Science 3210 001 : Introduction to Astronomy

Lecture 5 : Mars, Earth, and The Outer Planets

Robert Fisher

Items

! Tutor Akiva Bhansali has hours Monday 4 - 6 PM, Tuesday 4 - 8 PM onthe 12th floor lounge (possibly 14th floor if 12th floor is crowded.)

! Solution sets 1-3 has been posted to the website.

! Problem set number 4 has been posted to the website.

! Midterm 1 is in two weeks.

! Everything through next week’s lecture 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 lecture.

! No homework will be due in two weeks on the day of the first midterm.

! First observational project will be distributed in two weeks, following theexam.

! 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

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 explanation is still

sometimes 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 -- which in turn inspired real planetary

scientists like Carl Sagan and many others.

Early Martian Astronomy…

A Flyover Overview of Mars

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

Asaph Hall, Discoverer of Phobos and Deimos

From The Observatory, 1877

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.

Mars Odyssey - How to Get to Mars

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

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 Lander on Surface of Mars

Viking 2 Tests for Life

! The Viking 2 mission to Mars contained a highly-sophisticated

scientific package with four experiments designed to detect the

presence of life in the Martian surface soil.

Where’s the Carbon?

! One experiment, the gas chromatograph - mass spectrometer -

took a scoop of Martian soil, vaporized the soil and analyzed the

composition of the resulting chemicals.

! The idea was to test for the presence of carbon-bearing

compounds, which are the hallmark of organic chemistry and life

on Earth.

! The result was a stunning negative - there is even less organic

material on Mars than in the lunar regolith.

The Labeled Release Experiment

! Perhaps the most clever of the Viking experiments was the

labeled release experiment, which put a drop of liquid nutrient

bearing seven organic molecules metabolized by microorganisms

on Earth.

! The trick was to tag each of the carbon molecules using a very

rare isotope of radioactive carbon 14.

! Amazingly, this test produced a measurable response.

How to Reconcile the Viking Measurements?

! How can one reconcile the two Viking measurements?

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 andMars, 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

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!

! We will begin to cover the Outer Planets.

! First midterm next week!