Physics update course December 2012

Saturday 15th December

Trip to Diamond Light Source Facility

Robotic planetary exploration – Revealing the mysteries of our Solar System


Dr Fletcher is a planetary scientist and Royal Society Research Fellow at the University of Oxford. Dr Fletcher specialises in exploring the formation, dynamics, meteorology and chemistry of planetary atmospheres, for both the gas and ice giants in our own Solar System, and for giant planets around other stars. He gained his PhD from Oxford in 2007 and conducted his postdoctoral research at the Jet Propulsion Laboratory in California.

His primary research involves the acquisition and analysis of images and spectra of the giant planets from the visible to the sub-millimetre from a variety of ground-based observatories, space-borne telescopes and planetary probes, notably the Cassini mission to Saturn and Titan. These data are used to understand the fundamental physics and chemistry that shape planetary atmospheres in our solar system and beyond.

The number of giant planets discovered around stars throughout our galaxy has exploded over the last decade, and we now know them to be common features of planetary systems.

Given that we’ll never be able to observe the physical and chemical conditions on these giant exo-worlds directly, we must now consider the gas and ice giants in our Solar System as the four closest examples of a whole class of astrophysical objects. Yet despite decades of exploration and telescopic observation, some of our most basic questions about the origins and physical phenomena at work on these planets remain unanswered …..

What tropospheric processes causes Jupiter’s stripes to change their appearance, such as the 2009-10 fade of the South Equatorial Belt? What powers the incredible planet-encircling storm systems, hexagonal waves and polar vortices, as observed by the Cassini spacecraft at Saturn? And why do the ice giants appear so different, with sluggish Uranus contrasted with the dynamic cloud variability on distant Neptune?

By investigating each of these seemingly isolated phenomena, we are coming closer to explaining how planetary how planetary atmospheres vary as a function of distance from the parent star, and how our planetary system formed and evolved over the past 4.5 billion years.

The lecture discussed: The latest Cassini discoveries at Saturn and plans for its end-of-mission in 2012; The challenges of building long-lived robotic spacecraft for the frigid outer reaches of our Solar System; And, the European Space Agency’s ambitious new mission to Jupiter and its collection of potentially-habitable icy satellites (JUICE).

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The above picture on the right is of the planets in our Solar System to scale in size (but not position).

Cosmic Vision is the current cycle of ESA’s long-term planning for space science missions including a mission to Jupiter. JUICE (Jupiter Icy Moons Explorer), a large (L-class mission) for launch in about 2022 (the project is expected to last until 2030). Jupiter’s largest moon is Ganymede and it is bigger than Mercury.

Saturn’s moon Titan is of interest because it has a substantial atmosphere (like the Earth once) and its investigation is part of the Cassini mission now.


The Moon is the furthest Man has got in person and that was over forty years ago.

Robotics allows views from space not available from other methods. The Voyager 1 spacecraft, at the request of Carl Sagan, took the famous picture known as the Pale Blue Dot. It is, in fact, a picture of the Earth from 6 billion kilometres away.

The picture below shows six different perspectives of planetary surfaces. Venus’ surface showing it has dry baked soil with a dried up lake.

Titan, as shown from the Cassini mission, has water ice pebbles with a brown sludge coating caused by atmospheric nitrogen along with a sort of brown gunge.


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Gas giants are believed to have the sort of atmospheres they do partly because they don’t have mountains.

The bands seen in the Jovian atmosphere are due to counter-circulating streams of material called zones and belts, encircling the planet parallel to its equator. The zones are the lighter bands, and are at higher altitudes in the atmosphere. They have an internal updraft, and are high-pressure regions. The belts are the darker bands. They are lower in the atmosphere, and have an internal downdraft. They are low-pressure regions. These structures are somewhat analogous to high- and low-pressure cells in Earth’s atmosphere, but they have a very different structure—latitudinal bands that circle the entire planet, as opposed to small confined cells of pressure. This appears to be a result of the rapid rotation and underlying symmetry of the planet. There are no oceans or landmasses to cause local heating, and the rotation speed is much faster than it is on Earth. There are smaller structures as well: spots of different sizes and colours. On Jupiter, the most noticeable of these features is the Great Red Spot, which has been present for at least 300 years. These structures are huge storms. Some such spots are thunderheads as well.

Best templates of exoplanets are the planets in our Solar System.


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Mars is the easiest planet for us to get to and land on.

The various missions to Mars have shown its surface to have impact basins and the largest volcano (Olympus Mons) in the Solar System. The polar ice caps consist of solid carbon dioxide. There is evidence that there has been water but where did it go?

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Dust devils on Mars are a problem as they affect the solar cells on robots.

Mars Curiosity Rover is nuclear powered and uses a laser to vapourise rock.

Skycrane was the method of producing a soft landing for the Rover.

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This artist’s concept shows the sky crane manoeuvre during the descent of NASA’s Curiosity rover to the Martian surface.

The Gale Crater is an enormous mound made up of sedimentary layers.

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Some planets/moons appear to be hotter than they should be considering their distance from the Sun. This could be due to tidal stretching and flexing caused by the gravitational pull of surrounding planets. This could make the moons around the gas giants habitable.

The great red spot on Jupiter seems to be shrinking.


Planets that have tilted axes have seasons. Saturn looks blue because of haze particles in the atmosphere.

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The rather hazy picture above right is a chart with each line representing a mission. Cassini took over seven years to reach Saturn.

Remote sensing combines wavelengths to reproduce model spectra. This in turn produces an astrometrical model.

The troposphere is the lowest portion of a planet’s atmosphere and has a large temperature change.

Ammonia ice is white and remote sensing can only investigate the top layer.

What causes the different colours in the planets’ atmospheres? An active troposphere and a stable stratosphere.

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The Cassini@Saturn had all the instruments bolted on.

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The above picture on the left shows images as the Huygens probe descended Titan. Huygens showed that Titan had a liquid methane cycle and seas of hydrocarbons (methane and ethane). A submersible boat used sonar to investigate.


On 21 December 2008, Cassini passed directly over Ontario Lacus at an altitude of 1900 km and was able to observe specular reflection in radar observations. The signals were much stronger than anticipated and saturated the probe’s receiver. The conclusion drawn from the strength of the reflection was that the lake level did not vary by more than 3 mm over a first Fresnel zone reflecting area only 100 m wide (smoother than any natural dry surface on Earth). From this it was surmised that surface winds in the area are minimal at that season and/or the lake fluid is more viscous than expected.

The high energy particle environment from Saturn is deflected down to its Moons by its magnetic field.

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“Tiger” blue stripes on Enceladus are caused by gushing gas propelling ice into space and some of this ice forms some of Saturn’s central rings. The south pole of this moon could be habitable.

A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. Fluid dynamics produce plumes causing lightning.

The Great White Spot is a unique but short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice. The next one is expected in 2017. The Cassini mission’s extension, which goes through September 2017, is named for this Saturnian summer solstice occurring in May 2017. The northern summer solstice marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere. Since Cassini arrived at Saturn just after the planet’s northern winter solstice, the extension will allow for the first study of a complete seasonal period.

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Microwaves are used to probe below Jupiter’s clouds and see the source of the turbulence.


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Uranus is believed to have only about ten cloud features. Eight hours of imaging is hoping to show that it is less boring than it appears. A new technique applied at the Keck Observatory, is bringing Uranus into sharp focus through high-resolution infrared images, revealing in incredible detail the bizarre weather of the seventh planet from the sun. New features found by the Wisconsin group include a scalloped band of clouds just south of Uranus’s equator and a swarm of small convective features in the north polar regions of the planet, features that have never been seen in the southern polar regions. At the moment astronomers are puzzled by this as the primary driving mechanism must be solar energy because there is no detectable internal energy source. “But the sun is 900 times weaker there than on Earth because it is 30 times further from the sun, so you don’t have the same intensity of solar energy driving the system,” explains Larry Sromovsky, a University of Wisconsin-Madison planetary scientist who led the new study using the Keck II telescope.

Neptune’s more varied weather compared to Uranus is believed to be due in part to its higher internal heating. Several possible explanations have been suggested, including radiogenic heating from the planet’s core, conversion of methane under high pressure into hydrogen, diamond and longer hydrocarbons (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection in the lower atmosphere that causes gravity waves to break above the tropopause.

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Neptune’s great dark spot is an anticyclonic storm but when the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms. However, an almost identical spot emerged in Neptune’s northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years.

Because of Neptune’s complicated weather systems it looks different every night.

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Pluto has (at least) five moons which might get in the way of the New Horizons robotic spacecraft.

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The above picture was taken by Voyager looking backwards towards the Sun and the Earth as the Pale Blue dot is just visible.

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