Hydrodynamic escape
Encyclopedia
Hydrodynamic escape refers to a thermal atmospheric escape
mechanism that can lead to the escape of heavier atoms of a planet
ary atmosphere through numerous collisions with lighter atoms.
The classical thermal escape mechanism is when individual molecule
s from the high velocity tail
of the Maxwell–Boltzmann distribution
reaches the escape velocity
and overcome the gravity field. This is known as Jeans escape and depend on the temperature of the planet's exosphere
and the strength of its gravity field. It can be shown that for cold giant gas planets such as Jupiter
and Saturn
there is no thermal driven atmospheric escape of significance while for smaller and warmer planets such as Earth only light atoms may escape in this manner (heavier atoms stay).
Hydrodynamic escape occurs if there is a strong thermal driven atmospheric escape of light atoms which through drag effects (collisions) also drive off heavier atoms - a bulk flow type of escape of the upper atmosphere, a so-called "blowoff". The heaviest species of atoms that can be removed in this manner is called the cross-over mass.
It requires a large source of energy at a certain altitude to maintaining a significant hydrodynamic escape. Solar radiation is seldom enough for known present day atmospheres in the solar system. It is speculated that early atmospheres of Earth, Venus and Mars have experienced periods of significant hydrodynamic escape due to the heat input from planetary accretion processes
. For comparison, Pluto may have lost 0.5% of its total mass over the age of the solar system due to hydrodynamic escape.
Exoplanets that are extremely close to their star, such as hot jupiter
s can experience significant hydrodynamic escape to the point that they cease to be gas giant
s and are left with just the core, at which point they would be called Chthonian planet
s.
Atmospheric escape
Atmospheric escape is the loss of planetary atmospheric gases to outer space.- Thermal escape mechanisms :One classical thermal escape mechanism is Jeans escape. In a quantity of gas, the average velocity of a molecule is determined by temperature, but the velocity of individual molecules varies...
mechanism that can lead to the escape of heavier atoms of a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
ary atmosphere through numerous collisions with lighter atoms.
The classical thermal escape mechanism is when individual molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s from the high velocity tail
Long tail
Long tail may refer to:*The Long Tail, a consumer demographic in business*Power law's long tail, a statistics term describing certain kinds of distribution*Long-tail boat, a type of watercraft native to Southeast Asia...
of the Maxwell–Boltzmann distribution
reaches the escape velocity
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
and overcome the gravity field. This is known as Jeans escape and depend on the temperature of the planet's exosphere
Exosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...
and the strength of its gravity field. It can be shown that for cold giant gas planets such as Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
there is no thermal driven atmospheric escape of significance while for smaller and warmer planets such as Earth only light atoms may escape in this manner (heavier atoms stay).
Hydrodynamic escape occurs if there is a strong thermal driven atmospheric escape of light atoms which through drag effects (collisions) also drive off heavier atoms - a bulk flow type of escape of the upper atmosphere, a so-called "blowoff". The heaviest species of atoms that can be removed in this manner is called the cross-over mass.
It requires a large source of energy at a certain altitude to maintaining a significant hydrodynamic escape. Solar radiation is seldom enough for known present day atmospheres in the solar system. It is speculated that early atmospheres of Earth, Venus and Mars have experienced periods of significant hydrodynamic escape due to the heat input from planetary accretion processes
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
. For comparison, Pluto may have lost 0.5% of its total mass over the age of the solar system due to hydrodynamic escape.
Exoplanets that are extremely close to their star, such as hot jupiter
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
s can experience significant hydrodynamic escape to the point that they cease to be gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s and are left with just the core, at which point they would be called Chthonian planet
Chthonian planet
A chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star...
s.