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Category Archives: 06. The terrestrial planets: Earth, Moon, and their relatives

U.S. government scientists released their climate science report on Nov. 2, 2017. Here is their executive summary:

The climate of the United States is strongly connected to the changing global climate. The statements below highlight past, current, and projected climate changes for the United States and the globe.

Global annually averaged surface air temperature has increased by about 1.0°C (1.8°F) over the last 115 years (1901–2016). This period is now the warmest in the history of modern civilization. The last few years have also seen record-breaking, climate-related weather extremes, and the last three years have been the warmest years on record for the globe. These trends are expected to continue over climate timescales.

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Credit: U.S. Global Change Research Program, Washington, DC

This assessment concludes, based on extensive evidence, that it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.

In addition to warming, many other aspects of global climate are changing, primarily in response to human activities. Thousands of studies conducted by researchers around the world have documented changes in surface, atmospheric, and oceanic temperatures; melting glaciers; diminishing snow cover; shrinking sea ice; rising sea levels; ocean acidification; and increasing atmospheric water vapor.

For example, global average sea level has risen by about 7–8 inches since 1900, with almost half (about 3 inches) of that rise occurring since 1993. Human-caused climate change has made a substantial contribution to this rise since 1900, contributing to a rate of rise that is greater than during any preceding century in at least 2,800 years. Global sea level rise has already affected the United States; the incidence of daily tidal flooding is accelerating in more than 25 Atlantic and Gulf Coast cities.

Global average sea levels are expected to continue to rise—by at least several inches in the next 15 years and by 1–4 feet by 2100. A rise of as much as 8 feet by 2100 cannot be ruled out. Sea level rise will be higher than the global average on the East and Gulf Coasts of the United States.

Changes in the characteristics of extreme events are particularly important for human safety, infrastructure, agriculture, water quality and quantity, and natural ecosystems. Heavy rainfall is increasing in intensity and frequency across the United States and globally and is expected to continue to increase. The largest observed changes in the United States have occurred in the Northeast.

Heatwaves have become more frequent in the United States since the 1960s, while extreme cold temperatures and cold waves are less frequent. Recent record-setting hot years are projected to become common in the near future for the United States, as annual average temperatures continue to rise. Annual average temperature over the contiguous United States has increased by 1.0°C (1.8°F) for the period 1901–2016; over the next few decades (2021–2050), annual average temperatures are expected to rise by about 2.5°F for the United States, relative to the recent past (average from 1976–2005), under all plausible future climate scenarios.

The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s and is projected to further increase in those regions as the climate changes, with profound changes to regional ecosystems.

Annual trends toward earlier spring melt and reduced snowpack are already affecting water resources in the western United States and these trends are expected to continue. Under higher scenarios, and assuming no change to current water resources management, chronic, long-duration hydrological drought is increasingly possible before the end of this century.

The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse gases (especially carbon dioxide) emitted globally. Without major reductions in emissions, the increase in annual average global temperature relative to preindustrial times could reach 5°C (9°F) or more by the end of this century. With significant reductions in emissions, the increase in annual average global temperature could be limited to 2°C (3.6°F) or less.

The global atmospheric carbon dioxide (CO2) concentration has now passed 400 parts per million (ppm), a level that last occurred about 3 million years ago, when both global average temperature and sea level were significantly higher than today. Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens to hundreds of millions of years. There is broad consensus that the further and the faster the Earth system is pushed towards warming, the greater the risk of unanticipated changes and impacts, some of which are potentially large and irreversible.

The observed increase in carbon emissions over the past 15–20 years has been consistent with higher emissions pathways. In 2014 and 2015, emission growth rates slowed as economic growth became less carbon-intensive. Even if this slowing trend continues, however, it is not yet at a rate that would limit global average temperature change to well below 2°C (3.6°F) above preindustrial levels.

Read the full report (Weubbles et al. 2017) here.

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From a report in the New York Times by Nicholas St Fleur:

Continents cruise in the slow lane. Moving just millimeters at a time, it took the ancient supercontinent Pangea hundreds of millions of years to break apart into today’s landmasses. But a study published Tuesday shows that the journey wasn’t always a leisurely drive. When under extreme strain, the tectonic plates hit the throttle and accelerated to speeds 20 times faster than they were traveling before.

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Credit: Sascha Brune

After analyzing seismic data from across the world and building a model, a team of geophysicists have discovered that plates move in two distinct phases: a slow phase and a fast one. During the slow phase, the continental crusts, which can be more than 20 miles thick, are stretched out little by little while remaining connected. But then suddenly, one or both of the continents step on the gas pedal. A critical point is reached when the connection between the two continents becomes so weak it can no longer resist the forces trying to pull it apart. This acceleration is directly related to the thinning of the crust.

Links: NYT article; computer simulation illustrating the movement of different continents.

A report published on June 30, 2016, in the journal Science indicates that the hole in the ozone layer above Antarctica shows signs of beginning to heal. Since its discovery in 1985 the ozone hole has grown bigger each Spring, reaching a size of 10.9 million square miles in 2015.

The main cause of the hole is man-made chemicals known as CFCs (chlorofluorocarbons), which were widely used as propellants, refrigerants and solvents until they were banned by an international treatment in 1987 – use of CFCs had to be phased out by 1996. However, CFCs in the atmosphere are long-lived, so it will be decades, possibly centuries, before the ozone layer can fully repair itself. Naturally-released sulphur gases, for example, from volcanic eruptions can also affect the ozone layer, acting to slow its recovery.

Links: LA Times article; Science news feature and video.

From a news article in Nature, April 12, 2016:

After an unplanned 5-year detour, Japan’s Venus probe, Akatsuki, has come back to life and provided new images of Venus. These include a detailed shot of streaked, acidic clouds and a mysterious moving ‘bow’ shape in the planet’s atmosphere.

venus-combined-sideview

Credit: ISAS/JAXA

Akatsuki, which means ‘dawn’ in Japanese, launched in 2010 and was supposed to enter into orbit around Venus later that year to study the planet’s thick atmosphere. The mission would include looking for signs of active volcanos and other geology. However, upon entry, a fault in a valve caused the probe’s main engine to blow, and the craft entered an orbit around the Sun. As Akatsuki passed near Venus in December, Japan Aerospace Exploration Agency (JAXA) engineers managed to salvage the mission by instructing the craft’s much smaller, secondary thrusters to push it into a looping elliptical orbit around the planet. Its suite of five cameras capture light ranging from infrared to ultraviolet.

A highly detailed shot show dense layers within Venus’s sulfuric acid clouds. The highest-quality infrared image of this view of Venus, it suggests that the processes that underlie cloud formation might be more complicated than previously thought.

From an article on Sky and Telescope by David Dickinson, originally posted on December 9, 2015.

The Japanese Aerospace Exploration Agency’s (JAXA) Venus Climate Orbitor Akatsuki is finally orbiting Venus – five years later than planned. After a catastrophic main engine failure in 2010 causing the spacecraft to fly past Venus instead of entering its orbit, scientists and engineers have salvaged the mission and put Akatsuki back on track.

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Credit: JAXA

On December 6th 2015, five years to the day of the original blip, four tiny reaction-control thrusters burned for more than 20 minutes to insert the spacecraft into Venusian orbit. Engineers had tested these thrusters to ensure it was possible back in 2011, before putting the spacecraft into hibernation to prolong its life. The first opportunity to execute the manouver came at the beginning of this month, and it proved to be second time lucky for JAXA’s spacecraft.

The six instruments aboard the Akatsuki spacecraft will probe Venus’s atmosphere, measuring its rotation and convection.Researchers also hope to detect evidence for Venusian lightning using a high-speed imager aboard the spacecraft. Viewing across radio, infrared, visible and ultraviolet wavelengths, the payload will also record heat radiated from the Venusian surface and may spot active volcanoes if they exist. A series of radio occultation experiments will also allow researchers to probe the depths of the Venusian atmosphere as the spacecraft makes successive passes behind the planet as seen from Earth.

In addition to the science payload, JAXA also teamed up with the Planetary Society to carry more than 260,000 people’s names and messages printed on aluminium plates aboard the spacecraft.

Whilst the new orbit time is much longer than originally planned, with a closest approach of 400km as opposed to the intended 300km, the JAXA engineers have saved the mission from disaster. Akatsuki, meaning ‘dawn’ in Japanese, will begin to send back its findings in 2016, and as it’s the first time that JAXA have managed to put a spacecraft in orbit around another planet, their wait will hopefully be rewarded.

For more information on the terrestrial planets, see Chapter 6 of The Cosmos.

Link to the original article here.

From an article on Space.com:

Mercury has possessed a magnetic field for billions of years, and that field may have once been as strong as the Earth’s.

As NASA’s MESSENGER spacecraft ended its four-year mission at Mercury, it travelled so low over the surface, at altitudes as low as 15 kilometers, that it was able to detect weak magnetism coming from surface rocks in terrain 3.7 to 3.9 billion years old.

mercury-magnetic-field-diagram

Credit: NASA/University of British Columbia

The discovery indicates that Mercury’s magnetic field, generated by liquid rotating in the planet’s core, was that old, helping to constrain scenarios for how Mercury has evolved over time. No trace of magnetism on Earth in rocks older than 3.5 billion years has been found.

Link: Space.com article.

From a New Scientist article, May 16, 2015:

The United Arab Emirates has announced details of its uncrewed Mars probe, which it plans to launch in 2020 to monitor the planet’s atmosphere from orbit. The spacecraft, named Hope, will be a big step up from the country’s previous space activities as it attempts to compete with other emerging space powers like India and China.

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Credit: © Mohammed bin Rashid Space Centre

The Mars probe will carry spectrometers for analysing infrared and ultraviolet signals, along with a digital camera. These will measure water, dust and other molecules in the planet’s atmosphere, in an attempt to learn how Mars transitioned from a wet, warm world to the dry, dusty one we see today.

The science goals are similar and complementary to those of MAVEN and MOM, two Mars probes launched last year by NASA and the Indian space agency ISRO.

Link: New Scientist article.

From a news report on Phys.org, April 20, 2015:

The Japan Aerospace Exploration Agency (JAXA) has unveiled the plan for a Moon lander. If successful, Japan will be the fourth country to send an unmanned probe to the moon after Russia, the United States, and China.

JAXA plans to launch the mission as early as 2018, with a development cost estimated at up to 15 billion yen ($126 million). The probe, named SLIM (Smart Lander for Investigating Moon), will be carried by the nation’s solid-fuel “Epsilon” rocket.

Link: Phys.org article.

NASA’s MESSENGER orbiter of Mercury ran out of fuel and crashed into Mercury on May 1, 2015, ending a very successful mission. The craft slammed into Mercury’s surface at about 8,750 mph and created a new crater on the planet’s surface.

MESSENGER’s demise went unobserved because the probe hit the side of the planet facing away from Earth, so ground-based telescopes were not able to capture the moment of impact. Space-based telescopes also were unable to view the impact, as Mercury’s proximity to the Sun would damage their optics.

MESSENGER had been in orbit more than four years and completed 4105 orbits around Mercury. Among its many accomplishments, the MESSENGER mission determined Mercury’s surface composition, revealed its geological history, discovered its internal magnetic field is offset from the planet’s center, and verified its polar deposits are dominantly water ice.

The movie below shows a NASA simulation of the spacecraft’s epic voyage.

Links: MESSENGER home, Sky & Telescope’s report, NY Times article, high-resolution image of the crash-site, map of gravity anomoalies measured by deviations of MESSENGER from its predicted orbit.

From an NRAO press release, March 9, 2015:

From earthbound optical telescopes, the surface of Venus is shrouded beneath thick clouds made mostly of carbon dioxide. To penetrate this veil, probes like NASA’s Magellan spacecraft use radar to reveal remarkable features of this planet, like mountains, craters, and volcanoes.

Credit: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo

Credit: B. Campbell, Smithsonian, et al., NRAO/AUI/NSF, Arecibo

Recently, by combining the highly sensitive receiving capabilities of the National Science Foundation’s (NSF) Green Bank Telescope (GBT) and the powerful radar transmitter at the NSF’s Arecibo Observatory, astronomers were able to make remarkably detailed images of the surface of this planet without ever leaving Earth. The radar signals from Arecibo passed through both our planet’s atmosphere and the atmosphere of Venus, where they hit the surface and bounced back to be received by the GBT in a process known as bistatic radar.

This capability is essential to study not only the surface as it appears now, but also to monitor it for changes. By comparing images taken at different periods in time, scientists hope to eventually detect signs of active volcanism or other dynamic geologic processes that could reveal clues to Venus’s geologic history and subsurface conditions.

Links: NRAO press release.