Mariana Trench Descent Lab: Pressure, Temperature, Darkness and the Depth Limit of Fish

Depth0 m

Pressure1 bar

Water-

Rate0 m/s

-- fps

Drag: orbit · Wheel: zoom · W/S: ahead/astern · A/D: turn · L: lamp · Space: pause

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Mariana Trench Descent Lab

Dive from the surface to Challenger Deep (about 10,900 m): pressure, temperature, light, sound, the vehicle, and the animals that stop where physiology says they must.

Dive and vehicle

Depth (drag to scrub)0 m

Time warp

Excess weight314 kg

Target rate1.00 m/s

Autopilot trims ballast to hold the target rate Rigid hull (buoyancy grows as seawater compresses)

Animal magnifierx3

Eye adaptation (log brightness; off = true linear light)

Presets

Instruments

True depth-

Gauge if rho*g*h-

Pressure (gauge)-

In atmospheres-

rho*g*h estimate-

Model minus rho*g*h-

Seawater density-

Temperature-

Sound speed-

Zone-

Red light left-

Green light left-

Blue light left-

Blue irradiance-

Descent rate-

Dive clock-

Excess weight-

Net force (down)-

Terminal speed-

Altitude over floor-

Lamp lit volume-

Sounder echo-

Depth profiles (live)

Dive log: depth (blue) and rate (amber) against dive time

Hull and window loads

Area under pressure

Force on it-

As mass (tonne-force)-

African bull elephants (5 t)-

Family cars (1.5 t)-

Hull membrane stress-

Share of Ti yield (830 MPa)-

Sphere shrinks by-

Wall90 mm Ti-6Al-4V

Thin-shell sphere estimate sigma = p r / (2 t). Real hulls are designed against buckling and flaws with large safety margins; this is an illustration, not a design check.

Life at this depth

Epipelagic life (plankton, tuna, whales) is not modelled here; see the Ocean World reef simulation.

Why fish stop near 8,200 m

Fish tissue osmolality-

Seawater-

TMAO in muscle-

Model crossing depth-

Leading hypothesis (Yancey et al. 2014, PNAS): the osmolyte TMAO stabilises proteins against pressure, but the more TMAO a fish carries the closer its tissues get to the salt concentration of seawater. Near 8,000 to 8,500 m fish would have to be isosmotic or hyperosmotic. The line here is a schematic fit through published anchor points, not a measurement; a snailfish has been filmed slightly deeper than the estimate (about 8,300 m), so treat the limit as approximate.

Experiments

A. Pressure: full model vs rho*g*h

The simple formula uses one constant density. Real seawater compresses by about 4% by the bottom, so pressure is higher.

B. Time to the bottom vs ballast mass

Each run drops the submersible from rest with a fixed excess weight. Terminal speed from drag: v = sqrt(m g / (0.5 rho Cd A)). A rigid hull gains buoyancy as the water densifies, so a light ballast stalls.

The science

Hydrostatic pressure with compressibility

dp/dz = rho(p) g and rho(p) = rho0 (1 + K' p / K0)^(1/K') (Murnaghan form), rho0 = 1024 kg/m3, K0 = 2.34 GPa, K' = 5, g = 9.78 m/s2. Integrated with RK4 at 1 m steps; a closed form p = (K0/K')(u - 1), u^a = 1 + a K' rho0 g z / K0, a = 1 - 1/K' is used as a check. Temperature and salinity effects on density are ignored (simplified), so the bottom value differs from published figures of about 1,086 to 1,100 bar by a couple of percent.

Temperature and sound

Temperature is a schematic western-Pacific profile: warm mixed layer, thermocline, then 1.5 to 2.7 C; below about 4,000 m in-situ temperature creeps up because compression heats water adiabatically. Sound speed uses Munk's canonical profile c = 1500 (1 + e (eta - 1 + e^-eta)), eta = 2(z - 1300)/1300, e = 0.00737: the minimum (the SOFAR axis) sits near 1,300 m and speed rises with pressure below it.

Light

Beer-Lambert per colour band, I = I0 exp(-Kd z), with Kd of 0.34, 0.065 and 0.022 per metre for red, green and blue in very clear ocean water (approximate). Blue at the surface is assumed to be 50 W/m2; dark-adapted deep-sea fish detect about 1e-10 W/m2, reached near 1,200 m, so below roughly 1,000 m sunlight is effectively gone.

The vehicle

A generic crewed submersible with a 90 mm titanium sphere, 11.7 m3 displaced volume, drag area Cd A = 6 m2 and added mass 0.4 rho V. m dv/dt = dm g - (rho(z) - rho0) V g - 0.5 rho Cd A |v| v; dm is the excess weight over neutral. Release weights (500 kg) to go up. Real vehicles pump variable ballast to cope with the buoyancy gain, which the autopilot trim mimics.

Hull loads

F = p A and sigma = p r / (2 t) for a thin spherical shell. At about 1,100 bar a pound coin area carries roughly one adult bull elephant of weight.

Hadal life

Pseudoliparis swirei, the Mariana snailfish, has been recorded from about 6,200 to 8,100 m. Species cards use approximate sizes and depths from published surveys; the osmolality line is a schematic fit and the TMAO limit is the leading explanation, not a proven one. Sizes shown in the 3D view are magnified by the slider.

Simplifications: no currents, no waves, temperature and salinity do not change density, the trench floor is a flat plain, the descent is purely vertical with lateral manoeuvring at a slow fixed speed.