Lecture 5 : Mars, Earth, and The Outer Planets Robert Fisher
Items • Tutor Akiva Bhansali has hours Monday 4 - 6 PM, Tuesday 4 - 8 PM on the 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 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 • 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.
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.
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
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.
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. Side View Rock/Liquid Water Rock/Ice
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 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 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.