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Mercury, being the closest to the Sun, with a weak magnetic field and the smallest mass of the recognized terrestrial planets, has a very tenuous and highly variable atmosphere (surface-bound exosphere) containing hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10 −14 bar (1 nPa). [2]
Mercury in the Solar System has a metallic core equal to 60–70% of its planetary mass, and is sometimes called an iron planet, [36] though its surface is made of silicates and is iron-poor. Iron planets are thought to form in the high-temperature regions close to a star, like Mercury, and if the protoplanetary disk is rich in iron. Icy planet
The Moon's atmosphere is extremely thin, consisting of a partial vacuum with particle densities of under 10 7 per cm −3. [115] Mars (1.38–1.67 AU) [D 6] has a radius about half of that of Earth. [116] Most of the planet is red due to iron oxide in Martian soil, [117] and the polar regions are covered in white ice caps made of water and ...
Neptune, for example, has an atmosphere made of hydrogen and helium (with just a tinge of methane), and it doesn’t really have a surface—or, at least, not what we think of as a surface.
Terrestrial planets are substantially different from the giant planets, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets have a compact, rocky surfaces, and Venus, Earth, and Mars each also has an atmosphere. Their size, radius ...
Second, the direct emission from a planet atmosphere may be detected by differencing the star plus planet light obtained during most of the planet's orbit with the light of just the star during secondary eclipse (when the exoplanet is behind its star). [citation needed] The first observation of an extrasolar planetary atmosphere was made in ...
The troposphere is thought to have a highly complex cloud structure; water clouds are hypothesised to lie in the pressure range of 50 to 100 bar (5 to 10 MPa), ammonium hydrosulfide clouds in the range of 20 to 40 bar (2 to 4 MPa), ammonia or hydrogen sulfide clouds at between 3 and 10 bar (0.3 and 1 MPa) and finally directly detected thin ...
Despite being the only moon in the Solar System with a substantial magnetic field, it is the largest Solar System object without a substantial atmosphere. Like Saturn's largest moon Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io, or the Moon due to its lower density compared to the three. [18]