Солнечная система и ее тайны

Планеты Созвездия НЛО

Atmosphere

The Earth’s atmosphere is a small but very significant component of the planet, especially when it comes to life. It shields the surface from the vacuum of inter-planetary space; its pressure allows for liquids, especially liquid water, in the surface environment; and it mediates geochemical cycles that establish the richness of the planet’s chemistry.

A planetary atmosphere is defined as the layer of gas that overlays the planet's interior (a solid rock or ice crust, a liquid ocean, or a high-pressure gas envelope) and surrounds the planet. Its lower boundary is at either a solid or liquid interface, or at a depth where no light or heat can escape directly to space. Its upper boundary is where gas molecules can move freely into space. Defining the boundaries in this way makes any atmosphere a gas layer of a relatively low mass compared to the total mass of a planet.

Most planets have atmospheres, and they are often the only planetary region available to direct remote investigation - in other words, they are all we could see with a telescope. Being the outermost layer, their low mass and light gas nature mean that atmospheres are continuously losing molecules (and mass). That mass loss can range from insignificant to catastrophic, depending on the impinging stellar heat, the gravitational pull of the planet, and the type of gas that composes the atmosphere.

Most planetary atmospheres depend on a reservoir that replenishes their lost gases. For gas giant planets like Jupiter and Saturn that reservoir is simply the planet itself, its interior being composed of the same mixture of gases that extends into the atmosphere. For rocky planets like the Earth, that reservoir is the mantle of the planet - the dynamic silicate layer below the crust that releases gases and water through volcanoes and vents.

Atmospheres evolve with time. These changes can be especially dramatic for smaller planets. Mars is a good example. Mars is a small planet, but is close enough to the Sun that the atmospheric loss of gas overcame the outgassing from the mantle. This happened very soon after volcanic activity died down sometime in Mars' geological past. Atmospheric changes involve compositional changes, not simply mass loss. For example, very light gases, like hydrogen, and light noble gases such as helium and neon, are lost first. Occasionally a very large moon at a safe distance from the heat and radiation of its star can retain a thick atmosphere over the lifetime of the planetary system. Titan, the largest moon of Saturn, has a stable thick atmosphere of nitrogen (more than 90% composed of N2) and methane.

Remote Analysis

How do we study planetary atmospheres remotely? Exoplanets can be studied only in integrated light, meaning that we have no way to resolve the surface image of an exoplanet directly. We must add up - "integrate" in the calculus sense - all the light that comes from the planet. That limits detection and study of their atmospheres to the tools of spectroscopy - looking for specific features in the light spectrum emerging from the planet that betray an atmosphere, and can be separated from the overwhelming stellar light. The two main approaches are transmission spectroscopy, and emission spectroscopy.

Transmission spectroscopy relies on seeing what kinds of light are absorbed by an atmosphere. It can be applied only in the case of transiting exoplanets - the ones we are lucky enough to see projected against the bright emitted light from their host stars. During the few hours of the transit some of the stellar rays are passing through the planet's atmosphere before arriving at our telescopes; those few stellar rays would now carry a faint imprint of the planetary gases they encountered in the planet's atmosphere. In 2002 this method provided the first evidence for gases in the atmosphere of an exoplanet - hot atomic sodium gas in the atmosphere of HD209458b, a so-called "hot Jupiter."

The second approach, emission spectroscopy, can be used in transiting exoplanets, like HD209458b, at about half an orbital period later than transmission spectroscopy can be used. This is the time when the dayside of the planet is now facing in our direction. We receive reflected light and emitted light from the dayside hemisphere of the planet - the latter mostly in the form of heat, i.e. in infrared light. Of course, the light is still not seen separately from that of the host star because our telescopes capture it all just a single point of light. So, we need a very careful and precise separation of the light spectrum emitted from the star. That is possible because for a couple of hours during its orbit, the planet is behind the star so its light is blocked completely and is missing from the signal. When successful, emission spectroscopy of the planet allows us to discern the hot upper layers of the planet's atmosphere and which gases dominate.

Emission spectroscopy of an exoplanet can be used very successfully for directly imaged planets, like the four-planet system HR8799. In such rare cases, we see the planets directly and record their spectra individually and not combined with stellar light (see figure below).

Солнечная система и ее тайны