50 Years of Mars Observations 8 May 2014, by Douglas Smith

50 years of Mars observations 8 May 2014, by Douglas Smith wishful thinking. The notion of a moist Mars began to evaporate at the turn of the 20th century. In 1909, Lick Observatory dispatched a team of astronomers to climb Mount Whitney—whose summit, at 14,500 feet, rises above some four-fifths of Earth's atmospheric water vapor. Pointing a small telescope at Mars, the team measured no water vapor in excess of that in the rarefied air around them, although observatory director William Wallace Campbell cautioned the Associated Press that their technique, "the only method known, is not a sensitive one." Campbell diplomatically noted that "the question of life under these conditions is the A map of Mars made by the Italian astronomer Giovanni biologist's problem rather than the astronomer's." Schiaparelli in 1888. Objects appear inverted when seen through a telescope, so the South Pole is on top. Bigger telescopes make for more sensitive (Schiaparelli called the linear features he observed measurements, and by the 1920s the world's canali, which is Italian for "channel," and he made no largest telescopes were just north of Pasadena at inferences about their origin; the word was mistranslated the Mount Wilson Observatory. In 1926, into English as "canals.") Credit: Wikimedia Commons observatory director Walter Adams and Charles St. John wrote in the Astrophysical Journal that "the quantity of water-vapor in the atmosphere of Mars, area for area, was 6 per cent of that over Mount (Phys.org) —In 1964, Caltech astronomy professor Wilson . This indicates extreme desert conditions Guido Münch and Jet Propulsion Laboratory space over the greater portion of the Martian hemisphere scientists Lewis Kaplan and Hyron Spinrad pushed toward us at the time." The 60-inch telescope they the world's second-largest telescope to its limits used was second in size and power only to the and dashed—at least for the next few decades—anyadjacent 100-inch Hooker telescope, with which hopes of finding liquid water on Mars. Adams revisited the question in 1937 and 1939 and revised his figures downward. In 1941 he wrote, "If Back in the late 1800s, it was widely assumed that water vapor lines are present . they cannot be Mars was a planet with abundant water, just like more than 5 per cent as strong as in the earth's Earth. Astronomers were mapping Mars's polar atmosphere and are probably very much less." caps, which advanced and retreated as the seasons changed; a dark "wave," apparently of vegetation, which swept from the pole toward the equator every spring; and even ruler-straight lines that might have been canals dug by an alien civilization. Today, we know that the ice caps grow larger because the winters are cold enough to freeze carbon dioxide right out of Mars's thin air; the seasonal darkening is a wind-driven redistribution of the dust that blankets the planet; and the canals were optical illusions enhanced by 1 / 4 lay the problem: the wide, black blots left on the plate by Earth's dense blanket of air made the thin, faint lines from the tenuous atmosphere of Mars hard to see, let alone measure. The best opportunities to find the lines occur at approximately two-year intervals. Earth travels in a tighter orbit around the sun than Mars does, and as we pass Mars on the inside track our close approach maximizes the apparent difference in our velocities. This shifts Mars's spectrum ever so slightly away from Earth's—if you have an instrument powerful enough to discern the separation. Unfortunately, some passes are closer than others. When Earth overtook Mars in 1963, the latter was at the point in its orbit most distant from the sun. Although the two planets were as close to each other as they were going to get that time around, the velocity effect was minimized—imagine looking out the window of a moving train at a distant farmhouse instead of the nearby telephone poles. But the Hooker's spectrograph had recently been upgraded; Kaplan and Spinrad were expert Guido Münch in a 1967 portrait on his election to the spectroscopists; and Münch was a wizard at National Academy of Sciences. Credit: James making very sensitive emulsions, so the trio McClanahan decided to look for the lines anyway. With little prospect for success, the experiment was allotted a set of low-value nights that began more than two months after Earth had passed Mars and started to The "lines" Adams referred to are spectral ones. pull ahead. At its closest approach, Mars had been The spectrum of light contains all the colors of the 62,000,000 miles away; by the time Münch and rainbow, plus wavelengths beyond, that we can't company got their turn at the telescope, that see. Every gas in the atmosphere—both Earth's anddistance had nearly doubled. Their telescope was Mars's—absorbs a specific collection of these colors.no longer the best available, having been overtaken Passing the light from a telescope through a device as the world's largest by the 200-inch Hale called a spectrograph spreads out the rainbow and telescope at Caltech's Palomar Observatory. Even reveals the missing wavelengths, allowing the the weather conspired against them; four nights of gases that absorbed them to be identified. work yielded exactly one usable exposure. In those days, spectra were usually recorded as But as Münch wrote in the January 1964 issue of shades of gray on glass plates coated with a light- the Astrophysical Journal, that "strongly sensitive emulsion—essentially the same technique hypersensitized" plate gave "a spectrogram of photographer Matthew Brady had used to excellent quality which shows faint but document the Civil War. Once the plates were unmistakable lines which have been ascribed to developed, the missing wavelengths showed up as H20 in Mars' atmosphere . After comparing our black lines that were painstakingly analyzed under plate with other ones found in the Mount Wilson a microscope. Each line's location indicated its files, we have convinced ourselves that ours is the wavelength, while its darkness and thickness were spectrogram of Mars with the highest resolving related to the absorber's abundance. And therein power ever taken." 2 / 4 Even so, the lines were barely strong enough to be would drop by as much as 20 percent twice every usable. The preliminary water-vapor calculation, Mars year. These predictions have since been announced in May 1963, had an error factor of 10. confirmed many times over, and form part of our It would take another six months to work out the basic understanding of how Mars works. definitive number—a figure equivalent to 0.01 ± 0.006 per cent of the amount of water vapor over And what of water on present-day Mars, which is Mount Wilson, and 100 times less than the 6 where this story began? Leighton and Murray wrote percent Adams and St. John had referred to as that "considerable quantities of water-ice "extreme desert conditions" 40 years earlier. permafrost may be present in the subsurface of the Furthermore, a slightly stronger carbon dioxide line polar regions" just a few tens of centimeters enabled a direct estimate of Mars's atmospheric down—permafrost that was finally discovered in pressure: 25 millibars (2.5 percent of Earth's 2002 by JPL's Mars Odyssey mission. surface pressure)—one-quarter of the best previous estimates. (Munch and his collaborators noted in passing that although their value for carbon dioxide Provided by California Institute of Technology was not itself surprising, "what would appear indeed surprising is that the . value for the atmospheric pressure [is] so low that CO2 itself becomes a major constituent"—entirely unlike Earth, where nitrogen and oxygen make up 99 percent of the air we breathe.) Based on these results, Mars was now officially as arid as the moon, and nearly as airless. Confirmation would follow in 1965, when JPL's Mariner 4 became the first spacecraft to visit Mars. The behavior of Mariner's radio signal as the spacecraft passed behind Mars revealed that its actual atmospheric pressure was lower still: 5 to 9 millibars, or less than 1 percent of Earth's. And the 20 televised pictures of Mars's cratered, moonlike surface—some shot from as little as 6,000 miles above it—cemented the comparison. Professor of Physics Robert Leighton (BS '41, MS '44, PhD '47), who had been the principal investigator on Mariner 4's Television Experiment, as it was called, and Associate Professor of Planetary Science Bruce Murray, a member of the TV team, would use Münch's and Mariner's data as cross-checks on a detailed thermal model of Mars that they wrote for Caltech's IBM 7094 mainframe computer—a pioneering feat in its own right. Their results, published in 1966, correctly predicted that most of Mars's carbon dioxide was actually not in the atmosphere, but instead lay locked up in the polar caps in the form of dry ice; the paper also made the unprecedented suggestion that seasonal advance of each polar cap would freeze out so much carbon dioxide that the atmospheric pressure 3 / 4 APA citation: 50 years of Mars observations (2014, May 8) retrieved 28 September 2021 from https://phys.org/news/2014-05-years-mars.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. 4 / 4 Powered by TCPDF (www.tcpdf.org).

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