Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 315 / 4

A close-in giant is often in synchronous rotation after tidal locking, giving persistent dayside heating and nightside cooling. Its contrast is controlled by the competition between radiative relaxation time in a planetary atmosphere, wind transport characterized by the atmospheric advection time, wave adjustment, and drag. When heat transport is fast compared with radiation, day-night heat redistribution lowers the contrast; when radiation is fast, each hemisphere stays closer to its local radiative balance.

For a rough atmospheric column estimate,At comparable pressure, higher planetary equilibrium temperature sharply shortens radiative relaxation, tending to increase the day-night contrast. Wind speeds, rotation, and magnetic drag can modify this trend; dissociation and recombination can carry additional heat in very hot atmospheres.

At higher altitude, lower pressure generally means shorter radiative relaxation and a larger contrast. Infrared bands with larger opacity probe these higher layers, while lower-opacity windows sample deeper layers with longer cooling times and more effective redistribution. A wavelength-dependent exoplanet thermal phase curve can therefore reveal how the contrast and hot-region displacement change with pressure.

Let $L_J$ be Jupiter's current total emitted luminosity. Under the stipulated split, its intrinsic luminosity after removal of sunlight is approximately $L_J/2$. If the accessible energy reservoir down to $T_{\mathrm{cold}}$ is $E_J$, the constant-luminosity estimate gives $\tau \simeq E_J/(L_J/2)$.

Assume the more massive giant has approximately the same radius as Jupiter, a comparable structure factor, and an accessible cooling reservoir scaling as $G{M}^2/R$, as in the Kelvin-Helmholtz cooling time. Assume also that the final cold state contributes negligibly to that reservoir and that the giant radiates at the constant stipulated luminosity $L_J$. ThenThereforeThis is a characteristic scaling under the stated assumptions, not a detailed evolutionary age. A different mass-radius relation, a different accessible internal-energy fraction, or time-dependent luminosity changes the result; the mass and luminosity alone do not uniquely determine a cooling time.

Let $F_{\mathrm{abs}}={\sigma }_{\mathrm{SB}}{T}_{\mathrm{eq}}^4$ be the globally averaged absorbed stellar flux. The single-layer greenhouse model has an atmosphere transparent to incoming visible light and with infrared absorptivity and emissivity $\alpha$. Treat the surface as a blackbody and neglect internal heating. The layer emits $\alpha {\sigma }_{\mathrm{SB}}{T}_a^4$ both upward and downward.

Atmospheric radiative balance givesSurface radiative balance givesEliminating $T_a$ yieldsThe transparent limit has $T_s=T_{\mathrm{eq}}$, and a fully infrared-opaque layer has $T_s=2^{1/4}{T}_{\mathrm{eq}}$. The model omits convection, wavelength-dependent opacity, and atmospheric absorption of starlight, so it captures the simplest greenhouse energy balance rather than a full surface-temperature prediction.

Assume a hydrogen-helium perfect gas with mean particle mass $2.3m_H$, Jupiter gravity $g\simeq 24.8\mathrm{m}{\mathrm{s}}^{-2}$, and mixing over one atmospheric scale height. At $T=1000\mathrm{K}$,The eddy mixing time is ${\tau }_{\mathrm{mix}}\simeq H^2/K_{zz}$. A chemical quench level at one bar requires ${\tau }_{\mathrm{mix}}\lesssim {\tau }_{\mathrm{chem}}=10^5\mathrm{s}$, hence the vertical eddy diffusion coefficient must be of orderThe number depends quadratically on the assumed mixing length; using a fraction of $H$ reduces it accordingly.

Above the chemical quench level, neglect photochemistry, condensation, and molecular diffusion. The quenched atmospheric mixing ratio $f_A=n_A/n$ is approximately constant, while the number density isThus the abundance fraction is frozen, but the absolute number density falls with pressure and altitude. Examples are carbon monoxide–methane quenching, through $\mathrm{CO}+3{\mathrm{H}}_2\rightleftharpoons {\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}$, and nitrogen–ammonia quenching, through ${\mathrm{N}}_2+3{\mathrm{H}}_2\rightleftharpoons 2{\mathrm{NH}}_3$.

First refine the ephemeris, planetary mass, stellar radius, and stellar variability using exoplanet transit photometry and the radial-velocity method. Then combine observations that probe different regions rather than relying on one spectrum. A present-day programme could use the following complementary measurements; in the 2019 setting of the paper, James Webb Space Telescope observations would have been a future capability.

Together these address aerosols, molecular composition, elemental enrichment, vertical thermal structure, horizontal heat transport, winds, and escape. Repeat key events and monitor stellar activity, since stellar contamination and instrumental trends can imitate atmospheric signals. Use actual brightness, saturation limits, and predicted feature amplitudes to choose observing modes and exposure times.

The wavelength ranges and time-series capabilities are documented in the NIRISS SOSS guide, NIRSpec overview, MIRI spectroscopy guide, and ESO's CRIRES description.