Astronomy

How much solar UV radiation would someone get who were on the surface of Titan?

How much solar UV radiation would someone get who were on the surface of Titan?


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The Saturn system is about 10 times as far from the Sun as the Earth. This question is concerning Titan for two reasons: Titan has an Earthlike atmosphere so you don't have to account for a higher radiation due to vacuum. Titan's surface pressure is 1.45 atm (21.5 psi) but imagine you're standing on the summit of a mountain where the pressure were 1 atm (14.7 psi). The 2nd reason is that due to Titan's Earthlike atmosphere you don't need a spacesuit but an oxygen mask and warm clothes only (that should cover your entire body if you don't wanna get frostbites).

Compared to the average UV dose on the surface of the Earth, how much radiation would you get on a mountain peak on Titan? Since Titan is 10 times farther from the Sun, does this mean you get 10 times less the radiation on Earth, or does it decrease with the square so you get a 100 times less radiation of what you get on Earth? Or something else?

Also, do you know a calculator that can determine UV radiation and account for all factors?


You are correct that the incident radiation density is 100X less - it's a matter of computing the area of a spherical surface, which goes as the square of the radius. So even in the absence of atmosphere, and assuming you wear some kind of spacesuit that's transparent in the UV, you won't get much of a tan.


Triggers vitamin D – UV from the Sun is needed by our bodies to produce vitamin D. Vitamin D helps strengthen bones, muscles and the body’s immune system. It may also lower the risk of getting some kinds of cancers such as colon cancer.

Helps some skin conditions – UV is used in the treatment of skin conditions such as psoriasis. This is a condition where the skin sheds its cells too quickly and develops itchy, scaly patches. Exposure to UV slows the growth of the skin cells and relieves the symptoms.

Helps moods – Research suggests that sunlight stimulates the pineal gland in the brain to produce certain chemicals called ‘tryptamines’. These chemicals improve our mood.

Helps some animals’ vision – Some animals (including birds, bees and reptiles) are able to see into the near UV light to locate many ripe fruits, flowers and seeds that stand out more strongly from the background. The fruits, flowers and seeds often appear quite different from how humans see them. For example, when seen in UV light, some flowers have different line markings, which may help direct bees and birds to the nectar.

Aids some insects’ navigation – Many insects use UV emissions from celestial objects as references for navigating in flight. This is why a light sometimes attracts flying insects by disrupting their navigation process.

Useful for disinfection and sterilisation – UV has positive applications in the fields of disinfection and sterilisation. UV can effectively ‘kill’ (deactivate or destroy) microorganisms such as viruses and bacteria, for example, when hanging cloth nappies, underwear and tea-towels outside on the clothesline. To destroy the microorganisms, UV rays penetrate the cell's membrane, destroying the DNA, and so stops its ability to reproduce and multiply. This destructive effect explains why we can use UV antibacterial lamps for disinfection and sterilisation. Mangere Wastewater Treatment Plant in Auckland use UVC light to disinfect wastewater.


7 solar system worlds where the weather is crazy

What's the weather like on other worlds? Expect methane rain, global haboobs and a 10,000-mile-wide hurricane.

Our solar system is home to some weird and wonderful weather, with storms more terrifying in scale than anything in Earth's recorded history. From centuries-old hurricanes on Jupiter to immense winds on Neptune, if you leave Earth you'll be shocked by what you find.

On Mars you will find immense dust storms that cover the entire planet, while Venus has an incredibly thick and fast-moving atmosphere that can form permanent vortices at its poles. On Jupiter and Saturn there are some huge storms &mdash bigger than the diameter of multiple Earths &mdash that have raged for decades or even centuries. On the ice giant Neptune you'll find the fastest winds in the solar system, and within Neptune and Uranus it may rain diamonds.

Thanks to recent missions into space, we have learned more about these fascinating weather systems than ever before. Scientists are also performing long-term studies of weather systems, such as storms erupting from the Sun that can have direct effects on Earth. As we continue to reach into the unknown, who knows what else there is to discover in the solar system?

Jupiter's Great Red Spot: An Earth-sized hurricane

This iconic storm has been raging on Jupiter for centuries, but it may not be around forever. The giant spinning storm is comparable to a hurricane on Earth, although it is considerably larger. It measures about 10,000 miles (16,000 kilometers across), which is roughly 1.3 times the width of our planet. Scientists think its roots go up to 100 times deeper into Jupiter than Earth's oceans. Recent evidence, however, suggests the storm may be shrinking, although it can devour other storms to gain a boost.

That's not the only extreme weather on Jupiter: Its north and south poles have strange arrays of cyclones arranged in a circle, while the intense radiation from the planet bathes some of its moons, such as Io and Europa.

NASA's Juno spacecraft, which entered orbit around Jupiter in 2016, has been collecting incredible data about this gas giant using an array of instruments. This includes a microwave radiometer to measure the deep atmosphere of Jupiter, ultraviolet and infrared cameras to take images of the planet's atmosphere and its aurorae, and JunoCam, which has also been busy snapping visible light images.

Saturn's lightning: 10,000 times more powerful than Earth's

Amazingly we've not only seen lightning on Saturn, but we've also heard it. NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, was able to spot lightning on the planet in the daytime, meaning it must have been incredibly intense &mdash some bolts are thought to be 10,000 times more powerful than those on Earth, according to NASA.

By observing radio emissions from the planet, Cassini was also able to 'hear' the storms discharging in the atmosphere. Saturn occasionally develops massive storms that extend more than 190,000 miles (300,000 kilometers), encircling almost the entire planet, while the gas giant's north pole plays host to a weird, permanent hexagon of clouds that extends deep into the planet.

Solar storms: Angry outbursts that knock out power grids

The sun can wreak havoc on our planet. Its solar storms consist of bursts of radiation and charged particles, which can seriously damage satellites that keep a close eye on the sun's activity and prepare for the worst, but occasionally, when a large storm heads our way, satellites and power grids need to be turned off so they can ride it out.

Despite our best efforts, every now and again a violent solar outburst can catch us off guard. In 1859, a powerful solar flare named after astronomer Richard Carrington caused widespread interruptions to global telegraph communications. The Carrington Event of 1859 also sparked incredible aurora displays that were visible as far south as the Caribbean.

In 1989 a solar flare ravaged the electric power transmission from the Hydro Québec generating station, causing a blackout that left six million people without electricity for nine hours.

Solar activity has even been suggested to be a possible cause for the sinking of the Titanic. As new research suggests a solar storm behind the impressive northern light show at the time of the sinking could have disrupted the ship's navigation and communication systems and severely hindered rescue operations.

Venus' vortex: A storm that moves faster than its planet

At the south pole of Venus is a large vortex the size of Europe swirling in the atmosphere. This vortex appears to have been around for a long time and is a result of some strange properties on the planet. The atmosphere on Venus moves faster than the planet, reaching speeds of up to 250 miles (400 kilometers) per hour &mdash 60 times faster than the planet rotates, according to the European Space Agency.

Venus is also the hottest planet in the solar system, but remarkably not the closest to the sun. Its hellishly dense atmosphere blankets the planet and traps heat in a runaway greenhouse effect. As a result, Venusian temperatures can reach 870 degrees Fahrenheit (465 degrees Celsius).

Even the rain on Venus offers no relief from the heinous climate. Corrosive sulfuric acid falls from clouds and evaporates before even reaching the ground due to the extreme surface temperatures.

Neptune's mega wind: Faster than the speed of sound

Neptune, the furthest planet from the Sun, has the fastest winds in the solar system. At the planet's highest altitudes, where methane gives Neptune its blue color, winds can reach speeds of more than 1,300 miles (2,100 kilometers) per hour or 1.6 times the speed of sound. These immense winds also give rise to some large storms, such as the famous "Great Dark Spot" seen by the Voyager 2 probe in 1989.

Scientists are still intrigued as to the cause of this fleeting storm which had vanished by the time NASA's Hubble Space Telescope turned its gaze to Neptune some five years after Voyager 2.

Since then Hubble has kept a watchful eye on Neptune's turbulent storms which rotate clockwise due to the planet's rotation (unlike hurricanes on Earth which are low-pressure systems and rotate counterclockwise). Over the years Hubble has noted the arrival and demise of many Neptunian storms, one of which has recently perplexed scientists.

This particular vortex had been observed sweeping southward toward Neptune's equator, following the path of various storms before it. Though unlike its predecessors this vortex made a sharp U-turn and began to drift back northwards, much to the surprise of researchers.

Mars dust storms: Tornados visible from space

In 2018 a huge dust storm engulfed the surface of Mars, obscuring much of its surface from our view. These storms, known as "haboobs" when they occur on Earth, are fairly regular on Mars, occurring every few years, but this one was particularly large. They are caused by the sun heating the atmosphere of the planet, lifting dust off the ground &mdash although scientists aren't sure how they grow so big, according to NASA. They pose problems for solar-powered rovers on the surface, which rely on the sun's light.

Mars also experiences dust devils &mdash miniature tornadoes that form and move across the surface. This phenomenon is not exclusive to the Red Planet, in fact, they're also observed on Earth.

Dust devils are formed when the ground heats up causing air close to the surface to also warm up and rise. Whilst the air rises it can come into contact with cooler small segments of air higher up which in turn cause the column of air to rotate.

We can see these dust devils due to the dirt they kick up off the ground. They're so visible they can even be seen from space! In 2012, the Mars Reconnaissance Orbiter spotted a colossal Martian dust devil standing 2,600 feet (800 meters) tall and 98 feet (30 meters) wide.

Titan's methane rain: You'd feel every drop

Saturn's largest moon Titan is one of the most enigmatic bodies in the solar system. This Earth-like body hosts liquid on its surface, possesses a truly bizarre climate, and has been intriguing scientists for years.

On Titan methane occasionally falls as rain, after it evaporates from the surface and forms thick clouds. Methane rain on the freezing-cold moon would fall very slowly, due to the low gravity and thick haze, so you'd feel every drop, physicist Rajani Dhingra at the University of Idaho told New Scientist in 2019.

Titan's hydrological cycle (where "hydro" relates to methane not water like on Earth), sculpts the landscape and feeds liquid methane and ethane into huge lakes such as Kraken Mare which is more than 1,000 feet (300 meters) deep.


Carving out rivers

Hydropower might also be problematic as Titan has little rain except rare downpours of intense flash floods every few decades. “It’s not exactly ideal for hydroelectric power generation,” says Hörst. “For a short period of time, rivers would have very, very fast flow, then they’d be dry again.”

Dams or waterwheels could generate power from hydrocarbons made liquid by Titan’s extremely low temperatures, but it could be difficult to get the liquid flowing as the largest lakes and seas are lower than surrounding terrain.

“The topography doesn’t make it impossible, it just makes it a very big engineering project to carve out a river that flows downhill out of the sea,” Hendrix says.

A better option could be to put turbines in the seas because Saturn creates strong tides on Titan. Its largest sea, Kraken Mare, experiences up to a metre of tidal change each day. Those tides all flow through a narrow constriction separating the northern and southern parts of the sea, Seldon Fretum, or as it is nicknamed, the Throat of the Kraken.

“The Throat of Kraken is basically the Strait of Gibraltar,” Lorenz says. “We’re pretty sure there’s a very strong flow of liquid back and forth every Titan day. If you want reliable power that you know is going to be accessible, that’s where I would go.”


10 Interesting Facts about Titan

Image Credit: Infrared composite image of Titan by NASA's Cassini spacecraft

Titan is Saturn’s biggest moon and a fully 50% bigger and 80% more massive than our own Moon, making it the second-biggest natural satellite in the entire solar system after Ganymede, a moon of Jupiter. With a radius of 2,576 km, Titan is also slightly bigger than Mercury, although it has only about 40% the planet’s mass. Below are some more interesting facts about Titan, the sixth moon to be discovered in the solar system:

– Titan was discovered by Christiaan Huygens.

Titan was discovered on March 25th, 1655 by Christiaan Huygens, with a telescope he had built with the help of his brother, Constantijn Huygens, Jr.

– Titan’s atmosphere is denser than Earth’s

Titan is the only solar system moon that is known to have a substantial atmosphere. In fact, Titan’s atmosphere is denser than that of Mars, and at a pressure of 1.6 bar, it is more than 50% denser than our own atmosphere. In terms of its composition, the primary component is nitrogen, with varying amounts of hydrocarbons such as ethane, hydrogen cyanide, and carbon dioxide. Titan’s atmosphere is also much higher than Earth’s, extending to height of roughly 600 km (370 miles) above the surface compared to 480 km (300 miles) for Earth.

– Titan’s shape changes

Repeated fly-bys of Titan by the Cassini probe has shown that Titans’ surface rises and falls by as much as 10 meters during a single orbit, which is enough to measurably change the moon’s shape as it orbits Saturn. These findings suggest that Titan’s relatively thin crust overlays a deep sub-surface ocean of liquid that may or may not be water, and that may or may not also decouple the crust from the core, which would allow for the high degree of observed deformation of the moon.

Titan has hydrocarbon lakes

Titan is the only body in the solar system other than Earth that is known to have liquid on its surface. The image below shows an extended system of lakes and drainage channels that stores liquid hydrocarbons (liquid methane and ethane) that precipitates from Titan’s atmosphere. While this is roughly analogous to the hydrological cycle on Earth, it is not certain how long the episodes of precipitation lasts, although it is suspected that each episode delivers several tons of precipitation each time it occurs. The white areas on this image are unmapped regions.

– Titan may have ice volcanoes

Although no ice volcanoes have been positively identified, it is thought that ice volcanoes are the only mechanism by which the high levels of methane in Titan’s atmosphere can be explained since there is not enough liquid methane on the surface to maintain the observed levels of atmospheric methane. While two possible ice volcanoes that seem to be spewing water and ammonia (the source of atmospheric methane) were identified in 2008, the discovery is not yet confirmed as being ice volcanoes.

– Titan does not have high mountains

While several mountains have been identified on Titan, none are very high, which at first glance, is strange given the fact that the moon is tectonically active. One possible explanation for the lack of high mountains is that Titan’s crust is very soft, which would prevent the formation of high fold mountains like the Himalayas on Earth. Nonetheless, the highest mountain on Titan is located in the Mithrim Montes range, and measures all of 3,337 meters tall.

– Mountains on Titan are named after mountains in Middle-Earth

All the mountains and collections of hills on Titan are named after mountains, or mountain peaks in Middle-earth, a fictional world created by J.R.R. Tolkien (1892-1973) and whose story is told in the Lord of the Rings trilogy and other works. Some examples include Angmar Mons, named after the Mountains of Angmar, Erebor Mons, named after Erebor [The Lonely Mountain], and Moria Mons, which is named after the Mountains of Moria.

– Life could arise on Titan

Experiments have shown that with the addition of UV radiation, polymers such as tholins and other complex organic molecules can be created from Titan’s atmosphere. In fact, the building blocks of DNA and RNA as well as several proteins and amino acids have been produced when energy was applied to a cocktail of gases similar to those in Titan’s atmosphere, However, the obstacles to life arising on Titan remain formidable, and any analogies to life as we know it are inexact. Chemical and environmental conditions in the moon’s cold hydrocarbon lakes are so different from conditions on Earth that even if life did arise on Titan, there is a possibility that we may never recognize it as such.

– Titan’s “sand” dunes consist of organic soot

While some features, such as extensive dune fields on Titan strongly resemble dune fields on Earth, the material of which the dunes are made is not silicate sand. Some investigators hold that the dunes consist of rock that was eroded by liquid methane, while others are of the opinion that the dunes consist of organic compounds that have rained down from the moon’s atmosphere, much like terrestrial snow. What is certain about the dunes is that they contain less water than the rest of Titan, which supports the theory that photochemical reactions in Titan’s atmosphere continuously form a sort of organic “snow” that rains down to produce the dunes.

– Titan’s has few impact craters

Few impact craters have been positively identified on Titan, which suggests that the surface has undergone, or is undergoing a continuous process of recycling. In fact, studies have shown that Titan’s surface is only about 100 million to 1 billion years old, although the moon itself is as old as the rest of the solar system. While the geological processes that appear to be obliterating the large impact craters on Titan are not yet understood, it is thought that the dense atmosphere of the moon has somehow been protecting the surface from small and intermediate-sized impactors.


How much more efficient would solar cells need to be, before we could power a car with solar, utilizing only the surface area of the car itself?

Now, I imagine you would need to take I to account lots of other details such as all the other components in the system. However, to keep this question somewhat straightforward, let’s mainly look at the specific limitation of the solar cells.

The power of solar radiation depends on where you are on the planet and the time of day, but 1 kW per m 2 is a decent enough estimate.

A typical car might have about 6-10 m 2 of surface area on the top (depending on the size of the car as well as the amount of surface area lost to windows and such). Lets say 8. That provides a continuous power of 8 kW at an impossible 100% efficiency.

8 kW is approximately 10 horse power. And that's very much below the typical power of even low power compact cars (which certainly don't hit the 8 m 2 usable surface area used in this approximation). Even the increased efficiency of an electric engine over an ICE won't make up for this.

So the sun simply doesn't provide enough power to continuously power a car without some kind of battery system.

It kind-of depends on your requirements for the car. The vehicles running in the World Solar Challenge run at about 1500W (single seaters) and twice that for the Cruisers. That's at 90 km/h, which is a decent cruising speed. With six square metres of solar panel at 23-ish % efficiency, the energy income can just about keep up with the consumption, at high summer noon.

Running without a battery is still hideously impractical, if only because of shaded areas, but also because you want extra power for acceleration and a place to dump energy when braking regeneratively. But, if we're talking battery systems anyway, our options get a lot better. Where I live, the average car drives less than 40 km per day. A conventional EV such as the Nissan Leaf would eat 6-8 kWh to cover that distance. The average insolation at our lattitude is 2.7 kWh per m 2 per day. So, three square metres of perfect efficiency would suffice to "fuel" the average car on the average day. A Leaf has a projected area of 8 m 2, so with a 37% efficient solar-roof youɽ get there. That's way more than standard silicon roof panels, but less than the best experimental cells.

All that said, I still think the best way to power electric cars by solar energy, is not to put the panels on the car, but on your carport or overhead the parking lot instead. Way easier design-wise, more space to put the panels and orient them toward the sun, way less wear and tear on the cells, and your car will be nice and cool in the shade as a bonus.


Solar Variability: Striking A Balance With Climate Change

The sun has powered almost everything on Earth since life began, including its climate. The sun also delivers an annual and seasonal impact, changing the character of each hemisphere as Earth's orientation shifts through the year. Since the Industrial Revolution, however, new forces have begun to exert significant influence on Earth's climate.

"For the last 20 to 30 years, we believe greenhouse gases have been the dominant influence on recent climate change," said Robert Cahalan, climatologist at NASA&rsquos Goddard Space Flight Center in Greenbelt, Md.

For the past three decades NASA scientists have investigated the unique relationship between the sun and Earth. Using space-based tools, like the Solar Radiation and Climate Experiment (SORCE), they have studied how much solar energy illuminates Earth, and explored what happens to that energy once it penetrates the atmosphere. The amount of energy that reaches Earth's outer atmosphere is called the total solar irradiance. Total solar irradiance is variable over many different timescales, ranging from seconds to centuries due to changes in solar activity.

The sun goes through roughly an 11-year cycle of activity, from stormy to quiet and back again. Solar activity often occurs near sunspots, dark regions on the sun caused by concentrated magnetic fields. The solar irradiance measurement is much higher during solar maximum, when sunspot cycle and solar activity is high, versus solar minimum, when the sun is quiet and there are usually no sunspots.

"The fluctuations in the solar cycle impacts Earth's global temperature by about 0.1 degree Celsius, slightly hotter during solar maximum and cooler during solar minimum," said Thomas Woods, solar scientist at the University of Colorado in Boulder. "The sun is currently at its minimum, and the next solar maximum is expected in 2012."

Using SORCE, scientists have learned that about 1,361 watts per square meter of solar energy reaches Earth's outermost atmosphere during the sun's quietest period. But when the sun is active, 1.3 watts per square meter (0.1 percent) more energy reaches Earth. "This TSI measurement is very important to climate models that are trying to assess Earth-based forces on climate change," said Cahalan.

Over the past century, Earth's average temperature has increased by approximately 0.6 degrees Celsius (1.1 degrees Fahrenheit). Solar heating accounts for about 0.15 C, or 25 percent, of this change, according to computer modeling results published by NASA Goddard Institute for Space Studies researcher David Rind in 2004. Earth's climate depends on the delicate balance between incoming solar radiation, outgoing thermal radiation and the composition of Earth's atmosphere. Even small changes in these parameters can affect climate. Around 30 percent of the solar energy that strikes Earth is reflected back into space. Clouds, atmospheric aerosols, snow, ice, sand, ocean surface and even rooftops play a role in deflecting the incoming rays. The remaining 70 percent of solar energy is absorbed by land, ocean, and atmosphere.

"Greenhouse gases block about 40 percent of outgoing thermal radiation that emanates from Earth," Woods said. The resulting imbalance between incoming solar radiation and outgoing thermal radiation will likely cause Earth to heat up over the next century, accelerating the melting polar ice caps, causing sea levels to rise and increasing the probability of more violent global weather patterns.

Non-Human Influences on Climate Change

Before the Industrial Age, the sun and volcanic eruptions were the major influences on Earth's climate change. Earth warmed and cooled in cycles. Major cool periods were ice ages, with the most recent ending about 11,000 years ago.

"Right now, we are in between major ice ages, in a period that has been called the Holocene,&rdquo said Cahalan. &ldquoOver recent decades, however, we have moved into a human-dominated climate that some have termed the Anthropocene. The major change in Earth's climate is now really dominated by human activity, which has never happened before."

The sun is relatively calm compared to other stars. "We don't know what the sun is going to do a hundred years from now," said Doug Rabin, a solar physicist at Goddard. "It could be considerably more active and therefore have more influence on Earth's climate."

Or, it could be calmer, creating a cooler climate on Earth similar to what happened in the late 17th century. Almost no sunspots were observed on the sun's surface during the period from 1650 to 1715. This extended absence of solar activity may have been partly responsible for the Little Ice Age in Europe and may reflect cyclic or irregular changes in the sun's output over hundreds of years. During this period, winters in Europe were longer and colder by about 1 C than they are today.

Since then, there seems to have been on average a slow increase in solar activity. Unless we find a way to reduce the amount of greenhouse gases we put into the atmosphere, such as carbon dioxide from fossil fuel burning, the solar influence is not expected to dominate climate change. But the solar variations are expected to continue to modulate both warming and cooling trends at the level of 0.1 to 0.2 degrees Celsius (0.18 to 0.26 Fahrenheit) over many years.

Future Measurements of Solar Variability

For three decades, a suite of NASA and European Space Agency satellites have provided scientists with critical measurements of total solar irradiance. The Total Irradiance Monitor, also known as the TIM instrument, was launched in 2003 as part of the NASA&rsquos SORCE mission, and provides irradiance measurements with state-of-the-art accuracy. TIM has been rebuilt as part of the Glory mission, scheduled to launch in 2009. Glory's TIM instrument will continue an uninterrupted 30-year record of solar irradiance measurements and will help researchers better understand the sun's direct and indirect effects on climate. Glory will also collect data on aerosols, one of the least understood pieces of the climate puzzle.

Story Source:

Materials provided by National Aeronautics and Space Administration. Original written by Rani Gran, NASA's Goddard Space Flight Center. Note: Content may be edited for style and length.


The U.S. Federal Aviation Administration (FAA)

Solar activity can cause the navigational equipment on commercial airplanes to report the location of planes incorrectly. Fortunately, there are systems available to pilots that are not affected by solar activity. Or, if navigators are alerted to a proton storm, they can switch to a backup system. The FAA routinely receives alerts of solar flares. These alerts allow them to be prepared for potential communication and navigation problems.

Solar Radiation Alert System (PDF) (28 pp, 977 K, About PDF)
This report provides information about the continuous monitoring and evaluation of proton activity in the solar system.

U.S. National Aeronautics and Space Administration (NASA)

NASA’s Solar Particle Alert Network (SPAN) consists of multiple radio and optical telescopes that stream continuous data on solar flare activity. Solar flare eruptions are difficult to predict. However, the instruments used by SPAN can provide some warning. They can detect solar material as it makes its way from the Sun to Earth. This information also allows astronauts in space, who lack the protection of Earth’s atmosphere, to move to well shielded areas of their spacecraft.

Gallery of Space Weather
This webpage provides printable pictures of solar and space weather.

What is a solar flare?
This webpage provides the definition of solar flares and describes how they work.

The Difference Between Flares and CMEs
This webpage discusses the physical, visual, and scientific differences between coronal mass ejections and solar flares.

Space Place: Solar Activity
This webpage provides activities for students to help learn about the solar cycle and solar activity.

U.S. Department of Commerce (DOC), U.S. National Oceanic and Atmospheric Administration (NOAA)

NOAA’s Space Environment Center provides real-time monitoring and forecasting of solar and geophysical events. They also develop techniques for forecasting solar and geophysical disturbances.

A Primer on Space Weather (PDF) (11 pp, 4,187 K, About PDF)
This webpage provides information about solar activity and its effects on Earth and space weather.

National Park Service (NPS), Denali National Park

The NPS has many parks in which people can view the Northern Lights. Some of their images are available on individual National Park websites.

Aurora Borealis and Star Gazing
This webpage includes a short video of the aurora borealis captured by 8,000 images taken over a three month period in Denali National Park.

National Environmental Education Foundation (NEEF)

NEEF is an independent non-profit organization that complements the EPA’s mission. Its mission is to secure a safer and healthier world for ourselves, our children, and for generations to come.

SunWise Exit
This webpage provides links to information and resources about Sun safety for kids and educators.


Contents

The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sydney Chapman in 1930. Ozone in the Earth's stratosphere is created by ultraviolet light striking ordinary oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen) the atomic oxygen then combines with unbroken O2 to create ozone, O3. The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an individual atom of oxygen, a continuing process called the ozone-oxygen cycle. Chemically, this can be described as:

About 90 percent of the ozone in the atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000 ft), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimetres ( 1 ⁄ 8 inch) thick. [6]

Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the sun. Extremely short or vacuum UV (10–100 nm) is screened out by nitrogen. UV radiation capable of penetrating nitrogen is divided into three categories, based on its wavelength these are referred to as UV-A (400–315 nm), UV-B (315–280 nm), and UV-C (280–100 nm).

UV-C, which is very harmful to all living things, is entirely screened out by a combination of dioxygen (< 200 nm) and ozone (> about 200 nm) by around 35 kilometres (115,000 ft) altitude. UV-B radiation can be harmful to the skin and is the main cause of sunburn excessive exposure can also cause cataracts, immune system suppression, and genetic damage, resulting in problems such as skin cancer. The ozone layer (which absorbs from about 200 nm to 310 nm with a maximal absorption at about 250 nm) [7] is very effective at screening out UV-B for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface, and is important for the skin's production of vitamin D.

Ozone is transparent to most UV-A, so most of this longer-wavelength UV radiation reaches the surface, and it constitutes most of the UV reaching the Earth. This type of UV radiation is significantly less harmful to DNA, although it may still potentially cause physical damage, premature aging of the skin, indirect genetic damage, and skin cancer. [8]

The thickness of the ozone layer varies worldwide and is generally thinner near the equator and thicker near the poles. [9] Thickness refers to how much ozone is in a column over a given area and varies from season to season. The reasons for these variations are due to atmospheric circulation patterns and solar intensity.

The majority of ozone is produced over the tropics and is transported towards the poles by stratospheric wind patterns. In the northern hemisphere these patterns, known as the Brewer-Dobson circulation, make the ozone layer thickest in the spring and thinnest in the fall. [9] When ozone is produced by solar UV radiation in the tropics, it is done so by circulation lifting ozone-poor air out of the troposphere and into the stratosphere where the sun photolyzes oxygen molecules and turns them into ozone. Then, the ozone-rich air is carried to higher latitudes and drops into lower layers of the atmosphere. [9]

Research has found that the ozone levels in the United States are highest in the spring months of April and May and lowest in October. While the total amount of ozone increases moving from the tropics to higher latitudes, the concentrations are greater in high northern latitudes than in high southern latitudes, with spring ozone columns in high northern latitudes occasionally exceeding 600 DU and averaging 450 DU whereas 400 DU consituted a usual maximum in the Antarctic before anthropogenic ozone depletion. This difference occurred naturally because of the weaker polar vortex and stronger Brewer-Dobson circulation in the northern hemisphere owing to that hemisphere’s large mountain ranges and greater contrasts between land and ocean temperatures. [10] The difference between high northern and southern latitudes has been increased since the 1970s due to the ozone hole phenomenon. [9] The highest amounts of ozone are found over the Arctic during the spring months of March and April, but the Antarctic has their lowest amounts of ozone during their summer months of September and October,

The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are natural sources for all of these species, the concentrations of chlorine and bromine increased markedly in recent decades because of the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons. [11] These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. By 2009, nitrous oxide was the largest ozone-depleting substance (ODS) emitted through human activities. [12]

The breakdown of ozone in the stratosphere results in reduced absorption of ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to reach the Earth's surface at a higher intensity. Ozone levels have dropped by a worldwide average of about 4 percent since the late 1970s. For approximately 5 percent of the Earth's surface, around the north and south poles, much larger seasonal declines have been seen, and are described as "ozone holes". [13] The discovery of the annual depletion of ozone above the Antarctic was first announced by Joe Farman, Brian Gardiner and Jonathan Shanklin, in a paper which appeared in Nature on May 16, 1985. [14]

Regulation

To support successful regulation attempts, the ozone case was communicated to lay persons "with easy-to-understand bridging metaphors derived from the popular culture" and related to "immediate risks with everyday relevance". [ citation needed ] The specific metaphors used in the discussion (ozone shield, ozone hole) proved quite useful [15] and, compared to global climate change, the ozone case was much more seen as a "hot issue" and imminent risk. [16] Lay people were cautious about a depletion of the ozone layer and the risks of skin cancer.

In 1978, the United States, Canada and Norway enacted bans on CFC-containing aerosol sprays that damage the ozone layer. The European Community rejected an analogous proposal to do the same. In the U.S., chlorofluorocarbons continued to be used in other applications, such as refrigeration and industrial cleaning, until after the discovery of the Antarctic ozone hole in 1985. After negotiation of an international treaty (the Montreal Protocol), CFC production was capped at 1986 levels with commitments to long-term reductions. [17] This allowed for a ten-year phase-in for developing countries [18] (identified in Article 5 of the protocol). Since that time, the treaty was amended to ban CFC production after 1995 in the developed countries, and later in developing countries. [19] Today, all of the world's 197 countries have signed the treaty. Beginning January 1, 1996, only recycled and stockpiled CFCs were available for use in developed countries like the US. This production phaseout was possible because of efforts to ensure that there would be substitute chemicals and technologies for all ODS uses. [20]

On August 2, 2003, scientists announced that the global depletion of the ozone layer may be slowing down because of the international regulation of ozone-depleting substances. In a study organized by the American Geophysical Union, three satellites and three ground stations confirmed that the upper-atmosphere ozone-depletion rate slowed significantly during the previous decade. Some breakdown can be expected to continue because of ODSs used by nations which have not banned them, and because of gases which are already in the stratosphere. Some ODSs, including CFCs, have very long atmospheric lifetimes, ranging from 50 to over 100 years. It has been estimated that the ozone layer will recover to 1980 levels near the middle of the 21st century. [13] A gradual trend toward "healing" was reported in 2016. [21]

Compounds containing C–H bonds (such as hydrochlorofluorocarbons, or HCFCs) have been designed to replace CFCs in certain applications. These replacement compounds are more reactive and less likely to survive long enough in the atmosphere to reach the stratosphere where they could affect the ozone layer. While being less damaging than CFCs, HCFCs can have a negative impact on the ozone layer, so they are also being phased out. [22] These in turn are being replaced by hydrofluorocarbons (HFCs) and other compounds that do not destroy stratospheric ozone at all.

The residual effects of CFCs accumulating within the atmosphere lead to a concentration gradient between the atmosphere and the ocean. This organohalogen compound is able to dissolve into the ocean's surface waters and is able to act as a time-dependent tracer. This tracer helps scientists study ocean circulation by tracing biological, physical and chemical pathways [23]

As ozone in the atmosphere prevents most energetic ultraviolet radiation reaching the surface of the Earth, astronomical data in these wavelengths have to be gathered from satellites orbiting above the atmosphere and ozone layer. Most of the light from young hot stars is in the ultraviolet and so study of these wavelengths is important for studying the origins of galaxies. The Galaxy Evolution Explorer, GALEX, is an orbiting ultraviolet space telescope launched on April 28, 2003, which operated until early 2012.

This GALEX image of the Cygnus Loop nebula could not have been taken from the surface of the Earth because the ozone layer blocks the ultra-violet radiation emitted by the nebula.


Contents

The term "tholin" was coined by astronomer Carl Sagan and his colleague Bishun Khare to describe the difficult-to-characterize substances they obtained in his Miller–Urey-type experiments on the methane-containing gas mixtures such as those found in Titan's atmosphere. [2] Their paper proposing the name "tholin" said:

For the past decade we have been producing in our laboratory a variety of complex organic solids from mixtures of the cosmically abundant gases CH
4 , C
2 H
6 , NH
3 , H
2 O , HCHO, and H
2 S . The product, synthesized by ultraviolet (UV) light or spark discharge, is a brown, sometimes sticky, residue, which has been called, because of its resistance to conventional analytical chemistry, "intractable polymer". [. ] We propose, as a model-free descriptive term, ‘tholins’ (Gk ϴὸλος, muddy but also ϴoλòς, vault or dome), although we were tempted by the phrase ‘star-tar’. [4] [2]

Tholins are not one specific compound but rather are descriptive of a spectrum of molecules, including heteropolymers, [6] [7] that give a reddish, organic surface covering on certain planetary surfaces. Tholins are disordered polymer-like materials made of repeating chains of linked subunits and complex combinations of functional groups. [8] Sagan and Khare note "The properties of tholins will depend on the energy source used and the initial abundances of precursors, but a general physical and chemical similarity among the various tholins is evident." [2]

Some researchers in the field prefer a narrowed definition of tholins, for example S. Hörst wrote: "Personally, I try to use the word 'tholins' only when describing the laboratory-produced samples, in part because we do not really know yet how similar the material we produce in the lab is to the material found on places like Titan or Triton (or Pluto!)." [4] French researchers also use the term tholins only when describing the laboratory-produced samples as analogues. [9] NASA scientists also prefer the word 'tholin' for the products of laboratory simulations, and use the term 'refractory residues' for actual observations on astronomical bodies. [8]

Tholins may be a major constituent of the interstellar medium. [2] On Titan, their chemistry is initiated at high altitudes and participates in the formation of solid organic particles. [9] Their key elements are carbon, nitrogen, and hydrogen. Laboratory infrared spectroscopy analysis of experimentally synthetized tholins has confirmed earlier identifications of chemical groups present, including primary amines, nitriles, and alkyl portions such as CH
2 / CH
3 forming complex disordered macromolecular solids. Laboratory tests generated complex solids formed from exposure of N
2 : CH
4 gaseous mixtures to electrical discharge in cold plasma conditions, reminiscent of the famous Miller–Urey experiment conducted in 1952. [10]

As illustrated to the right, tholins are thought to form in nature through a chain of chemical reactions known as pyrolysis and radiolysis. This begins with the dissociation and ionization of molecular nitrogen ( N
2 ) and methane ( CH
4 ) by energetic particles and solar radiation. This is followed by the formation of ethylene, ethane, acetylene, hydrogen cyanide, and other small simple molecules and small positive ions. Further reactions form benzene and other organic molecules, and their polymerization leads to the formation of an aerosol of heavier molecules, which then condense and precipitate on the planetary surface below. [11] Tholins formed at low pressure tend to contain nitrogen atoms in the interior of their molecules, while tholins formed at high pressure are more likely to have nitrogen atoms located in terminal positions. [12] [13]

These atmospherically-derived substances are distinct from ice tholin II, which are formed instead by irradiation (radiolysis) of clathrates of water and organic compounds such as methane ( CH
4 ) or ethane ( C
2 H
6 ). [3] [14] The radiation-induced synthesis on ice are non-dependant on temperature. [3]

Some researchers have speculated that Earth may have been seeded by organic compounds early in its development by tholin-rich comets, providing the raw material necessary for life to develop [2] [3] (see Miller–Urey experiment for discussion related to this). Tholins do not exist naturally on present-day Earth due to the oxidizing properties of the free oxygen component of its atmosphere ever since the Great Oxygenation Event around 2.4 billion years ago. [15]

Laboratory experiments [16] suggest that tholins near large pools of liquid water that might persist for thousands of years could facilitate the formation of prebiotic chemistry to take place, [17] [4] and has implications on the origins of life on Earth and possibly other planets. [4] [15] Also, as particles in the atmosphere of an exoplanet, tholins affect the light scatter and act as a screen for protecting planetary surfaces from ultraviolet radiation, affecting habitability. [4] [18] Laboratory simulations found derived residues related to amino acids as well as urea, with important astrobiological implications. [15] [16] [19]

On Earth, a wide variety of soil bacteria are able to use laboratory-produced tholins as their sole source of carbon. Tholins could have been the first microbial food for heterotrophic microorganisms before autotrophy evolved. [20]

Sagan and Khare note the presence of tholins through multiple locations: "as a constituent of the Earth's primitive oceans and therefore relevant to the origin of life as a component of red aerosols in the atmospheres of the outer planets and Titan present in comets, carbonaceous chondrites asteroids, and pre-planetary solar nebulae and as a major constituent of the interstellar medium." [2] The surfaces of comets, centaurs, and many icy moons and Kuiper-belt objects in the outer Solar System are rich in deposits of tholins. [21]

Moons Edit

Titan Edit

Titan tholins are nitrogen-rich [22] [23] organic substances produced by the irradiation of the gaseous mixtures of nitrogen and methane found in the atmosphere and surface of Titan. Titan's atmosphere is about 97% nitrogen, 2.7±0.1% methane and the remaining trace amounts of other gases. [24] In the case of Titan, the haze and orange-red color of its atmosphere are both thought to be caused by the presence of tholins. [11]

Europa Edit

Colored regions on Jupiter's satellite Europa are thought to be tholins. [17] [25] [26] [27] The morphology of Europa's impact craters and ridges is suggestive of fluidized material welling up from the fractures where pyrolysis and radiolysis take place. In order to generate colored tholins on Europa there must be a source of materials (carbon, nitrogen, and water), and a source of energy to drive the reactions. Impurities in the water ice crust of Europa are presumed both to emerge from the interior as cryovolcanic events that resurface the body, and to accumulate from space as interplanetary dust. [17]

Rhea Edit

The extensive dark areas on the trailing hemisphere of Saturn's moon Rhea are thought to be deposited tholins. [14]

Triton Edit

Neptune's moon Triton is observed to have the reddish color characteristic of tholins. [22] Triton's atmosphere is mostly nitrogen, with trace amounts of methane and carbon monoxide. [28] [29]

Dwarf planets Edit

Pluto Edit

Tholins occur on the dwarf planet Pluto [30] and are responsible for red colors [31] as well as the blue tint of the atmosphere of Pluto. [32] The reddish-brown cap of the north pole of Charon, the largest of five moons of Pluto, is thought to be composed of tholins, produced from methane, nitrogen and related gases released from the atmosphere of Pluto and transferred over about 19,000 km (12,000 mi) distance to the orbiting moon. [33] [34] [35]

Ceres Edit

Tholins were detected on the dwarf planet Ceres by the Dawn mission. [36] [37] Most of the planet's surface is extremely rich in carbon, with approximately 20% carbon by mass in its near surface. [38] [39] The carbon content is more than five times higher than in carbonaceous chondrite meteorites analyzed on Earth. [39]

Makemake Edit

Makemake exhibits methane, large amounts of ethane and tholins, as well as smaller amounts of ethylene, acetylene and high-mass alkanes may be present, most likely created by photolysis of methane by solar radiation. [40] [41] [42]

Kuiper belt objects and Centaurs Edit

The reddish color typical of tholins is characteristic of many Trans-Neptunian objects, including plutinos in the outer Solar System such as 28978 Ixion. [43] Spectral reflectances of Centaurs also suggest the presence of tholins on their surfaces. [44] [45] [46] The New Horizons exploration of the classical Kuiper belt object 486958 Arrokoth revealed reddish color at its surface, suggestive of tholins. [8] [47]

Comets and asteroids Edit

Tholins were detected in situ by the Rosetta mission to comet 67P/Churyumov–Gerasimenko. [48] [49] Tholins are not typically characteristic of main-belt asteroids, but have been detected on the asteroid 24 Themis. [50] [51]

Tholins beyond the Solar System Edit

Tholins might have also been detected in the stellar system of the young star HR 4796A using the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) aboard the Hubble Space Telescope. [52] The HR 4796 system is approximately 220 light years from Earth. [53]

Models show that even when far from UV radiation of a star, cosmic ray doses may be fully sufficient to convert carbon-containing ice grains entirely to complex organics in less than the lifetime of the typical interstellar cloud. [3]