How could such a large lake disappear just like that? Where can we find evidence of life on Mars?

In the long evolution of a planet, stones are both products and witnesses.

There are tens of thousands of rocks in nature, and different rock types correspond to completely different formation mechanisms and record completely different evolution stories . This is especially true for extraterrestrial bodies that have extremely low exploration levels and are extremely difficult to obtain data on.

A rock on Mars can largely determine whether the next time you look up at it, you will see a boring little bright spot or a second cradle of life that can eliminate human loneliness.

However, Perseverance has complicated this matter.

Too long to read

Perseverance was sent to Mars to search for evidence of life;

It landed near a river delta, which may have been a large lake in ancient times;

As we all know, water is the source of life, and if Mars has ever had life, the evidence may be in the sedimentary rocks at the bottom of the lake;

As a result, Perseverance discovered that the rocks at the bottom of the lake are not sedimentary rocks, but igneous rocks, formed by solidification of magma;

As we all know, life cannot exist in magma at a temperature of thousands of degrees Celsius;

Oh no? Things are getting complicated...

Searching for Life on Mars

Perseverance is a Mars rover launched by NASA in 2020. In February 2021, Perseverance successfully landed in the scheduled target exploration area, the Jezero Crater on Mars. It is a "Mars rover" equipped with wheels that can move freely on the surface of Mars.

Generally speaking, when a huge amount of money is spent to launch a probe to an extraterrestrial body, the hope is that it can "make big news." Therefore, in addition to ensuring the basic premise of landing success rate, it is also necessary to carefully evaluate the potential scientific value of the landing area.

But we can't just guess about this. The general practice is to first use a remote sensing satellite deployed in the orbit of Mars to take pictures of the terrain, and based on existing knowledge, determine where there is great potential for exploration value, and then send the Mars rover down to actually run around and officially start verification.

The two biggest scientific questions that humans are targeting when exploring Mars are: the early evolution of the solar system and whether there is extraterrestrial life.

There are densely packed craters on Mars. There is an important reason why NASA scientists chose the Jezero crater as the exploration area for Perseverance - it has been flooded .

The word "flood" has been used so frequently in daily conversation that we often overlook its original meaning - the spread and siltation of water.

At the edge of the Jezero crater, remote sensing images clearly show the geomorphic features of the river channel and delta . A delta is formed when the rushing river water carries silt and flows out of the river mouth. At the river mouth, the river water disperses, and the silt it carries loses kinetic energy instantly, forming a fan-shaped sediment body at the river mouth. This is the delta.

On Earth, the Yellow River, Yangtze River, Nile River, etc. all have deltas at their mouths. Although there is no liquid water on Mars today, the sand and mud carried by rivers will not evaporate out of thin air. The deltas they piled up have been preserved as geological relics, clearly indicating that free-flowing water once flooded into the Jezero crater in ancient times, forming a huge lake.

However, the existence of water itself is not the ultimate answer that people seek. There is a greater meaning behind water: it is the source of life .

If there was life on Mars in its early days, it would most likely have existed in water (think about Earth, where life only accomplished the feat of landing billions of years after its birth).

Even though Mars is no longer suitable for sustaining life today, the evidence of the existence of ancient life can be frozen in time in some way: fossils.

Fossils are generally preserved in sedimentary rock layers. If there was life in the early surface waters of Mars, their remains would probably be buried in sedimentary layers built up by mud and sand in the water.

As long as a conclusive fossil sample of Mars is found, humans will no longer be alone in the vast universe. The feat of discovering extraterrestrial life for the first time can also be successfully imprinted with the name of Perseverance and passed down through the ages.

The battle between water and fire

Now, almost two years have passed since Perseverance landed on the edge of the Jezero Crater Delta. Several papers in Science and Science Advances have been published, reporting Perseverance's major discoveries:

The rocks it detected at the edge of the delta are most likely not sedimentary layers .

...not sedimentary rock layers...

——That is to say, the rock type is wrong!

——What kind of discovery is this!

The article provides evidence that the rocks in the Perseverance landing area (Seitah area) are likely igneous rocks . They are the product of solidification of hot magma and have nothing to do with water.

The evidence given in the article is also very convincing: Perseverance's analytical instruments detected a large amount of olivine in the rocks in the area. Olivine [ (Mg,Fe)2SO4 ] is a simple silicate of magnesium and ferrous ions, a unique product of igneous rocks. It is very unstable in surface water environments and therefore rarely appears in sedimentary rocks.

Moreover, Perseverance further discovered that the crystals of these peridotite are very large (millimeter level, which is already very large for rock-forming minerals), and can only grow inside the crust with good insulation conditions. Even the magma flowing on the surface may not be able to grow such large crystals.

In a word: they are not products of the earth at all .

At this point, things are already on the brink of embarrassment, because igneous rocks and sedimentary rocks are two completely different types of rocks at their "roots".

In the history of the development of geology, humans even started a fierce argument over this issue, which may have ended with a field seminar where people gathered with hammers for "friendly exchanges". This is the famous "water-fire dispute" in the history of geology.

In short, one group of people insisted that rocks were all "water-formed", while another group insisted that rocks were all "fire-formed". Everyone wanted to unify the "song of water and fire", but ended up with the "division of water and fire". In the end, geologists discovered that rocks can be formed under two completely different mechanisms , which are two completely irreducible true subsets.

One type of rock continues the name "igneous rock", which refers to rocks formed by direct solidification of high-temperature magma;

The other type, "sedimentary rocks", was later renamed sedimentary rocks, which refers to rocks formed by the burial and compaction of loose deposits on the surface (such as solid debris such as mud and sand, or chemical precipitates dissolved in water bodies).

Of course, in addition to these two categories, it was later discovered that there is actually a third basic category in the rock world - metamorphic rocks, which will not be discussed in this article.

Compared to igneous rocks, fossils can obviously only exist in sedimentary rocks . The reason is simple. According to the current human cognitive system, life obviously cannot exist in high-temperature magma of thousands of degrees Celsius.

In other words, if you are exploring with the goal of "finding traces of life", then the moment you discover that the target rock is igneous rock, this proposition is equivalent to being sentenced to death.

Mars' (brief?) history of water

But scientists are very clever creatures. They can tell everyone that discovering igneous rocks is also important! After all, compared with sedimentary rocks, igneous rocks have an important advantage - they have radioactive isotope dating capabilities! (Oh.) It can provide important age milestones for sorting out the evolutionary history of Mars! (Oh.)

The igneous rocks in the Perseverance landing area are estimated to be 3.8-4 billion years old . This age is much older than the Great Lakes and even older than the formation of the Jezero Crater! In other words, these igneous rocks were already there when there was no crater, not to mention the various possibilities of the crater filling with water and forming a lake.

The "ghost story" is not over yet, and what's worse is yet to come: the olivine in these igneous rocks is too "fresh" .

As mentioned earlier, olivine is not stable in surface water environments and is easily hydrated to form serpentine. If water had ever entered the Jezero crater, then these olivines formed earlier would have been submerged by water in the form of lake beds. However, their freshness does not prove that they had ever had a long-term "intimate contact" with water.

That could only mean that there might not be that much water in the pit, or that the water existed for too short a time .

No matter how terrible something is, there must be an explanation. Faced with ironclad facts, scientists can only give a complete explanation.

First, why did the igneous rocks in the crust come to the surface? This is easy to explain. The Jezero impact broke the surface, and the things underground were turned out.

Then there is water. After all, meanders and deltas are still there, solid evidence that flowing water once existed. Even the source of the water is easy to explain - it is the product of the heavy bombardment in the late Hadean period. At that time, when the large planets in the solar system adjusted their orbits, countless small celestial bodies were ejected, covering the face of Mars (and the Earth, the Moon...). Most of these small celestial bodies were ice, which brought abundant liquid water to the surface of the rocky planets, eventually forming rivers and oceans.

After a brief period of abundance, the water on Mars quickly disappeared .

After all, Mars is too small and cools quickly. Once the liquid iron in the core loses its fluidity, the planet's magnetic field will disappear. Without the protection of the magnetic field, the stream of charged particles radiated by the sun (solar wind) can easily blow away the atmosphere, completely halting the water cycle between the surface of Mars and the atmosphere.

When the water cycle chain is broken, water will evaporate and disperse into the universe, never to return. The rivers, lakes and seas on the surface will naturally dry up quickly, including the rolling river that flows into the Jezero crater.

So far, Perseverance has not found any signs of life on Mars in its first two years of its journey . But don't be disappointed too soon, as there will be two more good news to come.

The good news is that the igneous rock area it explored is at the edge of the delta, not inside. The second good news is nonsense: Perseverance has wheels. It is still moving forward, heading towards the interior of the delta.

What new things can be discovered in the second half of the exploration? We might as well continue to wait patiently. After all, any scientific discovery is a process of bold exploration and careful verification. Moreover, the other side of the Martian horizon is an area that humans have never set foot on.

As for how everything turns out, let’s wait and see the next episode for details.

Author: Eagle

Editor: Steed

Cover image source: NASA's Goddard Space Flight Center

Guokr (ID: Guokr42)

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