Flights of fancy: exploring Uranus and landing on Venus

24 July 2020

Have you ever wondered what it would be like to fly through the atmosphere of Uranus or land on Venus? Buckle up and join Professor Patrick Irwin from Oxford University’s Department of Physics as he takes you on a space odyssey…

NASA Goddard Space Flight Center (GSFC) is working on a new instrument, the Advanced Ice Giants Net Flux Radiometer, for possible inclusion in a future atmospheric entry probe to Uranus, launching in and around the 2030s. Such an instrument, mounted on the probe, would observe the Uranian sky above, below and to the side of the probe as it parachutes down through the atmosphere. Its aim? To measure where in the atmosphere solar and thermal radiation is emitted or absorbed by measuring the difference between the upwards and downwards flux in seven different spectral filters. In collaboration with Dr Shahid Aslam, Dr Conor Nixon and several others at NASA/GSFC, Professor Irwin has been modelling what the instrument might see and what selection of spectral filters will optimise the science return.

What would a human observer see?

‘While working on a proposal for the instrument, I got to thinking about what a human observer might see if they were to follow the probe down into Uranus’s atmosphere and it gave me the idea to simulate that in a video,’ explains Professor Irwin.

Watch Approach to Uranus.

Professor Irwin used the NEMESIS (Non-linear optimal Estimator for MultivariatE Spectral analysIS) radiative transfer and retrieval tool to model the descent. Developed originally to process observations of Saturn and Titan returned by the Composite Infrared Spectrometer or CIRS instrument – that was part-built and part-designed in Oxford – on the NASA Cassini spacecraft, NEMESIS was designed from the start to be generally applicable to any planetary atmosphere. The tool has since been applied to analyse observations of the atmospheres of all the solar system planets and has underpinned the results of hundreds of papers in the peer-reviewed literature, led by authors here in Oxford and also collaborators from many other institutions in the UK, continental Europe and the USA. More recently NEMESIS has been extended to additionally model the spectra of exoplanets.

Essential modelling

‘Our modelling is an essential stage in the development of a new instrument, but this simulation video is a fascinating bonus,’ continues Professor Irwin. ‘We simulated the visible spectrum seen by such a probe at every level in the atmosphere and averaged it over the response functions of the red, green and blue receptors in the human eye to reproduce a true colour view of Uranus’s sky – which can be seen towards the end of the video. The video provides a 360˚ view in the main panel, a hemispheric view up at the top left and a hemispheric view down in the top right. Since the atmosphere gets darker as the probe descends into Uranus’s atmosphere, the brightness is continually adjusted, with the “sky brightness” indicating the brightness of the brightest areas of scattered sunlight from both upward and downward hemispheres. In this simulation the sun is at 10˚ above the horizon, which is the likely geometry for some of the probe mission scenarios currently under consideration. In reality, an observer would also see some varied cloud structures on Uranus that we haven’t captured in our model.

‘We also took the liberty of simulating an orbit about Uranus first before final descent and landing, which would not in fact take place as the probe would go straight into the descent phase as the main spacecraft went into orbit. The appearance of Uranus from space in this simulation is similar to pseudo-true colour images taken by the Voyager spacecraft in 1986.’

Landing on Venus

Watch Descent to Venus.

The same calculation was also made for a descent through Venus's atmosphere, with the help of Dr Colin Wilson from Oxford’s Department of Physics, Dr Jo Barstow from The Open University and Professor Paul Byrne from North Carolina State University. The sun was simulated to be at an altitude of 35˚ above the horizon, similar to the conditions experienced by the Pioneer Venus lander in 1978. Due to the enormously thick Rayleigh-scattering atmosphere, the dark, volcanic surface of Venus can be discerned, at visible wavelengths, only once the spacecraft gets within a couple of kilometres of the surface. The surface imagery used was chosen to be similar to that observed at Venus, but is in fact from a location in Australia, scaled to match the darkness of basaltic surface seen at Soviet Venus probe landing sites.

Dr Wilson explains: ‘As was the case for the Uranus video, this simulation uses an “average” cloud profile that is assumed to apply everywhere on the planet, so cloud variations are not simulated. Apart from slightly brighter clouds over the poles, Venus really does look pretty featureless in visible light when viewed from space, due to Venus’s highly reflective sulphuric acid cloud droplets. One of the very few colour pictures of Venus in visible light taken from space, by the Mercury Messenger spacecraft on its way to Mercury, confirmed that Venus does indeed look a lot like a ping pong ball – very much like its appearance in this simulation. Most images of Venus are false-colour images using ultraviolet or infrared wavelengths, at which more cloud contrasts can be seen. The circular feature seen in many frames in the direction exactly opposite to the sun is a real scattering effect of the clouds known as the “glory”’, and has been observed by orbiting spacecraft, most recently the Venus Express.’

Furthering our understanding

The Venus descent simulations will be used in a number of ways. First of all, net flux radiometers like the ones proposed for the Uranus entry probe are also in development for Venus, so simulations like this are needed in order to understand how they can be optimised and which atmospheric parameters can be measured. Other potential users include those developing descent imaging cameras, and those seeking to generate electricity from solar panels at the surface and on cloud-level balloons.

For Venus, there is the advantage that real measurements made by American and Soviet entry probes are available, allowing the calculations to be checked against real data. The simulated light levels were found to be in generally good agreement with those measured from Venus descent probes. Although analysis is ongoing to minimise remaining differences, this result gives confidence to the Uranus simulations and underlies one of the key advantages of the NEMESIS project, namely that it can be applied to any planet and hence lessons learnt from one study can immediately be applied to another.

The videos were made with the assistance of Dr Maarten Roos-Serote of Lightcurve Films.