Guess the Planet 47: Pole

Here is this week's guess the planet image. What do you think this false colour image is showing us? and which planet does it depict?

Check back later in the week for the answer!

Frost Covered Dunes and Phase Changes

This week’s image takes us back to Mars, and because it comes from the HiRISE instrument we can have a look at a full colour version in wonderful detail. The addition of colour makes this scene even more spectacular than the black and white version. What we are looking at here is part of the circumpolar dune field, which surrounds the northern icecap of Mars. 

The North Pole itself is covered by a large icecap, made up primarily of water ice. This remains frozen year round, but during the winter a thin layer of Carbon Dioxide ice forms on top of it. This seasonal layer is only around a meter thick. Carbon Dioxide is also responsible for the bright streaks in our image. The glistening patches are covered by CO₂ frost, which accumulated during the northern winter. As the HiRISE team explain in the caption to this image, the satellite captured this scene during the early spring. This means that the seasonal frost layer was disappearing at this time. The feathery patterns which we can see in this image indicate places where the frost is sheltered from the sun, and so has yet to evaporate. 

Martian frost doesn’t melt, as water ice does on Earth, but rather goes straight from the solid state to a gaseous one, in a process called sublimation. Sublimation is very important on Mars, because the temperature and pressure conditions there mean that several of the substances which shape the martian landscape aren’t stable in their liquid phase. This is particularly true of water, which can only exist as a liquid for short periods of time on Mars, or under unusual conditions. It is fairly intuitive that the cold temperatures on Mars will make the liquid water freeze again, but what is equally important is the low pressure. 

Pressure has a large effect on the position of the freezing and boiling points of a substance. These are the temperatures at which it will change phase from a solid to a liquid and from a liquid to a gas, respectively. We are used to seeing these phase changes take place at set temperatures on Earth. With our fairly high atmospheric pressure water almost always freezes at 0^oC, and boils at 100 ^oC. The fact that these are nice round numbers is no coincidence. The Celsius, or centigrade, scale was deliberately calibrated based on these commonly observed properties. 

Although the modern centigrade scale is named after him, it doesn’t work quite the same way as the system which Swedish physicist and astronomer Anders Celsius first proposed in the early 1740’s. Celsius made measurements in the opposite direction, so that zero degrees represented the boiling point of water while 100 was calibrated to fall at the freezing point. The system we use today is actually based on a scale that was first proposed by the French physicist Jean-Pierre Christin. He appears to have come up with the idea independently of Celsius, at around the same time. The ascending scale took off when Celsius’ scale was reversed by another Swedish scientist; Carolus Linaeaus for use in his botanical research. The 0-100 degree system quickly became the most popular and is now a scientific standard. 

If he had lived on Mars, Celsius would likely have had a much harder time calibrating his temperature scale. The low atmospheric pressure means that there is no easily defined interval in which liquid water is stable. It will begin to boil the moment it is exposed to the thin atmosphere. This naturally produces water vapour, but ironically it can also turn the bulk of the boiling water back into ice. Evaporation requires energy, and this is drawn from the boiling liquid. When very hot water boils there is plenty of energy to go around, and the liquid will keep on evaporating until none is left. When cold water boils this energy is in short supply, and so the liquid gets colder and colder until it freezes again. This process is called evaporative cooling, and could result in ice covered rivers, where the exposed water boils away, until a cap of more stable ice forms above it. When we look at outflow channels on Mars, or the debris flows left by martian gullies, we need to take this process into account, as it will have a substantial effect on the appearance of the landscape such flows of water leave behind. 

Image Credit: NASA/HiRISE/University of Arizona https://www.uahirise.org/ESP_050703_2560

Guess the Planet 46) Speckles

Check out this week's image. What do you think we are looking at here, and what solar system body does this image come from?
Check back later in the week for the answer!

Farewell to Cassini

This week’s guess the planet image comes from Saturn and shows the swirl of clouds around the north pole of the distant gas giant. This image was taken by the Cassini Spacecraft as it began its final descent into Saturn. Today the Cassini mission comes to an end, as the spacecraft crashes into the planet it has spent more than a decade studying. This is the end of an era, and it will be a long time before we have another probe to send back marvellous pictures of Saturn and its moons.  I want to look back at this fantastic spacecraft, and some of the things it has accomplished over the course of the 13 years at Saturn. 

The Cassini-Huygens mission was launched in 1997 as a joint operation between NASA, who built the orbiter, and ESA who constructed the Huygens lander. It spent the next seven years travelling to the outer solar system, and the vicinity of Saturn and its moons. Unlike the Voyager probes which had flown past Saturn on their way to the worlds beyond, Cassini was there to stay and went into orbit around the gas giant in 2004. This allowed it to study Saturn in unprecedented detail, and for far longer than any fly by mission could. Cassini was placed into an elliptical orbit which would allow it to also perform regular flybys of several of Saturn’s moons, including Titan and Enceladus.

Titan was of particular interest to the mission, as it was the destination of the Huygens lander. This small probe touched down on the surface of Titan in January of 2005. This was the first time a lander had touched down on the surface of such a distant moon, and it sent back a lot of valuable data about the surface conditions on Titan. This moon has a thick atmosphere, which has shrouded the surface, hiding its features from view. In addition to deploying the Huygens lander Cassini also used Radar to peer through those clouds and return satellite images of the surface. In doing so it discovered vast hydrocarbon lakes and rivers, which we’ve talked about before on this blog. 

The Surface of Titan, from the Huygens Lander. 

Cassini’s observations of Enceladus were also very valuable. The probe detected a thin atmosphere of ionised water vapour, and observed the geysers that periodically erupt from the small moon. 

By flying through these geysers it was able to determine that they contain organic compounds from the subsurface ocean beneath the icy world. This is significant because, as the name suggests, organic compounds are a vital precursor to life. If organisms like those on Earth are to evolve in an extraterrestrial environment it will have to be one with organic compounds, so this discovery makes Enceladus and moons like it a prime target for astrobiological study. The presence of organic compounds doesn’t necessarily mean that life will evolve there, but it gives it a better chance.

Cassini didn’t stop at investigating the moons which we already knew about, but discovered six more during its time at Saturn. Naturally it also made numerous observations of Saturn’s famous ring system, including observing spoke like patterns in the ring system which had previously been detected through telescopes and by the Voyager probes. It made numerous observations of the structure of the rings, and the sizes of the particles that comprise them. 

Cassini also turned its attention to the atmosphere of Saturn, observing storms in the gas giant, and studying the composition of the atmosphere. It observed the “great white spot” storm that recurs every 30 years at Saturn, and has observed a stable hurricane at the planet’s South Pole. 

 The Cassini mission has encompassed far too many discoveries to cover them all in detail here. In the 13 years it has spent at Saturn it has massively expanded our understanding of this distant world. The sailing hasn’t always been smooth, in particular there were communication problems surrounding the Huygens landing, which required the ingenuity of the team behind the spacecraft to solve. However, despite the occasional setback, Cassini has been a dependable spacecraft for over a decade, sending back the most, and best, data we have ever had about this distant world.

Cassini will continue to record data as it plunges into the atmosphere of Saturn, although NASA do not expect that much of this will be received. Nonetheless the run up to the spacecraft’s destruction has allowed the team to perform multiple close flybys of the rings, the inner moons and the planet itself. The “grand finale” of the Cassini mission has already been a spectacular show.

Cassini will be sorely missed, but the contributions it has made will keep planetary scientists busy for decades!
 

Image credit: NASA/JPL-Caltech/Space Science Institute

https://www.nasa.gov/image-feature/jpl/pia21343/top-of-the-world

ESA/NASA/JPL/University of Arizona

https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#/media/File:Huygens_surface_color_sr.jpg

NASA / JPL / Space Science Institute

https://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#/media/File:Saturn_during_Equinox.jpg

Guess the Planet 45: Swirl

Here is this week's guess the planet!
What are we looking at here? and which planet is this an image of?

Check back on Friday for the answer!

Detecting Exoplanets

This week’s image is not a true colour photograph. In fact it isn’t a photograph at all, but rather the results of a model of atmospheric temperature. The planet being modelled is further away than any of the objects I’ve talked about before on this blog. This is the snappily named HD 80606b a “hot Jupiter” gas giant planet in orbit around the star Struve 1341B. This means that it is similar in size and mass to Jupiter, but orbits much closer to its sun. If it were in our solar system it would be inside the orbit of the Earth. It is located in the constellation of Ursa Major and is approximately 190 light years from Earth. The model shown above was created using data from NASA’s Spitzer Space Telescope. Exoplanets, those which orbit other suns, are too far away for us to clearly image them as we would with a planet in our own solar system. Nonetheless it is amazing how much information we can gather using a variety of astronomical techniques.

For a long time it was uncertain whether there actually were planets orbiting other stars. We presumed that our solar system was the norm, but had very little evidence to back this up. This all changed in the mid 1990’s when astronomers confirmed the presence of extrasolar planets, since then we have detected, and confirmed the existence of more than 3000 exoplanets of varying sorts. These include small objects, around twice the mass of Earth’s Moon, to massive objects which dwarf Jupiter. HD 80606b is four times as massive as Jupiter, although its radius is slightly smaller. This means that although it is also a gas giant, it is much denser. 

So how do we tell whether planets orbit a distant star? There are numerous techniques for detecting exoplanets, HD 80606b was detected using the transit method. All planets orbit their host stars, and this means that if the solar system is angled correctly they will occasionally pass between their star and Earth. By observing the brightness of the star over long periods of time, we can detect periodic dimming events, caused by the planet briefly blocking out some of the light. The extent to which the light from the star is dimmed can tell us a lot about the physical properties of the planet, and from the period at which dimming events occur we can determine the length of time it takes for a planet to orbit its star. Of course a planet that is far from its star won’t transit very often, and so it might not be possible to determine the period of the orbit using this method if it will be hundreds of years before the next transit occurs! As some of the light from the star passes through the planet’s atmosphere during a transit it can also give us information about that. 

Luckily there are other methods which can tell us a lot about an exoplanet. For example we can measure changes in the radial velocity of a star in response to an orbiting planet. All objects in space respond to the gravity of the bodies around them. A star tends to be much more massive than its orbiting planets, so the effect which they have on it is tiny compared to the effect it has on them. Nonetheless it is often detectable, although it takes very careful measurements to do so. The Earth only causes a 12 cm/s difference in the speed at which the sun moves relative to the centre of mass of the solar system, this is tiny, but can be measured. This means that Earth sized objects can potentially be detected. 

HD 80606b has another interesting property, which brings us back to the model results in this week’s image. It has one of the most eccentric orbits of any known planet. According to NASA “The planet spends most of its time far away from its star, but every 111 days, it swings extremely close to the star, experiencing a massive burst of heat.” This leads to massive variations in temperature during the course of the orbit, and this is what is being modelled in the image above. If you follow this link,  they discuss hot Jupiter planets in more detail, and have a video showing the changes in temperature across the planet as it moves through its orbit. The image I shared above comes from just after the closest approach, but the planet soon cools down as it travels further from the star.

 Apologies for the delay in posting this week's blog. There will not be a guess the planet in the first week of September, as I'm still rushing around madly this weekend. Things should get back to normal from next Monday, or at least I hope they will!

Image Credit: NASA/JPL-Caltech/MIT/Principia College

https://www.nasa.gov/feature/jpl/investigating-the-mystery-of-migrating-hot-jupiters

https://www.nasa.gov/image-feature/jpl/simulated-atmosphere-of-a-hot-gas-giant