The ESPRESSO planet-hunter looks at Jupiter

The ESPRESSO spectrograph – on the Very Large Telescope (VLT) in Chile – is primarily a planet-hunter. It’s designed to search for planets orbiting distant stars. But researchers said in late December 2023 they’d used this instrument to look toward the largest planet in our solar system, Jupiter. Researchers at the Institute of Astrophysics and Space Sciences in Portugal said they used ESPRESSO to peer into Jupiter’s dense atmosphere, and to study its powerful winds It was the first time astronomers have used such a tool to examine Jupiter.

The researchers published their peer-reviewed paper on November 24, 2023, in the journal Universe.

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ESPRESSO observes Jupiter

ESPRESSO searches for and characterizes exoplanets orbiting other stars, rocky planets in particular. However, astronomers can also use it to study planets in our own solar system. And that’s just what they did for five hours in July 2019. The researchers pointed the Very Large Telescope (VLT) at Jupiter. More specifically, they focused on the equatorial region of the planet, where there are light clouds at high altitude. In addition, the telescope also looked at the north and south equatorial belts of the planet. Lead author Pedro Machado, from the Institute of Astrophysics and Space Sciences and the University of Lisbon, said:

Jupiter’s atmosphere, at the level of the clouds visible from Earth, contains ammonia, ammonium hydrosulfide and water, which form the distinct red and white bands.

The upper clouds, located in the pressure zone of 0.6 to 0.9 bars, are made of ammonia ice. Water clouds form the densest, lowest layer, and have the strongest influence on the dynamics of the atmosphere.

The research team developed a new technique called Doppler velocimetry. It measures the amount of visible light reflected off the clouds in a planet’s atmosphere. Essentially, the wavelength of the light is bent in proportion to the speed the clouds are moving, relative to the observing telescope. Astronomers can then calculate the wind speed at that moment. ESPRESSO measured winds on Jupiter ranging from 37 to 266 miles per hour (60 to 428 km/h), with an uncertainty of less than 22 miles per hour (35 km/h). The paper stated:

We present a new study of Jupiter’s atmosphere dynamics using for the first time the extremely high-resolution capabilities of VLT/ESPRESSO to retrieve wind velocities in Jupiter’s troposphere, with a dedicated ground-based Doppler velocimetry method. This work is primarily a proof-of-concept for retrieving Jupiter’s winds using VLT/ESPRESSO Doppler velocities.

Challenging observations

Even though the Jovian observations are much closer than those of exoplanets, they can still be challenging. For example, one issue was due to the telescope having such high resolution. As Machado noted:

One of the difficulties centered on ‘navigation’ over Jupiter’s disk, that is, knowing exactly which point on the planet’s disk we were pointing to, due to the enormous resolution of the VLT telescope.

In the investigation itself, the difficulty was related to the fact that we were determining winds with an accuracy of a few meters per second when Jupiter’s rotation is on the order of ten kilometers per second [six miles per second] at the equator and, to complicate matters, because it is a gaseous planet, and not a rigid body, it rotates at different speeds depending on the latitude of the point we observe.

The researchers also compared the new data to previous observations – most of which were done in space – to help verify its accuracy.

A helping hand from Cassini

NASA’s Cassini spacecraft played a role in the new Jupiter study, even though it was a Saturn mission. The researchers used images it took when it passed by Jupiter in December 2000. That data is also consistent with the new observations from the VLT and ESPRESSO, further validating them. The paper said:

These results are complemented by a re-analysis of Cassini’s data from its fly-by of Jupiter in December 2000, performing cloud tracking at visible wavelengths, for cross comparison with Doppler velocimetry results, along with previous cloud-tracking results.

Continued monitoring of Jupiter

Overall, the positive results show that Doppler velocimetry can indeed be used to monitor winds on Jupiter using Earth-based telescopes. Astronomers want to see how the winds change over time. Then, they can build a model of the global circulation of the entire atmosphere.

All this will help scientists better understand how Jupiter’s deep, turbulent atmosphere behaves. It’s a complex system, with storms larger than Earth, fast-moving atmospheric belts, lightning and auroras.

The researchers at the Institute of Astrophysics and Space Sciences will continue using ESPRESSO and the VLT. By doing so, they can observe more of Jupiter’s total atmosphere, measure winds at different altitudes and collect wind data for an entire rotation period. Jupiter takes about 10 hours to complete one rotation on its axis.

Later, the same technique can be used for other giant planets as well, like Saturn. And that, in turn, will help scientists learn more about exoplanets, too. So ESPRESSO can continue its exoplanet mission, even when its gaze is sometimes turned back onto our own planetary neighbors.

Bottom line: For the first time, astronomers have used an instrument designed to study exoplanets – ESPRESSO, on the Very Large Telescope – to observe the planet Jupiter.

Source: Jupiter’s Atmosphere Dynamics Based on High-Resolution Spectroscopy with VLT/ESPRESSO

Via Institute of Astrophysics and Space Sciences

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