00:00These cumulus clouds that you see in the sky
00:04weigh the equivalent of a Boeing 747 or a 100-elephant.
00:08If you doubt my words, let's do the math.
00:11Researchers have determined that the density of water inside a cumulus
00:15was about 0.5 g per cubic meter.
00:18Clouds take various shapes,
00:21but a cloud of this type usually has a cubic shape
00:24and a volume of 1 billion cubic meters.
00:27By multiplying this volume by the density,
00:29we get an impressive weight of more than 500 tons.
00:32And to answer your eventual curiosity,
00:35this giant remains suspended in the air
00:37because its density is lower than that of the atmosphere around it.
00:42Giraffes are 30 times more likely to be struck by lightning than you are.
00:46Between 1996 and 2010,
00:49five deadly cases involving giraffes struck by lightning were reported.
00:54But since there were about 140,000 of them in the world during this period,
00:58this mortality rate for 1,000 giraffes per year is relatively high.
01:03Lightning prefers to strike objects that are higher up,
01:06which makes giraffes easy targets.
01:08However, scientists think they may have learned to avoid this situation
01:13by seeking refuge in thunderstorms
01:15or by moving quickly to densely wooded areas.
01:19The oldest animals on Earth are glass sponges.
01:22Their life expectancy can reach 15,000 years.
01:25Scientists have discovered the oldest glass sponge in the Ross Sea,
01:29a region of Antarctica.
01:31In the East China Sea,
01:33other researchers have found the skeleton of a glass sponge
01:36that would have lived for about 11,000 years.
01:39These sponges could not have existed during the last ice age.
01:42They spend their entire existence fixed to solid surfaces,
01:46filtering water to feed on bacteria and plankton.
01:51Earth's rotation is slowing down,
01:54which leads to an extension of the days.
01:56A long time ago, a terrestrial day lasted only 19 hours,
01:59but this delay has extended to 24 hours,
02:02mainly due to the gravitational attraction of the Moon,
02:05which slows down our planet.
02:07Extreme events, such as earthquakes,
02:10can temporarily influence the speed of this rotation.
02:14A Chilean earthquake in 2010 probably slightly shortened our days.
02:19However, since 2020, the days tend to become longer again.
02:25Scientists still can't agree on whether the water is really wet.
02:31Some say that the water itself is not wet,
02:34but that it can make other objects wet by sticking to it.
02:37Others consider that water is wet because it is a liquid and contains humidity.
02:42Water can also seem wet when it cools the skin by evaporating.
02:46The humidity of an object depends on two forces.
02:49The cohesive forces that hold water droplets together,
02:52and the adhesive forces that allow water to stick to other surfaces.
02:56Water does not adhere well to impermeable tissues,
02:59which remain dry due to the domination of cohesive forces in this particular case.
03:03There is no world map that is perfectly precise,
03:06unless it is on a real scale.
03:08Indeed, our planet is spherical while the paper is flat.
03:12In the 16th century, Gerardus Mercator developed a cylindrical cartographic projection,
03:17still used today, and which bears his name.
03:20Although this projection is convenient for navigation,
03:23because the North is always at the top,
03:25it deforms the high and low parts of the map a lot.
03:28Thus, Groenland seems 14 times larger than it really is,
03:32while Alaska is represented by the size of Brazil.
03:36About 4 billion years ago,
03:38the Earth's atmosphere was radically different from the one we know today.
03:42But volcanoes have played a decisive role.
03:45Many volcanoes erupted across the globe,
03:48releasing carbon dioxide that filled the sky
03:51and constituted the essence of the atmosphere.
03:54Tiny primitive plants used this carbon dioxide
03:58to develop by photosynthesis, thus producing oxygen.
04:02One theory suggests that the steam released by volcanoes
04:06would be cooled and transformed into water, thus forming the oceans.
04:10It took about 2 billion years for oxygen to diffuse enough in the air
04:15to allow animal life.
04:18If, for some reason, you decided one day to stack all the bacteria
04:22and germs of the planet on top of each other,
04:25it would extend over 10 billion years of light.
04:28However, you would not perceive this thread,
04:31which would be about 75 times thinner than a human hair.
04:35If you wound this long chain of bacteria around our galaxy,
04:39the Milky Way, it would go around it more than 20,000 times.
04:43Shattering, isn't it?
04:46It turns out that there is rust on the Moon,
04:48and it is possible that it has traveled from Earth to get there.
04:52Rust usually forms in the presence of oxygen and water,
04:56two elements that the Moon has in negligible quantities.
04:59However, when this star crosses the magnetic tail of the Earth,
05:03terrestrial oxygen can reach it thanks to solar winds.
05:06This oxygen could then combine with low amounts of water and iron
05:11at the lunar surface to form rust.
05:16Even a small asteroid can take our planet out of its orbit.
05:20But don't worry, you probably won't notice any difference.
05:23The impact will depend on many factors,
05:26such as the resistance of the asteroid,
05:28its speed and the type of terrain it touches.
05:31In general, when space rocks hit the Earth,
05:34they disintegrate or turn into gas before reaching the ground.
05:38However, the larger ones,
05:40like the one that caused the extinction of the dinosaurs,
05:43can cause catastrophic phenomena,
05:45such as giant waves, earthquakes and climate change.
05:50The Earth is not as solid as a rock inside,
05:53but rather similar to soft clay
05:55due to the intense heat and pressure exerted under the surface.
05:59This flexibility allows the mantle to move slowly over millions of years,
06:03which explains the occurrence of earthquakes,
06:05volcanic eruptions and the drift of continents.
06:09During the last glacial period,
06:11huge glaciers covered the Earth's surface,
06:14exerting pressure on the malleable mantle below.
06:17This made the mantle slightly sink.
06:19When the glaciers melted, the mantle began to rise again,
06:22and the Earth began to rise.
06:24Even today, in regions such as Canada, Greenland and Scandinavia,
06:29the Earth continues to rise because of this phenomenon.
06:33Our planet is covered with cosmic dust.
06:36Every year, about 5,000 tons of this matter falls on Earth.
06:40These interstellar particles are smaller than a grain of sand
06:44and often thinner than a human hair.
06:46Scientists go into the depths of Antarctica,
06:49where there is little precipitation and where the snow is extremely pure,
06:53to detect these tiny particles.
06:55Most of this dust comes from comets.
07:00If you are a big introvert, Greenland is the perfect place for you.
07:04It is the least populated region in the world,
07:06with only 0.14 people per square kilometer.
07:10Wow, I've never really seen a tenth of a person.
07:13Anyway, most of Greenland is covered with ice,
07:17so that its inhabitants live mainly on the west coast.
07:21And for your information, it has been inhabited for more than 4,500 years,
07:25so it must not be so bad at the bottom.
07:29The ground on which you walk is actually recycled.
07:32First, hot and melted magma rises to the surface
07:35and cools down to become solid rock.
07:38Then, thanks to the tectonic forces, this rock is pushed up.
07:43Over time, the wind, the rain and other forces
07:46separate pieces of this rock.
07:49These pieces are taken and end up being deposited in layers.
07:53The latter are compressed, forming sedimentary rocks like sandstone.
07:57Sometimes, if these sedimentary rocks are pushed deeply under the surface of the Earth,
08:02the heat and pressure transform them into a new type of rock,
08:05called metamorphic rock.
08:07A long time ago, life on Earth could have been purple instead of green.
08:12Shildas Sarma, a molecular biologist at the University of Maryland,
08:16thinks that the first microbes used a special molecule
08:20to capture sunlight, which made them purple.
08:23Today, the plants of Greenland are green
08:26because they use chlorophyll to capture sunlight
08:29and transform it into energy.
08:31But before the existence of chlorophyll,
08:33it was retinal molecules that fulfilled this function.
08:36They absorbed the green light of the sun
08:39and reflected the red and purple light,
08:42which gave the microbes a purple hue.
08:44Like little grapes of grapes, really.
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