Barren rock world

A barren rock world is a rocky planet with no real atmosphere. Its surface lies open to space: starlight heats it directly, the night side cools freely, and meteorites, cosmic rays and the stellar wind strike the ground unimpeded. What gas it has is an exosphere, a scattering of atoms so thin that they bounce between surface rocks without ever colliding with one another. Mercury is the Solar System's example, and the James Webb Space Telescope has now found several bare rocks around other stars. They matter as the baseline against which any rocky planet's atmosphere has to be measured.

Characteristics

Why some rocky planets have no air

A planet loses gas when molecules in its upper atmosphere move fast enough to escape its gravity, and when stellar radiation and wind strip gas away. Whether an atmosphere survives therefore depends on two numbers: how strongly the planet holds on, measured by its escape velocity $v_\mathrm{esc} = \sqrt{2GM/R}$, and how much energy it receives from its star. Zahnle and Catling (2017) plotted every body in the Solar System on those two axes and found a sharp dividing line, which they called the cosmic shoreline. Bodies with atmospheres lie on one side and airless bodies on the other, with the boundary running roughly as

$$I \propto v_\mathrm{esc}^4,$$

where $I$ is the insolation ^[1]. Mercury, with an escape velocity of 4.3 km/s at 0.39 AU from the Sun, falls on the airless side; Titan, with a lower escape velocity but far from the Sun, keeps a thick atmosphere. Planets close to red dwarfs sit near the line, because those stars emit strongly in X-rays and ultraviolet and flare often, and they are also the easiest rocky planets to observe, so that is where the line is being tested.

Temperature without an atmosphere

With no air to move heat around, a barren world's temperature is set point by point by the sunlight it absorbs. Mercury's surface reaches about 700 K at noon and falls to about 100 K before dawn, the widest swing of any planet ^[2]. Its equilibrium temperature, the average from the formula on Planet classification, is 440 K, a figure no place on its surface actually holds for long.

For a tidally locked exoplanet the simplest prediction is for the dayside. If the ground re-radiates heat where it absorbs it and none reaches the night side, the dayside's average temperature is

$$T_\mathrm{day} = \left(\tfrac{2}{3}\right)^{1/4} T_\star \sqrt{\frac{R_\star}{a}}\,(1 - A_B)^{1/4},$$

about 1.28 times the planet's equilibrium temperature. An atmosphere carries heat to the night side and lowers the dayside temperature, so measuring the dayside's glow is a way to test for one ^[3].

Surfaces

Without wind or water, the surface records its history. Craters accumulate, and the soil, called regolith, is a layer of rock ground up by impacts. Space weathering, the bombardment of the soil by micrometeorites and charged particles, darkens it over time by coating grains with tiny particles of metallic iron. Mercury's surface is also darkened by graphite, possibly the remains of its original crust ^[4], and its Bond albedo, the fraction of all sunlight reflected, is only 0.068 ^[2].

The planet's interior still shapes the surface. As Mercury's large iron core cooled over four billion years, the planet shrank, and its crust buckled into thrust faults, cliffs hundreds of kilometres long called lobate scarps. More than 5,900 such landforms have been mapped, recording a shrinkage in radius of up to about 7 km ^[5]. MESSENGER also found hollows: shallow, bright, flat-floored pits that look fresh, apparently formed where a volatile component of the rock is escaping to space today ^[6].

Interior and magnetic field

Mercury is dense for its size, 5.43 g/cm³, almost as dense as Earth, which is far larger and more compressed. Its iron core is about 2,020 km in radius, some 83 per cent of the planet's radius, under a silicate shell only about 400 km thick ^[7], and part of the core is solid ^[8]. That core still drives a weak magnetic dynamo, about 1 per cent of the strength of Earth's field at the surface, with its centre offset about 480 km north of the planet's centre ^[9]. Magnetised rocks in the crust show the dynamo has run for at least 3.7 billion years ^[10].

Formation

A barren rock world can start barren or become barren. A small planet may never gather much gas, and a planet close to a young, active star loses whatever it has to stellar X-rays, ultraviolet light and wind.

Mercury's large core is the part that needs explaining. The leading idea is a giant impact early in the Solar System's history that stripped away much of a larger planet's rocky mantle ^[11]. MESSENGER complicated the story. Mercury's surface has a ratio of potassium to thorium, a measure of volatile elements, similar to the other rocky planets, which rules out models in which Mercury was heated so strongly that volatiles boiled off ^[12]. Its surface is also rich in sulfur and poor in iron, a sign that it formed from unusually oxygen-poor material ^[13]. How Mercury ended up with so much iron, and yet kept its volatiles, remains open.

How we know

Mercury. Mariner 10 made three flybys in 1974 and 1975. NASA's MESSENGER orbited Mercury from 2011 to 2015 and mapped its surface, chemistry, gravity and magnetic field. Neutron measurements showed that craters near the poles, whose floors never see sunlight, hold water ice, despite Mercury being the planet closest to the Sun ^[14]. The exosphere of sodium, potassium, calcium, magnesium, oxygen, hydrogen and helium has been watched from Earth and from spacecraft, with a total pressure below about $5 \times 10^{-15}$ bar ^{[15] [2]}. ESA and JAXA's BepiColombo, launched on 20 October 2018, is due to reach Mercury orbit at the end of 2026, with routine science from April 2027 ^[16].

Exoplanets. A bare rock around another star is found by elimination. When a transiting planet passes behind its star, the drop in infrared light measures the dayside's glow, and a dayside as hot as the bare-rock formula above leaves no room for an atmosphere to carry heat away. The first such result came from the Spitzer Space Telescope: LHS 3844 b, a planet 1.3 times Earth's radius on an 11-hour orbit around a red dwarf, has a dayside of about 1,040 K, as hot as a bare rock can be, ruling out any atmosphere thicker than about 10 bar ^[17].

JWST has since measured several more. TRAPPIST-1 b's dayside, about 500 K at 15 micrometres, first suggested bare rock ^[18]; adding a second wavelength showed that the data fit either a bare, dark rock surface or a carbon dioxide atmosphere with a warm upper layer, so the question is still open ^[19]. Its neighbour TRAPPIST-1 c, at about 380 K, has no thick carbon dioxide atmosphere like Venus's, though a thin one is possible ^[20].

Notable examples

GJ 367 b is a super-Mercury. Its density of about 10 g/cm³ implies an iron core making up most of its mass ^[21], and JWST found a dayside at about 1,700 K with no sign of heat being carried to the night side, consistent with bare rock and no atmosphere ^[22]. See Exotic worlds for iron planets.

See also

• Lava world
• Arid world
• Exotic worlds
• Atmospheric escape
• Red dwarf
• JWST and rocky exoplanet atmospheres
• Sol
• Planet classification

References

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