I have done a few interviews on the BBC in the last week about NASA’s Juno space probe; it is great to see the mission getting such press coverage. You can listen to my BBC Radio Cymru interview here, and my BBC Radio Wales interview here. With all the press coverage there have inevitably been a few misunderstandings, so I thought I would try and explain as clearly as I can what Juno hopes to accomplish and how it will do it.
Some background on Jupiter
Jupiter is by far the largest planet in the Solar System. All the other planets together would fit into it, and the Earth would fit into it over 1,300 times! Because it is the largest planet in the Solar System, we believe that it would have dominated the formation of the planets. Once the gas in the central part of the solar nebula (the cloud of gas and dust from which the Sun and Solar System formed) had collapsed to form a nascent star, the disk of material around the still-forming Sun would have started clumping together under gravity and collisions to form the planets.
Because Jupiter is the largest planet, it sucked up most of the material in the disk of the solar nebula. It is mainly hydrogen and helium, as that is what the Sun and most of the Universe is made up of; about 75% hydrogen and 24% helium. But, the details of Jupiter’s composition are mainly based on theory rather than any hard observations.
What are Juno’s (main) scientific goals?
According to NASA’s Juno webpage (click here to go to it), the main objectives of Juno are
- Determine how much water is in Jupiter’s atmosphere
- Look deep into Jupiter’s atmosphere to measure composition, temperature, cloud motions and other properties
- Map Jupiter’s magnetic and gravity fields, which will reveal the deep structure of the planet
- Explore and study Jupiter’s magnetosphere near the planet’s poles, especially the aurorae, and provide new insights into how the planet’s enormous magnetic field is generated and how it affects the planet’s atmosphere
I will blog about each of these four points over the next few weeks, so let me start with the determination of how much water is in Jupiter’s atmosphere.
How much water is there in Jupiter’s atmosphere?
The reason this is an important question is that the two most popular theories for how Jupiter formed predict different amounts of water. Jupiter is thought to have either formed (i) from the collapse of a massive fragment of the Solar nebula, or (ii) from the build-up of planetesimals. In the first theory, the amount of water would be less than in the second theory, as the rocky planetesimals in the second theory would have been been coated in water-ice and ammonia-ice.
If you look at an astronomy textbook the interior model of Jupiter shows a solid core, but we have never actually observed this core.
Therefore, measuring the amount of both water and ammonia should help us decide which theory is closer to the truth. Water, ammonia, carbon dioxide and methane are examples of what we call ‘ices’ in astronomy, as in the environment of the Solar System all of these compounds can exist as gases but also as solids.
The water and ammonia will be measured by a microwave radiometer. This instrument consists of six antennae measuring the radiation at 600 MHz, 1.2, 2.4, 4.8, 9.6 and 22 GHz. These are the only microwave frequencies which are able to pass through the thick Jovian atmosphere. These radiometers will measure the abundance of water and ammonia down to a pressure of 200 bar, which corresponds to a depth below the cloud tops of 500 to 600 km. This is a small fraction of the radius of Jupiter, which is about 70,000 km, but it is still further below the cloud-tops than we have so far been able to study.
In the next blogpost on Juno, I will talk about how it will measure the gravitational and magnetic fields of Jupiter.