Title: Why don’t Venus and Mercury have moons?
Let’s get straight to the point: the reason why Venus and Mercury don’t have moons is:
they are too close to the Sun. Of course, this might not be the only reason,
but it’s perhaps the most plausible one, according to scientists.
In the solar system, you will find interesting moons: some examples are Jupiter with Io, Europa,
Ganymede and Callisto, Saturn with Titan and Enceladus, Neptune and its Triton, Pluto,
and Charon…but no moon has ever succeeded in becoming a moon of Mercury or Venus.
To understand why this is, let’s take a look at other planets first, and let’s see
how their moons formed or got captured, ending up orbiting around the planets in our solar system.
==============
The planets Mercury and Venus do not have moons of their own,
unlike other planets in the Solar System.
As Mercury and Venus are so close to the Sun, their gravitational fields are not
strong enough to hold onto any moons that may have formed in the early Solar System.
Although these 2 planets do not have moons, our Solar System is full of other objects that are
in orbit around them. The most notable of these objects are a group of asteroids known as the
Venus-Mars Trojans. These asteroids are in orbit around the two planets and share their orbits.
However, it is really hard to spot Venusian asteroids, because of their close proximity
to the Sun. A few searches in the Trojan region of Venus have so far reported no detection of stable
Venus asteroids around the L4 and L5 Lagrangian points, called trojans. Most authors concluded
that any population of primordial Trojans around Venus would have disappeared by now.
However, there are few studies on the dynamic stability and possibility of their existence.
Using direct numerical integrations under the same gravitational model, scientist Ćuk,
found that a substantial number of asteroids appear to be stable over 1 billion years.
In any case, all of the moons that we know of in the Solar System are in orbit around the
other planets. There are two main mechanisms to form moons around a planet: the collision
of the planet with another massive object, or the capture, mainly due to
a combination of gas drag and gravitational pull. Let’s see then how these planets got their moons,
in order to understand why it wasn’t possible for them to be brought around Venus and Mercury.
========= EARTH
Perhaps the most beautiful moon of all is our Moon, a satellite of the Earth.
Our natural satellite has captivated humans since ancient times. It's been an integral
part of our culture, with the mythology of many cultures being based upon it. The Moon's light,
ever-changing phases, and the tides it creates have a profound effect on our lives.
The Moon is, on average, 238,855 miles (384,400 kilometers) from the Earth, and has a diameter
of 2,159 miles (3,476 kilometers). It has no atmosphere, so on its surface, there is no air,
no water, and no life. But how did it form?
The Moon is believed to have formed around 4.5 billion years ago shortly after the formation
of the Earth. It is believed to have formed out of the debris created when a Mars-sized
object collided with the Earth, resulting in the debris being ejected into orbit around the Earth.
Over time, the debris collected and slowly became the Moon that we know today.
In this brand-new video released by NASA, you can appreciate the most
detailed simulation of the moon's origin. https://www.youtube.com/watch?v=kRlhlCWplqk
The Moon has a unique relationship with the Earth, as it is tidally locked in its orbit. This means
that the same face of the Moon is always facing us and that the same amount of its surface is
exposed to the sometimes intense rays of the Sun. The Moon is also responsible for the rhythm of
the tides, as its gravitational pull interacts with the Earth's water.
This influence is also responsible for the phenomenon of high and low tides.
The Moon's beauty and mysterious qualities will surely continue
to spark the imaginations of people for many years to come.
As far as we know, Mercury and Venus never underwent such collisions after they formed,
and if they did, they were never able to keep material in orbit around them.
In the case of Mercury, some studies have shown it might have undergone a colossal but
glancing smashup with a roughly Earth-size planet that could have stripped away much
of proto-Mercury's rocky mantle, explaining why the tiny, sun-scorched world has such a
huge iron core today. Such a collision might have created the planet Mercury,
as well as Mars, and at least some of the larger asteroids such as Vesta and Psyche.
Where would the larger body which hit today's Mercury have come from? And where is it now?
Well…this is poorly understood, and these are questions for future studies.
=========
What about Mars? MARS
The Red planet has two small moons that are named Phobos and Deimos.
Both moons have a heavily cratered surface and appear to be captured asteroids.
What scientists think actually happened is that, at the beginning of the solar system, when planets
were still forming, a huge quantity of gas was available all around. Some minor bodies such as
asteroids gradually lost their angular momentum because of their interaction with this gas,
acting like a breaking force along their path. Due to this mechanism, called gas drag, some asteroids
got closer and closer to their planet, eventually being captured by its gravitational pull.
This hypothesis is supported by observational evidence. For instance, the orbits of both
moons are highly inclined to the Martian equator, which suggests that they were not
formed with Mars but were instead asteroids that were captured by the planet’s gravitational pull.
Phobos is the larger of the two moons with a diameter of 22 km
and it orbits Mars at a distance of 9,377 km. Deimos, the smaller moon,
measures just 12 km across and orbits Mars at a distance of 23,460 km.
Due to tidal forces, it is estimated that Phobos is likely to be broken up by Mars’s gravitational
pull within the next 50 million years. The same mechanism that allowed the capture will keep going
until this fragile moon will be torn apart. Both of Mars’ small moons have had great
scientific importance since the first mission to Mars by Mariner 4 in 1965,
which took pictures of them. These images were further improved and clarified by
subsequent flybys by the Mariner 9, Viking 1 and 2, and the Mars Global Surveyor missions.
Mercury is the closest planet to the Sun and has a very weak gravitational field.
It is much too weak to hold a moon of its own. Venus, on the other hand,
has a much stronger gravitational field but is still probably too weak to hold onto a moon.
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=========== JUPITER
Jupiter is basically the King of moons in the solar system.
We know about 80 known moons of Jupiter, not counting the number of moonlets likely shed
from inner moons. The most massive
of the moons are the four Galilean moons: Io, Europa, Ganymede, and Callisto,
which were discovered in 1610 by Galileo Galilei. It is thought that Io, Europa, Ganymede,
and Callisto most probably formed out of material left over from the formation of Jupiter itself.
Like the rest of the Solar System planets, Jupiter formed out of the disc of dust and
gas surrounding the young Sun. Once this gas and dust had condensed to form Jupiter,
the leftover material coalesced and grew over time to form the Galilean moons.
These moons are as older as the solar system.
Needless to say, Jupiter is the king of moons because of its huge gravitational pull. It is
the most massive planet in the Solar System! Much, much more massive than Mercury or Venus.
SATURN Also, Saturn is a really massive gaseous planet,
and therefore we expect it to own a lot of moons. It has 63 confirmed moons.
There's Daphnis:
Calypso:
Dione:
Enceladus:
Hyperion:
Mimas:
and a bunch of other spectacular moons. By the way, you can find all the pictures on
the NASA website in the description. (to the videomaker: add this link to the description
NEPTUNE
Neptune has at least 14 moons and six known narrow rings. Each of the myriad particles
that constitute the rings can be considered a tiny moon in its own orbit. The four
moons nearest the planet orbit within the ring system, where at least some of them
may interact gravitationally with the ring particles, keeping them from spreading out.
So, just a quick recap. Venus and Mercury don’t have
moons most likely because they are too close to the Sun. Any moon with too great a distance from
these planets would be in an unstable orbit and be captured by the Sun. If they were too close to
these planets instead, they would be destroyed by tidal gravitational forces. The zones where
moons around these planets could be stable over billions of years are probably so narrow that
nobody was ever captured into orbit, or created in situ when the planets were first being accreted.
Despite the lack of moons orbiting them, both Mercury and Venus remain fascinating objects in
the Solar System. They have captivated mankind since the dawn of astronomy and are currently
the focus of several ongoing exploration projects. With the help of these projects,
we may one day discover new objects orbiting these two planets, including moons of their own.
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