the original article.

Rover detects ancient organic material on Mars – and it could be trace of past life

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The Curiosity rover on Mars has been busy.

By Monica Grady, The Open University

It was to a great fanfare of publicity that researchers announced they had found evidence for past life on Mars in 1996. What they claimed they had discovered was a fossilised micro-organism in a Martian meteorite, which they argued was evidence that there has once been life on the Red Planet. Sadly, most scientists dismissed this claim in the decade that followed – finding other explanations for the rock’s formation.

While we know that Mars was habitable in the past, the case demonstrates just how hard it will be to ever prove the existence of past life on its surface. But now new results from NASA’s Curiosity rover, including the discovery of ancient organic material, have revived the hope of doing just that. Understandably, the authors of the two papers, published in the journal Science, are very careful not to make the claim that they have discovered life on Mars.

While the 1996 discovery has never been verified, it hasn’t ever been conclusively disproved either. What the study has done, though, is to propel the search for life on Mars higher up the list of international space exploration priorities – giving space agencies ammunition to argue for a coordinated programme of missions to explore the Red Planet.

Mineral veins on Mars seen by Curiosity.

Curiosity is the latest rover to trundle across the gritty sands of Mars. It has been tacking across the floor of Gale Crater on Mars for five years, returning stunning images of Martian landscapes, with vistas opening up to show rocky outcrops seamed with mineral veins. Close up, the veins have the appearance and chemistry of material that has been produced by reaction of water with the rocks, at a time when water was stable at the surface for extended periods of time. Such reactions could create enough energy to feed microbial life.

Ancient rocks

One of the papers reports the discovery of low levels of organic carbon in mudstones from Gale Crater. This might not sound like much carbon – but finding it at all is a big deal, since organic material could be traces of decayed living matter.

The sediments, analysed by the SAM instrument on Curiosity, come from just below the surface, where they have been shielded from most of the UV radiation that would break down organic molecules exposed on the surface. The organic material discovered on Mars is rich in sulphur, which would have also helped to preserve it.

However, the environment in which the mudstones were deposited – a 3.5-billion-year-old lake bed – would have been altered in other ways as the sediments settled and compressed to become rock. Over the intervening years, fluid flowing thought it would have initiated chemical reactions that could have destroyed the organic matter – the material discovered may in fact be fragments from bigger molecules. In rocks on Earth, such reactions – which causes living matter mainly from plants and microbes to degrade – produce an insoluble material known as kerogen.

Excitingly, the material discovered on Mars is similar to terrestrial kerogen. But that doesn’t necessarily mean it is biological in origin – it is also similar to an insoluble material in tiny meteorites that rain down on the surface of Mars.

At this point, we simply don’t know whether the origin is biological or geological. But it is the preservation of the material that is important – if there is this much organic matter preserved close to the surface, then there should be even better protected material at greater depths. What is needed to find more clues is a mission to Mars with a deep drill. Luckily there is one: ESA’s ExoMars rover, scheduled for launch in two years’ time.

Mysterious methane

The second paper investigates a problem that has been disturbing Mars scientists for several years: the abundance of methane in Mars’ atmosphere. Earth-based telescopes, spacecraft orbiting Mars and now Curiosity, have measured episodic sudden increases in the background methane content.

While this might be taken as a signature of biological activity – the main producers of methane on Earth are termites and bovine gut bacteria – non-biological mechanisms, such as weathering of Martian rocks or release from ancient ice, are possible too.

Gale crater on Mars.

The new results represent the longest systematic record of atmospheric methane, with measurements taken regularly over five years. What the authors have found is a systematic variation in methane concentration with season, with the highest concentrations occurring at the Gale Crater towards the end of the northern summer. This is the period when the southern icecap – which freezes carbon dioxide out of the atmosphere, but not methane – is at its biggest, so enhanced methane is not unexpected. However, the abundances of methane measured are greater than models predict should occur, meaning we still don’t know exactly how they are produced.

The team also found several spikes where methane abundance suddenly jumped to be higher than average during the year. The authors conclude that this must be related to surface temperature. They therefore suggest that methane could be trapped at depth, gradually seeping to the surface. Here it is retained by the soil until the temperature increases sufficiently to release the gas.

However the paper states that, despite this, there “remain unknown atmospheric or surface processes occurring in present-day Mars”. While the authors do not specify biology as one of those unknown processes, it remains an intriguing possibility. This, to me, is a cue for further measurements – and fortunately, we may know soon. ESA’s Trace Gas Orbiter is now in place at Mars, and has just started recording data.

So, what can we conclude after reading these two papers? That even with the superb instrument array carried by Curiosity, and detailed modelling and interpretation of the results, we are still left looking for evidence of life on Mars. Is it a romantic yearning to discover that we do have companions within the solar system (even if they are likely to be very small and uncommunicative)? Or is it that our theories of how life arose on Earth cry out to be verified by a “second genesis”?

The ConversationWhatever the reason, there is still much to be discovered on Mars. Luckily, a series of missions planned well into the next decade will help us make those discoveries. These include the return of Martian samples to Earth, where we can carry out even more detailed analyses than Curiosity.

Monica Grady, Professor of Planetary and Space Sciences, The Open University

This article was originally published on The Conversation. Read the original article.