Mars at Opposition

Mars will be at opposition on May 22nd, 2016.

From a previous post, we know that opposition occurs when the planet and the Sun are 180° away from each other in the sky. What this means is that when the Sun is setting, the planet will be rising. In the next couple of days, if you look to the east at sunset, Mars will be rising above the horizon. A great time to view Mars would be at midnight when it is near the apex of its path across the sky. Mars will be in Scorpius (the constellation opposite the Sun's location in Taurus) as seen here.

Because Mars is at opposition, it is the closest to us, but is still about 0.4 AU (60 million kilometers or 37.2 million miles) away from us.

Amor Asteroids

A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt. The brown band covering area between the Earth and just outside of Jupiter's orbit are the location of Amor asteroids.

Amor asteroids are near-Earth asteroids with perihelions outside of Earth's orbit, i.e. they never cross the orbit of Earth). However, they can cross the orbit of Mars (and in some cases, Jupiter), so it is believed that Phobos and Deimos may have been Amor asteroids captured by Mars. These class of near-Earth asteroids are named after the first asteroid defined to be an Amor asteroid, 1221 Amor.

Amor asteroids are defined by three things:

1. It must have an orbital period of greater than one year. Since Kepler's third law of planetary motion says that the square of the period of the orbit in years must equal the cube of the semi-major axis of the orbit in AUs, the semi-major axis must be greater than one AU.
2. To be a near-Earth asteroid, recall that the asteroid must come within 0.3 AUs of Earth's orbit. This the is the closest Venus and Earth can theoretically get.
3. To be an Amor asteroid, it cannot come closer to Earth than Earth's aphelion because it cannot cross any part of Earth's orbit. Earth's aphelion is 1.017 AU.

In reality, the third definition trumps the first definition since obviously, 1.017 AU is greater than 1.0 AU. By these definitions, for an Amor asteroid, the semi-major axis must be greater than 1.017 AU  and the perihelion of the asteroid must be between 1.017 AU and 1.3 AU. There are 3729 known asteroids that fall into this category, 580 of which are numbered, and 75 with proper names. The most-well known Amor asteroid is 433 Eros which is the first asteroid to be orbited and landed on. The spacecraft NEAR Shoemaker visited and flew by twice before landing in 2001.

433 Eros rendering from NEAR Shoemaker visit

via Astronomy Picture of the Day



 

Amor asteroids can be further subdivided into four subgroups:

• Amor I are Amor asteroids with semi-major axes between Earth and Mars, i.e. 1.017 AU
• Amor II have semi-major axes between 1.523 AU and 2.12 AU. The average semi-major axis for main asteroid belt asteroids is 2.12 AU. They have perihelions of less than 1.523 AU as all Amor asteroids must have. Again, they may impact Mars.
• Amor III have semi-major axes between 2.12 AU and 3.57 AU. They may be considered main belt asteroids, but have high enough eccentricities to bring them within 0.3 AU of Earth. Some may have high enough eccentricities to bring them close to Jupiter (5.2 AU). Amor asteroid 5370 Taranis is actually a Jupiter crosser.
• Amor IV have the most extreme eccentricities since their semi-major axes are greater than 3.57 AU but still have perihelions between 1.017 AU and 1.30 AU. Their aphelions are greater than 5.2 AU (the size of Jupiter's orbit) and some may even have aphelions nearing Saturn (9.6 AU). All these asteroid not only cross the orbit of Mars, but also cross the orbit of Jupiter. So far, no Saturn crossers in this subgroup have been discovered. Only two Amor IV asteroids have been discovered: 3552 Don Quixote and (85490) 1997 SE5.

Again, these are asteroids that we do not have to worry about as they do not come closer than 0.017 AU of Earth. But they will be concern for any future crewed missions to Mars and beyond.

Note: The Moon is 384,400 km or 0.00257 AU, so there is no danger of these asteroids impacting the Moon, either.

Asteroids

Asteroids are believed to be remnants from the formation of the Solar System. The majority of these objects are found in the space between Mars and Jupiter called the asteroid belt.



As seen in the above image, there are also a few asteroids that are 60° ahead of Jupiter called "Greek" asteroids and some that are 60° behind Jupiter called "Trojan" asteroids. Not seen, but will be discussed are the three-A asteroid groups: Amor, Apollo, and Aten. These are near-Earth asteroids that are a concern for scientists and are searched for and catalogued by astronomers.

In our previous post about Phobos and Deimos, we learned a little about C-type and D-type asteroids. There are two other main types (and no, they are not A-type and B-type) called S-type and M-type. As mentioned before, C-type asteroids (and to an extent, D-type) have strong carbon lines in their spectra. S-type asteroid are stony asteroids, made of silicates, with densities similar to Earth's density, and make up about 17% of all asteroids. M-type asteroids are metallic, with the most common metal found in them iron and some having nickel. This is one reason why mining asteroids might be a lucrative business in the future, when travelling easily from Earth to the outer reaches of the Solar System will be achieved.

253 Mathilde - C-type Asteroid

NASA

15 Eunomia - S-type Asteroid

Astronomical Institute of the Charles University

16 Psyche - M-type Asteroid

Astronomical Institute of the Charles University

 



 Asteroids, generally, come in many sizes, with the largest being 1 Ceres at 950 km in diameter. However, most asteroids are only a few miles in size and are irregularly shaped. It is typically believed that asteroids are planetessimals (baby planets) that were not able to accrete into a planet because of the presence of Jupiter. Measurements of the mass of the asteroid belt show that there is only enough mass there to create a planet with the quarter of the size of a planet.

Asteroids have two parts to their names, a number designation and a proper name. The first few asteroids were just given names, but as more and more were discovered, astronomers started placing a number in front to give an indication of the sequence of discovery. So 1 Ceres was the first asteroid discoved and 253 Mathilde was the 253rd. As of 2013, there are several hundred thousand asteroids named with more than a million probably out there. Many of those asteroids are in the asteroid belt, but as mentioned above, there are thousands leading and trailing Jupiter in its orbit and thousands that are near Earth's orbit. We will learn more about Amor, Aten, and Apollo asteroids in a future post.

Most asteroids orbit independently around the Sun, but there are some that mutually orbit each other.  The most well known pair is Ida and Dactyl, where Dactyl was the first satellite discovered around an asteroid.  The Solar System is a strange place.

Martian Canals?

Hubble Image of Mars (Mercator Projection)

 

By the mid 19th century, telescopes had improved enough that surface features on Mars could be easily observed and examined. In 1877, Italian astronomer Giovanni Schiaparelli noted what he thought were long straight lines on the surface which he named after rivers on Earth and identified them on maps he drew of Mars as "canali" which is Italian for channels or grooves. English speakers misinterpreted canali as canals and thought that the channels on Mars were artificial, created by Martians (which is the correct term for someone from Mars - if there were such a creature).

Schiaparelli Map of Martian "canals"

Percival Lowell Map of Martian "canals"

The Martian canals led rise to a plethora of science fiction which believed that Mars was inhabited by intelligent creatures including "War of the Worlds" by H.G. Wells and "The Martian Chronicles" by Edgar Rice Burroughs (who also created Tarzan the Apeman). 

As telescopic resolution improved with new technology, it became clear that the channels on Mars were not artificial, but were created naturally in the ancient past by running water. In a previous post, we learned that water once flowed freely on the surface, but as the atmosphere was stripped away, allowing the planet to cool and the air pressure to drop, surface water no longer exists on Mars.

The Satellites of Mars

Mars has two satellites, Phobos and Deimos. They are named after creatures summoned by Ares (the Greek equivalent of the Roman war god Mars) in the Iliad and their names mean Fear (Phobos - where the word Phobia comes from) and Fright (Deimos). Unlike the Moon, these satellites did not form in the same location in the solar nebula as Mars but rather formed elsewhere (likely, the asteroid belt between Mars and Jupiter) and wandered too close to Mars and were captured by its gravity.

AreaVoices.com

 

Phobos is an 11-km diameter, irregularly shaped object that only orbits 6000 km from the surface of Mars. Compare that to the Moon, which orbits 384,400 km from the surface of the Earth. If the Moon orbited only 6000 km from the Earth, not only would our tides be much higher (see post on tides) but the Moon would appear 64 times bigger in the sky making it about 32° across. At that apparent diameter, it would fill up a sixth of the sky!  Because Phobos is so close to Mars, it only takes about seven and a half hours to orbit Mars, which means that it crosses the Martian sky twice in one Martian day, taking only about four and a quarter hours to cross the sky.  It also orbits retrograde around Mars, meaning that it rises in the west and sets in the east.

 

Deimos is smaller than Phobos, being only 6.2 km in diameter, but orbits much farther away, at 23,500 km from the Martian surface.  At that distance, Deimos takes about 30.3 hours to complete one orbit around Mars, or about a Martian day and a quarter.

 

Both Phobos and Deimos were discovered in August of 1877 by Asaph Hall at the United States Naval Observatory in Washington, D.C. (Quick fact - the USNO is home to the official Master Clock for the US and is also the official residence of the Vice President.)  Despite being smaller, Deimos was actually discovered first on August 12th and Phobos was discovered on August 18th.  The names were suggested by Henry Madan from the Iliad.

 

The reason why Phobos and Deimos are believed to be captured asteroids is because they are similar in composition, albedo, densities of C- or D-type asteroids.

• C-type asteroids (carbonaceous asteroids)
• The most common type of asteroids (make up about 75% of all asteroids
• They have a low albedo which means they do not reflect a lot of light, almost appearing black
• Their compositions are similar to the early solar nebula except for the lack of volatile elements (gases, water, etc) but do contain hydrated minerals (water-containing minerals)
• D-type asteroids
• They have a lower albedo than C-type asteroids
• Their spectra are the strongest toward the red end of the electromagnetic spectrum
• They contain organic, carbon, and anhydrous (lacking-water) silicates
• However, they may have water ice cores

 

The Atmosphere of Mars

Mars has a very tenuous atmosphere.  As mentioned before, at the surface, the pressure is only 0.636 kPa, 200 times less than that of Earth.  Because of the low pressure, any liquid water on the surface of water would be immediately vaporized.  Mars may at one time had a more substantial atmosphere, but over time, the gas was stripped by the solar wind because of the lack of a magnetosphere.  Other gases escaped because their average speed is faster than the escape velocity required to leave Mars.  We do know that Mars had a thicker atmosphere in the distant past because of evidence of flowing water (see previous post).

The atmosphere of Mars is mostly carbon dioxide, much like Venus.  However, that is where the similarities end. Venus' atmospheric pressure is 90 times that of Earth.  Because Mars is so much thinner, it did not undergo a runaway greenhouse effect.  Besides carbon dioxide, Mars' atmosphere also contains argon, nitrogren, oxygen, carbon monoxide, and trace water vapor.

Mars would have difficult time maintaining a thick atmosphere because of the lack of a magnetosphere. 

The magnetosphere prevents solar wind from reaching the outer layers of the atmosphere and stripping the gas in Earth's atmosphere.  The magnetosphere is created by the rotation of the molten core in Earth's interior.  Our molten core is made of iron and nickel which are ferromagnetic materials, i.e. they can become magnetic.  As the core rotates, it turns the Earth into a giant magnet which pushes the solar wind away from Earth.  Mars' core does not rotate, so it does not have a magnetosphere.  Not only does the magnetosphere keep Earth safe, but it also creates the beautiful aurora we see near the poles of Earth.

If we wanted to terraform Mars, not only would we have to find a way to thicker the atmosphere, we would also have to find someway to get the core to rotate again or some other way of creating a giant magentosphere around Mars to save the atmosphere from the solar wind.

The Geography of Mars

Technically, the correct term for the title of this post is Areology, from Ares (the Greek god of war) and -logy (the study of something).
Mars has mountains much like Earth.  However, unlike many of the mountain ranges on Earth, Martian mountains were not created by plate techtonics.  Olympus Mons, Ascraeus Mons, Arsia Mons, and Elysium Mons are all taller than Mount Everest. Pavonis Mons is slightly shorter, but has a much wider base.

All five of those mountains are shield volcanoes, which erupt but have low viscosity lava. The lava flows down the sides of the volcano, which lead to wide bases and low profiles.  For the Martian volcanoes, lack of plate tectonics lead to all the shield volcanoes on Mars to get really wide and really tall.

Olympus Mons is the largest of these volcanoes.  It is 21.4 km tall (from the peak to average surface elevation around its locality)and as shown in the picture below, the base has an area equivalent to the size of Arizona.  This means that the slope of Olympus Mons is only 5 degrees from base to peak.  Olympus Mons is not the tallest mountain in the solar system. That honor belongs to Rheasilvia on the asteroid Vesta. It has a height from base to peak of 22 km, though only 12 km are above the average surface elevation of Vesta.  Mauna Kea and Mauna Loa are two shield volcanoes in the Hawaiian island chain. They have peaks only 4.2 km above sea level, but from base to peak they are 10.2 km.


Olympus Mons

Tharsis Montes

Elysium Mons

Mars is also home to one of the most extensive canyon systems in the solar system.  Valles Mariners is a huge scar running across the landscape in the southern hemisphere.  If we were able to place it on Earth, it would run from New York to Los Angeles.  It is approximately 4000 km long send has a maximum depth of 7 km.  Compare this to the Grand Canyon in Arizona, which is "only" 446 km long and 1.8 km deep.  The Grand Canyon was shaped by the Colorado River, but it is unknown what formed the Valleys Marineris.  It is believed that liquid water or volcanism formed it, but it could be a combination of both.

Valles Mariners



Lastly, Mars may have at one time liquid oceans. It does not anymore because of the atmospheric pressure (future post) and the low temperature. Images from Martian probes show evidence of ancient shorelines.

Astronomy Picture of the Day 

Mars Global Surveyor

 



Mars

Space.com



 

Mars is the fourth planet from the Sun, the third largest terrestrial planet, and the seventh largest planet in the Solar System (only Mercury is smaller). 

• Distance from the Sun: 1.5 AU
• Solar Day equivalent to 24.5 Earth hours
• Tropical Year (how long it takes to complete one orbit around the Sun: 687 Earth days 
• Inclination of 5.65° from the Solar equator and 1.85° from the ecliptic
• Martian density is 3.9 g/cm³ or about 3.9 times that of water
• Mass is 10.7% of Earth and Radius is 53.2% of Earth
• Gravity on Mars is 0.376 times that of Earth (a 100-lb person would weigh 37.6 lbs on Mars)

Mars has a geography that you would find on Earth, but to a larger scale.  Mars boasts the largest mountain in the solar system, the longest and deepest canyon, and strange polar caps.  Mars is also well known for its reddish color, which you can see above, but is also apparent when seeing it in the sky.  Another strange feature is that Mars is criss-crossed by a multitude of channels, has high spots and low spots, and has features that on Earth, were created by flowing water.

 

Mars has two satellites that did not form in the same location as Mars.  Phobos and Deimos were probably captured by Mars and will be discussed in a later post.

 

Mars has an atmosphere, which can be seen as the bluish ring in the above photo of Mars. The major component of its atmosphere is carbon dioxide, like Venus, but that is where the similarities end.  It has an atmospheric pressure at the surface of 0.636 kPa, compared to Earth's surface pressure of 101.3 kPa and Venus' surface pressure of 9.2 MPa (9,200 kPa).  Despite the low pressure, the surface of Mars can reach 35°C (95°F) in direct sunlight, but drops to as low as -143­­­°C (-225.4°F) at night.  Unlike Earth and Venus, it does not retain heat as well because of its thin atmosphere.  The average surface temperature of Mars is -63°C (-81.4°F) so liquid water does not really exist on the surface.