Castles in the Sky

scope on the skies

Castles in the Sky

Science Scope—March/April 2023 (Volume 46, Issue 4)

By Bob Riddle

Most if not all of us have a condition known as pareidolia. Serious? No, but pareidolia is a rather imaginative thing. The word comes from two Greek words meaning “wrong image.” It is a human condition that has us seeing familiar patterns or images as we look around us. Think of the “man in the Moon” or the “face” on Mars as some familiar examples of pareidolia. And for most of us, regardless of our age, when looking at clouds, especially the large and puffy cumulus clouds, we often see shapes resembling dogs, castles, faces, even a winged flying horse (see Figure 1).

Figure 1 What do you see in this cloud formation? — Figure 1

Imaginary cloud shapes like we see in our atmosphere are probably not limited to Earth; any world (planet or moon) with an atmosphere should have clouds if the atmosphere temperatures are in the range for condensation of atmospheric gases. Except for Mercury, Pluto, and other known dwarf planets, the eight “classical” planets and the moon Titan have relatively thick atmospheres. Other worlds may have periodic or seasonal thin atmospheres and some cloud formation. For example, it is thought that Pluto may develop a temporary atmosphere when it is at perihelion, closest to the Sun, and the surface warms enough so that some of the surface ices sublimate into gases. However, we’ll have to wait until the next Pluto perihelion in 2237 to know for sure!

Clouds on the different worlds in our solar system show a wide range of features, but clouds are, well, clouds. Clouds, as we know them, form as atmospheric temperatures either cool down or are at the temperature at which gas changes to liquid form—collecting in amorphous shapes of small droplets and ice crystals suspended in an atmosphere. On our planet, the upper atmosphere has water vapor condensing into clouds, and in a similar fashion, the upper atmospheres of the gas giant planets Jupiter and Saturn have temperatures and pressures that allow for the condensation of ammonia into clouds (see Online Resources).

Typically, there is a circulation pattern wherein atmospheric gases flow upward due to temperature differences. Clouds form as the gases reach the altitude at which the temperature and pressure allow for condensation to occur. In some locations, it is the relief of a landmass, perhaps a mountain range that forces flowing air to rise, cool, and condense into clouds. This is known as the orographic effect and is the source of regular cloud-forming patterns seen along the mountainous parts of the Pacific Ocean coastline in both North and South America, as well as in other locations where local geography, for example, interrupts the flow of atmospheric gases (see Figure 2).

Lenticular clouds forming over the Patagonian Andes. — Figure 2

Cloudy skies

While the clouds of other worlds are perhaps colorful, all are active and dynamic, yet we only see them from an orbital perspective, looking down on the topmost layers of their atmosphere. However, as surface dwellers on Earth who will someday send people to Mars who will observe and study clouds from the planet’s surface, this is perhaps more relevant. From a surface-based perspective, the appearance of clouds and how they are forming could be a signal that some sort of atmospheric change is coming. On Mars that could be a global-sized dust storm, while on Earth it could be a variety of weather patterns depending on a variety of conditions from storms to drought conditions.

With a cell phone app from the Globe Observer website (see Online Resources), students could participate in the Globe Observer Clouds program. Participants observe their local cover cloud and surface conditions. These observations are coordinated with two of NASA’s Earth observing satellites, Terra and Aqua, as each satellite flies over the observer’s location—Terra usually during the morning between 10 a.m. and 12 p.m., while the Aqua overpasses are usually between 1 and 3 p.m. Students may check for overpass times at the Globe website. Students will collect data to be used with the Clouds and the Earth’s Radiant Energy System (CERES) instrument and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the two satellites.

Clouds play a major factor in conditions on the surface of our planet, on the surface of Mars, and undoubtably on Venus with its very thick and heavy atmosphere. Clouds of different types, shapes, and heights all influence incoming solar radiation as well as local and regional weather. We may be placing people on Mars in the close future so it is important that we are able to understand clouds and weather on Mars and other worlds with atmospheres. Students can help NASA by gathering observational data about clouds on Mars by participating in the Cloudspotting on Mars project. Students will have access to data collected by an instrument onboard NASA’s Mars Reconnaissance Orbiter. The instrument, the Mars Climate Sounder, observes and takes images of the planet’s horizon in visible and infrared wavelengths as the spacecraft orbits Mars. What shows up in these images are clouds, which because of their height above the horizon, are referred to as mesospheric clouds. Martian clouds are interesting as they form at high altitudes at temperature where carbon dioxide condenses into clouds. As the spacecraft orbits toward and then passes a cloud formation, the images sort of distort into an arch shape. The peak of the arch represents the altitude of the cloud. Students will analyze images for the cloud heights to be added to location and other data generated by the orbiter. The outcome will be the data collected by students helping to answer questions about these Martian clouds and “to create citizen scientist curated maps of mesospheric clouds across Mars” (see Online Resources).

Clouds in space

Yes, there are clouds in space—quite a lot. We know these clouds collectively as nebula or nebulae. There are various types of nebulae with the colorful and brightly glowing emission nebula, like the Great Orion Nebula (see Online Resources), the most familiar. Nebulae are mostly made up of hydrogen, oxygen, helium gases, other gases, and dust. Space is also dusty! A nebula could be leftovers from a star that went nova, or a collection of dust and gases slowly coming together gravitationally to possibly form a star or planets. Depending on their composition and interaction with nearby stars, nebula display themselves in different beautiful colors, or not. For example, the colors in the Orion Nebula come from ionized gases of hydrogen (orange), oxygen (green), and sulfur (red). Energy from nearby stars or stars embedded within the nebula ionizes the gases (see Figure 3).

There are a variety of types of nebulae and nebulae grouped together as either emission nebula (ionized gases), reflection nebula (reflected starlight), planetary nebula (looks like a planetary disk), supernova remnants (clouds of gases, possibly planet fragments), and dark nebula (blocks background starlight; see Online Resources). To astronomers of previous centuries, some nebulae looked similar to a planetary disk, so these objects became known as planetary nebula. As telescope technology improved, astronomers realized that these were not planets but rather compact forms of nebula. Other than the dark nebula, many of these clouds of dust and gases are visible to the unaided eye or with optical aids like binoculars or telescopes, and certainly a camera’s time-exposure images.

Dark nebulae, or absorption nebula, is like sunlight blocking thick clouds in our atmosphere in that the dark nebula block and absorb the light from stars in their respective background. These are curious celestial features as some appear as a hole in the sky—a dark spot surrounded by stars like shown in a picture of Barnard 68, a 7-minute time exposure of the star field surrounding Barnard 68 (see Figure 4). Dark nebula and their surroundings have shapes that appear differently depending on the wavelength of light they are imaged by. The combination of dark obscuring dust and gases with the colors of ionized gases and reflected starlight often give rise to the cosmic version of pareidolia. For example, dark nebula and starlight combine to form the spectacular Horsehead Nebula near the Belt Stars of Orion. Students could explore the many shapes of nebulae with some online resources and compare the views in different wavelengths of light (see Online Resources).

For students

Become a citizen scientist and join NASA or the GLOBE Observer program for cloud spotting on Mars, or cloud cover and patterns in our atmosphere (see Online Resources).
Science fiction from the last century portrayed Venus as having a thick cloudy atmosphere that gave the planet a hot and humid “equatorial, jungle-like” climate. However, Venus has been described as a twin of Earth, but the “Bad Twin”! What do these two planets have in common that gives Venus this reputation? What traits do they not share?
Explore nebulae and other deep-sky objects by requesting images in different wavelengths from either the Aladin Lite or NASA’s SkyView website (see data table and Online Resources).