Rover offers daily taste of Mars weather

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One of the primary instruments on the rover Curiosity is the Rover Environmental Monitoring Station, a weather station to help us better understand the climate of Mars.

We already know that the atmosphere and climate of Mars are both similar and different from those of Earth.

A day on Earth is 24 hours, while on Mars it is 24 hours 40 minutes, so both planets have daily weather cycles.

The axis of rotation of Mars is tilted only a little more than the Earth’s, 25.2 degrees for Mars versus 23 degrees for Earth, so both planets have seasons. But Mars is farther from the Sun and receives only about half the sunlight of Earth, so Mars is much colder. And because of the greater distance from the Sun, it takes about two Earth years for Mars to complete one orbit around the Sun, so the Martian seasons are twice as long.

The Earth’s orbit around the Sun is almost circular, whereas the orbit of Mars is more like an oval. This varying distance to the Sun during the orbit causes greater seasonal temperature changes. And there is no known liquid water on the surface of Mars—no rivers, no lakes, and no oceans.

Another difference in the atmospheres of these planets is their chemical makeup. The Earth’s atmosphere is composed of mostly nitrogen and oxygen, a roughly 80-20 split, whereas the atmosphere of Mars is 95 percent carbon dioxide with the other 5 percent being nitrogen and argon. Both atmospheres also contain small amounts of other gases.

Mars has a very thin atmosphere. We can measure the amount of atmosphere by the surface pressure on the planet. The average surface pressure on Mars is just 6.1 mbar, whereas on Earth it is 1013 mbar. This means that Mars has about one one-hundredth the atmosphere of Earth.

The REMS instruments are adding to our understanding by collecting data on atmospheric pressure, humidity, ultraviolet radiation at the surface, wind speed and direction, and air and ground temperature around the rover.

Most of the sensors are located in two booms that extend out from the rover mast at about 1.5 meters above the surface. Boom 1 has a set of wind sensors and the ground temperature sensor. Boom 2, which points in the driving direction, has wind sensors and the relative humidity sensor. Both booms have air temperature sensors.

Other weather sensors are located on other parts of the rover. The UV sensor is located on the rover deck, while the pressure sensor is located inside the rover body and is connected to the outside atmosphere via a tube. The total weight of the REMS system is less than three pounds.

Data from REMS has already helped scientists better understand transient whirlwinds (dust devils), track daily and seasonal changes in air pressure including thermal tides, and observe the relationship between changes in radiation and daily atmospheric changes.

In the first 12 weeks after landing, 20 atmospheric events were observed with at least one characteristic of a dust devil. Evidence for dust devils includes a brief dip in air pressure, a change in wind speed, a change in wind direction, a rise in air temperature, or a dip in UV light. Two of the 20 atmospheric events included all five of these indicators. These are the first surface measurements of these weather events on Mars.

Another observed weather event is daily thermal tides. Mars is a cold world, with highs this time of year in the 30s and lows near -100 degrees Fahrenheit. The cycle of higher pressure in the mornings and lower pressure in the evenings is caused by the daytime heating of the atmosphere by the Sun. This solar heating causes the atmosphere to expand and create greater surface pressure in the mornings, and as it cools in the evening the pressure decreases. With Mars’ daily rotation, this wave of expanded atmosphere moves around the planet.

It is now mid-spring at Gale Crater, Curiosity’s home, which is south of the Martian equator. In the coming months REMS will record the seasonal changes on Mars.

You can observe those changes, too. The latest Mars weather data can be found at marsweather.com/data. This Web page will give you high and low daily air temperatures, the wind speed and direction, and atmospheric pressure for any sol (Mars day), from landing to the most current data. The sunrise and sunset times are given in local Mars time.

Next time you want to start a conversation with someone about the weather, ask them about how cold it was yesterday on Mars.

Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at marty.scott@wallawalla.edu.

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