Solar System Surprises: But Why? Quiz

Answer: Fine dust scattering sunlight in the atmosphere

The main difference is what scatters the sunlight: tiny gas molecules dominate the effect in Earth's clear air, while fine dust plays the leading role in this Martian sunset. On Earth, air molecules scatter blue light more strongly than red light, helping make the daytime sky blue. At sunset, sunlight travels through more atmosphere. Much of the blue light is scattered out of its direct path, leaving the setting Sun looking orange or red.

Mars's dust grains are much larger than gas molecules and scatter light differently. They keep scattered blue light closer to the Sun's direction, while red and yellow light spreads more widely across the sky. This produces the blue glow near the setting Sun. Curiosity photographed this view in Gale Crater in 2015, and the imaging team calibrated its colours to remove camera effects.

Answer: Permanent shadow on parts of the crater floors

Mercury's axis has very little tilt, so the Sun stays low in the sky near its poles. Deep crater walls keep parts of their floors permanently shaded, creating cold traps where water ice can survive despite the heat elsewhere on the planet. The mosaic combines images taken by NASA's MESSENGER spacecraft; the dark region near its center includes the north pole. Radar observations from Earth and measurements made by MESSENGER have provided strong evidence for ice in Mercury's polar regions.

Answer: Sending radio waves and recording their echoes

Magellan used radar: it sent radio waves towards Venus and recorded their echoes from the ground. Unlike visible light, these waves could pass through the thick clouds. Computers turned the returning signals into images of the terrain. The globe shown here was constructed mainly from those radar images, with simulated colours based on photographs from Soviet Venera landers. Magellan also studied Venus's gravity, but radar revealed the volcanoes, plains and impact craters in this view.

Answer: Repeated stretching and squeezing by gravity

Jupiter's gravity repeatedly stretches and squeezes Io as the moon moves along its slightly oval orbit. The moons Europa and Ganymede help keep that orbit from becoming circular through their gravitational pulls. This repeated flexing produces heat inside Io, a process called tidal heating.

Heat left from formation and radioactive decay, in which unstable atoms release energy, are important for many worlds, but they do not explain Io's enormous heat output. Galileo photographed the volcanic plumes shown in this composite in 1997.

Answer: Loosely packed rock fragments held together very weakly

The sampling head sank into Bennu much more easily than scientists had expected. Although the surface looked rocky, its particles were loosely packed, with plenty of empty space between them and very little holding them together. The spacecraft would have continued sinking if its thrusters had not fired to pull it away.

A burst of nitrogen gas stirred up dust and small stones so the sampler could collect them. This brief touch-and-go encounter revealed a surface more like a loose pile of rubble than a solid slab of rock.

Answer: Methane and ethane

Methane and ethane are compounds of carbon and hydrogen that are gases at ordinary temperatures on Earth. Titan is cold enough for them to collect as liquids in lakes and seas, while water is frozen into hard ice. Methane also forms clouds, falls as rain and flows through river channels, giving Titan a weather cycle with familiar features but a different liquid.

The proportions of methane and ethane vary between lakes. This mosaic combines near-infrared images, made using light just beyond visible red to see through Titan's haze.

Answer: Hot plasma rising and cooler plasma sinking

The bright cells are called granules. Hot plasma, a gas containing electrically charged particles, rises in their centres, spreads out and cools. Cooler material sinks along the darker edges, completing a heat-carrying cycle called convection. The pattern continually changes as the material moves.

The Sun's energy comes from nuclear fusion deep in its core, where hydrogen nuclei (the centres of hydrogen atoms) combine to form helium and release energy. These reactions do not take place inside the visible cells.

The Inouye Solar Telescope recorded this view using near-infrared light, displayed here in colour.

Answer: Charged particles flowing from the Sun with its magnetic field

Sunlight can knock electrons out of atoms and molecules in a comet's gas, leaving electrically charged particles called ions. These interact with the solar wind, a stream of charged particles flowing out from the Sun and carrying a magnetic field. That interaction draws the ion tail generally away from the Sun.

A comet can also have a broader, curved dust tail, shaped mainly by the pressure of sunlight and the motion of its dust grains. In this photographic negative, bright objects appear dark, which is why the tail looks like a dark streak.

Answer: Water ice

The hills are thought to be blocks of water ice carried by glaciers across Sputnik Planitia, the broad plain within Pluto's heart-shaped region. At Pluto's low temperatures, water ice is hard enough to form rugged uplands, while nitrogen ice can slowly deform and flow. Scientists think pieces broke away from those uplands and were carried out onto the plain.

Water ice is less dense than the surrounding nitrogen ice, allowing the blocks to float within it like icebergs. New Horizons photographed chains and clusters of these hills during its July 2015 flyby. The comparison with icebergs does not mean there is an open sea of liquid water here.

Answer: The Moon rotates once per orbit, in the same direction as it orbits

The Moon rotates once in the same time it takes to orbit Earth, and both motions are in the same direction. This is called synchronous rotation. It keeps essentially the same half of the Moon facing us, so Earth stays in roughly the same part of the sky for someone standing well within that half. Small apparent shifts, called libration, mean Earth is not perfectly still. An orbiting spacecraft can see Earth rise because its viewpoint is moving.

The Lunar Reconnaissance Orbiter (LRO) imaging team combined observations and adjusted the contrast to create this view.