PH 205

Mars

How is Mars similar to Earth? How is it different? How did the atmosphere of Mars evolve? What is a reverse greenhouse effect?

Mars is our neighboring planet, lying farther from the Sun, at about 1.5 AU.

This photograph shows the huge canyon system called Vallis Marineris, which stretches about a fifth of the way around the planet. Scientists are not sure how it formed, but the leading hypothesis is that the huge crack formed as the planet cooled.

In the late 1800's and early 1900's, many people believed that an intelligent civilization existed on Mars. An Italian astronomer, Giovanni Schiaparelli, first reported seeing a complex system of canals on the surface of Mars in 1877, and others confirmed the sighting later. Intricate maps were drawn of the canal system and theories developed about the civilization. It was popular belief that while Venus was lush and tropical because of its proximity to the Sun, populated with friendly natives, Mars was inhospitable and the Martians were much more warlike.

Of course, we now know that there is no canal system on Mars. The Mars Express Orbiter is one of the probes that has extensively mapped the surface of Mars. We now know a great deal about Mars and the seasonal changes seen on the planet. Even though the atmosphere is thin, very strong winds arise on the surface, raising dust storms that can cover the planet. There has been much resurfacing due to erosion, so that the surface of Mars does not support many impact craters, like what is seen on Mercury and Earth's moon.

Mars is considerably smaller in size than Earth. The smaller size and greater distance from the Sun means that Mars cooled off faster. We believe that Mars does possess an iron core, but that it has solidified, so that there is no dynamo effect in present-day Mars, resulting in a very weak magnetic field.

The relatively small mass of Mars also means it is not as hard for molecules to reach escape velocity, resulting in a thinner atmosphere.

This time-lapse sequence of photographs taken by NASA's Mars Reconnaissance Orbiter of Newton Crater on Mars from late spring to early fall shows evidence that there is liquid water flowing on the surface of present-day Mars. Scientists speculate that the water must be salty, since pure water would remain frozen at these temperatures. Later measurements by NASA's Mars Reconnaissance Orbiter detected signatures of water in similar locations. The confirmation of liquid water on the surface of Mars has profound implications for the possibility of finding some form of life, present or extinct, and for future manned missions to Mars.

Ancient floodplain deltas like this one, found in Eberswalde Crater, offer evidence that vast amounts of water existed on Mars in the distant past, billions of years ago. It may be possible that conditions allowed life to arise on this wet planet.

What happened to the plentiful water on Mars? As depicted in this NASA video, the water may have been lost due to the cessation of the magnetic dynamo when the core cooled and solidified. The weakened magnetosphere did not protect Mars from the solar wind, which stripped away the atmosphere, and depleted the surface water.

The present-day atmosphere on Mars is mostly made of carbon dioxide, with a small amount of nitrogen and other elements, at a pressure that is about 1/150 that of Earth. The thin atmosphere means that the surface temperature can vary a great deal, from 300 K (80 degrees Fahrenheit) on the hottest days to 200 K (-100 degrees Fahrenheit) at night.

Weather exists on Mars, with strong seasonal winds, and clouds of water ice and CO2 ice. Frozen polar icecaps are seen, mostly made of CO2 (dry ice).

It is believed that the atmosphere of Mars underwent an evolution that was similar to that of Earth in the early stages, with a primary atmosphere and a secondary atmosphere that was thick enough to moderate the surface temperature and support a greenhouse effect. However, the loss of the magnetosphere was a main factor in the evolution of the atmosphere, resulting in what is known as a reverse greenhouse effect.

Olympus Mons is a large, extinct shield volcano on Mars, sixteen miles high and as big as the state of Arizona. Volcanic activity in early Mars helped supply the atmosphere with greenhouse gases. With the cooling of the planet and cessation of volcanic activity as well as a lack of plate tectonics, the greenhouse gases ceased being replenished as they are on Earth.

Atmospheric evolution on Mars

• Primary and secondary atmospheres like Earth
• Most of the atmosphere later disappeared
  • No magnetospere
  • Weak gravity
  • Impacts
• Reverse greenhouse effect
  • Carbon dioxide steadily declined
  • Carbon dioxide absorbed into rocks and water
  • No plate tectonics
• Greenhouse effect diminished
• Temperature lowered
• Water vapor froze out
• Carbon dioxide froze out

Please view this tutorial for more information on atmospheric evolution on Mars and the reverse greenhouse effect.

Exploration of Mars

Rovers have been used to explore the surface of Mars since 2004, when the Opportunity rover landed on the planet. This animation depicts the unusual airbag method used to land the Spirit and Opportunity rovers on the surface.

The Opportunity rover was originally planned to remain active for 90 Earth days, and has continued to function at least 12 years longer than expected. It is still active (as of 12/21/2016) and continues to send data back to Earth. It has survived intense dust storms and rough terrain to explore the surface of Mars. It has provided us with varied views of Martian terrain and features, such as the image below from the rim of Victoria Crater.

This image of the rim of Victoria Crater shows evidence of stratification in the surface rock of Mars. This is a large impact crater, about 730 meters wide. The descent into the crater was a risky undertaking, because of the steepness of the crater walls, and was initially delayed by a six-week dust storm. Once inside the crater, Opportunity created a panoramic view of the interior.

Preparation for the next robotic mission to Mars is underway, scheduled for 2020. The rover is based on the design of the Curiosity rover, currently active on Mars. The mission is focused on more in-depth geological surveys and the search for evidence of life. It also prioritizes the advancement of knowledge regarding natural resources for use by future human visitors to the planet.

The Mars 2020 Perseverance Rover will be searching for signs of past microbial life and habitable conditions on Mars.

Moons of Mars

Mars has two moons, Phobos (left) and Deimos (right). The moons are small and irregularly shaped, compared to Earth's moon. It was thought that they were probably captured asteroids, but the low density of Phobos suggests otherwise. We now think that Phobos could have been created from material ejected from a large impact on Mars.

Phobos is somewhat larger than Deimos, and both are heavily cratered. Phobos is considerably closer to Mars than Deimos. Phobos is less than 3 Mars radii away from the red planet, while the orbital radius of Deimos is a bit less than 7 Mars radii.

Both moons have prograde orbits, but Phobos has an orbital period that is much less than a Martian day. If you were on the surface of Mars looking up at Phobos, it would look like it was moving backward in the sky. Deimos has an orbital period that is longer than a Martian day, so it would appear more normal, moving in the same apparent direction as the sun across the sky.

Phobos and Deimos are both tidally locked to Mars, having the same side always facing the planet. Phobos, with its relatively low orbit, has a bit of drag from the higher gases in the Martian atmosphere. This drag gradually slows down the orbital speed of Phobos. Eventually, as the orbit decays, we expect Phobos will fall inward and impact the surface of Mars.