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

Guess the Planet 44) Orb

Check out this image! 
Which planet do you think is shown here? And what does this image tell us about it?

Check back later in the week for the answer.

What can we learn from an eclipse?

The eclipse in this week’s guess the planet image is quite similar to the one which was visible from some parts of the world on Monday.. The main difference is that in this image, it is the Earth which is blocking out the sun. The observer is not on the moon, but rather in orbit around it. This image was captured by the Kaguya spacecraft of the Japanese Space Agency’s (JAXA), when the Earth passed between it and the sun. A film of this eclipse, including a spectacular diamond ring effect can be found on the JAXA website. 

Eclipses have been observed since antiquity and our ability to predict them has always been very good. We know the movements of the planets with great precision, and can easily project those movements into the future. There are few sources of uncertainty which can affect these motions, so phenomena such as eclipses, the recurrence of comets, and the movements of the planets across the night sky can all be predicted quite reliably. This is also true of periodic meteor showers such as the Perseids and Leonids which occur when the orbit of the earth intersects fields of debris, generated by the passage of comets. As a comet moves along its orbit it leaves debris behind. This means that whenever the earth crosses the comet’s path it hits some of this material, even if the comet itself is a very long way away at the time. This debris burns up in the atmosphere producing shooting stars, which can always be seen in the same parts of the sky at certain times of year. 

We have quite a few images of eclipses from other planets. The image below was captured by NASA’s surveyor 3, from the surface of the moon in the 1960s. The image isn’t as high quality as the more recent observation from Kaguya, but is still an impressive example of how our home planet can block out the sun. 

The image below was captured by the New Horizons spacecraft as it passed Pluto. Here the dwarf planet is silhouetted against the sun. At first glance this might not seem like a particularly useful image, after all none of the surface of the small world is visible from this angle. However, it wasn’t the surface which the New Horizons team were interested in when they took this image. The blue glow around Pluto is its atmosphere, and from this angle the light of the sun passes directly through the “limb” of the atmosphere, allowing measurements of its properties to be captured. 

You can learn a lot about the chemical composition of a material from the way light interacts with it. Different elements preferentially absorb and reflect light of different wavelengths. This is why materials appear to be different colours, and looking at the colours, or regions of the spectrum, that interact with materials is a good way to determine their chemical makeup. This process is called spectroscopy and can be used at a range of scales. On the very small scale tiny amounts of material can be superheated to see what wavelengths of light they emit. On the very large scale the wavelengths emitted by stars and galaxies can be analysed to look for peaks in certain parts of the spectrum which indicate their chemical composition. Other materials won’t emit light, but will reflect or scatter it, and again some colours of light will be reflected more strongly than others. This is the case in this image of Pluto’s atmosphere.

Examining the interaction of light with the atmosphere revealed a lot of information and allowed the New Horizon’s scientists to work out how Pluto’s haze probably formed. NASA’s description of this image states that: “…the haze is a photochemical smog resulting from the action of sunlight on methane and other molecules in Pluto's atmosphere”. One interesting thing about this image is the blue colour. On Earth we are used to seeing a blue sky, and it appears that colour because our atmosphere preferentially scatters light with a blue wavelength. The new Horizons team note that the same is true of the particles that form the haze on Pluto leading to this characteristic colour.  

Image Credits:

Japanese Space Agency (JAXA) (C) JAXA/NHK, http://space.jaxa.jp/movie/20090218_kaguya_movie01_j.html

NASA https://science.nasa.gov/science-news/science-at-nasa/2009/25feb_kaguyaeclipse

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute https://photojournal.jpl.nasa.gov/catalog/PIA21590

Guess the Planet 43: Eclipse

Check out this image of an eclipse somewhere in the solar system. This isn't our moon, so which solar system body is blocking out the sun? And where was this image taken?

Check back later in the week for the answer!