Io – that’s perhaps the smallest name of any celestial object known to humans. It is one of the 67 moons of Jupiter and is quite an interesting object hanging out there in the void. 

In this article on Io facts, we are going to share some of the most mind-blowing information about this interesting Jovian moon. So, sit tight. It is going to be one hell of a read…

Io Facts: 1-5

1. Io was discovered on January 8, 1610. The discoverer was Galileo Galilei. It was one of the four Jovian moons that were discovered by Galileo and hence, it is often referred to as the Galilean Moon of Jupiter. The other three moons that were discovered by Galileo are:

2. If we are to compare the volume and mass of the four Galilean moons of Jupiter, they ranking they get is:

  • First – Ganymede
  • Second – Castillo
  • Third – Io
  • Fourth – Europa

3. If we are to arrange all 67 moons of Jupiter in order of distance from Jupiter, Io takes the 5th spot. However, if we are to arrange the four Galilean moons in order of distance from Jupiter, Io takes the 1st spot.

4. Actually, Io was not discovered on January 8. It was discovered on January 7, 1610 by Galileo. However, that night, Galileo failed to distinguish between Europa and Io. Things became clear to him on the next night, that is, 8th January. Hence, it is said that the discovery was done on 8th January.

5. Discovery of Io, Europa, Callisto and Ganymede was the first ever discovery of any moon orbiting any other planet but Earth.

Io Facts: 6-10

6. Galileo’s discovery was stunning. It was his discovery of these moons that eventually led people to believe that planets orbit Sun and NOT the solar system orbiting Earth. Galileo gets a lot of credit for this.

7. The truth however is that it was an ancient Indian mathematician-astronomer by the name Aryabhata (born 470 CE) who actually told that planets orbit the Sun and not the solar system orbiting Earth. 

His observations and studies are all recorded in his work ‘Aryabhatiya’ – an astronomical treatise. Still, to this day, West refuses to give him the credit he deserves.

8. Nonetheless, coming back to Io, Galileo actually named the Jovian moon as Jupiter I (because it was the first Jovian moon he discovered). The name he gave was used till the mid 1800’s when it became clear that using numbers as names will only create a lot of confusion.

9. So, in the mid 1800’s the name of Jupiter I was changed to Io. Io comes from Greek mythology where Io was the priestess of Hera – the wife of Zeus. Io was the daughter of Inachus, who was in turn, the King of Argos.

10. According to Greek mythology, Zeus fell in love with Io. However, he was afraid that Hera would catch him with Io. So, to avoid being caught, Zeus turned Io into a cow.

Io Facts: 11-15

11. Age of Io: Io is just as old as Jupiter itself. This means that Io is around 4.5 billion years old and was formed around the same time when Jupiter was born.

12. Distance between Io and Jupiter: Io hangs at a distance of 421,700 kilometers. This is the mean distance. The farthest Io goes from Jupiter is 423,400 kilometers (Apoapsis) and the closest it gets to Jupiter is 420,000 kilometers (Periapsis). This is known as the orbital distance of Io from Jupiter.

13. Orbital time and speed: Io orbits around Jupiter in 42.45936086 hours. That is equivalent to 1.769 (or ~ 1.77) Earth days. The moon travels around Jupiter at a speed of 17.334 km/s.

14. Measurements of Io: The measurements here refer to the following:

  • Mean Radius of Io: 1,821.6 ± 0.5 kilometers.
  • Surface Area of Io: 41,910,000 km2.
  • Mass of Io: (8.931938 ± 0.000018)×1022 kg.
  • Volume of Io: 2.53×1010 km3.
  • Mean Density of Io: 3.528 ± 0.006 g/cm3.
  • Minimum Surface Temperature: 90 K or -183.15°C.
  • Maximum Surface Temperature: 130 K or -143.15°C.
  • Mean Surface Temperature: 110 K or -163.15°C

15. Gravity and escape velocity on Io: Surface gravity on Io stands at 1.796 m/s2. That is less than the gravity on Earth. Earth has a gravity of 9.807 m/s2 while our Moon has a gravity of 1.622 m/s2. Escape velocity for Io is 2.558 km/s.

Io Facts: 16-20

16. Based on data available from various spacecrafts like Galileo and Voyager, scientists have used several computer models to determine that the crust of Io and its mantle are rich in silicate. It’s core on the other hand is either rich in just iron or it is rich in iron-sulfide.

17. 20% of the mass of Io comes from its metallic core. Scientists say that the radius of the core is dependent on the sulfur amount present in the core. Here are the two estimates that scientists give:

  • If the core is made entirely of iron, the radius of the core is between: 350 km and 650 km.
  • If the core is made of a combination of sulfur and iron, the radius of the core is between: 550 km and 900 km.

18. Scientists say that 75% of the mantle of Io is made of forsterite – a mineral that is rich in magnesium. The iron content in the mantle is higher than what is present in mantles of our Earth and our Moon. The iron content in Io’s mantle is greater than that of Mars.

19. The surface of Io lacks impact craters. Rather, its surface is covered with smooth plains and lots of tall pits and mountains that significantly vary in size and shape. The surface is also covered with volcanic lava flows.

20. The surface of Io is covered with a wide range of colorful materials that are basically sulfur compounds. Scientists compare the surface of Io with either a pizza or a rotten orange.

Io Facts: 21-25 

21. Scientists speculated that the absence of an impact crater on Io’s surface is actually caused by continuous lava flows that cover those craters. This speculation was eventually confirmed when scientists received images from Voyager I. That spacecraft managed to capture 9 active volcanoes on the surface of Io.

22. Io’s surface has a very colorful appearance and that’s caused by volcanism that deposits material on its surface. The materials that are deposited on its surface by its volcanoes include sulfur dioxide, sulfur and various silicates such as orthopyroxene.

23. If there is something that is found widely across the surface of Io is the sulfur dioxide frost. Large regions of the surface of Io are covered with gray or white material.

24. Many yellow regions or yellow-green regions seen on the surface of Io are basically formed by sulfur. However, the sulfur deposits that are found in polar regions and mid-latitude regions often get radiation damage, breaking up the sulfur and creating the red-brown polar regions.

25. The surface of Io also gets painted with silicate materials and sulfurous materials that come out in the form of umbrella-shaped plumes during explosive volcanism. Depending on the sulfur dioxide and sulfur content of the plume, the plume may look white or red.

Io Facts: 26-30

26. Io is dotted with several mountains as well. So far anywhere between 100 and 150 mountains are assumed to be present on its surface with an average height of 4 miles or 6 kilometers. The Boösaule Montes located at the south reach a maximum height of 17.5 ± 1.5 kilometers of 10.9 ± 0.9 miles.

27. In terms of length, the mountains are pretty long with the average length stretching over 98 miles or 157 kilometers.

28. Since the mountains on Io lack any global tectonic pattern that we see on Earth, scientists believe that these mountains are actually not made of sulfur. Rather, the hypothesis is that these mountains are made of silicate rocks.

29. Interestingly the Ioian mountains are tectonic structures. They are formed by compressive stresses at the lithosphere base resulting in uplifting of the crust and thus, formation of mountains. No, volcanoes on the Jovian moon do not form the mountains.

30. Another important feature of the Ioian mountains is they have a lot of plateaus (flat tops). Funny that most of the mountains on Io go through degradation via mass wasting – a phenomenon in which large landslides lead to deposits at the base of the mountains.

Io Facts: 31-35

31. Since our own Moon has no atmosphere, it is logical to think that this Jovian moon will not have an atmosphere of its own. Quite the contrary, Io has its very own atmosphere. And yes, it is unlike what we have on our Earth.

32. Atmosphere of Io is thin, really thin. The gases that make up Io’s atmosphere include sulfur dioxide as the primary component. SO2 is the primary component. This means there are other components as well.

33. The other components that make up Io’s atmosphere include SO or sulfur monoxide, NaCl or Sodium Chloride (yes our table salt) and there’s atomic oxygen as well as atomic sulfur present in the atmosphere.

34. Well, no need to get too excited here. Though there’s oxygen in Io’s atmosphere, it is too scanty to support aerobic life. And scientists don’t yet know if there is anaerobic life present on Io or not.

35. One really interesting thing about Io’s atmosphere is its temperature and density variation depending upon several factors like volcanic activity, latitude, time of the day, abundance of surface frost etc.

Io Facts: 36-40

36. Wondering what’s the maximum atmospheric pressure out there on Io? Scientists say that it ranges anywhere between 3.3 x 10-5 pascals and 3 x 10-4 pascals.

37. This maximum atmospheric pressure on Io is present on the anti-Jupiter hemisphere of the Jovian moon and also at its equator. This maximum atmospheric pressure is also seen when the temperature of surface frost is at peak and even during early afternoons.

38. That’s the maximum. What about the minimum? The minimum atmospheric pressure comes at night. That’s when the atmospheric pressure dips all the way down to between 0.1 x 10-7 pascals and 1 x 10-7 pascals.

39. Here is something really interesting. The magnetosphere of the host planet, that is, Jupiter, actually strips Io’s atmosphere of the gas.

40. The gas that escapes the atmosphere of Io can land in two possible places:

  • It can land in the neutral cloud that is known for surrounding Io.
  • It can land in the plasma torus of Io.

Wondering what plasma torus is? Plasma torus is a ring created by ionized particles. This ring is known for sharing not only the orbit of Io but also known for co-rotating with Jupiter’s magnetosphere.

Now the question is, what really is a torus? It is a concept in geometry. It is a surface of revolution that is generated by revolving a circle in a 3D space ensuring that the plane of the circle and the axis of the circle are on the same plane (called coplanar). See the image below:

torus example

Io Facts: 41-45

41. Did you know that the magnetosphere of Jupiter manages to strip out nearly 1 ton of material from the atmosphere of Io?

42. So, the atmosphere must be quickly replenished, right? Exactly that’s what happens. The volcanoes on Io manage to pump in around 104 kilos of sulfur dioxide into the atmosphere of Io every single second. That’s far more than what is stripped off.

43. Here is yet another interesting thing. The SO2 that comes out of the volcanoes are frozen and they fall down as frost. This frost undergoes sublimation (changing state directly into gas from solid) by sunlight.

44. Scientists can say that the atmosphere is mostly sublimation-driven because the atmosphere is densest:

  • In Io’s anti-Jupiter hemisphere.
  • When Io is closer to the Sun.

45. The other constituents of the atmosphere of Io comes from three sources namely:

  • Chemical breakdown of SO2 by ultraviolet radiation of the Sun.
  • Direct volcanic outgassing.
  • Sputtering (bombardment with particles from the magnetosphere of Jupiter) of deposits found on the surface of Io.

Io Facts: 46-50

46. Io has been imaged when the moon was in eclipse. The high resolution images show that Io (during eclipse) has aurora-like glow. This happens because charged particles from the magnetic field of Jupiter hit the atmosphere of Io.

47. Unlike in the case of planets, the aurorae of Io are actually far brighter near the equator. This happens because the Jovian moon lacks its own intrinsic magnetic field.

48. The multi-colored aurorae of the Jovian moon is actually caused by the emission from different components of the moon’s atmosphere. For instance, blue comes from emissions from volcanic gases like SO2.

49. The red color is emitted by the oxygen content of the atmosphere of Io. Green on the other hand is because of sodium.

50. Because Io doesn’t have a thick atmosphere, if probes are sent by any space agency for landing on Io, the probe will not require an aeroshell-style heatshield encasement. Rather, retrothrusters for soft landing will suffice.

Io Facts: 51-55: Some Io Trivia for You

51. Io is tidally locked with Jupiter. This means that one side of Io always faces Jupiter just as one side of our Moon always faces our Earth.

52. Io along with Earth and Venus are the only celestial bodies in our very own Solar System which is known for have active volcanoes that spew out basaltic lava.

Note: There are two type of volcanoes that you can find in the Solar System. They are the normal volcanoes as we know on Earth, and then there are colder versions of volcanoes known as Cryovolcanoes.

Cryovolcanoes are those volcanoes that do not spew out basaltic lava. Instead they spew other volatile material like water, ammonia etc. Here is a list of different celestial bodies where you can find volcanoes (normal and cryovolcanoes):

Moon (our moon) – There are volcanoes on moon but they are inactive now.

Mars – There are volcanoes but they are inactive.

Earth – There are active volcanoes that spew basaltic lava.

Venus – Astronomers believe that there are active volcanoes on the planet. The probe called Venus Express detected various spots on the planet where temperatures have risen and fallen rapidly, suggesting lava flows that cause the brief temperature spikes.

Titan (the largest moon of Saturn) – It may have cryovolcanoes that erupt ammonia, water and other volatile liquids but not basaltic lava.

Enceladus (another moon of Saturn) – There are over 100 geysers that spew out water and various other chemicals periodically. So, it does have active cryovolcanoes.

Triton (the largest moon of Neptune) – It has very active cryovolcanoes and they spew out ice and other material but not basaltic lava.

53. Io has 400 active volcanoes on its surface. This makes Io the most volcanically active celestial body in our Solar System. Even Earth’s volcanoes cannot match Io.

54. The volcanic eruptions are pretty violent. Some volcanoes on Io can spew materials as high as 330 kilometers above the surface of Io.

55. Pioneer 10, Pioneer 11, Voyager 1, Voyager 2 and Galileo spacecraft have flown by Io. However, Voyager 1 was the first one to send back Io’s images. The next ones to do so were Voyager 2 and Galileo spacecraft. So, all the information we have on Io actually comes from Voyager and Galileo spacecrafts.


Categorized in: