Which method lets a ship measure water depth far below it?

• A telescope photographs the bottom through deep water and measures depth from its brightness.
• Sonar sends sound toward the bottom and times the returning echo to estimate depth.
• A microphone records animal calls and measures depth without sending a pulse toward the bottom.

How do Alvin and a tethered ROV differ?

• Alvin carries people; a tethered ROV has no one aboard and is piloted from a ship.
• A tethered ROV carries people; Alvin has no one aboard and is piloted from a ship.
• Both Alvin and a tethered ROV carry people, with pilots steering from inside each vehicle.

A global seafloor map means every part of the bottom has already been surveyed in detail by sonar.

Which list describes conditions a vehicle faces in much of the deep ocean?

• High water pressure, cold surroundings, and little or no sunlight.
• Very low water pressure, warm surroundings, and constant sunlight at the bottom.
• High water pressure, hot surroundings everywhere, and constant sunlight at the bottom.

How does a ship normally send commands to a tethered deep-sea ROV and receive its video?

• Through radio signals sent directly across deep water, without using the tether cable.
• Through sound echoes from the bottom, without sending any signals along the tether.
• Through signals carried by the tether cable connecting the ship and the ROV.

For a sonar estimate, treat sound speed as a constant 1500 meters per second. An echo returns 4 seconds after the pulse is sent straight down. What is the water depth, in meters?

Mia says, 'Orbiters can map the Moon because it never gets cold or dark.' Which reply corrects her explanation?

• The Moon is always warm and sunlit, and ships map the deep seafloor using the same sunlight.
• The Moon has never been mapped from orbit, so its maps cannot be compared with ocean surveys.
• The Moon has cold and darkness too; orbiters do not need to look through seawater.

Which task matches the design of NASA's Europa Clipper mission?

• Land on Europa, drill through its ice, and steer a submarine to collect organisms from its ocean.
• Use flyby observations to assess whether Europa has conditions that could support life.
• Land on Enceladus, collect ocean organisms, and bring them back to Earth for identification.