Archive | October, 2016

Evolution of Atmospheres D.11.01

31 Oct

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.

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Planetary Atmospheres D.10.31

30 Oct

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.

The Gas in the Atmosphere D.10.30

29 Oct

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.

Definition of Atmosphere D.10.29

28 Oct

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 in planets such as gas giants 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.

Atmosphere Component D.10.28

27 Oct

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.

Lights in the Distance D.10.27

26 Oct

Light from a distant object is bent by gravity as it travels. Our telescopes therefore see the distant object at a different location from where it actually is.

It may also appear brighter or dimmer, depending on whether we receive more or less of the light than normal, and often appears distorted.

Stars versus Planets D.10.26

25 Oct

Stars are incredibly bright and distant. Planets, on the other hand, generate very little light of their own (primarily infrared) and mostly reflect light from their home star. From our point of view, most stars are only a single pixel on our telescope’s detectors, and the planets’ light falls into that pixel as well. We can’t separate the planets’ light from their star’s light.

However, for some nearby systems, we have a chance of detecting a planet this way because our telescopes can detect more detail. Astronomers use various image-processing techniques to remove the light of the star while leaving the light from the planets.