Planetary Atmospheres & Introduction to Stars

IS Symposium• Just a reminder that all IS students are encouraged to attend and participate

in next week's campus conversation "Complicating Normal--A Campus Conversation on Dis/Ability."  Several programs are featured including:

• LeDerick Horne, April 1, 1:15 p.m. in Riley Auditorium.  A gifted word poet, playwright and disability advocate, LeDerick's talk is titled "Beyond Classification."  An informal reception follows the presentation.

• Panel on Disability Experience, April 1, 7 p.m. in Towers 110• Lennard Davis, April 2, 3:30 p.m. in Roush 114.  Dr. Davis is a professor of

English, professor of disability and human development and professor of medical education at the University of Illinois at Chicago.  He directs Project Biocultures, a think tank devoted to issues found at the intersection of disability, culture, medicine, biotechnology and biosphere.  His talk is "The End of Normal:  Disability, Diversity and Neoliberalism."

Mercury

• Small, bright but hard to see

• About the same size as the moon

• Density about that of Earth

• Day ~ 59 Earth days• Year ~ 88 Earth days

Venus

• Bright, never very far from the sun– “Morning/Evening star”

• Similar to Earth in size and density

• Day ~ 243 Earth days (retrograde!)

• Year ~ 225 Earth days

Venus

• Very thick atmosphere, mostly CO2

• Heavy cloud cover (sulfuric acid!)– About 90 times the pressure

of Earth’s atmosphere– Very strong greenhouse

effect, surface temperature about 750 K

• No magnetic field

Surface Features

• Two large “continents”– Aphrodite Terra and

Ishtar Terra– About 8% of the

surface • Highest peaks on

Aphrodite Terra rise about 14 km above the deepest surface depression– Comparable to Earth’s

mountains

Hothouse Venus: 850 °F

Mars• Fairly bright, generally not too hard to see

• Smaller than Earth• Density similar to that

of the moon• Surface temperature

150–250 K• Day ~ 24.6 hours• Year ~ 2 Earth years• Thin atmosphere,

mostly carbon dioxide– 1/150 the pressure of

Earth’s atmosphere• Tiny magnetic field, no

magnetosphere

Mars

• Northern Hemisphere basically huge volcanic plains– Similar to lunar maria

• Valles Marineris – Martian “Grand Canyon”– 4000 km long, up to 120

km across and 7 km deep– So large that it can be seen

from Earth

Martian Volcanoes• Olympus Mons

– Largest known volcano in the solar system– 700 km across at base– Peak ~25 km high (almost 3 times as tall as Mt. Everest!)

Martian Seasons: Icecaps & Dust Storms

Martian Surface Iron gives the characteristic Mars color: rusty red!

View of Viking 1 1 m rock Sojourner

Water on Mars?

Mars Louisiana

Outflow ChannelsRunoff channels

Life on Mars?

• Giovanni Schiaparelli (1877) – observed “canali” (channels) on Martian surface

• Interpreted by Percival Lowell (and others) as irrigation canals – a sign of intelligent life

• Lowell built a large observatory near Flagstaff, AZ

(Incidentally, this enabled C. Tombaugh to find Pluto in 1930)

• Speculation became more and more fanciful– A desert world with a planet-wide irrigation system to carry

water from the polar ice caps?

– Lots of sci-fi, including H.G. Wells, Bradbury, …

• All an illusion! There are no canals…

Viking Lander Experiments (1976)

• Search for bacteria-like forms of life

• Results inconclusive at best

Atmospheric Histories

• Primary atmosphere: hydrogen, helium, methane, ammonia– Too light to “stick” to a planet unless it’s very

big Jovian Planets

• Secondary atmosphere: water, CO2, SO2, …

– Outgassed from planet interiors, a result of volcanic activity

Atmospheric Histories - Venus

• Venus is closer to Sun than Earth hotter surface

• Not a lot of liquid water on surface initially

• CO2 could not be absorbed by water, rocks because of higher temperatures

run-away Greenhouse effect: it’s hot, the greenhouse gases can’t be be stored away, it gets hotter …

Earth’s Atmospheric History

• Volcanic activity spews out water steam• Temperature range allowed water to liquify• CO2 dissolves in oceans, damping greenhouse effect • More water condenses, more CO2 is absorbed• If too cold, ice forms less cloud cover more

energy• No oxygen at this point, since it would have been

used up producing “rust”• Tertiary atmosphere: early life contributes oxygen

– 1% 800 Myrs ago, 10% 400 Myrs ago

Mars – Freezing over

• Mars once had a denser atmosphere with liquid water on the surface

• As on Earth, CO2 dissolves in liquid water• But: Mars is further away from the Sun temperature drops below freezing point

inverse greenhouse effect • permafrost forms with CO2 locked away• Mars probably lost its atmosphere because its

magnetic field collapsed, because Mars’ molten core cooled down

Stellar Parallax

• Given p in arcseconds (”), use d=1/p to calculate the distance which will be in units “parsecs”

• By definition, d=1pc if p=1”, so convert d to A.U. by using trigonometry

• To calculate p for star with d given in lightyears, use d=1/p but convert ly to pc.

• Remember: 1 degree = 3600”• Note: p is half the angle the star

moves in half a year

Our Stellar Neighborhood

Scale Model

• If the Sun = a golf ball, then– Earth = a grain of sand

– The Earth orbits the Sun at a distance of one meter

– Proxima Centauri lies 270 kilometers (170 miles) away

– Barnard’s Star lies 370 kilometers (230 miles) away

– Less than 100 stars lie within 1000 kilometers (600 miles)

• The Universe is almost empty!

• Hipparcos satellite measured distances to nearly 1 million stars in the range of 330 ly

• almost all of the stars in our Galaxy are more distant

Luminosity and Brightness

• Luminosity L is the total power (energy per unit time) radiated by the star, actual brightness of star, cf. 100 W lightbulb

• Apparent brightness B is how bright it appears from Earth– Determined by the amount of

light per unit area reaching Earth– B L / d2

• Just by looking, we cannot tell if a star is close and dim or far away and bright

Brightness: simplified

• 100 W light bulb will look 9 times dimmer from 3m away than from 1m away.

• A 25W light bulb will look four times dimmer than a 100W light bulb if at the same distance!

• If they appear equally bright, we can conclude that the 100W lightbulb is twice as far away!

Same with stars…

• Sirius (white) will look 9 times dimmer from 3 lightyears away than from 1 lightyear away.

• Vega (also white) is as bright as Sirius, but appears to be 9 times dimmer.

• Vega must be three times farther away

• (Sirius 9 ly, Vega 27 ly)

Distance Determination Method

• Understand how bright an object is (L)is (L)• Observe how bright an object appears (B)appears (B)

• Calculate how far the object is away:

B L / d2

So

L/B d2 or d √L/B

Homework: Luminosity and Distance

• Distance and brightness can be used to find the luminosity:

L d2 B

• So luminosity and brightness can be used to find Distance of two stars 1 and 2:

d21 / d2

2 = L1 / L2 (since B1 = B2 )

i.e. d1 = (L1 / L2)1/2 d2

Homework: Example Question

• Two stars -- A and B, of luminosities 0.5 and 2.5 times the luminosity of the Sun, respectively -- are observed to have the same apparent brightness. Which one is more distant?

• Star A

• Star B

• Same distance

Homework: Example Question

• Two stars -- A and B, of luminosities 0.5 and 2.5 times the luminosity of the Sun, respectively -- are observed to have

the same apparent brightness.   How much farther away is it than the other?   

• LA/d2A = BA =BB = LB/d2

B dB = √LB/LA dA

Star B is √5=2.24 times as far as star A