Oct. 24, 2021

Episode 7 - Planets part 1

Welcome to Untangling Science, a podcast about science that is for everyone, with me Darragh Ennis. You probably know me as The Menace from ITVs quiz show The Chase, but my day job is as a scientist at the University of Oxford. In this podcast I want to bring the world of science to people who think it’s too complicated to understand in a way that is fun and straightforward. We have a website, www.untanglingscience.com and you can follow us on Twitter @untanglings . I have a blog on the website that I leave useful information, links and diagrams from each episode in, so check that out. This time we will be heading back into space to talk about some of the planets in our solar system. There’s too much to cover in one episode so this will be a two parter. In this episode we will talk about the planets closest to the Sun, apart from Earth, Mercury, Venus and Mars. Then in the next episode we will move out into the colder parts of the solar system to consider Jupiter, Saturn, Uranus and Neptune. Hope that sounds good, and let’s start with the smallest of the planets Mercury


The solar system is really big. Now I know you already think that it’s big, but it’s even bigger than that. From one side to the other is not far off 300 billion kilometres. Human brains are not very good at handling massive numbers like this, there needs to be some context so we can really appreciate how big that is. To give you an idea 300 billion seconds is not very far off 10,000 years. So rather than throwing about giant numbers we are going to use a bigger measurement of distance called an astronomical unit. This might sound complicated, but it’s just the average distance the Earth is from the sun and is about 150 million kilometres. It takes light a little over 8 minutes to travel this distance so it’s a lot, but it’s a good way to keep the numbers under control so I’m going with it. Now, let’s get to Mercury. Humans have known about Mercury for quite a long time. The first record we have dates back over 3400 years to the Assyrian empire as it is readily visible without special telescopes. In particular it could be seen crossing between the Sun and the Earth in what is known as a transit. But what is Mercury actually like? The closest planet to the Sun is the smallest, but surprisingly it’s not the hottest. More on that in a minute. It is on average 0.4 astronomical units from the Sun, so less than half the distance from the Sun to the Earth. Mercury is very small for a planet, its diameter of under 5000km means it is in fact smaller than some of the larger moons in the solar system. One of the interesting things about the planets is that the closer they are to the Sun, the faster they have to move. This is because the massive gravity of the Sun draws any slower moving object towards it, so Mercury moves extremely quickly around the Sun, almost 50% faster than the Earth does and 9 times faster than Neptune. It is partly down to this speed of movement that Mercury got its name after the speedy messenger of the Roman gods.  This speed and the closeness of Mercury to the Sun means that it’s year is incredibly short, just 88 earth days. Mercury takes much longer than earth to rotate on its axis, about 59 of our days. I have quite deliberately not talked about a day on Mercury as it’s not as simple as the planet revolving which it does in 2/3 of the mercurian year. This is not a coincidence that it is 2/3 as Mercury is locked into a tidal relationship with the Sun. This sounds complicated, but luckily the moon is in a similar relationship to the Earth which makes it easier to understand. Because of the speed of rotation of the Moon we always see the same side of it from earth. So while it is going around us, it is also spinning around and the same side of the moon points towards the earth. Mercury is similar to the Sun, but what has this got to do with the day on Mercury? Well if we were to land a rover on Mercury and it could record the time it takes from noon one day to noon on the next it would actually take 176 days, or two Mercury years. If you’re still confused check out the blog post for this episode where I’ve posted a nice animation that explains it all (https://www.youtube.com/watch?v=OJrl733eyfY)

One of the things these very long days result in is a VERY big temperature difference in night and day areas on Mercury. During the day temperatures can go as high as 420 degrees Celsius and at night time they can be as low as -170. The reason for this is that Mercury has almost no atmosphere. The earth’s atmosphere is not just there to supply us with air to breathe, it also acts as an insulator that stops us getting too cold and reflects some of the heat from the Sun to stop us overheating. As well as being either freezing cold or scalding hot Mercury is a pretty barren place, covered in craters much like our moon is. Despite these conditions it is likely that there is some water on Mercury, with strong indications that there is ice in some of the craters near the permanently cold polar regions. Much of what we know about Mercury comes from data collected by two space probes. In the mid 1970s Mariner 10 was able to map almost half of the planet’s surface when it flew close by Mercury after it visited Venus. It made three close approaches but the same face of the planet was close by each time. In 2008 the Messenger probe performed a fly by and was able to eventually enter into orbit around Mercury. The much more sophisticated probe was able to map the surface of the planet in great detail before crashing into the surface in 2015.


Ok, that’s a whole heap of information, so let’s do a recap:


  • The solar system is immense, so we’ll use the distance from the earth to the Sun as a way to measure things, and that’s an astronomical unit.
  • As far as planets go Mercury isn’t all that big, it’s smaller than some of the bigger moons.
  • It moves around the Sun very quickly, a mercury year takes about 88 days.
  • Mercury spins around about every 59 days but because of how it revolves a day on the surface of mercury takes 176 days, or twice as long as it takes to go around the sun.
  • Having no atmosphere to regulate temperature means the half of the planet facing the sun is extremely hot while the dark side is very cold indeed.
  • Most of what we know in detail about mercury comes from two NASA probes, Mariner 10 and Messenger


Ok, do we feel all caught up? Good so, on to our next planet Venus. Venus is the brightest and by far the hottest of all the planets and both of these are for the same reason, its atmosphere. So, despite being much further away from the sun than Mercury, 0.7 astronomical units compared to Mercury’s 0.4, is, it is much hotter and the reason for this is its atmosphere. Venus has a completely runaway greenhouse gas atmosphere filled with carbon dioxide and it is covered with thick clouds made up of sulphuric acid. So not the most pleasant of places. Because of this, temperatures at the surface are around 500 degrees Celsius, which is hot enough to melt lead. Added to this is the immense pressure caused by this atmosphere meaning that it’s about 90 times more than on earth, or equivalent to being a kilometre under the sea. One other effect of the layer of thick clouds completely covering Venus is that it reflects a lot of sunlight back into space, making it one of the brightest things in the night sky. This meant that is was very easy to see and track and has been studied by humans for over 4,000 years. The shining brightness of this planet was the reason why it was named after the Roman goddess of love and beauty and throughout history it has been sacred to gods of many religions. But not only is it easy to see from Earth, it also passes fairly close to us (in solar system terms anyway) so it was the first planet we sent probes to. The Mariner 2 probe visited Venus as early as 1962 and the soviet mission Venera 7 landed there in 1970, becoming the first probe ever to successfully land on another planet. The incredibly hostile environment meant that the probe did not function for long, but the Magellan orbiter in the 1990s was able to finally give us detailed information of the surface of Venus. Plans to send rovers onto the surface seem unlikely due to the conditions on the surface, which is why more energy has been dedicated to missions to Mars. But more about that in a minute. In terms of size and general makeup of the planet, Venus is extremely similar to Earth. It is a rocky planet with a diameter of slightly over 12,000km so less than 1000km smaller than earth is across. It has been suggested that Venus at one point in its history had water oceans on its surface but they’ve been lost due to runaway greenhouse gases in the atmosphere. Some scientists such as Carl Sagan have used Venus as a warning of what could happen to Earth should greenhouse gases spiral much further out of control. Despite the hostile environment it has been speculated that there may be life forms in Venus atmosphere where temperatures are cooler but still very acidic. Certain gases that are indicators of life have been detected in the atmosphere but those results have been disputed and are far from conclusive. If there is life on Venus it would have to be extremely tough to survive in clouds of sulphuric acid! Unlike Earth, Venus has no moons and it spins the other way, meaning the sun rises in the west. It also has the longest day of any of the planets at 243 earth days while a trip around the sun only takes it 224 days. In recent years we have learned a lot more about this very hostile planet with probes from the European Space agency and Japan orbiting Venus and scanning the surface and more missions planned in the next decade. So before we move on to Mars, a quick catch-up about Venus


  • Venus is super bright and the hottest of all the planets in our atmosphere
  • This is because of Venus having a runaway greenhouse atmosphere made up largely of carbon dioxide covered by thick clouds of sulphuric acid.
  • This means the temperature at the surface is about 500 degrees and the pressure is like being a kilometre under the sea.
  • Venus is very similar in size to earth, but has no moon and it spins the other way.
  • A day on Venus is longer than a year at 243 earth days, while a year on Venus is about 224 earth days long.


Now we move on to Mars, a world that is hugely different to Venus. Mars is about one and a half times as far from the Sun as the earth is and unlike Venus it doesn’t have a thick atmosphere to trap heat, in fact its atmosphere is incredibly thin. It was believed to have an atmosphere more similar to Earth’s long ago but this has been gradually stripped away. Part of the reason for this is that Earth has a strong magnetic field that helps retain its atmosphere and Mars had something similar but it seems to have been lost billions of years ago. This means that charged atoms in what is called solar wind that streams through space is very gradually taking away atoms from Mars atmosphere. So without a strong greenhouse effect to retain heat, Mars is now quite a cold planet with average temperatures of about -60 degrees Celsius though temperatures of up to 35 degrees have been recorded. As well as being cold Mars is a dry dusty planet, though this was not always the case. There is lots of evidence for large bodies of water on Mars at some point in its history, with detailed images of what is believed to be gullies and canyons carved out by flowing water and glaciers. Some of these are absolutely vast with one, the Madim Vallis being 700km long and over 2km deep which makes the Grand Canyon look almost like a pothole in comparison. But what happened to all of this water? It goes back to Mars atmosphere, which is so thin that air pressure is extremely low. This means that liquid water is very difficult to form as it would turn to steam even at very low temperatures and the water is then lost into space as the atmosphere gets stripped away. What there is though is ice and quite a lot of it. The polar regions of Mars hold large reserves of ice, enough to cover the whole planet to over 10 metres and there are believed to be underground ice reserves of immense size too as well as huge craters filled with ice such as the 2km deep ice in the Korolev crater. Another thing most people know about Mars is that it’s red, indeed this blood red colour is why it’s named after the Roman god of war. This is because of large amounts of iron in the soil which gives it a rusty red colour. This red dusty surface covers most of the planet which is also covered in impact craters much like our moon is because the thin atmosphere means meteors don’t get burned up on entry. Really, we should be a lot more grateful for our atmosphere, it does an awful lot for us. But speaking of the moon, Mars has two of those. We are now far enough away from the awesome gravity of the Sun that planets can have moons and Mars two are called Phobos and Deimos. These are the twin sons of the god Mars Greek equivalent Ares and their names mean fear and dread. As moons go they’re pretty unremarkable. They are quite small at 22 and 12km across and both are not quite round. They also won’t orbit Mars forever with Phobos expected to crash into the red planet in about 10 million years, while Deimos is eventually expected to drift into space. Mars moons are much smaller than earth, and so is the planet itself. It is about half the diameter of earth and gravity there is 38% that found on Earth. A day on Mars is slightly longer than earth at just over 24 hours while a year is 687 earth days. But despite these differences, conditions at the surface of Mars are easily the closest to what we find here on Earth, and along with the presence of water ice make it the only really plausible candidate for colonisation by humans. Because of these conditions and the hope of maybe landing some humans on it in the near future, there have been a lot of missions to Mars including landing several amazing robotic rovers. But before we go into that, a quick recap.


  • Mars is one and a half times further from the Sun that Earth
  • A day is a little bit longer than on earth and a Mars year is 687 earth days
  • Mars is cold and has hardly any liquid water due to its very thin atmosphere
  • Mars at one point had lots of water with many water carved features on its surface
  • There is still quite a lot of ice, especially in the polar regions
  • Mars has two small moons that will either crash or drift away in the future



For the final part of the podcast I like to cover some research aspect of what we are talking about and today it will be research largely done by robots. We know more about Mars than any other planet than earth largely because we have been able to send lots of research missions to not just orbit the planet but to roam around on the surface. For this section I will ignore those that orbited Mars and talk about the wonderful rovers that have told us so much about the red planet. The first rover to land on Mars was the Sojourner rover which touched down in 1997 as part of NASAs Pathfinder mission. Weighing just 11.5kg about the weight of a medium sized dog, Soujourner was expected to complete 7 days of operations on the Martian surface, with two cameras and some basic scientific equipment. In fact this hardy little rover was able keep going for almost three months and travelled about 100m before it stopped functioning. In this short time period Sojourner was able to carry out analysis of rocks on the Martian surface, giving critical information about the geology of Mars and insights into the history of the planet particularly to the history of water and its role in rock formation. All of this data was transmitted by the Pathfinder lander back to earth, which if you saw the movie The Martian a few years ago was what Matt Damon’s character used to communicate with NASA.


The next rovers on Mars were Opportunity and Spirit. These twin rovers were far more ambitious pieces of kit. Weighing in at 185kg, about the weight of an adult lion, Opportunity and Spirit landed on opposite sides of the planet in January 2004 with planned missions of about 90 Mars days. Spirit continued to function for over 5 years before getting stuck and it ceased communications with NASA in 2010. Opportunity amazingly continued to work for 57 times its expected life span and only stopped communications in June 2018. Spirit travelled almost 8km during its mission while Opportunity managed a whopping 45km, which isn’t bad since they were designed to travel approximately 100m each. Both of the rovers had much more sophisticated cameras and were able to take highly detailed panoramas of the Martian surface as well as high resolution closeups of rocks and geological formations. They were able to analyse the rocks they encountered which told scientists a lot about the geology of Mars, its previous volcanic activity and about the history of water on the planet, a key factor in searching for evidence of life.


NASA significantly stepped up the sophisticated nature of their rovers with the next mission, and one I’ve been following on social media as Curiosity has it’s own Twitter account. On the anniversary of its landing each year it plays Happy Birthday to itself and so far it has done that 9 times, with the tenth a few weeks away from when I recorded this. I know you shouldn’t pick favourites but I’m quite happy to say that Curiosity is my favourite robot of all time, with Bender from Futurama a close second. About the size of a small car and weighing as much as a decent sized walrus, Curiosity touched down on Mars in November 2011. Like the other rovers, Curiosity has far exceeded it’s intended lifespan and its two year mission has been extended indefinitely. Equipped with an array of scientific instruments and cameras, Curiosity is essentially a mobile lab that is remote controlled from millions of kilometres away. Curiosity’s mission was more focused on searching for potential signs of current or historical life on Mars while still looking at physical, atmospheric and geological properties of the red planet. The rover has been able to take and analyse soil and rock samples as well as studying the martian atmosphere and potential water cycle. Curiousity has not as yet covered as much distance as Opportunity and is about 23km from it’s landing site but it has been able to transmit much more data back to scientists on earth.


The latest NASA rover mission to Mars is called Perseverance. It landed in April of 2021 and its design is very similar to Curiosity with one super cool and notable exception which is very appropriately called Ingenuity. Ingenuity, in case you didn’t know, is a tiny helicopter that has to date made 12 flights on another planet! This is so so amazing. Just over 120 years since the first powered flight, humans have an aircraft flying around on Mars. It’s millions of miles away! It is just so cool that this is a real thing. Perseverance itself will add to the curiosity mission and help us better understand Mars while allowing us to see it from the sky thanks to it’s ingenious friend.


The latest rover to land on Mars is not a NASA mission but is instead from the Chinese National Space Administration and it is called Zhurong. The rover landed in May 2021 and it is quite a bit smaller than the latest NASA rovers but the exact dimensions and weight are not publicly known. Zhurong has lots of cameras and scientific instruments on board with which it is analysing the rocks and atmosphere of Mars. Like Perseverance, Zhurong has ground penetrating  radar, which is used on earth for surveying and archaeology to try and see what’s going on underground without excavating. Zhurong’s collected data will be used exclusively by Chinese scientists for the first 6 months after it is acquired after which it will be released to the scientific community, so exactly what it has found should become public early next year.


Well that’s it for this episode. I’d like to end by thanking Neal from PodKnows productions for editing the episode and all his advice on how to do a podcast and thanks also to Paul Farrer for his amazing original theme tune. The next episode which will be on the other planets in our solar system will be out soon. Please remember to subscribe and share the podcast if you’ve enjoyed it and leave comments and ratings wherever you get your podcasts from. Thanks for listening.