Let it snow….

Winter in the northern hemisphere is synonymous with snow (at least in films and Victorian novels). The Earth is unusual because it has water can commonly be found on its surface in solid, liquid and gas form, and both solid and liquid water can rain/snow/sleet/drizzle from the sky. So what happens on other bodies in the solar system?

 

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Clouds of ice crystals on Mars – Ready to snow? (NASA/JPL)

 

 

Most water on the surface of Mars is now trapped in its glaciers/ice caps, but what about snow? Whilst there is less water vapour in the Martian atmosphere than the Earth’s, Mars still has clouds made of ice crystals. These ice crystals can fall slowly out of the sky, but at night under the right conditions, snowstorms can occur. Most of this snow will vaporise before it hits the ground, and any that do make it to the ground will likely vaporise during the day.

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Maxwell Montes the highest mountain on Venus, appears brighter than the surroundings because of  sulphide frost (Image NASA/JPL)

Venus has the hottest surface temperature of any planet in the solar system. Water snow is not possible. The atmosphere of Venus contains lots of sulphur and temperatures hot enough to melt lead, it is so thick we can’t see the surface of the planet so we have collected radar data to understand what it looks like. One feature of this data is that on the tops of some high mountains there is an increase in the number of radio waves reflected, giving the appearance that the tops of mountains are brighter, just as snow on the top of mountains is brighter than the surrounding rock.

Whilst it is far too hot for snow on Venus, it is thought that these mountain tops are covered in a form of frost caused by metal sulphides, which forms only at high elevations and it’s this which makes is appear more reflective to radar.

Mercury, my main planet of study, doesn’t have snow-capped peaks, being too hot over the majority of the planet and no atmosphere. So the nearest feature to snow is ice in the craters close to the north and south poles, these craters are permanently in shadow protecting the ice from the boiling Sun. Without an atmosphere there isn’t any form of precipitation other than a daily micrometeorite shower.

 

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The south pole of Mercury, the black areas are permanently in shadow and thought to contain water ice. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

 

Titan, a moon of Saturn is much to far out from the Sun to have water snow, and any water near the surface, just like in the bleak midwinter, is like a stone. It does snow on Titan, near the poles hydrocarbons can crystallize out in the atmosphere and then fall as snow down onto the surface.

Another moon of Saturn, Enceladus, is probably the most visually spectacular example, geysers on the south pole can be seen erupting water vapour into space. Some of this water is lost in orbit and forming the E ring of Saturn, but some of the water falls back down onto the surface as ice crystals. As this fall back it is happening without an atmosphere it doesn’t really count as snow but is a great excuse to show the stunning photos of the plumes.

 

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Jets of ice crystals coming from Enceladus  (NASA/JPL/Space Science Institute)

 

There are lots of forms of snow around the solar system however maybe Earth still has some of the best snow views in the solar system. Happy Christmas/ Winter Solstice/ Saturnalia to all my readers!

 

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Snow in the Sierra Nevada mountains

 

 

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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.

 

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!