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Cassini gave an epic final show with a series of high-risk maneuvers to get where no space probe had gone before. Here are some interesting things we learned in this final.
An artistic Cassini design plunging into the gap between Saturn and its rings. NASA / JPL
After 13 years in orbit around Saturn, the Cassini mission ended on September 15, 2017 with a dive into the atmosphere of the planet.
But during the final phase of the operation, while the spacecraft was practically working on smoke, NASA engineers developed a series of daredevil maneuvers designed to probe previously uncharted territory. During his Grand Final, Cassini passed several times between the rings of the planet and the atmosphere, grazed the outer edges of the rings and traveled to the high latitude regions where he observed the aurora of the planet. .
He has also toured the planet more often, providing more regular observations of changing characteristics such as the planet's busy magnetosphere. "The orbits of the grand finale have also allowed us to access unexplored time scales," says Elias Roussos (Max Planck Institute for Solar System Research, Germany). "This type of data lends itself to discoveries, and I think there will be many more to come."
October 5th Science, six new research reports detail what researchers have learned over the past year from Cassini's latest show. Here are some takeaways.
1. The rings consist of a frozen chemical soup
Previous remote measurements had long ago revealed that the rings of Saturn are mainly composed of grains of water ice. However, the complete recipe was a mystery. During his Grand Final, Cassini collected and analyzed some of the grains that fall from the rings on the planet, according to a phenomenon called "ring rain".
"We were expecting steam and grains, and we saw methane, molecular nitrogen, and carbon dioxide," said Hunter Waite (Southwestern Research Institute). ), first author of one of the new articles. Cassini also detected ammonia and organic compounds bound to tiny grains of ice water, on the nanogram scale.
Researchers do not yet know whether this composition is representative of all or only the most internal rings, which could have been polluted by interactions with the planet or by other bodies such as comets, such as comets. Alternatively, the rings could be a reservoir of the original disk of gas and dust surrounding the sun from which the planets were formed. Or maybe there are unknown processes that form and maintain the rings, feeding them with these chemicals.
2. More than rain, it's a shower.
This image of Cassini reveals the ephemeral D ring of Saturn, the innermost ring barely visible next to the rest of the system.
NASA / JPL-Caltech / Institute for Space Science
The amount of material falling from the rings in Saturn is much higher – at least 10 times more – than expected. Most of these materials seem to come from the inner regions of the D ring, the closest to the planet. When the rings spin around the planet, they throw tons of ice-grains covered with the mixture of chemicals previously described. Astronomers have calculated a precipitation of 10 tons of material per second. For millions of years, this rain could change the composition of the equatorial atmosphere of Saturn.
The researchers also observed that the seeds appeared to have a preferential source: the D68 loop, an illuminated region of the D ring that could have been altered by a collision with a passing object, such as a comet.
This has led astronomers to revisit their ideas about the longevity of the ring. "At the current rate of trade, the D ring, it's not replenished by the C ring, would disappear in a few million years," says Waite. This means that at some point, the rings could disappear, unless they are restored by an unknown mechanism.
3. There is a radiation belt between the rings and the atmosphere
Most planets with a magnetic field, such as the Earth, are surrounded by radiation belts that trap charged particles. In the case of Saturn, researchers already knew the radiation belts outside the rings. Scientists suspected the existence of an additional radiation belt between the planet and the rings, but they had never been able to detect it until now.
"We knew that some processes could generate high-energy particles near Saturn," Roussos says. "But any charged particle that is injected between Saturn and its dense rings must fight against the impacts with the atmospheric molecules of the planet and with the dust in the Saturn D ring that exhausts its energy."
An illustration of Saturn's radiation belts. The radiation belt inside the rings (inset) was
observed during the grand finale of Cassini.
MPS, JHU-APL
Nevertheless, Cassini detected a proton belt loaded just above the atmosphere. This new ring is split into two segments due to the presence of the D68 ring absorbing the protons. However, there is another loop called D72, which does not seem to influence the belt at all. "We did not think these buckles were important when we built our radiation belt models several years ago. We now see how strong their influence is and how diverse they are, "says Roussos.
4. As on Earth, auroras can produce radio broadcasts
During the passage of Cassini in the magnetosphere of Saturn, the probe measured the radio emissions of the particles generating the auroras. The aurora borealis resemble in some respects those of the Earth: the charged particles move along the magnetic field lines and excite certain atoms. On Saturn, aurorae emit ultraviolet light from excited hydrogen atoms, rather than oxygen and nitrogen.
On Earth, the charged particles that create the aurora can feed a different phenomenon when they are higher in the atmosphere. At a certain altitude, they encounter a magnetized plasma. Auroral magnetic field lines trap electrons, which then transmit their energy to radio waves.
Researchers have now shown that these radio broadcasts also existed on Saturn. They measured low frequency radio sources between 10 kHz and 20 kHz, generated a few hundred thousand kilometers above the atmosphere. They also observed that these emissions change over time, revealing that plasma density is changing locally. "Why does plasma density vary so much with time? The question remains open until now, "says Laurent Lamy (Observatoire de Paris, France). "But it is interesting to note that radio broadcasts provide a diagnosis of local plasma conditions."
The references:
Mr. K. Dougherty et al. "The magnetic field of Saturn revealed by the Cassini Grand Final." Science. October 5, 2018.
E. Roussos et al. "An energy proton radiation belt located between Saturn and its rings." Science. October 5, 2018.
L. Lamy et al. "The low-frequency source of Saturn's kilometric radiation." Science. October 5, 2018.
H.-W. Hsu et al. "In situ collection of dust grains falling from the rings of Saturn into its atmosphere." Science. October 5, 2018.
D. G. Mitchell et al. "The dust grains fall from Saturn's ring D into the upper equatorial atmosphere." Science. October 5, 2018.
J.H. Waite et al. "Chemical interactions between the atmosphere of Saturn and its rings." Science. October 5, 2018.
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