The North Pole

 Here is the full version of this week's guess the planet picture: 

Several readers speculated that this week’s guess the planet post might be the north pole of a planet, the question being which one. We don’t have to look massively far from home to find the region depicted here, as it is the north pole of Earth. The bright material covering much of this image is sea ice in the arctic sea. 

This image was acquired by the MODIS instrument on NASA’s Terra satellite in orbit around Earth. NASA shared this image in their visible earth gallery, and note that it is rare to see a true colour image of this area, as it is frequently blanketed in clouds. Some clouds are visible in this image being the irregular bright features in the lower half of the image. The dark regions are open water, and the cracks are “leads” or fissures in the ice. 

Sea ice is common in the cold oceans of Earth’s poles, where vast sheets of ever shifting ice float on the surface of the Arctic and Antarctic Oceans. However the impact of anthropogenic climate change on this region is dramatic. The wintertime extent of arctic sea ice has reached a record low for the second year running in 2016. This trend is likely to continue as the ocean temperatures continue to rise. In March this year NASA’s Goddard institute released a time lapse video of the growth of winter sea ice across the arctic throughout the winter of 2015-2016. It remains to be seen whether this trend will continue through the winter of 2017 as well. 

In many ways water ice is quite a peculiar material. The fact that it floats on liquid water demonstrates that it has a lower density in its solid form than it does as a liquid. For most materials this is the other way around, with the solid being denser than the liquid. 

The reason for water’s odd behaviour are hydrogen bonds. The hydrogen and oxygen atoms that make up a water molecule have opposite electrical charges. The oxygen atoms are highly electronegative, while the hydrogen atoms have a positive charge. Within the molecule itself these charges cancel out, but because of the shape of a water molecule one end tends to be slightly positive, while the other is slightly negative. The result is that adjacent water molecules experience an intermolecular attraction. This phenomenon affects the boiling point of water, and produces some interesting effects when phase changes occur. 

When water freezes to form ice, the hydrogen bonds result in a crystalline lattice forming. The water molecules become locked into a hexagonal network with large gaps in between. The result is a material with very low overall density, compared to its liquid form. The hexagonal structure of ice crystals results in another phenomenon which we see a lot of depictions of at this time of year. Small ice crystals grow outwards, forming complex patterns, and the hexagonal structure at the microscopic scale results in striking symmetry as it does so. The result is the six sided snow flake.  

 I hope that you all enjoy your winter celebrations, whether they involve snow, ice or water. There won't be a post next week, but Guess the planet will resume in the first week of January.

References:

NASA timelapse and more information on sea ice extent. https://www.nasa.gov/feature/goddard/2016/2016-arctic-sea-ice-wintertime-extent-hits-another-record-low

Image Credits:

NASA Modis image of the north pole in 2000: http://visibleearth.nasa.gov/view.php?id=55099

Public domain photograph of a snowflake by Wilson Bentley via Wikipedia https://en.wikipedia.org/wiki/Snowflake#/media/File:Bentley_Snowflake8.jpg

Guess the Planet 10: Bright Material

Welcome to this week's "guess the planet". What do you think this bright region is, and on which planet can it be found?

Earth as seen from Mars

Earth and Moon

This week’s guess the planet image is fairly obviously a view of the Earth and Moon from a considerable distance away. This image was actually captured by the HiRISE camera on the Mars Reconnaissance Orbiter. This isn’t the usual purpose of this instrument, as it is usually pointed down towards the martian surface, producing the highest resolution images of that planet. However on a few occasions it has been used to image other targets. Let’s take a look at a few of these images, and consider the advantages of using a mars based camera. 

As well as numerous images of Mars the HiRISE instrument has been used to image Phobos and Demos the two martian moons, producing amazing images of those small bodies. Unlike Earth's moon these objects are very small, and so do not have sufficient gravity to become spherical. They look more like irregular asteroids.

Deimos

Phobos

I plan to talk more about Phobos and Deimos in the future, so watch out for these moons in a future guess the planets blog. 

The HiRISE camera is effectively the most powerful telescope to have been sent out of Earth orbit, and can thus produce some amazing astronomical images on the rare occasions when it is pointed away from Mars. This image of Jupiter and its associated moons was acquired as a calibration exercise for the camera. NASA reports that the fact that Mars was so much closer to Jupiter at the time the image was taken means that it has a similar resolution to a Hubble space telescope image acquired from Earth orbit would. 

Jupiter

 This very wide image includes Jupiter and several of the Galilean satellites, check out the link below for higher resolution versions, as I can't include it here at a resolution that does it justice. 

Image credits: NASA/JPL/ University of Arizona

The Earth as seen from Mars

https://www.nasa.gov/mission_pages/MRO/multimedia/mro20080303earth.html

image of Jupiter from Mars

http://www.uahirise.org/jupiter.php

Phobos

http://www.uahirise.org/phobos.php

Deimos 

 http://www.uahirise.org/deimos.php

Guess the Planet 9: Looking Back

It might be a little obvious which planet (and associated moon) is pictured here. So this week the question is: Where was this image taken from?

The Far Side of the Moon

This week’s guess the planet image is a photograph of the far side of the moon. Credit goes to the crew of Apollo Eight. They were the first humans to orbit the moon in December of 1968, bringing back some phenomenal images of its far side. Their most famous photograph shows the earth, rising above the limb of the moon, giving a unique perspective on our home planet. 

 For most of human history the far side of the moon was something of an enigma. The surface of our satellite always looks the same in the night sky, because the same hemisphere is always facing the Earth. For thousands of years astronomers had no way of viewing the far side, until the soviet Luna 3 spacecraft orbited the moon for the first time in 1959. It sent back humanity’s first view of this area which surprisingly looks quite different to the near side. 

Our side of the moon is dominated by the dark lunar “maria”, Latin for “seas”. These regions are actually dark planes covered by expanses of basaltic lava. The far side of the moon has far fewer maria, instead it is covered by brighter, heavily cratered terrain.

So why does one face of the moon always point towards Earth? 

The moons far hemisphere is sometimes referred to as its “dark side” however this is a misnomer. The moon spins on its axis as it orbits the Earth. This means that it experiences day and night, which can be clearly seen as it changes shape throughout the month. If the moon didn’t spin then we would see different sections of its surface as it gradually moved around us, however the moon is “tidally locked” with the Earth. This means that the rate at which it spins and that at which it moves through its orbit exactly match. It rotates at just the right speed to keep the same face pointed towards the Earth. This is no coincidence; the tidal forces which the Earth and the Moon exert on one another have gradually brought them into line.  

The same is true elsewhere in the solar system. Pluto and its large moon Charon are both locked to one another. The same side of Charon is always visible from Pluto, but unlike Earth’s moon the reverse is also true, and the same side of Pluto always faces its satellite. This means that Charon doesn’t move through the night sky in the same way that our moon does, but would always appear in the same place. It was long thought that mercury was tidally locked to the sun. However better observations revealed that this wasn’t the case, Mercury actually rotates three times for every two revolutions around the sun.

 This ratio is called the spin-orbit resonance. Everything in the solar system interacts gravitationally with its neighbours, so numerous resonances occur. Some are more complicated than others, and all produce different orbital arrangements. 

Image Credits: 

• Apollo Eight image of the lunar far side GPN-2000-001127. Public Domain image from NASA via https://en.wikipedia.org/wiki/File:The_Lunar_Farside_-_GPN-2000-001127.jpg
• "Earthrise" Photographed by astronaut Bill Anders from Apollo 8. via https://en.wikipedia.org/wiki/File:NASA-Apollo8-Dec24-Earthrise.jpg

More information about:

The Luna 3 spacecraft 

Apollo Eight

Orbital Resonance

Guess the Planet 8: Lots of Craters

What is the significance of this crater strewn area, and which solar system body is pictured here?

Shadow of Rosetta on Comet 67P

This week’s “guess the planet” image comes from comet 67P/Churyumov-Gerasimenko. Credit for this image goes to ESA’s Rosetta Team.

Rather than talking about the geology in this image I drew your attention to the dark smudge like feature in the bottom part of the image. This feature is entirely man made as it is actually the shadow of the Rosetta spacecraft, cast on the surface of the comet during a close flyby in 2015. ESA report that the shadow is 20 by 50 metres, indicating the scale of the image.

At the time this image was taken the Rosetta orbiter came within six kilometres of the comet’s surface allowing it to take images of an unprecedentedly high resolution. As ESA outline on their website the fact that the spacecraft is casting a shadow during its close approach is also very significant. This means that Rosetta was passing between the comet and the sun, and thus the sun was directly overhead. 

This meant that the surface features didn’t cast shadows across the image and allowed the Rosetta team to determine the reflectance of the materials which make up the comets surface. As planetary scientists we are often interested in the “albedo” of a surface. This is the proportion of light which it reflects, and can tell us a lot about the sort of materials the surface is made of. However the lighting conditions at the time an image is taken will have just as much of an effect on the brightness of an imaged surface as its albedo. Because there are few shadows in this image (apart from the obvious one) the Rosetta team know that the reflectance of the surface materials reflects their true albedo. 

The angle of the sun is very important when examining remote sensing images. A couple of weeks ago I talked about how you can use the direction of shadows to distinguish between positive and negative relief features. The length of shadows can also be used to add a third dimension to what would otherwise be a flat image. So long as you know the angle of the sun you can use some basic trigonometry to determine the height of the feature casting the shadow. 

The sun is thus a very useful tool in interpreting the surface of another planet. 

Image credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The Original image, and information about it's properties can be found here:

http://imagearchives.esac.esa.int/picture.php?/60930/category/252