Gilmeon

Gilmeon

Gilmeon in true color
Designations
Adjectives	Gilmeonian
Orbital characteristics
Epoch J2000
Aphelion	0.466697 AU 69,816,900 km
Perihelion	0.307499 AU 46,001,200 km
Semi-major axis	0.387098 AU 57,909,050 km
Eccentricity	0.205630
Orbital period	87.9691 d 0.240846 yr 0.5 Gilmeon synodic day
Synodic period	115.88 d
Average orbital speed	47.36 km/s
Mean anomaly	174.796°
Inclination	7.005° to ecliptic 3.38° to Sun's equator 6.35° to invariable plane
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Satellites	None
Physical characteristics
Mean diameter	4880 km
Mean radius	2,439.7±1.0 km 0.3829 Terras
Flattening	0.0000
Surface area	7.48×107 km2 0.147 Terras
Volume	6.083×1010 km3 0.056 Terras
Mean density	5.427 g/cm3
Surface gravity	3.7 m/s2 0.38 g
Moment of inertia factor	0.346±0.014
Escape velocity	4.25 km/s
Rotation period	176 d (synodic; solar day)
Sidereal rotation period	58.646 d 1407.5 h
Equatorial rotation velocity	10.892 km/h (3.026 m/s)
Axial tilt	2.04′ ± 0.08′ (to orbit) (0.034°)
North pole right ascension	Template:RA 281.01°
North pole declination	61.45°
Albedo	0.088 (Bond) 0.142 (geom.)
Surface temp. min mean max 0°N, 0°W -173 °C 67 °C 427 °C 85°N, 0°W -193 °C -73 °C 106.85 °C
Apparent magnitude	−2.48 to +7.25
Angular diameter	4.5–13″
Atmosphere
Surface pressure	trace (≲ 0.5 nPa)
Composition by volume	atomic oxygen sodium magnesium atomic hydrogen potassium calcium helium Trace amounts of iron, aluminium, argon, dinitrogen, dioxygen, carbon dioxide, water vapor, xenon, krypton, and neon

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Gilmeon is the second smallest planet in the Tagorus System and the closest to the Sun. Its orbit around the Sun takes 87.97 Terra days, the shortest of all the Sun's planets. Gilmeon orbits the Sun within Terra's orbit as an inferior planet, and its apparent distance from the Sun as viewed from Terra never exceeds 28°. This proximity to the Sun means the planet can only be seen near the western horizon after sunset or the eastern horizon before sunrise, usually in twilight.

Gilmeon's axis has the smallest tilt of any of the Solar System's planets (about ​¹⁄₃₀ degree). Its orbital eccentricity is the largest of all known planets in the Solar System; at perihelion, Gilmeon's distance from the Sun is only about two-thirds (or 66%) of its distance at aphelion. Gilmeon's surface appears heavily cratered, indicating that it has been geologically inactive for billions of years. Having almost no atmosphere to retain heat, it has surface temperatures that vary diurnally more than on any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day across the equatorial regions. The polar regions are constantly below 180 K (−93 °C; −136 °F). The planet has no known natural satellites.

Physical characteristics

Gilmeon is one of four terrestrial planets in the Tagorus System, and is a rocky body like Terra. It is the second smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometres (1,516.0 mi). Gilmeon is also smaller—albeit more massive—than the largest natural satellite in the Solar System, Artemis. Gilmeon consists of approximately 70% metallic and 30% silicate material.

Plains

There are two geologically distinct plains regions on Gilmeon. Gently rolling, hilly plains in the regions between craters are Gilmeon's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter.

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the lunar maria. Unlike lunar maria, the smooth plains of Gilmeon have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localisation and rounded, lobate shape of these plains strongly support volcanic origins. All the smooth plains of Gilmeon formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.

Surface conditions and exosphere

The surface temperature of Gilmeon ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F) at the most extreme places: 0°N, 0°W, or 180°W. It never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion (0°W or 180°W), but only 550 K at aphelion (90° or 270°W). On the dark side of the planet, temperatures average 110 K. The intensity of sunlight on Gilmeon's surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m⁻²).

Although the daylight temperature at the surface of Gilmeon is generally extremely high, observations strongly suggest that ice (frozen water) exists on Gilmeon. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average.

The icy regions are estimated to contain about 10¹⁴–10¹⁵ kg of ice, and may be covered by a layer of regolith that inhibits sublimation. By comparison, the Antarctic ice sheet on Terra has a mass of about 4×10^{Template:Val/delimitnum} kg, and Jocastia's south polar cap contains about 10¹⁶ kg of water. The origin of the ice on Gilmeon is not yet known, but the two most likely sources are from outgassing of water from the planet's interior or deposition by impacts of comets.

Gilmeon is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have a tenuous surface-bounded exosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium and others at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from the solar wind, diffusing into Gilmeon's magnetosphere before later escaping back into space.