The four innermost worlds of our solar system—Mercury, Venus, Earth, and Mars—are known as the terrestrial planets due to their rocky composition. During their formation, these planets were enveloped by massive, primordial atmospheres captured directly from the solar nebula, composed almost entirely of the lightest elements, hydrogen (H) and helium (He). However, the atmospheres surrounding these rocky bodies today bear little resemblance to that initial composition. The central question in planetary science is why these original, thick envelopes of gas vanished so quickly from the inner solar system.
Defining Primary and Secondary Atmospheres
The distinction between primary and secondary atmospheres is crucial for understanding the loss of the early gaseous envelope. A planet’s primary atmosphere is the initial gaseous layer gravitationally captured from the surrounding solar nebula during the planet’s core accretion phase. This type of atmosphere is characterized by a composition dominated by hydrogen and helium.
The secondary atmosphere, which Venus, Earth, and Mars possess today, forms later through entirely different processes. This newer atmosphere originates from internal sources like volcanic outgassing of volatiles trapped within the planet’s mantle and crust, or from external delivery by impacting comets and asteroids. Consequently, secondary atmospheres are rich in heavier molecules such as water vapor (H2O), carbon dioxide (CO2), and nitrogen (N2).
The High-Energy Environment of the Young Sun
The primary driver for this atmospheric stripping was the hyperactive state of the Sun during its youth, known as the T-Tauri phase. This stage lasted for the first few million years of the solar system, during which the Sun was far more luminous and energetic in certain wavelengths than it is now. It produced dramatically elevated levels of high-energy radiation, specifically X-rays and extreme ultraviolet (XUV) radiation.
This intense XUV flux was readily absorbed by the hydrogen and helium in the upper layers of the primary atmospheres, causing massive heating. Simultaneously, the young Sun ejected a far more furious and dense stellar wind than the modern solar wind. The T-Tauri wind was estimated to be up to a hundred times more vigorous, moving at high velocities and consisting of a powerful stream of charged particles that slammed into the nascent atmospheres of the inner planets. This combination provided the necessary energy to overcome the planets’ gravitational hold on their light-element atmospheres.
Physical Mechanisms Driving Atmospheric Loss
The energy from the young Sun fueled the most potent escape process: hydrodynamic escape, often called “blow-off.” This mechanism occurs when the intense XUV radiation heats the upper atmosphere so much that the gas expands rapidly and flows outward like a continuous wind, exceeding the planet’s escape velocity. Because the atmosphere escapes as a bulk flow, the lighter hydrogen atoms drag along heavier atoms and molecules through viscous collisions, effectively stripping the entire upper layer. This was the dominant process responsible for the wholesale removal of the hydrogen and helium primary atmospheres.
A less significant, yet still contributing, thermal process is Jeans escape, where only the fastest-moving individual gas molecules in the upper atmosphere spontaneously achieve escape velocity. Since the mean velocity of a gas molecule is inversely related to its mass, this mechanism preferentially removed the lightest elements (H and He) over billions of years. However, during the early epoch, the rapid, bulk action of hydrodynamic escape far outpaced Jeans escape.
A third, non-thermal process is solar wind stripping, which targets planets lacking a strong global magnetic field. The charged particles of the young, intense solar wind directly collided with and ionized atmospheric gas particles in the upper layers. These ionized particles were then swept away into space by the solar wind’s magnetic field, contributing significantly to the atmospheric loss on planets like Mars.
Planetary Characteristics Determining Gas Retention
While the Sun provided the energy for escape, the varying retention rates among terrestrial planets were governed by their individual physical properties.
Planetary Mass and Gravity
Planetary mass and corresponding gravitational force were critical, as a higher escape velocity requires more energy for a gas molecule to depart. Earth and Venus, being more massive, exerted a much stronger gravitational pull. This allowed them to retain a larger fraction of their early atmospheres compared to the smaller, lower-gravity Mars and Mercury.
Distance from the Sun
Distance from the Sun also played a role, directly influencing the temperature and the intensity of solar radiation exposure. Planets closer to the Sun, such as Mercury and Venus, experienced greater heating from the XUV radiation. This accelerated the atmospheric molecules to higher velocities and amplified the efficiency of hydrodynamic escape. This proximity factor is why Mercury, the smallest and closest planet, has virtually no atmosphere today.
Magnetic Field Presence
The presence of a global magnetic field, or magnetosphere, acts as a protective barrier against solar wind stripping. Earth’s strong magnetic field deflects the solar wind’s charged particles, shielding its atmosphere from direct erosion. In contrast, Mars lost its global magnetic field early in its history, leaving its atmosphere vulnerable to the relentless solar wind, which contributed to its thin, tenuous state today.