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.
NASA/JPL

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.

How we discovered 840 minor planets beyond Neptune – and what they can tell us

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The Canada-France-Hawaii Telescope (CFHT) at sunset, which observed the OSSOS survey.
wikipedia, CC BY-SA

By Michele Bannister, Queen’s University Belfast

Our solar system is a tiny but wonderfully familiar corner of the vast, dark universe – we have even been able to land spacecraft on our celestial neighbours. Yet its outer reaches are still remarkably unmapped. Now we have discovered 840 small worlds in the distant and hard-to-explore region beyond Neptune. This is the largest set of discoveries ever made, increasing the number of distant objects with well known paths around the sun by 50%.

These little icy worlds are important as they help us tell the solar system’s history. They can also help us test the idea that there’s a yet unseen planet lurking in the outer solar system.

Our planetary system as we see it today is not as it formed. When the sun was newborn, it was surrounded by a massive disk of material. Encounters with tiny, growing planets – including some of the worlds we’ve just discovered – moved the giant planets outward from the sun until they settled into their present locations. The growing planets, on the other hand, went everywhere, scattering both inward and outward.

Planetary migration also happened in far away systems around many other stars. Fortunately, the celestial bodies in our own planetary system are comparatively close by, making it the only place where we can see the intricate details of how migration happened. Mapping the minor planet populations that are left over from the disk lets us reconstruct the history of how the big planets were pushed into place.

Mapping the sky

The new discoveries were made as part of a five year project called the Outer Solar System Origins Survey (OSSOS). The observations, conducted in 2013-2017, used the imaging camera of one of the world’s major telescopes – the Canada-France-Hawaii Telescope on Maunakea in Hawaii. The survey looked for faint, slow-moving points of light within eight big patches of sky near the plane of the planets and away from the dense star fields of the Milky Way.

With 840 discoveries made at distances between six and 83 astronomical units (au) – one such unit is the distance between the sun and the Earth – the survey gives us a very good overview of the many sorts of orbits these “trans-Neptunian objects” have.

Earlier surveys have suffered from losing some of their distant discoveries – when too few observations occur, the predicted path of a minor planet in the sky will be so uncertain that a telescope can’t spot it again, and it is considered “lost”. This happens more to objects with highly tilted and elongated orbits, producing a bias in what’s currently known about these populations.

Our new survey successfully tracked all its distant discoveries. The frequent snapshots we made of the 840 objects over several years meant that each little world’s orbit could be determined very precisely. In total, more than 37,000 hand-checked measurements of the hundreds of discoveries precisely pinned down their arcs across the sky.

We also created an accompanying software “simulator” (a computer model), which provides a powerful tool for testing the inventory and history of our solar system. This lets theorists test out their models of how the solar system came to be in the shape we see it today, comparing them with our real discoveries.

Strange new worlds

The new icy and rocky objects fall into two main groups. One includes those that reside on roundish orbits in the Kuiper belt, which extends from 37au to approximately 50au from the sun. The other consists of worlds that orbit in a careful dance of avoidance with Neptune as it travels around the sun. These “resonant” trans-Neptunian objects, which include Pluto, were pushed into their current elongated orbits during Neptune’s migration outwards.

In the Kuiper belt, we found 436 small worlds. Their orbits confirm that a concentrated “kernel” of the population nestles on almost perfectly round, flat orbits at 43 to 45au. These quiet orbits may have been undisturbed since the dawn of the solar system, a leftover fraction of the original disk. Soon, we will see a member of this group up close: the New Horizons spacecraft, which visited Pluto in 2015, will be flying by a world that’s about the size of London on New Year’s Day 2019.

The dwarf planet candidate 2015 RR245 is on an exceptionally distant orbit, but is one of the few dwarf planets that could one day be reached by a spacecraft mission.
Alex Parker/OSSOS, CC BY-SA

We found 313 resonant trans-Neptunian objects, with the survey showing that they exist as far out as an incredible 130au – and are far more abundant than previously thought. Among these discoveries is the dwarf planet 2015 RR245, which is about half the size of Britain. It may have hopped onto its current orbit at 82au after an encounter with Neptune hundreds of millions of years ago. It was once among the 90,000 scattered objects of smaller size that we estimate currently exist.

Are there more planets?

Among the most unusual of the discoveries are nine little worlds on incredibly distant orbits, never coming closer to the sun than Neptune’s orbit, and taking as long as 20,000 years to travel around our star. Their existence implies an unseen population of hundreds of thousands of trans-Neptunian objects on similar orbits.

Artist’s concept of Planet Nine.
NASA/JPL-Caltech/Robert Hurt, CC BY-SA

How these objects got on their present paths is unclear — some orbit so far out that, even at their closest approach, they are barely tugged by Neptune’s gravity. One explanation that has been put forward is that a yet unseen large planet, sometimes called “Planet Nine”, could be causing them to cluster in space. However, our nine minor planets all seem to be spread out smoothly, rather than clustering. Perhaps the shepherding of such a large planet is more subtle – or these orbits instead formed in a different way.

The ConversationThe history of our solar system is just beginning to be told. We hope this new set of discoveries will help piece together the story.

Michele Bannister, Research Fellow, planetary astronomy, Queen’s University Belfast

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

A giant ‘singing’ cloud in space will help us to understand how star systems form

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The dark band is the Dark Doodad Nebula, a place where new stars and planets can form.
Flickr/cafuego, CC BY-SA

By Aris Tritsis, Australian National University

We know that the birthplaces of stars are large molecular clouds of gas and dust found in space.

But what exactly determines the number and kind of stars and planets that are formed in these clouds? How was our Solar system nursed and how did it emerge from such a cloud billions of year ago?

These are mysteries that have been puzzling astronomers for decades, but research published today in Science adds an extra dimension to our understanding.

A 3D approach

Knowledge of the 3-dimensional structure of these clouds would be an important leap in our understanding of how stars and planets are born.




Read more:
From pancakes to soccer balls, new study shows how galaxies change shape as they age


The physics responsible for the formation of stars is also responsible for shaping the clouds. But even with the most advanced telescopes in the world we can only see the two-dimensional projections of clouds on the plane of the sky.

Thankfully, there is a way around this problem. A recently discovered type of structure in molecular clouds, called striations, was found to form because of waves.

Here enters Musca, a molecular cloud that “sings”. Musca is an isolated cloud in the Southern sky, below the Southern Cross, that looks like a thin needle (see top image). It is hundreds of light years away and stretches about 27 light years across, with a depth of about 20 light years and width up to a fraction of a light year.

Musca is surrounded by ordered hair-like striations produced by trapped waves of gas and dust caused by the global vibrations of the cloud.

3D model of Musca molecular cloud.
Aris Tritsis, ANU, Author provided

Trapped waves act like a fingerprint – they are unique and can be used to identify the sizes of the boundaries that trapped them. Boundaries are naturally created at the edges of clouds where their physical properties change abruptly.

Just like a cello and a violin make very distinct sounds, clouds with different sizes and structures will vibrate in very different manners – they will “sing” different “songs”.

A ‘song’ in the cloud

By using this concept and calculating the frequencies seen in observations of Musca it was possible to measure for the first time the third dimension of the cloud, the one that extends along our line of sight.

The frequencies found in the observations were scaled to the frequency range of human hearing to produce the “song of Musca”.

A “singing” molecular cloud.

The results from this method were amazing. Despite the fact that Musca looks like a thin cylinder from Earth, the true size of its hidden dimension is not small at all. In fact, it is comparable to its largest visible dimension on the plane of the sky.

No longer a thin cylinder when the extra dimension is revealed (Aris Tritsis)

Musca is not actively forming stars. It will be millions of years before gravity can overcome all opposing forces that support the cloud.




Read more:
Signals from a spectacular neutron star merger that made gravitational waves are slowly fading away


As a result, with its structure now determined, Musca can be used as a prototype laboratory against which we can compare our models and study the early stages of star formation.

The ConversationWe can use Musca to better constraint our numerical models and learn about our own Solar system. It could help solve many mysteries. For example, could the ices found in comets have formed in clouds rather than at a later time during the life of our solar system?

Aris Tritsis, Postdoctoral Fellow, Australian National University

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