Science 3210 001 : Introduction to Astronomy

Lecture 4 : Mars, Earth, and The Outer Planets

Robert Fisher

Items

Solution set 3 has been posted to the website.

Midterm 1 is next week. Everything through today’s lecture on the inner solar system (but not the outer

solar system) will be on the exam. Exam will be multiple-choice and true/false questions. The exam will be one hour long. After the exam, we’ll take a break and return to the outer planets.

No homework will be due next week.

First observational project will be distributed next week, following the exam.

Late Homeworks

Sample Midterm Question

Which of the following inner solar system bodies is most similar to Mercury in terms of surface properties? A) Venus B) Earth C) Moon D) Mars

Sample Midterm Question

Which of the following statements is false? A) At the location of Chicago, the sun is never visible at the zenith. B) At the location of the North Pole, all visible stars are circumpolar

stars. C) At the location of the Equator, all stars are visible at some point in

the year. D) At the location of the South Pole, the celestial equator is at the

Meridian.

Review Week 3

Kepler’s Three Laws

Newton’s Three Laws

Spectra -- Continuum, Absorption, Emission

Review Week 4

Solar System Overview Sun Planets Moons/Rings Dwarf Planets, Asteroids, Kuiper Belt Objects, Comets, Meteors

Mercury

Venus

Today’s Material

Mars, Earth

The Outer Solar System

Mars

Mars

The “red planet” Mars is the current focus of NASA’s unmanned interplanetary missions, because it is believed to have once harbored a warm, moist Earth-like phase -- possibly even life.

There are several similarities between Earth and Mars.

Mars orbits the sun at 1.5 AU.

Its axis is tilted at 25 degrees.

Its day is nearly identical to one Earth day.

A Visual Comparison of Earth and Mars

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

A Flyover Overview of Mars

QuickTime™ and aYUV420 codec decompressor

are needed to see this picture.

Mars vs. Earth

Mars is much smaller than the Earth, with a radius about half that of Earth, and a mass of about a tenth the Earth’s.

The surface temperature today is far below the freezing point of water.

Even if one could warm water ice on Mars today, it would go directly into a gaseous state without becoming liquid because of the thin atmosphere.

It has two tiny moons, Phobos and Deimos, with properties radically different than Earth’s moon.

While tilt is similar to that of Earth today, the tilt angle oscillates wildly over tens and hundreds of millions of years.

It has only a weak magnetic field in its crust, and lacks a magnetic core.

Phobos and Deimos : The Moons of Mars

Mars has two tiny moons located very near the planet’s surface -- the closest moons to any planet in the solar system.

It is thought they were asteroids intersecting the Martian orbit captured via drag through an early, thicker Martian atmosphere.

Phobos Deimos

From The Observatory, 1877

Asaph Hall, Discoverer of Phobos and Deimos

Phobos and Deimos

An orbiting body at one specific radius has an orbital period equal to the rotational period of the planet -- a geosynchronous orbit. From the planet, the body would appear to be stationary.

Phobos

Phobos orbits well inside Martian Geosynchronous orbit, and so

appears to rise in the west and set in the east

Deimos

Deimos orbits outside of Martian geosynchronous orbit, and remains visible for two nights in a row.

Olympus Mons, The Largest Volcano in the Solar System

Olympus Mons is roughly three times the height of Mount Everest, but is much broader, with shallower sides.

Digitally-Reconstructed Flythrough of Valles Marineris from Mars Odyssey

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

Crevasses on Martian Polar Icecap, Revealed by Martian Global Surveyor

What Can Cause These Variations in the Martian Climate?

The leading explanation for the stratification in the Martian polar cap is the variation in the Martian rotational and orbital properties.

All planets are “perturbed” in the orbits about the sun by gravitational influences from the other planets, particularly Jupiter

Mars is particularly susceptible to these perturbations because It is the closest planet to Jupiter

It lacks a large moon (like Earth) to “dampen” out the effects

Variation in Martian Obliquity and Orbital Eccentricity

Researchers have found that both the angle of inclination of the Martian rotation (obliquity) and the eccentricity of its orbit vary wildly over a timescale of millions of years -- leading to alternating epochs of warm and cold climate

Today

Earth Mars

Variation in Martian Obliquity and Orbital Eccentricity

Researchers have modeled the effects of the variation of Martian obliquity and orbital eccentricity, and found that they in fact do naturally lead to variations in the amount of power the Martian surface receives, on the timescale of millions of years

Water on Mars

Multiple lines of evidence compiled over many years strongly suggest that Mars had abundant liquid water on the surface in the distant past, and may even have frozen water just beneath the surface today.

One line of evidence comes from images of the surface -- suggestions that the morphology, or shapes, suggests the presence of water.

Another line of evidence comes from direct surface measurements made by the Rovers sent to the surface.

A third line of evidence comes from imaging instruments on orbiters which detect hydrogen -- a key component of water.

Riverbeds on Mars

Many regions on Mars show what appear to be signs of meandering, dry riverbeds.

Evidence for Flooding on Mars

Evidence for massive erosion from floods can be seen on the surface of Mars today, for instance in the Ares Vallis.

Catastrophic Floods on Earth

Similar catastrophic floods have occurred on the Earth as well, for instance in the Washington State Scablands. These were believed to have been formed from massive floodwaters a thousand feet (!!) deep.

Seepage Channels

Various craters and valleys on Mars show signs of runoff in the recent past.

Newton Crater

Seepage Channels

While liquid water cannot exist on the surface of Mars today, it is possible that these runoff regions develop only after subsurface liquid water has burst through a “dam” of frozen surface water.

This water would be boiling away violently, and so these events must develop suddenly and disappear rapidly.

Similar behavior occurs in ice flows in Antarctica on the Earth.

Rock/Liquid Water Rock/Ice Side View

Where Did All That Water Go?

Very good evidence exists that a LOT of liquid water once ran on the surface of Mars in the past. Where did all of that water go?

Because the atmospheric pressure is so low on Mars today, any water on the surface of Mars today will evaporate in the first global warming cycle

Some water may be buried in layers of CO2 ice at the poles of Mars

However, the leading explanation has been that the water has become frozen beneath the surface of Mars.

Permafrost on Earth

The situation on Mars is analogous to permafrost on Earth, where regions (mostly inside the arctic circles) have water frozen in the surface year-round.

Mars Odyssey Neutron Maps

In 2002, Mars Odyssey imaged Mars in neutrons, scanning for hydrogen-rich material just beneath the surface.

Odyssey’s Hydrogen Map of Mars

Odyssey found bands of hydrogen-rich material around both the north and south poles of Mars -- possibly due to frozen water.

Surface Water on Mars

In the very distant past -- billions of years ago -- Mars appears to have had abundant surface liquid water. It is possible that the lowest-lying areas on the surface, particularly in the Northern hemisphere, were submerged in a giant ocean.

Mars’ climate eventually became unsuited to liquid water at the surface, and most of it was probably lost over time to atmospheric evaporation.

The remaining water became frozen into the surface in a kind of permafrost, similar to that on arctic regions on Earth.

Life on Mars?

Because there is excellent evidence suggesting that large amounts of surface water existed in the past on Mars, it is natural to think that life may have existed on Mars as well.

One of the biggest questions that one can ask today is whether life existed on Mars in the past, and may possibly even exist today.

The pioneering Viking 2 lander, launched by NASA in the 1970s, tested directly for the existence of life on the surface of Mars.

Question

How would you construct a test for life on another planet?

Viking 2 Lander Model

Viking 2 Tests for Life

Canals on Mars??

In 1877, Italian astronomer Giovanni Schiaparelli described features he saw on Mars as “canali,” which is probably best translated as “channels”.

This phrase became mistranslated as “canals,” which suggested to some astronomers that the features seen were artificially-created.

Later space missions have uncovered tons of evidence for water on Mars, including channels like those Schiaparelli claimed to have seen. They are, however, far too small to be visible from Earth, even with the largest telescopes available.

Schiaparelli’s Drawing of Mars

Although primitive photographic plates existed at that time, Schiaparelli recorded his observations in drawings, which he believed to be more accurate than photographic plates.

Canals on Mars??

Other astronomers (most noteably Percival Lowell) became fascinated with the concept, and astronomical research of Mars has flourished since.

Despite the body of work, it is likely that Schiaparelli’s canals were a physiological fluke, though the topic is still debated today.

These “canals” gave rise to the wealth of Martian science fiction -- from Edgar Rice Burroughs to H.G. Wells to Ray Bradbury, and many, many more.

Early Martian Astronomy…

The Earth

“We shall not cease from exploration, and at the end of all our exploring will be to arrive where we started and know the place for the first time.”

-- T.S. Eliot, Little Gidding

Earth

Earth is superficially similar in many respects to both Venus and Mars, in terms of its composition, size, and so on.

The primary features which distinguish Earth is

Existence of a major moon.

Relatively strong magnetic field.

Abundant surface water.

Life.

Greenhouse Effect Demo

Coriolis Demo (2/2)

Circulation Demo

Greenhouse Effect

Molecules in the atmosphere are transparent to visible light, but absorb radiation in the infrared.

Fundamentally, this is because individual atoms in molecules can rotate and vibrate at energies much lower than electrons in atoms.

These lower-energy transitions typically occur in the infrared portion of the spectrum.

Diatomic Molecule

Coriolis Demo (1/2)

Long-Term Stability of Earth’s Climate

Physicists and other scientists will often characterize a system by its equilibrium and stability.

For instance, a ball may be instantaneously in balance -- in equilibrium -- but that balance may be either stable or unstable.

Unstable Stable

Earth’s Climate

Up until now, Earth’s climate has been stable…

Earth’s Climate

The trend of global warming has been pronounced in the last one hundred and fifty years.

Carbon Dioxide Emissions

The trend of heating is correlated with increased greenhouse gas emissions.

Astronomical Context

We have seen that Earth is unique in the solar system.

Runaway greenhouse heating resulted in the hellish world of Venus.

Insufficient greenhouse heating resulted in the frigid world of Mars.

Has mankind upset the delicate climate equilibrium on Earth?

Outer Planets

Gravitational Slingshot Effect

If a spacecraft were to encounter a stationary body, its final speed must equal its initial speed.

Gravitational Slingshot Effect

The same encounter, viewed from the reference frame of the sun, looks very similar, except that the spacecraft has acquired the motion of the planet. It has gotten an “assist” from the planet.

Planetary Grand Tour

An rare alignment of the planets (occuring once every 200 years) permitted the Voyager 1 and 2 space missions a “Grand Tour” of the solar system using the gravitational slingshot effect.

Gas Giant Planet Interiors

Each of the gas giants is thought to have a rocky core interior about the size of the Earth or greater, overlaid by thick gaseous atmospheres.

Next Week -- Midterm!

Next week we will continue our discussion of Mars and the Earth, comparing the geology of the two planets.

We will discuss the consequences of enriching our atmosphere with carbon dioxide -- is Earth destined to become like Venus?

We will also begin to cover the Outer Planets.

First midterm next week!