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This article comes from the official Wechat account: Earth knowledge Bureau (ID:diqiuzhishiju), text: planets do not glow, revised manuscript: gu Hanying / Editor: moth

Original title: "Great Discovery, there may be Life on this Planet"

In October 2022, the Hao Jihua team of the University of Science and Technology of China simulated the chemical composition of Enceladus's underground ocean through a computational model, revealing for the first time that Enceladus's seawater may be rich in phosphorus.

On May 17, 2023, Villanueva and others used the James Webb Space Telescope (JWST) to observe Enceladus. The results show that Enceladus's plume is as long as 10000km, about 20 times the diameter of Enceladus (figure 2).

The process of Enceladus's organic molecules and ice particles entering the plume

(figure: NASA / JPL) ▼

The plume is mainly composed of water vapor, in which about 30% of the material ejected into space enters Saturn's E-ring and 70% is scattered in other parts of Saturn's system.

On June 14, 2023, the journal Nature published the findings of Kanagan Kangren's team. This study analyzed the chemical composition of particles in Saturn's E-ring collected by the Cassini probe and found that the ice particles in the E-ring are rich in sodium phosphate. It is estimated that the phosphorus concentration in Enceladus's underground ocean is about 1-20 mmol / kg, at least 100 times that of Earth's ocean.

Cassini's cosmic dust particle analyzer obtained their spectra through the analysis of nine single ice particles. At the bottom right of the picture is the superimposed spectrogram, and there is an obvious peak in the corresponding position of sodium salt and phosphate, indicating that the E ring is rich in phosphorus.

(figure: Yasuhitoet al2023) ▼

With the discovery of phosphorus on Enceladus, planetary scientists and the public began to refocus on the cold moon. Why does Enceladus have liquid water oceans in such a cold environment? What does the discovery of phosphate on Enceladus mean?

In addition, where does the current human understanding of Enceladus come from? Will scientists' attention to planets that may have extraterrestrial life lead to further exploration plans?

Enceladus, which looks like this (photo: wiki) ▼

Cassini probe and Enceladus at present, the human understanding of Saturn system mainly comes from Cassini-Huygens (Cassini-Huygens). The mission of the space probe is carried out jointly by NASA, ESA and ASI.

Cassini-Huygens was launched on October 15, 1997 and reached Saturn's orbit in July 2004. On December 25 of the same year, the Cassini and Huygens separated.

On January 14 of the following year, Huygens successfully landed on Titan and sent back data. Cassini, on the other hand, has been in orbit for 13 years and has sent back a large amount of data, which is the main source of information for humans to understand the Saturn system.

Cassini-Huygens launches (photo: wiki) ▼

During Cassini's 13 years in orbit, a large amount of data has been sent back, and planetary scientists have not yet finished analyzing the data. Let's take a brief look at the process of human understanding of Enceladus:

In November 1.1980, Voyager 1 found Enceladus in the densest position of Saturn's E ring.

In August 2.1981, Voyager 2 found both ancient impact craters and young geomorphological structures on Enceladus's surface.

Damaged impact Crater (Picture: wiki) ▼ on Enceladus

3. In February 2005, Cassini's cosmic dust analyzer recorded thousands of impacts from tiny dust or ice particles, confirming that the E-ring is a wide ring of ice particles.

Enceladus and Saturn's E Ring (Picture: NASA) ▼

4. In July 2005, Cassini obtained images of Enceladus's Antarctic region. The image shows that there is active geological activity on its surface, and huge and high-temperature cracks are found, also known as "tiger lines".

Thermal images observed by Cassini in the 12-16 micron band

There are four cracks with significantly higher temperatures in Enceladus's Antarctic region.

(figure: NASA / JPL / GSFC / SSI) ▼

5. In 2006, Cassini observed Saturn's E-ring, proving that the geyser ejection on Enceladus was the main source of the E-ring. In the same year, Cassini's high-resolution images combined with other data speculated that there was a liquid water ocean beneath Enceladus's Antarctic.

6. In October 2007, data from the coincidence infrared spectrometer carried by Cassini showed that almost all geysers came from near four high-temperature cracks in Enceladus's Antarctic.

The formation of geysers in the Antarctic region of Enceladus

(diagram, figure: NASA) ▼

7. In March 2008, the Ion and Neutral Particle Mass Spectrometer (Ion and Neutral Mass Spectrometer, INMS) on Cassini analyzed Enceladus's plume and detected water vapor, carbon dioxide, carbon monoxide and organic molecules.

(mostly simple organic compounds with less than 50 atomic units of mass).

8. In August 2008, Cassini located exactly where plumes erupted from Enceladus's surface. The image shows that the crack on Enceladus is about 300 m deep and the inner wall is V-shaped. On the outside of the crack, there are a large number of fine sediment and tens of meters of ice. In December of the same year, more geological activities on Enceladus, such as the directional diffusion of the ice crust, were discovered.

Schematic diagram of cracked cross-section structure of Enceladus

(figure: NASA / JPL) ▼

9. In June 2009, Cassini detected sodium salts in ice particles in Saturn's outermost rings. In September of the same year, ammonia was found in the plume.

10. In February 2010, Cassini's team found that the area with the highest pressure on Enceladus did not completely overlap with the hottest area on the infrared map, speculating that Enceladus's rotation axis wobbled slightly during rotation.

Enceladus ejected plumes (Picture: wiki) ▼

11. In April 2014, Less et al drew a map of its internal structure using the Doppler shift data of Cassini flying over Enceladus at 02:00. The results show that Enceladus has a 10-kilometer-deep liquid ocean under 30-40 km of ice, and the underground ocean extends from Antarctica to south-central latitudes, with a water content equivalent to that of Lake Superior.

In July of the same year, 101 geysers on Enceladus were found to be connected to its underground ocean, which provided a sample for studying the livability of the underground ocean.

12. In March 2015, Cassini's Cassini cosmic dust analyzer data showed that tiny rock particles rich in silicon were formed when mineral-dissolved hot water in Enceladus moved upward and came into contact with colder water.

Geysers on Enceladus (imaginary, NASA) ▼

13. In 2016, scientists used Cassini's seven-year observations to accurately determine the rotation of Enceladus and speculated that there were global liquid water oceans rather than local polar liquid water oceans under Enceladus's ice.

14. In April 2017, Cassini's ion and neutral mass spectrometer detected large amounts of hydrogen in Enceladus's plume, indicating a continuous heating process on Enceladus.

In September of the same year, Cassini crashed into Saturn's atmosphere and burned out in order to avoid the pollution of Saturn's moons by microbes that might be attached to Earth.

The fall of the Cassini (imaginary picture, picture: NASA) ▼

15. In November 2017, the team of planetary scientists Gail Joe Brett at the University of Nantes in France simulated the internal structure and liquid water cycle of Enceladus using data returned by the Cassini spacecraft and engineering experiments on Earth. the plume phenomenon in Enceladus's Antarctic region is explained.

Cassini traverses Enceladus's plume (photo: NASA) ▼

16. In 2018, Postberg et al analyzed the data of Enceladus plume detected by Cassini's cosmic dust particle analyzer and found more complex macromolecular organic compounds (molecular weight more than 200atomic mass units).

At present, planetary scientists have determined that Enceladus has not only six basic elements of life-carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus-but also an ocean of underground liquid water suitable for life. As a result, Enceladus has become one of the most likely celestial bodies in the solar system to have life.

Enceladus's Night Sky Imagination (Picture: wiki) ▼

Enceladus (Enceladus) is Saturn's sixth largest moon, ranking 14th from Saturn from near to far.

In August 1789, William Herschel first discovered the second Saturn moon through a 1.2m telescope, so it is called Enceladus, with an average diameter of about 505 kilometers, which is relatively small.

He is also the discoverer of Uranus (photo: wiki) ▼

Enceladus's surface has a geometric albedo of 138% and is located in the snow line of the solar system (also known as the freezing line, which in astronomy or planetary science refers to a specific distance from the central protostar, where it is cold enough to allow volatile compounds such as water, ammonia, methane, carbon dioxide and carbon monoxide to condense into solid ice particles. In the solar system, the snow line lies between the asteroid belt and the orbit of Jupiter. ), which makes the average nighttime temperature on its surface only-196 ℃, colder than other Saturn moons.

On the other hand, a satellite of this size is usually unable to store or generate enough heat internally, so it is supposed to be a completely frozen solid, but Enceladus is not. There is an ocean of liquid water under the cold ice crust.

Size comparison between Enceladus and the UK (photo: NASA) ▼

The Genesis of Enceladus liquid Water Ocean the main reason for the existence of underground liquid water ocean on Enceladus is tide

Heat it. When Enceladus moves around Saturn, because of the eccentricity of its orbit, Saturn's gravity will periodically "squash and pinch" Enceladus, causing it to be "squeezed and pulled" internally to produce a large and continuous amount of heat.

In addition, due to Enceladus's irregular shape, Saturn's gravity will produce a net moment on Enceladus, forcing Enceladus to wobble, which will also generate a lot of heat inside Enceladus, which is mainly concentrated in Antarctica.

The scientists simulated this gravitational perturbation through the model and found that the corresponding surface maximum stress range of the wobble is exactly the same as that of the Antarctic region.

In the thermal image, Enceladus's heat is mainly concentrated in Antarctica.

(figure: NASA / JPL) ▼

In order to better explain Enceladus's heat production mechanism. In November 2017, Gail Joe Brett, a planetary scientist at the University of Nantes in France, hypothesized that Enceladus's rock core was highly porous and soft, similar to a spongy core. Under the action of Saturn's tidal gravity, it will produce continuous and stable friction heat (tidal heating is a relatively stable heating process. If the magma in the rock core is sticky, the tidal gravity produces more friction heat, and if the temperature increases, the magma in the rock core is easy to flow, and the friction heat produced by tidal gravity decreases).

Computer simulations show that it generates as much heat as 10GW, enough to keep Enceladus's underground ocean liquid for tens of millions to billions of years.

The simulation also shows that the poles of the core should correspond to the thinner regions of the ice crust. This could well explain the activity of Enceladus in the South Pole, but not why there are no similar plumes in the North Pole.

Enceladus's geysers caused by tidal heating

(figure: NASA / JPL / SSI) ▼

Dr. Dennis Matson of the Jet Propulsion Laboratory (JPL) and others believe that tidal heating is still not enough to explain why it can produce liquid water and form water vapor to spray into space, and more heat is needed to generate and maintain the temperature and jet process observed by Cassini.

They introduced a new model. The model holds that Enceladus's heat comes from radioactive decay and tidal heating. Radioactive decay is thought to lose most of the heat shortly after the formation of the solar system, and Enceladus retains that heat for some reason. The model also predicts that Enceladus is still in a cooling phase, which could last about a billion years.

Surface texture of Enceladus (Picture: NASA) ▼

What does the discovery of phosphate on Enceladus mean? Phosphorus is one of the important components of genetic material DNA / RNA, energy storage compound adenosine triphosphate / ATP and cell membrane. Phosphorus was found on Enceladus, making up the last jigsaw puzzle needed for life. This means that Enceladus has almost all the conditions necessary for life to exist.

More importantly, Enceladus's underground liquid water ocean is rich in soluble phosphorus, which can easily give birth to life. At present, it is estimated that the concentration of phosphorus in Enceladus's liquid water ocean is 10-1000 times higher than that in Earth's ocean. The concentration of phosphorus in water affects the growth and reproduction of organisms, for example, eutrophication lakes can lead to the proliferation of algae.

For example, satellite images of Lake St. Clair taken by Landsat 8

The picture is full of green algae (figure: NASA) ▼

In Earth's oceans, it is because of the existence of a large number of organisms that the phosphorus in the oceans is consumed to a very low concentration; if life does exist on Enceladus, then why is the phosphorus content in its liquid water oceans so high?

This may suggest that there is no life on Enceladus, or that life can only be metabolized at a very slow rate.

However, this kind of liquid water ocean with such high phosphorus content is good news for any potential alien organisms, and it may also be one of the best transit stations or supply stations for mankind in the future.

The structure of Enceladus, as shown in the figure

(photo: Jihua Hao, et al. 2022) ▼

It is important to note that the detected phosphate comes from Saturn's E-ring, not directly from Enceladus's plume or Enceladus's surface. Although it is almost certain that phosphates exist in Enceladus's underground liquid water ocean, we still need to be cautious.

In the future, if there are new detectors to detect and sample Enceladus, it can not only verify Enceladus's geothermal mechanism, but also confirm whether there is life in Enceladus's underground liquid water ocean. This is of great significance in the search for extraterrestrial life.

Enceladus and the size of the Earth and the Moon (Picture: wiki) ▼

Extraterrestrial life is possible in the solar system, including but not limited to Venus, Mars, Enceladus, Titan and Europa.

After the discovery of water on Mars, planetary scientists carried out the Mars exploration mission Yi Li; when phosphine molecules were discovered on Venus, planetary scientists designed the Leonardo da Vinci probe (DAVINCI+), the Venus-D mission and the VERITAS mission.

Explore the endless (photo: NASA) ▼

Today, phosphate that can be absorbed by life has been found on Enceladus, and the corresponding detection plan has not yet been put on the agenda. The only constant in planetary scientists'"new love and old love" is their obsession with exploring and studying extraterrestrial life.

Although there is no successor to Cassini after retirement, the Ice Moon Project JUICE was launched on April 14, 2023, with the main goal of exploring the Jupiter system, including, but not limited to, the underground liquid water oceans of Europa, Ganymede and Callisto.

The itinerary of the Ice Moon Project Juice probe

(figure: EAS) ▼

Enceladus and Europa have similar environments, and the detection of Europa will help humans understand the underground liquid water oceans of these ice satellites and the existence of extraterrestrial life.

Reference:

1. Villanueva G. L., Hammel H. B., Milam S. N., et al. JWST molecular mapping and characterization of Enceladus' water plume feeding its torus. Nature Astronomy, 2023

2. Frank P., Yasubito S., Fabian., et al. Detection of phosphates originating from Enceladus's ocean. Nature, 2023. DOI:10.1038/s41586-023-05987-9.

3. Hao J. H., Christopher R. G., et al. Abundant phosphorus expected for possible life in Enceladus's ocean. Earth, Atmospheric, and Planetary Sciences. 2022. DOI:10.1073/pnas.2201388119

4. Https://solarsystem.nasa.gov/news/12916/cassini-at-enceladus-a-decade-plus-of-discovery/

5. Https://www.wikiwand.com/zh-hans/%E5%8D%A1%E8%A5%BF%E5%B0%BC%E5%8F%B7%E6%8E%A2%E6%B5%8B%E5%99%A8

6. Http://news.bbc.co.uk/2/hi/science/nature/4895358.stm#map

7. Https://www.nasa.gov/mission_pages/cassini/whycassini/cassini20100708-b.html

8. Https://www.jpl.nasa.gov/news/cassini-spacecraft-reveals-101-geysers-and-more-on-icy-saturn-moon

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