Put a ring on it

Some of the most spectacular images from within our Solar System come from Nasa’s Cassini spacecraft as it orbits Saturn. One of the reasons for this is that many of the vistas are dominated by the planet’s rings.

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Saturn, its rings (and their shadow), and the moon Tethys (NASA/JPL/SSI)

 

The rings of Saturn are made up mainly of lumps of ice (with a small amount of dust), which while individually are only millimeters up to about a kilometer in size, add up to whole rings structures which whilst only a kilometer deep extend laterally for hundreds of kilometers.

There is still a lot of debate about how they formed and how long they will last. Cassini’s  final orbits are designed to answer some of these questions by flying between the planet and the rings and we should get more answers in the months and years after the end of the mission once the data is analyzed. Current theories suggest that the rings formed around the same time as Saturn did; either from the same planetary nebula which formed the planet or from a moon which was torn apart by Saturn’s gravity. Other researchers have suggested that they are only 100 million years old and transient features which will eventually fall into the planet (over very long time periods). whilst the majority of the evidence points to them being very long lived so far. The exception to this is the outer E rings, these are known to have been generated by the ice volcanoes of Enceladus throwing out icy material into the orbit of Saturn and form a ring.

Rings are dynamic with transient features such as “spokes” of dust caught in the magnetic field chasing around jus above the surface of the rings, disturbances within them like propellers formed by the gravitational distortion of moonlets, or the ripples generated by Daphnis as it travels within a gap in the rings.

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Daphnis a small moon causing ripples within the rings of Saturn (NASA/JPL/SSI)

Pan, a moonlet within a gap in the rings has made the gap by collecting up the ice particles, forming a band around its middle.

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Pan, which orbits within the rings, has built up ice within its middle (NASA/JPL/SSI)

It is not just Saturn which has rings, Neptune, Uranus, and Jupiter all have rings and even the 250 km diameter small planetoid 10199 Chariklo has a ring system around it. Temporary rings probably occur on all planets as comets and asteroids get too close are torn apart by the gravitational pull. forming a ring of debris which would eventually fall to the surface, Mars may develop a transient ring system, when Phobos is torn apart by tidal forces as it slowly spirals towards the surface. This ring will be transient as the material itself falls onto the surface of Mars.

 

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Infrared image of the rings of Jupiter

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The marks on Mars

Looking at Earth, Venus, and Mercury so far has shown different systems of tectonics. The last terrestrial planet Mars has some familiar features but all is not as simple as it seems.

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Mars Magnetic crust as seen by Mars Global surveyor (Nasa/JPL)

Magnetic images of Mars show band like repetitions of positive and negative anomalies at its southern hemisphere. These bands look similar to magnetic bands seen in Earth’s oceanic crust.

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Bands of magnetic anomalies in the crust of the Atlantic ocean

The bands on Earth’s ocean floors represent the gradual formation of crust over time; as the plates move apart and magma rises up to form new crust it records the magnetic field as it forms, over time the periodic flipping of the Earth’s magnetic field leads to an alternating pattern.

Mars doesn’t currently have an active magnetic field, and crater dating shows that the south is older than the northern 1/3 of the planet which is topographically 3-6 km lower than the southern portion of the planet. This northern lowland does not show magnetic banding. Theories suggested for this lowland include proto-tectonics like that on Earth but which did not occur until after the planet’s magnetic field stopped and/or large impacts causing the substantial topographic difference between the two regions.

Another feature which provides clues about the nature of Martian tectonics is Tharsis Plateau, this is a vast volcanic system close to the equator and includes the 22 km high Olympus Mons, the highest volcano in the solar system. This volcano was able to grow much bigger than the hotspot volcanoes seen on Earth due to a lack of movement, if the plates had been moving then the volcano would not have developed to the same extent. This shows that Mars the plates were not moving unlike on Earth.

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Olympus Mons (JPL)

The huge weight of the plateau put a lot of stress on the crust around it and led to the formation of  Valles Marineris. The extra load on the crust mean that large valleys formed close to the edge of Tharsis Plateau as the weight caused the crust to buckle and break and shear.

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Valles Mariners, a crack caused by the weight of nearby volcanic plateau (NASA)

The magnetic bands suggest that early in Martian history it may have had some form of spreading ridge generating crust in a magnetic field. Both of these processes stopped as Mars cooled. These bands and the north-south topographic divide hint at an early active tectonic history, however, there are few signs of large-scale subduction which would allow recycling of plates so it seems unlikely that Mars ever developed a fully functioning multiple plate tectonic system, though an understanding of its history is far from complete.

 

Scarred faces

The Colorado River slowly cutting its way through rock over millions of years has formed a 450 km long, and 1.8 km deep canyon called the Grand Canyon. Whilst up close and even from space it looks impressive, in reality, it’s fairly small compared to other features which scar the planets across the solar system. Today, we’ll look at the three largest rifts and canyons in the solar system.

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The Grand Canyon in the south if the image at seen from the ISS (NASA)

Valles Marineris on Mars at 4000 km long is 9 times as long as the Grand canyon and stretches 1/4 of the way around the entire planet. There are a few theories about how it formed, the most prominent is that the nearby Tharsis bulge (an area which hosts the tallest volcano in the solar system) caused tension in the crust, which lead to it tearing and pulling apart, in a process known as rifting. Erosion then would have deepened the valley further and has produced outflow channels at the end of it.

 

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Valles Mariners, Mars (NASA)

 

One Venus, Baltis Vallis is even longer at nearly 7000 km long, its ends are covered so its original length is unknown. This feature, like the Grand Canyon, was also likely formed by erosion. Instead of water, high-temperature lava flows would have torn up, melted, and dissolved the surfaces as they flowed over them, slowly wearing them away in the same way rivers do on Earth. Similar features were probably once present on the early Earth, Komatiite lavas, found in Australia also show signs of eroding the rocks beneath them and forming channels as they flow through an area.

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a 600km segment of the 7000 km Balits Valley from Radar data collected by Magellan, the channel can be seen as a meandering line going diagonally across the image arrow to arrow (NASA/JPL_

However, the largest in the solar system is right here on Earth the 10’000 km long and up to 8.5 km deep Atlantic Ocean is a large cut into the Earths Surface. Like Valles Marineris, this was formed by rifting rather than a valley. Tectonic forces pulling and pushing Europe and Africa away from the Americas. Plate tectonics allowed the rift to develop much further and volcanism generated new oceanic crust was generated in between the two.

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Bathymetric image of the Atlantic ocean (NOAA)

This quick overview of the largest valleys and rifts in the solar system highlights two main processes which can form them; erosion and tectonics. Volcanism on Venus and Mars, extensional forces tearing the crusts show these planets have been tectonically active. In the next few posts I’ll examine tectonics work across the solar system.

Tis’ the reason for the season

Happy Solstice everyone! Today in the northern hemisphere is the shortest day (longest in the southern hemisphere).

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If you imagine a stick through the Earth around which the planet spins on a daily basis this is the Earth’s axis. If this axis was vertical, then the lengths of day would not change, throughout the year, however it actually lies out at an angle of  23 degrees from the vertical. At different points in its orbit the north will be pointing either in the direction of the sun or further away. Today the northern hemisphere is pointing directly away from the sun. This means that in the northern hemisphere the days are much shorter and the nights longer due to spending more time facing away from the Sun than towards it. In addition due to the curvature of the Earth the beams of light hitting the surface is more spread out towards the poles than the equator and so the amount of incoming energy spreads out, these two mean that the climate gets colder during the winter. What about other planets, are there seasons and how are they manifested?

Nether Jupiter or Mercury have seasons due to there low angles of axial tilt, Mercury has the smallest axial tilt of any planet, an upshot of this is that there are many craters on the poles which are in constant shade, this allows for permanent water ice on the surfaces of the closest planet to the Sun.

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The north pole of Mercury, some of the craters are permanently in shadow (NASA/ John Hopkins/Carnegie Institution)

There are huge temperature variations on Mercury related to the eccentricity of its orbit (how elliptical it is rather than circular) linked with a 3:2 ratio of years to days but these do not cause temperature changes in latitude.

Venus has a tilt of 177 degrees, what this means is that it is completely flipped over when you look at its rotation (it spins clockwise whilst the other planets spin anticlockwise)

That being said it means that the axis is only about 3 degrees off the vertical and with the very efficient heat transport in the dense atmosphere the temperature is fairly constant over the whole globe and that Venus doesn’t have a strong seasonal changes.

With a similar axial tilt to Earth (25 degrees) Mars also has seasons, which are about twice as long as on Earth (due to the longer year). This leads to growing and shrinking of carbon dioxide ice caps and temperature changes just as on Earth. Intriguingly images from the Mars reconnaissance orbiter have shown linear features called recurring slope lineae forming on crater edges, these features grow during the warmest months then disappear during the coolest month. They are thought to be  formed from brines (very salty water) which melt and run down the slope, they do not appear in the winters due to it being too cold for these brines to melt, although the source of the water is not currently certain.

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Water flows at Newton Crater, Mars (NAS/JPL/Univ. Arizona)

One other seasonal feature of mars is dust storms , which are known to cover the whole planet at times which most commonly occur during the spring and summer.

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Martian dust storm (Nasa/JPL-Caltech/MSSS)

 

Saturn has seasons which last around 7 Earth years, changes in cloud composition and occurs during this transition and there are increased storms during spring

Uranus is lying on it side, meaning that the axial tilt is just of the equator which means that the poles experience 42 years of day light followed by 42 of darkness, the change in temperature between the side facing the sun and the side facing away the sun probably has an effect on its climate however as it has only been briefly visited by the Voyager 2 probe little is known about the long term seasonality ice giant.

Finally Neptune has a similar axial tilt to Earth of 28 degrees, at the moment a lack of observational evidence makes it difficult to say if it has any strong seasonal effects although an increase in cloud cover has been noticed by Hubble as it transitions into a 40 year long summer.

Happy Holidays!

Something old or something new

A recent paper on the lunar impact craters has shown that meteorites crash into its surface of the Moon more frequently than previously estimated, this not only has implications for future exploration of the lunar surface but also may adjust when we think the events on other planets and moons in the solar system happened.

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A new ~34  m diameter crater from Nasa’s LROC, image is 1000 m wide NASA/GSFC/Arizona State University

In order to understand how geological events area dated in the solar system, lets look at how we date events in on Earth; geologists have two styles of dating: absolute & relative. Relative dating is done by looking at the relationships between two different geological features to work out which is older and which is younger. For example, a layer of sandstone on top of limestone, we know the limestone came first and the sandstone was deposited on top of this. A volcanic magma is younger than the existing rocks it cuts through on its way to the surface. This form of dating enables working out the order of events but does not give an actual date, amount of time or how long has passed between the two events.

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A volcanic dyke (yellow) cuts across preexisting volcanic ash layers (blue) in Tenerife

In contrast, absolute dating uses the radioactive decay of elements in to give a range of dates when something could have formed, based on the time that has passed. Measuring the amount of the parent (the initial element) and the daughter (what it decays into) allows the calculation of the age of the rock using the decay rate. Many different elements such as lead or potassium can also be used, as elements with vary in abundance  in a sample and have different decay rates vary, different methods have to be used for different situations.

These methods, alone or as a combination can be used to estimate when and in what order something happened on Earth, however, for other bodies in our solar system we have only undertaken limited exploration and whilst it is possible to do relative dating from a distance, absolute dating requires clean labs and high tech equipment, so how do we get dates for the events on other worlds?

Most of the exploration of the rest of the solar system has been done remotely by satellite, the features are not directly sampled. Where we have sent probes to the surface of planets so far no lander or rover has been built with the ability to carry out dating as it is done in an earth based lab (limited dating has been done on Mars but as the equipment was not designed for this so gives a wide range of possible dates and so the data is of limited use). The few meteorites found on Earth which come from other planets, can be used to get the date those samples, however, it is often unclear of the context (where on the planet they originated from) and so can’t be used to date regions areas and features.

In order to date features on other planets, a modified form of relative dating is used to provide dates. Meteorite impacts have occurred throughout the history of the solar system and continue to happen, the older the geological feature, the more impact craters are likely to have impacted this surface. Larger impacts happen less often than smaller impacts so surfaces with larger craters tend to be older than younger ones.

In order to give a date to this, the rate of cratering has estimated, whilst each impact event is random, a curve based on the predicted impacting rate for a certain size crates can be calculated for a given surface since the late heavy bombardment 4.1 billion years ago. Count the number of craters of a certain size in a given area and an estimation can be made of the age of that feature.

These curves are then calibrated to dates using the impact craters on the Moon lined tied to the limited number of moon rock samples retrieved by the Apollo astronauts. So all planetary bodies within the solar system are linked to the craters and the cratering rate of the moon.

The paper takes data from the  Nasa’s Lunar Reconnaissance Orbiter which has been orbiting the moon for 9 years and shows that the frequency of impact craters is higher than expected, so craters are forming more often, suggesting that surfaces with craters on them may be younger than previously thought.

This has implications for the possibility of previous life on Mars  which is linked to the presence of liquid water on mars, evidence for this is given by floodplains dated through impact craters, if these are slightly younger than previously estimated it then Mars may have had longer time period where mars had liquid water, the longer the time, the greater the opportunity for life to start.

The exceptions to this are Earth and Io. The rocks on Earth are easily available to get to labs for analysis, the surface is often very active with geological activity causing many smaller impacts to be destroyed. Io is so volcanically active that it its surface is covered by 1 cm of new material each year, which is faster than impact craters form.

It will be interesting to see how the new findings adjust the impact rates else where in the solar system and what effects that will have on the timing of events on our planetary neighbours.