New Horizon’s arrival at Pluto this year, after a nine year-long traverse through the Solar System has revealed many new things about this little icy planet. Pluto used to be the ninth planet but was demoted in 2006 to the status of dwarf planet. Until the New Horizons mission started sending back great images of Pluto in July 2015, the other earlier images captured by the Hubble Space Telescope were low resolution and pixelated. Since receiving these new images, we now know more about the surface of Pluto. Instead of an old, dead, scarred surface, Pluto has smooth plains, suggesting it must be geologically active. So while we wait for more images from Pluto, let’s look at some of the planetary surfaces across the Solar System.
Mercury on first glance looks a little bit like our Moon. It has no atmosphere to protect it; therefore it has been bombarded by asteroid impacts. It is also incredibly difficult to observe as this planet is very close to the Sun, ranging from 29 million miles (47 million km) to 45 million miles (72 million km) as it completes its orbit in just 88 days. Mercury is the most heavily cratered planet in our Solar System. It not only has many craters but also features called basins. A basin is a crater which exceeds 300km in diameter. The largest impact basin is the Caloris Basin which is 1550 km (960miles) wide.
Only two spacecraft have visited Mercury, Mariner 10 in 1973 and the MESSENGER spacecraft in 2008. The Mariner 10 managed to map around 40% of the planet and MESSENGER was able to provide much higher resolution images. In 2017, the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) spacecraft BepiColombo will begin its journey towards closest planet to the Sun. However, using the Earth and Venus for gravity assist manoeuvers will mean that Bepi Columbo will not be in orbit around Mercury until 2024. This planned four year mission will seek to find out if Mercury is still geologically active, what its core might be made out of as well as any differences in the planet’s features compared to images provided by Mariner.
Venus is similar in size to our Earth; however its surface is constantly hidden by a thick atmosphere of mostly carbon dioxide (CO2) and thick clouds of sulphuric acid. Temperatures on Venus reach 500 degrees C and the atmospheric pressure is over 90 times that of the Earth’s. Before mapping of Venus was conducted, it was believed that Venus was a tropical rainforest planet because of its thick clouds. Mariner 2 conducted a flyby over Venus in 1962 and deduced that Venus was incredibly hot and unlikely to harbour life. The NASA Magellan mission launched in 1989, used radar to map the 98% of the surface of Venus. More recently ESA’s Venus Express in 2006 confirmed that Venus was once humid but water has been lost but it showed signs that perhaps there is still some volcanic activity at this planet.
Because of the harsh conditions on Venus, no spacecraft has survived much longer than one hour on the surface. In 1975, Soviet probes Venera 9 and 10 deployed landers which provided some of the first images of the surface of Venus. However both craft suffered the same fault, when the lens cap failed to come off limiting images to 180o rather than the planned full 360o panoramic. The images revealed a slope with relatively little sand and some large flat rocks. Venera 10 determined the rock density was similar to basaltic rocks here on Earth.
Mars is the planet that we have traversed more so than any other. From the first successful flyby by Mariner 4 in 1964 to the 7 current missions that are still operational on Mars, we have a greater idea of what Mars is really like, enough to inspire planned manned missions the Red Planet potentially within the next decade. A day on Mars last just over 24 hours and it has seasons just like the Earth. In the late 19th and earth 20th Centuries, ideas that channels of water or canals flowed on Mars was made popular by Italian Astronomer Giovanni Schiaparelli. Better observations in subsequent years disprove this theory but the successful landing of rovers on the surface has helped determine that water once flowed but is now frozen under the surface or at the ice-caps.
The Curiosity rover successfully landed on Mars in August 2012. It has discovered so far that Mars once had fresh water at its surface and has imaged rocks that have been eroded by flowing water. Red dust blown around in dust storms has created ripples on the surface sand and more interestingly Curiosity has discovered traces of methane in the atmosphere and signs that there are organic materials present. This doesn’t mean that there is life or even was ever life on Mars but it does provide us with a better idea about the current conditions on Mars, if perhaps humans do may the voyage to the Red Planet, however this will still have to content with a carbon dioxide atmosphere, extreme cold and UV radiation.
Jupiter, Saturn, Uranus and Neptune are collectively known as the gas giant planets. Although they are likely to have a small rocky core, they are primarily made of gas and have no solid ground to land on. Jupiter would only have needed to be 80 times as massive to be considered a star. Saturn is the second largest planet, Uranus and Neptune while larger than the rocky planets are smaller than the other Gas Giants. All have an atmosphere of mostly hydrogen and helium, and on Jupiter and Saturn the surface is mostly covered by thick clouds. On Jupiter the red/brown clouds cover oceans of liquid metallic hydrogen and on Saturn the gases have been under some much pressure that beneath the clouds the hydrogen and helium have been turned to liquid form. Uranus and Neptune have larger traces of methane ice causing their blue colour, causing them to be sometimes called the ice-giants.
The first mission to study all of the gas giants was that of the Voyager spacecraft. Launched in 1977, two identical craft Voyager 1 and 2 headed off for missions to the gas giants. The Voyager mission discovered a ring system around Jupiter and that the range in size of the particles that causes Saturn’s rings varied from dust to house sized. As well as 10 new moons at Uranus and detected the fastest wind speeds at Neptune. So despite not having solid ground to land on, the Gas Giants have quite distinctive properties.
As we patiently wait for more images to be released of Pluto and its moons from the New Horizons mission, we are left wondering how interesting and different the planets in our Solar System are and how much more there is still to discover.
(Article by Martina Glass, Senior Education Support Officer)