Humans have scribbled "We were here" all over the solar system. From footprints on the moon to burnt-out landers on Titan, the visible records of our excursions past Earth are badges of pride — not to mention unique opportunities to do science. "Aside from just a curiosity, it allows us to know exactly where all the lunar samples came from relative to one another," said planetary scientist Jeff Plescia of the Johns Hopkins University Applied Physics Lab, who uses the Lunar Reconnaissance Orbiter spacecraft to look for human artifacts on the moon. But our tracks won't last forever. A new paper in the Journal of Geophysical Research traces how tracks left by the Mars rovers Spirit and Opportunity are wiped away by the wind, usually within a Martian year. "It's a little humbling," said planetary scientist Paul Geissler of the U.S. Geological Survey, lead author of the new paper. "Mars will just clean up after us, and wait for the next visitors." This gallery takes a quick tour through the solar system through the visible marks we've left behind. Above: Mars Rover Looks Back One of the Mars exploration rovers looks back at where it's been. Image: NASA/JPL-Caltech/Cornell

Spirit Tracks Disappear The rover Spirit has been watched from orbit since the instant it landed in Gusev Crater in 2004. Although the wheels are barely more than 6 inches wide, Spirit's tracks around its landing site were clearly visible to the Mars Global Surveyor's cameras in early 2004. (Tracks are marked by white arrows above.) But when astronomers looked again in 2006 with the HiRISE camera on the Mars Reconnaissance Orbiter (which took over after the Mars Global Surveyor failed in late 2006), the tracks were completely gone. Some were replaced with dust devil tracks (marked "D" below). Image: NASA/JPL/Malin Space Science Systems/University of Arizona/AGU

Wind Wipes Opportunity Tracks Spirit's sister rover, Opportunity, has also been under constant surveillance since it landed in Meridiani Planum three weeks after Spirit's landing. Although the soil at Meridiani Planum didn't show marks as well as the dust-covered sand in Gusev Crater, the Mars Global Surveyor managed to see Opportunity's tracks in April 2004. It also caught a glimpse of the landed (which touched down perfectly in the middle of a tiny crater), the parachute and heat shield that the rover shed as it plunged through the atmosphere, blasts from the rover's landing rockets, and the rover itself (above). But as at Spirit's landing site, these tracks were mostly gone (below) by 2006. Rover scientists concluded that the tracks were mostly erased by wind over the course of a Martian year (687 Earth days), and not filled in with settling dust particles. Image: NASA/JPL/Malin Space Science Systems/University of Arizona/JGR

Martian Dust Chronicles A global dust storm on Mars in July 2007 showcased the destructive power of Martian winds. Spirit took these images through its rear hazard-avoidance cameras while it spent nearly an Earth month sitting still, waiting out the storm. The gales were so severe they contaminated Spirit's cameras with dust. The rover's tracks are clear and sharp in the top image. But 22 Martian days later, they were completely obliterated, filled in with new soil blown in from somewhere else. Image: NASA/JPL-Caltech/Cornell/JGR

Opportunity Knocks Three Times Oddly, Opportunity's tracks weathered the same dust storm much better. The rover sat out the storm at the edge of Victoria Crater, which the HiRISE camera captured in several stunning portraits. Streaks of dark sand crowned the crater's northern edge before the dust storm began (above, left), but were smudged out by the storm's end (above, middle). Over the next few months, the streaks returned to normal, as dark material from inside the crater blew up over the rim. But throughout all this, Opportunity's tracks remained mostly visible. They're still visible in HiRISE images from mid-2009. Geissler and colleagues think the tracks survived because the rover pushed dark hematite marbles called blueberries down into the soil as it drove, leaving a trail brighter than the blueberry-covered surrounding area that was harder to fill in. "It's easy to blow away or replace a layer of sand, but it's not so easy to replace those blueberries," Geissler said. "I'm guessing those tracks are going to remain visible, although faded, until we get enough erosion to exhume those blueberries." Opportunity crossed the same point three times in its tenure on Mars (below). The oldest tracks in this image are from late 2006, when Opportunity first arrived at Victoria Crater. The fresher tracks in the foreground are from August 2008, when the rover emerged from exploring inside the crater. Aside from a bit of color fading, the tracks look more or less the same. Images: NASA/JPL/Cornell/University of Arizona/JGR

Mars From Above This spectacular shot of Opportunity from orbit was captured by the HiRISE camera in 2006. The rover, its tracks and even the shadow of the camera mast are clearly visible (zoomed above, wide below). Members of the Mars rover team at Cornell hung a version of this image in the elevator of the Space Sciences building with a label saying, "You are here." Geissler happened to be in charge of driving Opportunity the day that image was downloaded. "Our experience of Mars has never been the same," he said. Images: NASA/JPL/Cornell/University of Arizona

Apollo Landing Sites Unlike Mars, the moon can potentially hold on to signatures of human activity for millions of years, Plescia said. The only thing that could bury astronaut footprints or rover tracks are tiny micrometeorites, which slowly churn up the lunar soil. "The weathering on the moon is incredibly slow," he said. "I think even the tracks will be there for a long time." In 2009, NASA's Lunar Reconnaissance Orbiter looked back at the Apollo landing sites — just in time for the 40th anniversary of the first human steps on the moon. You can even see footprints left by the Apollo 14 astronauts in 1971 (below). Image: NASA/Goddard Space Flight Center/Arizona State University

Lunokhod 2 LRO also photographed the final resting place of the Soviet lunar rover Lunokhod 2. This image from March 2010 shows the tracks the rover made trekking 23 miles during five Earth months in 1973. "We can't see all the little details, but the tracks are still so clear," Plescia said. "You can clearly see the wheel tracks over the whole traverse — backing up, turning around, doing different things." Below: a model of Lunokhod 2 in all its 1970s sci-fi glory. Image: 1) NASA/GSFC/ASU/Sergei Gerasimenko/Sasha Basilevsky.

2) Moscow University

Lunokhod 1 The Lunokhod 1 rover, the first robot to successfully trek across another world, was thought to be lost in 1971. But images from LRO (above) helped astronomers pinpoint its location in April 2010. Both Lunokhod rovers were equipped with special reflectors for lunar laser-ranging experiments, which test Einstein's general relativity by measuring the distance between Earth and the moon with extreme precision. When astronomers aimed lasers at Lunokhod 1, the reflection from its mirror was at least 5 times brighter than the signals from Lunokhod 2. "It’s got a lot to say after almost 40 years of silence," said U.C. San Diego physicist Tom Murphy in a press release. In our professional opinion, the Lunokhod rovers (below) may have been the most adorable robots ever sent into space. Images: 1) NASA/GSFC/Arizona State University.

2) NASA

Venera 13 The Soviet Venera 13 lander touched down on Venus in 1982. It was the seventh lander in the Venera series to successfully make a soft landing on the planet's surface. The first six either never left Earth or crashed. Despite temperatures of 855 degrees Fahrenheit and atmospheric pressures 89 times that of the Earth, Venera 13 survived and transmitted data for 127 minutes, more than an hour and a half longer than its planned lifetime of 32 minutes. The lander has probably been corroded by the extreme conditions in the Venusian atmosphere. Image: NASA/USSR